Groundwater Modelling to Assist Well-Field Design and Operation … › publications › technical2004 › tr27-04.pdf · 2005-10-14 · Groundwater Modelling to Assist Well-Field

Groundwater Modelling to Assist Well-Field Design and Operation for the ASTR Trial at Salisbury, South Australia

Paul Pavelic

1, Peter Dillon

1 and Neville Robinson

2 1 CSIRO Land and Water, Adelaide 2 School of Chemistry, Physics and Earth Sciences, Flinders University, Adelaide

CSIRO Land and Water Technical Report No. 27/04 September 2004

Copyright and Disclaimer © 2004 CSIRO To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO Land and Water.

Important Disclaimer: CSIRO Land and Water advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO Land and Water (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it.

Groundwater Modelling to Assist Well-Field Design and Operation for the ASTR Trial at Salisbury, South Australia Paul Pavelic1, Peter Dillon1 and Neville Robinson2

1 CSIRO Land and Water, Adelaide 2 School of Chemistry, Physics and Earth Sciences, Flinders University, Adelaide

Technical Report No. 27/04 September 2004

CSIRO Land and Water Page i

Acknowledgements

This work was made possible through the financial support of the City of Salisbury and the support of the project partners: United Water International, SA Water, Northern Adelaide and Barossa Catchment Water Management Board, Delfin Lend Lease, SA Land Management Corporation and the City of Salisbury. Technical information and feedback was provided by the following individuals: Stuart Lane, Mark Purdie, Chris Kaufmann and Colin Pitman (City of Salisbury), Nabil Gerges (NZG Consultants), Zac Sibenaler (AGT Consultants) and Kelly Wescombe, Stephanie Rinck-Pfeiffer and Uwe Kaeding (United Water International) and John Van Leeuwen (CSIRO Land and Water). The layout of the six-well system was conceived during initial discussions with Nabil Gerges.

CSIRO Land and Water Page ii

Executive Summary “Aquifer Storage Transfer and Recovery” (ASTR) describes a new concept intended to be trialled at the Greenfields Railway Station site at Salisbury, SA. This will establish if wetland-treated urban stormwater injected into a brackish aquifer can be recovered from separate wells to create safe and reliable drinking water supplies.

This study has involved the use of groundwater flow and solute transport modelling techniques to identifying the optimal number and arrangement of injection and recovery wells for a proposed ASTR trial at an operational scale of 400-600 ML/yr. The design of the well-field must take into account two constraints: 1) that the salinity of the recovered water does not reach levels that would limit its productive use, and 2) that the effective residence time in the aquifer is sufficient for the required attenuation of pathogens and organics to meet the Australian drinking water guidelines.

Simulations with the FEFLOW groundwater simulation package were performed to explore alternate well-field designs involving two-, four- and six-well arrangements. A six-well system, consisting of four injection wells positioned in a diamond shape, and two inner recovery wells, was found to meet the mixing and residence time constraints over the long term at full-scale with inter-well separation distances of 75 and 100 m (values of 50 and 150 m were also tested). The analysis also took into account:

• the volume of water available from the wetland • the timing of the demand for recovered water • an injection sequence to maximise flushing of the ‘transfer zone’ in the first stage of

operation • uncertainties in aquifer properties (notably porosity and dispersivity) • the effect of operational scheduling on travel time • the orientation of the well field with respect to regional groundwater flow • the effect of the ambient hydraulic gradient • the ratio of recovered to injected volumes • the relative efficiency of an alternative six-well arrangement

A semi-analytical model was also developed based on the Theiss solution for well drawdown and tracking of moving fronts of injected and recovered water. Comparisons between the numerical and semi-analytical methods demonstrate that the FEFLOW results are accurate in defining fronts at all stages of injection and recovery. The semi-analytical method is a useful and robust tool for assessing the distribution of injected water bodies in confined aquifers for a range of well configurations.

Simulations of measured solute breakthrough data from an observation well at the nearby Parafield ASR site suggest the target aquifer is heterogeneous, and that the effective porosity may be smaller than the base-case used to model the ASTR well-field. If so, this would imply shorter travel times to recovery wells and larger volumes of water to flush ambient groundwater from the transfer zone.

A separation distance of 75 m is recommended to best address both constraints. The proposed layout for the ASTR well-field at Salisbury is presented. Recommendations are given on the location of, and principles for siting observation wells, and on the next stage of groundwater modelling to deal with a range of potential operational issues and to assist in the interpretation of the trial and ASTR management over the longer-term.

CSIRO Land and Water Page iii

Table of Contents Acknowledgements i Executive Summary ii List of Figures iv List of Tables v 1 INTRODUCTION 1 2 REVIEW OF RELEVANT LITERATURE 2 3 SITE DESCRIPTION 4 4 WELL–FIELD DESIGN CRITERIA 6

4.1 Definition of two primary constraints 6 4.1.1 Salinity 6 4.1.2 Residence time 7

4.2 Water availability constraints 7 5 METHODS OF ANALYSIS 9

5.1 Calculation of minimum and effective travel times 9 5.1.1 Minimum travel time 9 5.1.2 Effective travel time 9

5.2 Numerical methods 10 5.2.1 Simulation package 10 5.2.2 Conceptual model 10

6 RESULTS AND DISCUSSION 12 6.1 Two-Well systems 12 6.2 Four-Well systems 16 6.3 Six-Well systems 18

6.3.1 Separation distance 18 6.3.2 Flushing volume 21 6.3.3 Increasing the proportion of water recovered 22 6.3.4 Rectangular versus rhombic configuration 23 6.3.5 Effect of well-field orientation relative to the direction of regional groundwater flow 23

6.4 Effect of injection and recovery scheduling on effective residence time 24 6.5 Uncertainties in aquifer characteristics 25 6.6 Analysis of solute breakthrough data from Parafield ASR scheme 27

7 CONCLUSIONS 30 8 RECOMMENDATIONS 31 9 REFERENCES 34 10 APPENDIX: Verification of FEFLOW 36

CSIRO Land and Water Page iv

10.1 Breakthrough of injected water at observation wells 36 10.2 Recovered water quality changes with time 37 10.3 Comparison with semi-analytic method 38

List of Figures Figure 1. Schematic representation of ASR and ASTR........................................................... 1 Figure 2. Site location map showing the Greenfield Railway Station study site

and the nearby Parafield and Greenfields North ASR sites .............................................. 5 Figure 3. Schematic illustration of effect of salinity and travel time constraints

on viable separation range between injection and recovery wells..................................... 6 Figure 4. Plan view of mesh design with expanded view of study area

(dark circular patch) showing location of wells for the 100 m separation case................ 11 Figure 5. Schematic layouts of the three well-configurations tested (spacing for each

configuration indicated in the key; regional groundwater flow is from right to left) .......... 12 Figure 6. Flow paths and travel times for two-well system, 200 m separation with

continual injection and recovery (no regional gradient) ................................................... 13 Figure 7. 2-well system breakthrough curves at recovery well and two observation

wells along the shortest flow path, 200 m separation with continual injection and recovery........................................................................................................................... 14

Figure 8. 2–well system breakthrough curves at recovery well for four scenarios (for the cyclic scenario injection is indicated by grey lines, recovery by black lines) ....... 15

Figure 9. 2-well system breakthrough curves of reactive tracer at recovery well for two scenarios ............................................................................................................. 15

Figure 10. 4-well system solute distributions for four scenarios: a) injection in 3 perimeter wells and no recovery, b) injection in 3 perimeter wells and recovery from central well at 5 L/s, c) injection in 3 perimeter wells and recovery from central well at 25 L/s, and d) injection into all wells ......................................................... 17

Figure 11. 4-well system breakthrough curves at recovery well for the four scenarios ......... 17 Figure 12. Effective travel time versus mixing fraction (year 3 onwards) for 50,

75, 100 and 150 m separations (the bars identify the maximum and minimum times and concentrations; grey shading indicates where both constraints are met) ....... 19

Figure 13. Solute distribution at two stages of flushing mode (125 and 365 days) and two stages of operational mode (1095 and 3650 days)............................................ 20

Figure 14. Steady-state piezometric head distribution during injection (left) and recovery (right) (contour interval = 2m; wells are indicated by hollow circles; note that the ambient piezometric heads is ~0 m at the ASTR site and varies from ~ +3 m on RHS to –4.5 m on LHS of figures) .............................................................................. 20

Figure 15. Tracer concentration and effective travel time isofringes late in the injection phase (a) and late in the recovery phase (b). Plot (c) shows the annual cycle of travel times at the recovery well. ..................................................................................... 21

Figure 16. Effect of volume of water injected in first year on recovered water salinity .......... 22 Figure 17. Effect of recovering 100% of injected volume (400 ML) on recovered water

quality .............................................................................................................................. 22

CSIRO Land and Water Page v

Figure 18. Effect of rhombic versus rectangular layout on recovered water quality ............... 23 Figure 19. Effect of orientation and magnitude of well-field relative to groundwater

flow direction on average f from year 4 onwards (maximum and minimum values indicated by the ‘error’ bars) ............................................................................................ 24

Figure 20. Effect of increasing and decreasing effective porosity relative to base-case value of 0.25 on recovered water quality (75 m well-separation) .............................................. 26

Figure 21. Effect of increasing and decreasing aquifer dispersivity relative to base case value of 5 m on recovered water quality (75 m well-separation)............................. 26

Figure 22. EC breakthroughs at the Parafield site over 12 month period since the commencement of ASR................................................................................................... 27

Figure 23. Measured and predicted solute breakthroughs for three values of porosity at the 50 m observation well (RN 20741) ........................................................................ 29

Figure 24. Measured and predicted solute breakthroughs for three values of dispersivity at the 50 m observation well (RN 20741) ..................................................... 29

Figure 25. Proposed layout of ASTR well-field at the Greenfields Railway Station site ................................................................................................................................... 31

Figure 26. Proposed location of observation wells and their functions in relation to the injection and recovery wells................................................................................... 33

Figure 27. Comparison of analytically and numerically determined breakthrough curves at a distance of 50 m from the ASR well.............................................................. 36

Figure 28. Comparison of analytically and numerically determined solute changes in the recovered water ..................................................................................................... 37

Figure 29. Comparison between FEFLOW solute isofringes and semi-analytical fronts for 6-well system at 125, 365, 1095 and 3650 days .............................................. 38

List of Tables Table 1. Model parameters and values used in the baseline simulations ............................. 11 Table 2. Effective travel times for base- and compressed-case injection/recovery

schedules (the rest period refers to the intervening period between injection and recovery in any given year) ............................................................................................. 25

CSIRO Land and Water Page 1 of 38

1 INTRODUCTION “Aquifer Storage Transfer and Recovery” (ASTR) describes a concept that is about to be trialled on the Northern Adelaide Plains to demonstrate if subsurface treatment of wetland-treated urban stormwater injected into an aquifer when recovered from wells can create safe and reliable drinking water supplies. Unlike ASR, which relies upon the same well for injection and recovery, the groundwater flow system established through ASTR offers more uniform residence time and travel distance in the aquifer, which is thus likely to lead to more predictable levels of chemical and microbial attenuation of contaminants necessary for the provision of water of potable quality (Figure 1).

ASR ASTRASR ASTR

Figure 1. Schematic representation of ASR and ASTR

One of the issues in establishing the project is to identify the optimal number and arrangement of injection and recovery wells that meet the various hydrogeological, operational and regulatory constraints at the site selected for investigation. Groundwater modelling was undertaken to test a range of possible ASTR well-field designs. An established numerical flow and solute transport model (FEFLOW) was used. Also a new semi-analytical model was developed specifically for this project to validate the FEFLOW results. The semi-analytical model involves tracking of movement of injected and recovered water fronts. Some key results from these two independent methods will be compared.

This report covers: • literature on movement and mixing of injected waters in aquifers • hydrogeological characteristics of the study site • local and external constraints that control feasibility of a given well-field design • development of a numerical model of the study site and its use to explore and select the

most suitable well-field design • sensitivity to various hydrogeological and operational factors and uncertainties on

feasibility of most suitable design • testing of key aquifer parameters used in this study through calibration at a nearby ASR

site • recommendations on further work

These sections are followed by a summary of the main findings and recommendations for advancement of the ASTR trial.


2 REVIEW OF RELEVANT LITERATURE Although ASTR offers a new way of conducting enhanced aquifer recharge, it is also true that there have been numerous related field and modelling studies that are of interest, and which have either directly or indirectly influenced this study. What follows is a description of those considered most relevant to this study.

Two-well tracer tests have long been conducted to assess aquifer parameters and processes. The most common approach involves pumping from one well and injection into another, whereby a tracer is added to the injectant stream once steady-state conditions have been established and monitoring occurs at observations wells and in the recovered water. For instance, Molz et al., (1986) conducted a two-well tracer test with a 38 m separation at the Mobile site in Alabama to better understand advective and dispersive processes in a sandy aquifer. McCarthy et al., (1986) studied the mobility of natural organic matter (NOM) present in a shallow coastal aquifer in South Carolina using a injection well and recovery well 5 m apart. Mean residence times and mass balances were used to estimate NOM removal and retardation along a transect of six sampling locations at two different depths and at the recovery well.

Studies conducted in Israel by Harpaz and Bear, (1963) and Bear and Jacobs, (1965), amongst others, have significantly extended our understanding of processes governing the migration and mixing of injected water plumes. Laboratory experiments involving Hele-Shaw models provided useful analogues for exploring the distribution of single and multiple interacting injected plumes subjected to pumping and regional flow. The considerable effort required for these experiments motivated the development of analytical models which built upon earlier studies for predicting the shapes of injected water bodies, travel times and solute breakthroughs for various well-arrangements and operational schedules. The techniques presented were found to be useful in field trials in the evaluation of aquifer properties. The conceptual and theoretical work by Bear and co-workers, which may be found in Bear, (1979), is the basis for the semi-analytical modelling being developed for this study, and further details are provided in the Appendix of this report.

Merritt, (1986) explored the factors affecting the recovery efficiencies during ASR of fresh water into a saline limestone aquifer through the use of a finite difference model that was developed as a part of the study. Various arrangements of ASR wells were tested, although results were only given for a 5-well, cross-hair arrangement. He demonstrated that the pattern of fresh water in the aquifer was dependent upon which of the wells the water was pumped into, and similarly, the distribution at the end of an ASR cycle differed according to which wells were pumped. During particular combinations of injection and recovery residual fresh water remained in isolated zones between the wells after the extracted water had become too saline for potable usage. The recovery efficiency for the case where all wells were pumped was lower than the case where only the central well was pumped by a factor of six percent. This was in the absence of a regional hydraulic gradient. When the central well was first targeted followed by the peripheral wells the recovery efficiency was 10 percent higher than when injection occurred in all wells simultaneously, whilst the residual injectant in the aquifer was noticeably more contiguous.

Trefry and Johnston, (1996) used the FEFLOW package to design efficient strategies for controlled injection of nutrient-rich solutions to stimulate microbial activity of a shallow aquifer contaminated by hydrocarbons. Schemes comprised of one-, two-, three-, four- and nine-wells were tested to identify the best ‘curtain’ of bioclogging agents with minimal outlay of amendment solution and hydrologic disturbance. Not surprisingly, it was found that the four- and nine- well systems yielded the most effective distributions of amendment, but at greater installation and operating costs than simpler systems.


Dillon et al., (2002) developed an approach for assessing the feasibility of alternative riverbank filtration schemes in alluvial sediments adjacent to surface water bodies, taking into account the removal of cyanobacterial toxins and the degree of mixing between fresh river water and more saline ambient groundwater. Using analytical expressions that predicted the minimum travel time between the surface water and the well, and the proportion of surface water extracted from the production well, a site was identified along the River Murray that met both criteria and minimised pumping costs. As will be shown later, these objectives are not too dissimilar to those of this study.

Fildebrandt et al., (2003) used a similar analytical approach to arrive at the appropriate separation distances between injection and recovery wells in the Bandung Basin in Indonesia, taking account of the travel time needed to achieve a given level of pathogen attenuation. A nomogram was developed that allowed prediction of the number of pathogen log removals from any set of design parameters. Miller et al., (2002) developed the ASR Risk Index (ASRRI), a computer program that evaluates the risk of contamination from trace organic or microbial contaminants based on the formulations and assumptions also used by Fildebrandt et al.

Dillon et al., (in press) evaluated some of the previously unreported data from the Talbert Gap saline intrusion barrier project in Orange County, California where 23 multi-completion wells have been installed along a transect adjacent to the coast since the mid 1970’s to protect adjacent groundwater resources. A blend of tertiary-treated reclaimed water and deep groundwater was injected into a complex multi-aquifer system and tracked during transport by matching temporal perturbations in chloride concentration in the injectant the travel times at observation wells over the 850 m span to the recovery well. This provided the basis for an analysis of the fate of injected contaminants.

Stuyfzand et al., (2002) investigated the geochemical processes arising from prolonged injection of potable water into a deep sandy pyrite-rich aquifer and its recovery 98 m away at the Someren site in the Netherlands. The oxic zone around the injection well, which developed following 2.5 years of uninterrupted flushing of the aquifer with injectant, vanished due to reactions with pyrite and organic matter with associated leaching of iron and ammonium. Deposits of iron(hydr)oxides and organic matter around the injection well led to some reversible well clogging.


3 SITE DESCRIPTION The study site is adjacent to the Greenfields Railway Station, situated in the northern metropolitan area of Adelaide in South Australia on parcels of land largely administered by the City of Salisbury (Figure 2). This site was chosen ahead of two nearby sites at Parafield and Greenfields North after weighing up considerations such as land use and availability, the number of existing monitoring wells, and the potential for interactions with existing ASR operations (Rinck-Pfeiffer, 2004; Rinck-Pfeiffer et al., 2004).

The aquifer targeted for study is known locally as the "T2" aquifer, the second of several Tertiary marine-deposited formations continuous across the Adelaide Plains and is composed of fossiliferous and marly limestones through to siliceous calcarenite (Gerges, 1999). Locally, the T2 aquifer is 52 m thick and encountered at depths of between 154 and 206 m below ground surface. The transmissivity is moderate (125 m2/day), with available evidence suggesting that flow is porous rather than through fissures or karst. However the aquifer is known to be heterogeneous with respect to depth, although the major layers may be continuous over the distances of tens to hundreds of metres (Pavelic et al., 2001). The effective porosity of the aquifer is assumed to be 0.25, based on a previous modelling study by NZG Consultants (Rinck-Pfeiffer et al., 2004). The ambient groundwater is brackish, with a salinity of ~1,900 mg/L TDS measured at the existing well RN 20328 (Figure 2).

The regional groundwater flow direction has been examined using data by Gerges, (1999), Zulfic, (2002) and other DWLBC potentiometric surface maps which reveal flow is approximately from due east to due west at an average hydraulic gradient of 0.0015. For the hydraulic data given above, this translates to an ambient groundwater velocity of 5 m/yr. Flow direction appears to be independent of season and sufficiently removed from the nearest T2 irrigation wells (10 km NW) and the Regency Park Golf Course and Coopers Brewery T2 wells (8km SW) to negate any major fluctuations from pumping centres (Hodgkin, pers. comm.). The Parafield ASR scheme (~1km NE) exerts a strong local influence and is likely that the this will dominate the flow direction at the study site. During injection at Parafield flow is towards the southwest and during recovery to the northeast. A local gradient of up to ~0.008, a factor of five higher than the regional gradient, was measured between an observation well at Parafield (RN 20741) and the existing T2 well at the Greenfields Railway Station (RN 20328) in winter 2003 when both ASR wells were injecting stormwater. (Note the Greenfields ASR scheme, 1 km SE, targets the T1 aquifer which is considered to be hydraulically isolated from T2, and has yet to commence operation).

Dispersivity is assumed to be 5 m for the dimension of interest for the plumes of injectant. This possibly conservatively over-estimates the extent of mixing and analyses for ASR systems in similar environments (Pavelic et al., 2002) and is a typical literature value over the 50-200 m scale range (Gelhar and Collins, 1971).


Figure 2. Site location map showing the Greenfield Railway Station study site and the nearby Parafield and Greenfields North ASR sites

##

#

##

#

#

#

# # ###

##

T2 Obs 18251

T2 Obs 18250

T2 Product ion 17760

T1 Obs 17761T2 Obs 18248

T2 Obs 18249

PW2 (T2) Well No. 20943

OW Q1 Well No. 20739

OW T1 Well No. 20742

REEBED

HOLDINGSTORAGE

IN-STREAMBASIN

PS1 PS2

SALIS

BURY

POR

T W

AK

EF

IEL D

RYANS

MAWSON LAKES

ELDER

RYANS

SALI

SBUR

Y

LANCASTER

BRAD

MAN

BARD

SLEY

OLDFIELD

CATALINA

MAR

TINS

GEO

RGE

BELL

FREE

GREENFIELDS

BENNETT

CRO

SS K

EYS

MAI

N N

ORTH

WYA

TT

SHEPHERDSON

JACK

SON

PW1 (T2) Well No. 20743


T1 Well No. 20329

T2 Well No. 20328

##

PARKDALE

# Parafield - Mawson LakesReclaimed Water Pipeline

Bolivar - Mawson LakesReclaimed Water Pipeline

T1 Well No. 16624

Greenfields NorthWetland PumpingStation

T1 Well No. 16625

Greenfields North Wetland

Mawson Lakes Sanctuary

Greenfields Railway Station

Mawson LakesRecycled WaterMixing Tank

Greenfields South Wetland

Mawson Lakes

Mawson Lakes

Parafield StormwaterHarvesting Facility

University of South Australia

N - S

Rai l

way

# RL 3.0

#RL 11.0

CadastreFuture Land DivisionsStreetsWater MainsCity of Sal isbury Land

0 1 KilometersN

EW

S

Figure 1 - Stormwater Harvesting and Supply Works and Groundwater Well LocationsParafield, Greenfields & Mawson Lakes

City of Salisburyre

Greenfields North ASR site


Parafield ASR site

##

#

##

#

#

#

# # ###

##

T2 Obs 18251

T2 Obs 18250

T2 Product ion 17760

T1 Obs 17761T2 Obs 18248

T2 Obs 18249

PW2 (T2) Well No. 20943

OW Q1 Well No. 20739


REEBED

HOLDINGSTORAGE

IN-STREAMBASIN

PS1 PS2

SALIS

BURY

POR

T W

AK

EF

IEL D

RYANS

MAWSON LAKES

ELDER

RYANS

SALI

SBUR

Y

LANCASTER

BRAD

MAN

BARD

SLEY

OLDFIELD

CATALINA

MAR

TINS

GEO

RGE

BELL

FREE

GREENFIELDS

BENNETT

CRO

SS K

EYS

MAI

N N

ORTH

WYA

TT

SHEPHERDSON

JACK

SON

PW1 (T2) Well No. 20743


T1 Well No. 20329

T2 Well No. 20328

##

PARKDALE

# Parafield - Mawson LakesReclaimed Water Pipeline

Bolivar - Mawson LakesReclaimed Water Pipeline

T1 Well No. 16624

Greenfields NorthWetland PumpingStation

T1 Well No. 16625

Greenfields North Wetland

Mawson Lakes Sanctuary


Mawson LakesRecycled WaterMixing Tank

Greenfields South Wetland

Mawson Lakes

Mawson Lakes

Parafield StormwaterHarvesting Facility

University of South Australia

N - S

Rai l

way

# RL 3.0

#RL 11.0

CadastreFuture Land DivisionsStreetsWater MainsCity of Sal isbury Land

0 1 KilometersN

EW

S

Figure 1 - Stormwater Harvesting and Supply Works and Groundwater Well LocationsParafield, Greenfields & Mawson Lakes

City of Salisburyre

Greenfields North ASR site


Parafield ASR site


4 WELL–FIELD DESIGN CRITERIA 4.1 Definition of two primary constraints

For the purposes of this study local considerations were reduced to just two primary constraints: 1) that the recovered water meets its target quality in terms of the maximum permissible salinity; and 2) that sufficient residence time occurs in the aquifer between injection and recovery wells to allow contaminants to degrade to levels below the regulated concentrations (other constraints are dealt with independently in section 4.2).

Identifying the viable separation distance between the injection and recovery wells under these two conditions must balance between the need to keep the separation small in order to flush the aquifer in the transfer zone with fresh water and to expand the separation to extend travel time to allow adequate time for contaminant attenuation. In many cases it is possible to find a viable separation range where both constraints are met, as demonstrated in Figure 3.

Separation between injection and recovery wells (m) 0

TDScontaminant concentration

Allowable TDS conc.

Allowable contam. conc

InjectantTDS

Native g/w TDS

Viable

rangeseparation

Separation between injection and recovery wells (m) 0

TDScontaminant concentration

Allowable TDS conc.

Allowable contam. conc

InjectantTDS

Native g/w TDS

Viable

rangeseparation

Figure 3. Schematic illustration of effect of salinity and travel time constraints on viable separation range between injection and recovery wells

4.1.1 Salinity If the salinity of the injectant is Cinj and the ambient groundwater is Camb, and that these concentrations are sufficiently different such that the proportion of injectant present in any mixture in the groundwater or recovered water (Cmix), otherwise known as the "mixing fraction", f can be estimated from the following mass balance equation:

injamb

mixamb

CCCC

f−−

= (1)

Given that the proposed injectant (recycled stormwater from Parafield wetland), has an average of 150 mg/L TDS; the local ambient groundwater is 1,900 mg/L TDS; and the maximum permissible concentration for the recovered water is 300 mg/L TDS, the minimum permitted value of f is ~0.9. That is, a mixture which contains no less than 90% injectant (no more than 10% ambient groundwater) can be tolerated before the salinity of the recovered water becomes excessive. The 300 mg/L limit has been established to meet acceptance criteria for recovered water entering the Mawson Lakes Mixing Tank.


4.1.2 Residence time

Urban stormwater runoff may contain a variety of constituents that can be of concern to human health or the environment. The broad classes include nutrients, metals, microbial pathogens, algal toxins, and trace organic compounds such as pesticides, pharmaceuticals, hormones and endocrine disruptors. From a reuse perspective, the potential risks associated with these constituents to public health and the environment must be considered.

For the purposes of this study, microbial pathogens represent the single greatest risk with respect to protection of human health and will be used as a benchmark for a broader range of contaminants. Survival of the pathogenic microorganisms such as bacteria, viruses, protozoan cysts and helminth eggs, introduced into groundwater diminishes with residence time because they are filtered, adsorbed, die-off or are degraded by native microorganisms to varying degrees in different hydrogeological settings (Pavelic et al., 1996; Toze and Hanna, 2002). Microbial levels reduce exponentially in aquifers. Typical inactivation rates in groundwater, defined as the time for a one-log removal (90% reduction), are in the order of 3 to 6 days for indicator bacteria (eg. E. coli ), but may be higher for other bacteria, and between 5 to 30 days for viruses. The effectiveness of aquifers to remove microbial pathogens depends on a wide range of complex and poorly understood factors. Studies are currently underway that address gaps in knowledge (eg. Toze and Hanna, 2002).

Thus public health can be safeguarded by ensuring an adequate minimum residence time of the injected water in groundwater. For example, in Europe it is common practice to allow 50 days residence time in groundwater between points of recharge of undisinfected water and points of extraction for potable supplies (van Waegeningh, 1985). In this case, a minimum average residence time of the injectant in the groundwater system of ten months (300 days) is proposed in order to ensure at least several log removals of the most persistent organisms. Whilst this criterion is perhaps overly-conservative, given typical die-off rates and criteria in use elsewhere, such a barrier was proposed from a risk management perspective until local data is collected that may allow it to be relaxed (Swierc et al., in prep.). For example, other constituents that may be present in stormwater may biodegrade more slowly than pathogens. This also allows a realistic time-frame for sampling and analysis of groundwater from intermediate observation wells to provide an early warning in the event of unforseen problems. A conservative approach also allows for a margin of uncertainty in aquifer characteristics, most particularly due to preferential flow, which causes faster-than-average- movement of water and contaminants through discrete parts of the aquifer.

4.2 Water availability constraints

An external constraint that strongly influences ASTR viability is the availability of water. The volume of surplus recycled stormwater for injection during winter months is determined by seasonal rainfall variability and demand for this water by other users.

There are two demands on the water from the Parafield wetland: Mawson Lakes Mixing Tank (non-potable urban supplies using stormwater shandied with reclaimed water from the Bolivar plant) and G.H. Michell and Sons (wool processing using stormwater). The Parafield Pipeline will transport recycled stormwater from the Parafield and Greenfields wetlands to Mawson Lakes (Figure 2). Flows to ‘Michell’ are transported from the Parafield wetland via an independent pipeline.

Wescombe and Furness, (2004) recently undertook a hydraulic modelling study of flows through the Parafield pipeline for four different scenarios encompassing ‘normal’ operations,


maximising ASTR inputs and outputs, and when either only Parafield or Greenfield wetlands are on-line. They identified that for the pumps selected, the pipeline has the capacity to transport a peak flow of 77 L/s from the Parafield wetland (with an operating pressure of 47 m during winter), which could be brought up to 154 L/s if the Greenfields wetland is also brought on-line. Further, the pipeline could handle a scenario of 400 ML/yr of injection at a rate of 31 L/s over a 150 day period and 320 ML/yr recovery also at a rate of 25 L/s over a 150 day period.

Given that the hydraulic modelling of Wescombe and Furness, (2004) only took into account the pipeline capacity, the City of Salisbury provided estimates of the volumes of water available for injection, drawn largely from the “Parafield drain scheme hydrologic study” by Richard Clark and Associates, (2001). An initial estimate that was used in many of the simulations, but which was subsequently revised, suggested that 1000 ML of water would be made available in the first year of operation, followed by around 400 to 600 ML/yr in subsequent years as demand on the stormwater increased.

Re-evaluation of the figures by City of Salisbury indicated the figure for the first year was an overestimate, and that only around 500 to 600 ML/yr would be available for ASTR in an average rainfall year. Only with engineering improvements to the Parafield and Greenfields systems could the volumes of water reach 1000 ML/yr, and even if achieved, is unlikely to be available in the initial stages of the trial.


5 METHODS OF ANALYSIS

5.1 Calculation of minimum and effective travel times

5.1.1 Minimum travel time

For a system involving one injection well and one recovery well in the presence of a uniform regional groundwater flow field, and where it is assumed that the rates of injection and extraction are equal and resultant local flow system has reached steady-state, the minimum residence time of injected water in the aquifer (tmin) is given by the following analytical equation (Miller et al., 2002):

do

e

vDLQ

Lnt

±=

π3min (2)

where:

tmin = minimum time for water to pass from injection to recovery well (days) ne = effective porosity of aquifer (m3/m3) L = distance from injection to recovery well (m) Q = rate of injection and recovery (m3/day) D = aquifer thickness (m) vdo = component of flow due to Darcian velocity in aquifer, positive in the direction from the injection well towards the recovery well (m/day)

Considering Eqn. (2) firstly without the effect of the regional gradient, it is apparent that tmin increases proportionally with porosity and the square of the separation distance and is inversely proportional to the flow rate. Regional effects can either increase or decrease tmin depending on the location of the wells with respect to the flow direction.

Whilst the minimum travel time offers a useful criterion, its practical benefits are limited since verification relies on having observation well data suitably located along the shortest groundwater flow path. Further, it is recognized that the water extracted from a recovery well is derived from numerous converging flow paths which provide an averaged “effective” travel time.

5.1.2 Effective travel time

The effective travel time was calculated in the numerical model by assuming a tracer is present at some defined concentration in the injected water but is not present in the ambient groundwater, and is subject to simple exponential decay, ie:

C(t) / C0 = exp (-λteff) (3) where:

teff = effective time for water to pass from injection to recovery well (days) λ = decay rate constant ( /day) C(t) = tracer concentration at some time, t C0 = tracer concentration in injectant


Consequently, concentrations can be converted to residence times by rearranging Eqn. (3) to solve for teff after steady-state or quasi steady-state conditions have been reached over the domain where C(t)>0.

Simulations were performed using an input concentration of 1.0 and a decay rate of 6.93x10-3

/day (equivalent to a half-life of 100 days) since this provided sufficient resolution over time-scales of interest and the outputs were sufficiently intuitive so that a visual assessment of the results could be made without the need to transform the data from concentration to travel time.

5.2 Numerical methods

5.2.1 Simulation package

The finite-element model FEFLOW (Version 5.1) was chosen to simulate flow and solute transport processes at the Greenfields Railway Station site (Diersch, 2004). FEFLOW is a three dimensional finite-element package capable of simulating contaminant and heat flow and transport. FEFLOW also has a built-in grid-design, problem editing and graphical post processing display modules that allow rapid model development, execution and analysis. A 32-bit SUN SPARC workstation running operated remotely via the X-Vision 97 interface to a PC was used as the hardware platform for the numerical simulations.

5.2.2 Conceptual model

The aquifer was modelled in 2D. The model domain is 10,000 m x 10,000 m in plan view, with the ASTR operation situated at its centre (Figure 4). The large overall scale of the domain arises from the desire to keep boundary effects to an absolute minimum and the ease with which this can be achieved in FEFLOW. Due north is towards the top margin for convenience. The constant head boundaries were set on the left (-7.5 m) and right side boundaries (+7.5 m) to reflect the average regional hydraulic gradient from east to west of 0.0015. The upper and lower margins are no-flow boundaries.

The finite element mesh was designed to minimise numerical dispersion by ensuring that the major flow paths associated with injection/recovery wells are where mesh sizes are smallest and gradually diminish towards the model boundaries (Figure 4). Additional mesh refinement was carried out in the area occupied by the injected water plume.

In the absence of local hydraulic data the aquifer was assumed to be homogeneous with a uniform thickness of 52 m. The ASTR wells are fully penetrating. Possible vertical leakage into and out of the aquifer was ignored. In section 4.1.1 the salinity contrast between the two end-member waters was shown to be insufficient to warrant inclusion of density effects. Temperature changes are also ignored.

Time-varying flux boundaries are specified at the ASTR wells to reflect the divergent, quiescent and convergent flow conditions associated with the different stages of the operating regime. Time-varying concentration boundaries at the ASTR recovery wells were set as inactive during ‘rest’ and recovery periods to deal with concentration changes. Simulations are transient and cover a period of 10 years.

Table 1 gives the baseline aquifer parameter values used in the simulations. Prior to conducting the simulations a representative model was verified against two analytical solutions, as detailed in sections 10.1 and 10.2 of the Appendix. Front lines generated by the


semi-analytical method are compared with FEFLOW simulations at several stages of an ASTR operation are also given in section 10.3 of the Appendix.

Figure 4. Plan view of mesh design with expanded view of study area (dark circular patch) showing location of wells for the 100 m separation case

Table 1. Model parameters and values used in the baseline simulations

Parameter Value

Aquifer thickness (b) (m) 52 Hydraulic conductivity (K) (m/d) 2.5 Transmissivity (T ) (m2/d) 130 Anisotropy ratio (x:y) 1.0 Storage coefficient (S) 2x10-4 Porosity (n) (m3/m3) 0.25 Longitudinal dispersivity (αl ) (m) 5.0 Transverse / longitudinal dispersivity ratio 0.1 Molecular diffusivity (Dm) (m2/s) 10-9 Injection and recovery rates (Qinj/rec) (m3/d) 2160 Regional hydraulic gradient (∂h/∂x) 0.0015 Period of injection (d) Vinj / x.Qinj

+ Period of recovery (d) Vrec / x.Qrec

+

+ Vinj/rec = total volume of water injected or recovered and x = number of injection or recovery wells


6 RESULTS AND DISCUSSION Three different well-field designs, comprising of two-, four- and six-well systems, are evaluated (Figure 5). Simulations of the two- and four- well systems are presented first and are mostly of an exploratory nature to establish a basic understanding on hydrodynamic processes before moving on to the more complex interactions associated with the six-well systems. The most detailed analysis, including realistic representations of operational scenarios for the ASTR trial, is reserved for the six-well system.

Figure 5. Schematic layouts of the three well-configurations tested (spacing for each

configuration indicated in the key; regional groundwater flow is from right to left)

6.1 Two-Well systems

A two-well system is an obvious starting point for understanding flow and transport processes as convergence towards an appropriate ASTR well-field design is reached (Figure 5a). The injection and recovery wells were positioned 200 m apart. Injection and extraction was at equal rates of 25 L/s and on a continuous basis. Initially the regional groundwater flow component was assumed to be zero.

Figure 6 presents the steady-state flow field along with an overlay of the evolving injected water fronts as a function of time, generated by the method of particle tracking based on advective velocity distributions (ie. assuming no dispersion nor diffusion). Steady-state conditions develop quickly - within a period of hours, and hence most of the simulations are for uniform flow conditions. As the injectant moves from source to sink the steady-state flow lines are symmetric about the axis which intersects the two wells. Path lines increase in curvature and length with distance perpendicular to this axis.

The injected water fronts change in shape with increased residence time. In plan view, water less than approximately several months old is essentially circular in shape, water less than around one year old is tear-shaped, and older water still returns to an approximately circular shape as it gradually envelopes the recovery well within three years.

By Eqn. (2) the minimum travel time is determined to be 252 days, which compares well with 251 days determined with FEFLOW, a difference of <0.5%. The effective residence time from

xa) 2-well c) 6-well

z

z

z

z

z

zz

injection wellrecovery well

x = 200 my = 150 m

z = 50,75,100,150 mSeparation distances tested:

b) 4-well

y

yy

xa) 2-well c) 6-well

z

z

z

z

z

zz

injection wellrecovery well

x = 200 my = 150 m

z = 50,75,100,150 mSeparation distances tested:

b) 4-well

y

yy


a reactive ‘tracer’ was 414 days (using Eqn. 3), or 65% greater than the corresponding minimum time and 114 days greater than the target residence time to ensure adequate pathogen attenuation of 300 days.

The solute breakthrough curves for a scenario identical in all respects to that described above except for the inclusion of a regional hydraulic gradient of 0.0015 from E→W is presented in Figure 7. Complete breakthrough at an observation well situated midway between the injection and recovery wells, occurs within 400 days as expected from the isochrones presented in Figure 6. However, breakthrough at the recovery well is much more sluggish, with the maximum mixing fraction reaching only 0.77, or ~550 mg/L TDS, which obviously exceeds the threshold level of 300 mg/L. Over the 10 year simulation period almost 7,900 ML of water was injected and that same amount recovered.

Figure 6. Flow paths and travel times for two-well system, 200 m separation with

continual injection and recovery (no regional gradient)

0

IR 1837 1095 days36530

100 200 m0

IR 1837 1095 days36530

100 200 m


Figure 7. 2-well system breakthrough curves at recovery well and two observation

wells along the shortest flow path, 200 m separation with continual injection and recovery

Mixing fractions at an observation well situated 0.4 m away (upgradient) exceeded the values in the recovered water by around 0.05, from the second year onwards. Recall that this is so because observation wells intersect a narrow band of flow paths, which in this case transports a greater proportion of injected water. Recovery wells draw from multiple flow paths.

Next, Figure 8 presents the results for three different scenarios on the recovered water quality. In the first case, the position of the wells was swapped such that the recovery well was situated upstream of the injection well, but made little difference on the breakthrough curve. Therefore the deterioration in the recovered water quality due to regional flow effects under these conditions is relatively small. In the second case, water was extracted from the recovery well at only half the rate of injection, which allowed the critical mixing fraction (f ) to be reached within 2600 days. However this still required 5600 ML of injectant, and the recovery efficiency was limited to only 50% thereafter. In the third case, annual, intermittent cycling of six months of injection followed by six months of extraction from the recovery well resulted in much more transient quality of recovered water, with mixing fractions varying by to 0.5 within any given cycle. Whilst the response at the recovery well is shown in continuity over the 3650 days, only over half the period does this actually reflect the quality of the extracted water, the remainder being analogous to observations made at the well during routine monitoring (note these two periods are clearly identified in this figure, but will not be as easily identifiable in many of the subsequent figures). It should also be noted that whilst the threshold value of f was reached from year four onwards, the durations were extremely short-lived before f fell dramatically. The minimum travel time under these conditions is estimated to be 510 days, whilst the effective travel-time varied from 550 to 720 days over each annual cycle.

Figure 9 presents the ‘tracer’ concentration in the recovered water for the standard case of continual injection and extraction and for the cycling case. For continuous injection/extraction the steady-state reactive tracer concentration of 0.057 is reached within 600 days. For cyclic injection/extraction a quasi steady-state is firmly established by year 3, with concentrations ranging from 0.008 to 0.021. This is firm evidence for the variance in travel times due to alternating injection/extraction strategies. The water extracted during the initial stages of

0

0.2

0.4

0.6

0.8

1

0 1000 2000 3000 4000Time (days)

Mix

ing

frac

tion,

fobs well at midway point (r=100m) obs well 0.4 m upgradient of recovery wellrecovery well

TDS<300 mg/L150

TDS

(mg/

L)875

1900 0

0.2

0.4

0.6

0.8

1

0 1000 2000 3000 4000Time (days)

Mix

ing

frac

tion,

fobs well at midway point (r=100m) obs well 0.4 m upgradient of recovery wellrecovery well

TDS<300 mg/L150

TDS

(mg/

L)875

1900


pumping has a higher concentration which indicates it is relatively young, whereas that extracted in the latter stages has a lower concentration and is older.

Clearly, the simulations performed indicate that the two-well strategy is an unsuccessful and inefficient means of recovering water with a suitably low TDS. Long time-frames and large volumes of water must be stored before a small proportion of sustained recovery can occur.

0

0.2

0.4

0.6

0.8

1

0 1000 2000 3000 4000

Time (days)

Mix

ing

frac

tion,

f

continual I&R (no cycling)

reverse hydraulic gradient

half strength extraction

cyclic I&R (extraction shownin black)

0

0.2

0.4

0.6

0.8

1

0 1000 2000 3000 4000

Time (days)

Mix

ing

frac

tion,

f









Figure 8. 2–well system breakthrough curves at recovery well for four scenarios (for

the cyclic scenario injection is indicated by grey lines, recovery by black lines)

Figure 9. 2-well system breakthrough curves of reactive tracer at recovery well for two scenarios

0

0.05

0.1

0 1000 2000 3000 4000Time (days)

continuouscyclic

Mix

ing

frac

tion,

f

0

0.05

0.1

0 1000 2000 3000 4000Time (days)

continuouscyclic

Mix

ing

frac

tion,

f


6.2 Four-Well systems

Having established that a two-well system provides inadequate buffering against the effects of mixing with ambient groundwater, it was apparent that modifications to the well-field configuration were necessary. A four well-system was chosen such that a recovery well is centred between three injection wells positioned at the corners of an equilateral triangle. The separation distance between the recovery well and the injection wells was 150 m, and the spacing between the injection wells was 260 m, as calculated trigonometrically (Figure 5b). The separation distance was reduced by 50 m from the 200 m used in the two-well system owing to the observation that the minimum effective travel time had been easily achieved, and therefore was also likely to be achieved for a more closely-spaced four-well configuration since the progression of the injection fronts would be retarded by interaction effects.

The number of scenarios tested with this configuration was limited to just four. Each scenario involved continuous injection into the three injection wells at 25 L/s and the only variation was the condition imposed upon the central ‘recovery’ well. The four conditions for this well were: a) no injection or extraction, b) continuous extraction at 5 L/s, c) continuous extraction at 25 L/s, and d) continuous injection (no extraction) at 25 L/s. For scenarios b and c the issue of disposal of the recovered water was not addressed. Solute distributions after one and three years are provided for each scenario in Figure10. The corresponding breakthrough curves at the ‘recovery’ well are presented in Figure 11.

The first case serves to demonstrate the effects of the three separate injected water plumes along the interface zones caused by the effect of opposing flow velocities from each well that effectively minimises the net velocity whilst migration proceeds unperturbed on the opposing boundaries of the plumes. However the absence of any withdrawls makes this an unrealistic scenario. It is noteworthy that after 2 years of injection (total volume injected ~5000 ML) none of the injectant had reached the recovery well, and the threshold value of f was not reached after 10 years. The effective travel time to the recovery well was determined to be in excess of 3 years. Extraction at 5 L/s, ie. the withdrawal of just 7% of the volume injected, induced a sufficient hydraulic gradient towards the centre of the domain such that the time taken to reach the threshold was reduced from >10 to <3 years, as signified by the more rapid development of the characteristic clover-leaf pattern. Increasing the extraction to 25L/s lessened this time to <2 years. The associated travel time was 237 days. The final case, although again unrealistic given a volumetric recovery of only 33%, serves to illustrate that it is conceptually feasible to displace the ambient groundwater from within the ASTR ‘transfer’ zone.

Whilst there was considerable opportunity for extending the analysis for the four-well system, this was not pursued since a single recovery well, that was assumed to be limited by the formation characteristics to 25 L/s, would not adequately meet the volumetric demand for extracted water during the ASTR trial.


Figure 10. 4-well system solute distributions for four scenarios: a) injection in 3

perimeter wells and no recovery, b) injection in 3 perimeter wells and recovery from central well at 5 L/s, c) injection in 3 perimeter wells and recovery from central well at 25 L/s, and d) injection into all wells

Figure 11. 4-well system breakthrough curves at recovery well for the four scenarios

(a)

(c)

(b)

(d)

(a)

(c)

(b)

(d)

0

0.2

0.4

0.6

0.8

1

0 1000 2000 3000 4000Time (days)

Mix

ing

frac

tion,

f

3 inject / 1 rec @ 25 L/s3 inject / 1 rec @ 5 L/s3 inject / 0 rec4 inject


6.3 Six-Well systems

A six-well configuration was devised by expanding the four-well system by reflection across the upped right-hand boundary of the triangular domain (Figure 5c). This design has a ratio of recovery to injection wells of 1:2 (higher than the four-well system), maintains uniform separation distances between the injection and recovery wells, and its modular structure easily accommodates further expansion, or contraction, as necessary. This well-field is contained within a rhombic (ie. diamond shaped) domain.

6.3.1 Separation distance

A schedule was devised to simulate two modes of operation during the trial. In the first year a greater than average volume of water is injected in order to flush the aquifer of ambient groundwater. No recovery occurs during this period as the object is to allow recovery efficiencies to approach optimal levels more rapidly. During the second mode, from the second year onwards, the operational sequence of injection and recovery is more typical. The volume of water available for flushing was limited to 1000 ML. Two plausible operational regimes were tested: one with 400 ML/yr injection, another with 600 ML/yr. In both cases only 80% is withdrawn, ie. 320 ML/yr and 480 ML/yr respectively. This ‘80% rule’ conforms with that used by Richard Clark and Associates, (2001) in their water balance modelling of the Parafield ASR scheme. The remaining 20% counteracts the loss of injected water due to regional drift. During the operational mode, injection periods commenced from the first day of the year at a rate of 25 L/s and progressed until the 400 ML or 600ML had been injected, a period of 46.3 or 69.4 days respectively. Recovery commenced exactly mid-year and progressed in a like manner. This ensured that the intervening periods between injections and recoveries were almost equally spaced.

Separation distances of 50, 75, 100 and 150 m were compared to determine the smallest scale at which the primary constraints could be met for both the 400 and 600 ML/yr operations. Figure 12 summarises the results in terms of the relationship between the effective travel times and mixing fractions. Salinities were averaged from year 3 onwards as the year after flushing often yielded a characteristic dip in mixing fractions prior to stabilisation (as will be described in more detail below). The plot clearly shows that the 75 and 100 m separations are the only spacings to meet both constraints. At the 100m scale increasing the volume injected from 400 to 600 ML/yr reduces the effective travel time, although the minimum value of 350 days still exceeds the target value. Reducing the separation improved the salinity of the recovered water, however the 50 m spacing failed to meet the travel time constraint. The largest separation tested failed the salinity constraint. Of the two constraints, salinity is by far the most important (due to the high cost of desalination), whilst travel time can be manipulated by operational management, to some degree, and the recovered water disinfected at relatively low cost if necessary.

Figure 13 shows the evolution of the injected water plume for a 100 m spacing at an operational scale of 400 ML/yr. The solute distribution is shown at two different stages of flushing: one at 125 days where injection into the four outer wells has just been initiated after completion of injection into two central wells, and the other at 365 days after 1000 ML of flushing has concluded. The steady-state piezometric heads during injection and recovery are given in Figure 14.

The effective travel time isochrones at 100 day intervals up to 500 days, given in Figures 15a&b, show that age distributions of the injected water are dependant on the patterns of injection and recovery. Figures 15c shows that travel times at the recovery wells over a one year period during operational mode. Each annual cycle is repeated (as was shown in Figure 9) such that ages increase in accordance with the duration of elapsed time for all of the year


except during pumping where ages, interestingly, diminish as recovery progresses. Oldest water is recovered at the beginning of the recovery period and the youngest water at the end of the period as the greatest proportion of recently injected water is drawn towards the recovery well. The pumping of older water during injection and younger water during recovery also agrees with the isofringe patterns around the recovery wells (Figures 15a&b).

Figure 12. Effective travel time versus mixing fraction (year 3 onwards) for 50, 75, 100

and 150 m separations (the bars identify the maximum and minimum times and concentrations; grey shading indicates where both constraints are met)

0.7

0.8

0.9

1

0.7

0.8

0.9

1

Mix

ing

frac

tion,

f

0 200 400 600 800 1000Travel time (days)

0 200 400 600 800 1000

100 m (600ML/yr)100 m (400ML/yr)

150 m (600ML/yr)

50 m (400ML/yr)75 m (400ML/yr)

100 m (600ML/yr)100 m (400ML/yr)

150 m (600ML/yr)

50 m (400ML/yr)75 m (400ML/yr)

0.7

0.8

0.9

1

0.7

0.8

0.9

1

Mix

ing

frac

tion,

f

0 200 400 600 800 1000Travel time (days)

0 200 400 600 800 1000

100 m (600ML/yr)100 m (400ML/yr)

150 m (600ML/yr)

50 m (400ML/yr)75 m (400ML/yr)

100 m (600ML/yr)100 m (400ML/yr)

150 m (600ML/yr)

50 m (400ML/yr)75 m (400ML/yr)


d) 3650 daysc) 1095 days

b) 365 daysa) 125 days

100m 100m

100m100m

d) 3650 daysc) 1095 days

b) 365 daysa) 125 days

100m100m100m 100m100m100m

100m100m100m100m100m100m

Figure 13. Solute distribution at two stages of flushing mode (125 and 365 days) and

two stages of operational mode (1095 and 3650 days)

Figure 14. Steady-state piezometric head distribution during injection (left) and recovery (right) (contour interval = 2m; wells are indicated by hollow circles; note that the ambient piezometric heads is ~0 m at the ASTR site and varies from ~ +3 m on RHS to –4.5 m on LHS of figures)

Injection Recovery

1000 m 1000 m

Injection RecoveryInjection Recovery

1000 m 1000 m 1000 m 1000 m

-1 0m20m

10m Injection Recovery

1000 m 1000 m

Injection RecoveryInjection Recovery

1000 m 1000 m 1000 m 1000 m

-1 0m20m

10m


Figure 15. Tracer concentration and effective travel time isofringes late in the injection

phase (a) and late in the recovery phase (b). Plot (c) shows the annual cycle of travel times at the recovery well.

6.3.2 Flushing volume

Having established that a 100 m spacing was one of the most favourable scenarios tested, an assessment was made at this scale on what effect altering the injected volume and sequence during the flushing mode would have on salinities during recovery. Figure 16a compares the average concentration history at the recovery wells for three scenarios. In the first and second 2400 and 1000 ML are injected uniformly into each well respectively, whilst in the third, 1000 ML is divided such that 500 ML is injected into the two ‘recovery’ wells first-up, followed by 500 ML into the four perimeter wells. For all cases there was a dip in mixing fractions in the second year, and in some cases the third and fourth years of operation, arising through the entrainment of small pockets of ambient groundwater where little or no flow had occurred (as observed in Figure 13b). When the greatest volume of injection (2400 ML, ie. 6 months of injection into 6 wells), this dip was smallest and least persistent, but increased in magnitude as the volume declined. For 1000 ML of flushing, it was found that injecting half the water into the ‘recovery’ wells followed by injection into the four perimeter wells lead to a substantial reduction in the volume of entrained groundwater and the quality of recovered water in subsequent years. This agrees with one of the findings of Merritt, (1986) described earlier.

Using the latter scheduling pattern once again, volumes injected in the first year of less than 1000 ML were tested (Figure 16b). This showed that only for 1000 ML would an average mixing fraction of 0.9 be achieved at virtually all times (except for part of the third year),

teff (days)>500

0100200300400500

100 m

teff (days)>500

0100200300400500

100 m100 m

300

400

500

600

700

0 100 200 300 400Time (days in calendar year)

Res

iden

ce ti

me

(day

s)

RecoveryInjection

(a) (b)

(c)

teff (days)>500

0100200300400500

100 m

teff (days)>500

0100200300400500

100 m100 m

300

400

500

600

700


Res

iden

ce ti

me

(day

s)

RecoveryInjection

300

400

500

600

700


Res

iden

ce ti

me

(day

s)

RecoveryInjection

(a) (b)

(c)


although injection of 800 ML would be sufficient from year 4 onwards. With 500 ML injection 6 years was needed for salinities to converge towards the other scenarios. Note that the plots shown in Figure 16a were for 600 ML/yr injection and in Figure 16b were for 400 ML/yr.

Figure 16. Effect of volume of water injected in first year on recovered water salinity

6.3.3 Increasing the proportion of water recovered

Annual recovery of only 80% of the injected volume has been used to ensure that the remaining 20% keeps the saline margins of the injected water plume away from the recovery well so that good quality water can be extracted in subsequent years. The sensitivity of this hydrodynamic balance was tested by increasing the proportion of recovery to 100%. Whilst this scenario maintains a volumetric balance between inputs and outputs, in terms of the mass balance, less stormwater is recovered than injected since a small fraction of the extracted water is derived from ambient groundwater. Is this sufficient to buffer against excessive mixing? Figure 17 suggests that this can probably be sustained for the first few years of operation but not indefinitely. Whilst mixing fractions at the upgradient well remain fairly uniform within the range from 0.90 to 0.95, the downgradient well exhibits a steady decline over time from 0.88 in year 2 to 0.85 in later years. This is because some of the water cannot be recovered. Note that in the absence of a regional gradient the concentration of recovered water from both wells would be identical. Enhancing recovery also leads to a 10 day reduction in the minimum, and 60 day reduction on the maximum, effective travel time to 400 and 640 days respectively (cf. Fig. 15).

Figure 17. Effect of recovering 100% of injected volume (400 ML) on recovered water quality

0.5

0.6

0.7

0.8

0.9

1

0 1000 2000 3000 4000Time (days)

Mix

ing

frac

tion,

f

2400 ML1000 ML500 ML in then 500 ML out

0.5

0.6

0.7

0.8

0.9

1

0 1000 2000 3000 4000Time (days)

Mix

ing

frac

tion,

f

500 ML in then 500 ML out500 ML800 ML

(a) (b)

0.5

0.6

0.7

0.8

0.9

1

0 1000 2000 3000 4000Time (days)

Mix

ing

frac

tion,

f


0.5

0.6

0.7

0.8

0.9

1

0 1000 2000 3000 4000Time (days)

Mix

ing

frac

tion,

f


0.5

0.6

0.7

0.8

0.9

1

0 1000 2000 3000 4000Time (days)

Mix

ing

frac

tion,

f


0.5

0.6

0.7

0.8

0.9

1

0 1000 2000 3000 4000Time (days)

Mix

ing

frac

tion,

f


(a) (b)

0.5

0.6

0.7

0.8

0.9

1

0 1000 2000 3000 4000

Time (days)

Mix

ing

frac

tion,

f

Rec well - u/gradientRec well - d/gradientAverage recovery


6.3.4 Rectangular versus rhombic configuration

The six-well configuration used throughout was compared to a rectangular arrangement to assess its relative efficiency. A 100 m spacing was used and all factors apart from layout were kept equal. The recovered water quality curves shown in Figure 18 demonstrate that after the initial flushing period is complete, and quasi steady-state conditions have established (notionally from year 4 onwards), the average mixing fractions (over extraction and non extraction periods) are 0.09 higher for the rhombic arrangement, with a virtual absence of the ‘spikes’ characteristic of the extraction periods for the rectangular scheme. Greater mixing arises from the rectangular layout since the two bodies of injected water which originate from the left and right side pairs of injection wells fail to merge during recovery and the resultant rift allows ingress of ambient groundwater from the north and south. This can be explained by the less compact grouping of wells than for the rhombic arrangement. If the total aquifer volume within the four perimeter wells is taken as a guide, the difference between the two configurations is 25%. Whilst this issue could be rectified by reducing the separation distance between the two recovery wells (thereby shortening the rectangle along its primary axis), this would lead to greater interference effects between the pumping wells. Thus the rhombic layout represents the more efficient of the two configurations.

Figure 18. Effect of rhombic versus rectangular layout on recovered water quality

6.3.5 Effect of well-field orientation relative to the direction of regional

groundwater flow

All of the simulations for the rhombic well configuration so far have involved a common ambient groundwater flow direction (E→W). That is, the angle between the regional flow-lines and the line of symmetry passing through the two recovery wells is 30o. Various orientations were tested by realigning the direction of regional flow rather than the more onerous task of modifying the grid discretisation to accurately reposition the wells for each case. Three different flow directions were tested: N→S, NE→SW, and E→W. A case without regional flow and another where regional flow is five times the magnitude of the base-case simulations were also tested. The results are summarised in Figure 19, which shows for each case apart from the latter case where the gradient is enhanced, the average value of f from

0

0.2

0.4

0.6

0.8

1

0 1000 2000 3000 4000Time (days)

Mix

ing

frac

tion,

f rectangular

rhombic

0

0.2

0.4

0.6

0.8

1

0 1000 2000 3000 4000Time (days)

Mix

ing

frac

tion,

f rectangular

rhombic

0

0.2

0.4

0.6

0.8

1

0 1000 2000 3000 4000Time (days)

Mix

ing

frac

tion,

f rectangular

rhombic


year 4 onwards (when solute concentrations had reached quasi steady-state), and the maximum and minimum values within each year (the separation distance was 100 m). The range in the average f between cases is only 0.03, whilst the maximum difference relative to the base-case is 0.02. The lowest f of the cases tested was for N→S flow as this is the case where the well-field domain is least streamlined to the groundwater flow, whilst the most streamlined case was for NE→SW flow. In the latter case only one injection well is positioned down gradient of the NE→SW flow field, whilst for N→S flow there are two down gradient wells.

The influence of regional groundwater flow at the velocity associated with the base-case hydraulic gradient of 0.0015 is not excessive. From simple water balance calculations it can be demonstrated that the annual through-flow across a 200 m strip of aquifer which easily covers the width of the injected water plume is 14 ML/yr, and so for the smallest scale ASTR operations of 400 ML/yr, the maximum volumetric loss of injectant through drift is 4%, and for 600 ML/yr is just 2%. This demonstrates why the effect is subdued and becomes most pronounced where time-scales are long. When the hydraulic gradient is increased (by a factor of five) to 0.0075 the losses due to regional drift become more pronounced and average mixing fractions decline to 0.81. This case was tested to gauge the potential effect of the nearby Parafield ASR scheme. A well-field orientation, whereby the main transect of wells is approximately aligned with the background gradient is recommended as this has the least impact on recovered water quality.

Figure 19. Effect of orientation and magnitude of well-field relative to groundwater flow direction on average f from year 4 onwards (maximum and minimum values indicated by the ‘error’ bars)

6.4 Effect of injection and recovery scheduling on effective residence time

The effective residence time of the injectant in the aquifer depends on the periods of injection and recovery when transport velocities are high and also on the intervening “rest” periods when velocities are much-reduced as there is only regional flow. These two components

0.7

0.8

0.9

1

N→S NE→SW E→W NA: no gradient E→W: 5X gradient

Direction of groundwater flow

Mix

ing

frac

tion,

f base case

0.7

0.8

0.9

1

N→S NE→SW E→W NA: no gradient E→W: 5X gradient

Direction of groundwater flow

Mix

ing

frac

tion,

f base case


depend on the number and sequencing of active wells, flow rate, and on changes in regional flow gradient and direction. As a means of demonstrating the dependence of the duration of the rest period on residence time, a simulation was carried out where recovery occurred sooner after the cessation of injection (ie. by bringing injection period of each year forward 100 days making that intervening rest period only 36 days). Note that this was for a 50 m separation at an operational scale of 400 ML/yr. The resultant travel times for this ‘compressed case’ is compared to the base-case in Table 2. This shows that the minimum and maximum travel times are reduced by 95 and 106 days, which approximately coincides with the time in which the rest period has been compressed, since groundwater velocities in the transfer zone during injection or recovery are more than an order of magnitude higher than ambient velocity during rest periods. Reducing in rest period between the end of injection and start of recovery can also be inferred from Figure 15c as resulting in an equal decline in the maximum residence time at the beginning of recovery, and minimum residence time at the end of recovery. In the example given above the 100 day difference in rest period between the two cases is a significant proportion of the effective travel times. It could be demonstrated that if the effective travel time were large with respect to each annual injection/recovery cycle, there would be less dependence on when the rest periods were distributed within each cycle.

Table 2. Effective travel times for base- and compressed-case injection/recovery schedules (the rest period refers to the intervening period between injection and recovery in any given year)

Case Rest period (days)

Minimum travel time (days)

Maximum travel time (days)

Base 136 221 538

Compressed 36 126 432

6.5 Uncertainties in aquifer characteristics

The aquifer is assumed to be homogeneous and isotropic although in practice the T2 aquifer is inherently heterogeneous and the injection fronts are unlikely to proceed in the idealised manner predicted. Preferential flow of injected water through more permeable parts of the aquifer but displaces ambient groundwater much more slowly in less permeable zones. The effective volume of aquifer occupied by the injected water is increased due to greater mixing along the interface and so recovery efficiencies are likely to be less than those predicted when the aquifer is assumed to be homogeneous. In the absence of detailed knowledge of the spatial distribution of aquifer hydraulic properties at the study site, this can only be taken into account through sensitivity analysis.

Based on our knowledge of the factors affecting recovery efficiencies of ASR systems, the two aquifer parameters that are not sufficiently well-defined and that influence ASTR viability are effective porosity and aquifer dispersivity. Effective porosity determines the storage volume in the aquifer, and hence the effective travel time and dimensions of the injected water plume for a given operational scale. Figure 20 compares, the recovered water quality with a porosity of 0.25 in the base-case (well spacing 100 m in a 6 well configuration) with a porosity of 0.4, which is close to the mean of measured values at the Bolivar ASR site (Pavelic et al., 2001), and a porosity of 0.1, which is the worst-case scenario identified from


the modelling of the Parafield ASR scheme detailed below. It demonstrates that for all porosities, recovered water quality would meet the mixing fraction target over the 10 year period for the 75 m separation, 600 ML/yr scenario presented. Salinity deteriorated slightly as a result of increased porosity and hence reduced plume size. Increasing the porosity from 0.25 to 0.4 also increased the effective residence time by approximately 100 days.

Dispersivity controls the extent of the mixing zone in the aquifer. The base-case value of 5 m is compared to 0.5 m and 50 m (Figure 21). The separation distance is 75 m and the scale of operation is 600 ML/yr once again. Clearly the lowest dispersivity case results in a highest average mixing fraction (0.999) and the highest dispersivity case the lowest mixing fraction (0.82), compared with the base-case (0.97). Although the highest value tested failed to meet the salinity target, a dispersivity of this magnitude would be considered extremely unlikely for the T2 aquifer (Pavelic et al., 2002).

Figure 20. Effect of increasing and decreasing effective porosity relative to base-case

value of 0.25 on recovered water quality (75 m well-separation)

Figure 21. Effect of increasing and decreasing aquifer dispersivity relative to base-

case value of 5 m on recovered water quality (75 m well-separation)

0.5

0.6

0.7

0.8

0.9

1

0 1000 2000 3000 4000Time (days)

Mix

ing

frac

tion,

f

alpha=0.5malpha=5malpha=50m

0.5

0.6

0.7

0.8

0.9

1

0 1000 2000 3000 4000Time (days)

Mix

ing

frac

tion,

f

n=0.4n=0.25n=0.1


6.6 Analysis of solute breakthrough data from Parafield ASR scheme

The confidence of the model predictions presented above would have been enhanced through the calibration of the model against measured data. Clearly this is not possible at the proposed ASTR site within the current scope of work, however at the nearby Parafield site, ASR operations that commenced in 2003 have produced useful solute breakthrough data to test the values of the critical parameters used in this study, and to begin to address issues associated with uncertainty in aquifer parameters as detailed above.

At the Parafield site there are two ASR (ie. dual purpose) wells situated 100 m apart in a NW-SE direction (locations are shown in Figure 2). An observation well has been drilled midway between these wells. All wells are fully penetrating and completed ‘open hole’. A second observation well is completed in the overlying T1 aquifer. Injection commenced in the upstream (SE) well (RN 20743) in mid-June 2003 and in the downstream well (RN 20943) in mid-July 2003. In the first winter season the total volume injected was 282 ML, of which 165 ML (59%) was via RN 20743 and 117 ML (41%) via RN 20943. The electrical conductivity (EC) of the source water and receiving groundwater were monitored. Breakthrough at the 50 m observation well (RN 20741) due to the low salinity injectant (median value = 180 µS/cm) from the initial value of ~3750 µS/cm commenced in mid-late July and was complete by mid-October (Figure 22).

Figure 22. EC breakthroughs at the Parafield site over 12 month period since the commencement of ASR

0

1000

2000

3000

4000

5000

Apr May Jun Jul Aug2003

Sep Oct Nov Dec Jan Mar Apr2004

May Jun Jul

µ S/c

m PWPS-1 T2 prod (20743)

PWPS-2 T2 prod (20943)

OWPS1-T2 obs (20741)


T1 well (20742)

T2 obs well (20741)(r=50m)

Injection started:

µ S/c

m

(20743)(20943)

0

1000

2000

3000

4000

5000



May Jun Jul

µ S/c





T1 well (20742)

T2 obs well (20741)(r=50m)

Injection started:

µ S/c

m

(20743)(20943)

0

1000

2000

3000

4000

5000



May Jun Jul

µ S/c





T1 well (20742)

T2 obs well (20741)(r=50m)

Injection started:

µ S/c

m

(20743)(20943)

0

1000

2000

3000

4000

5000



May Jun Jul

µ S/c





T1 well (20742)

T2 obs well (20741)(r=50m)

Injection started:

µ S/c

m

(20743)(20943)


If we assume that the aquifer is homogeneous and isotropic, the regional gradient is absent, and that the interaction between the two injecting wells is minimal such that the injectant migrates out as a cylindrical body, then the radius of the advective front at any time during the injection phase, R' (t), may be defined as:

ebnQt

tRπ

=)(' (4)

where:

Q = injection rate (m3/day) t = duration of injection (days) D = thickness of aquifer (m) ne = effective porosity of aquifer (m3/m3)

Since injection was unevenly weighted between the two wells (as explained above), it is reasonable to assume that breakthrough at the 50 m observation well was the result of one well only (RN 20743). From Eqn. (4) and based on our determination that 81 ML of injectant that was needed for the advancing injection front to reach the observation well (identified by when a mixing fraction of 0.5 is reached), an effective porosity of ~0.20 is calculated. In hydrogeological terms, this is relatively close to the base case used in the numerical simulations of 0.25. The data also reveals small EC kick-backs (increases) in early August and again in September 2003 when there were pauses in injection. This has been observed previously in the T2, such as at the Andrews Farm ASR site (Pavelic et al., 2000), and suggests that some ambient groundwater entrained in less mobile parts of the aquifer has diffused into the fresher parts that are better connected to the ASR well. The total porosity of the aquifer must therefore exceed 0.20.

Numerical simulations of this operation were also performed to revise the estimate of effective porosity by taking into account the interactive effect of the two ASR wells and regional groundwater flow on the geometry of the injected water plumes. The parameterisation used was identical to the models developed for the ASTR simulations (Table 1) unless otherwise stated. For comparative purposes the EC data was converted to mixing fractions by Eqn. (1) since EC can be assumed to be quite conservative in this system.

Observed and predicted solute breakthrough for values of porosity ranging from 0.1 to 0.25 are presented in Figure 23 for a common dispersivity value of 5 m. The intermittent injections, due to the breaks in the availability of stormwater, produced an observed breakthrough curve which is discontinuous. The simulations also reflect this, although not to a high degree of accuracy, possibly since the volumetric data was only available on a monthly basis (injections were distributed about the mean of each month).

The results demonstrate that the best-fit value of porosity is less than the value of 0.2 derived from the analytical equation. A value in the order of 0.15 best reflects the initial breakthrough of injectant (roughly up to f = 0.3), however a value as low as 0.1 is needed to ensure the later stage of breakthrough occurs by around day 70. Figure 24 shows the effect of three different dispersivities on breakthrough using the approximate best-fit porosity value of 0.15. Over the first 70 days, when the majority of breakthrough occurs, the match is better reflected by the lower dispersivities (0.5-5m) than by the highest dispersivity (50m).

The "fully-penetrating" 50 m observation well could be assumed to intersect numerous horizontal layers. The permeability of the layers determines how rapidly the injected water migrates to the observation well, and hence a succession of breakthrough fronts from the


different layers will reach the well at different times. During breaks in injection the low salinity water that occupies the more permeable parts of the aquifer mixes with the brackish ambient groundwater in less permeable parts, resulting in salinity increases with time. Knowledge of heterogeneity at this site is poor and could not be reliably incorporated into this modelling. Downhole flow metering in the well and pump tests with piezometric monitoring can be used to provide the necessary information. This analysis serves to demonstrate that accurate characterisation of the permeability distribution in the aquifer at the ASTR will be needed to develop a reliable understanding of flow and transport processes.

Figure 23. Measured and predicted solute breakthroughs for three values of porosity

at the 50 m observation well (RN 20741)

Figure 24. Measured and predicted solute breakthroughs for three values of dispersivity at the 50 m observation well (RN 20741)

-0.1

0.1

0.3

0.5

0.7

0.9

1.1

0 30 60 90 120 150 180

Time (days since 01.06.03)

Mix

ing

frac

tion,

f

observedn=0.25n=0.15n=0.1

-0.1

0.1

0.3

0.5

0.7

0.9

1.1

0 30 60 90 120 150 180


Mix

ing

frac

tion,

f

observedn=0.25n=0.15n=0.1

-0.1

0.1

0.3

0.5

0.7

0.9

1.1

0 30 60 90 120 150 180


Mix

ing

frac

tion,

f

observed

alpha=50m

alpha=5m

alpha=0.5m

-0.1

0.1

0.3

0.5

0.7

0.9

1.1

0 30 60 90 120 150 180


Mix

ing

frac

tion,

f

observed

alpha=50m

alpha=5m

alpha=0.5m


7 CONCLUSIONS This study has used modelling techniques to identifying the optimal number and arrangement of injection and recovery wells for a proposed full-scale ASTR trial of 400-600 ML/yr. Two primary constraints at this site dictate the feasibility of any given well-field: 1) that the recovered water quality does not exceed 300 mg/L TDS (no more than a 10% contribution from ambient groundwater) to allow full productive use of the water, and 2) that the effective residence time in the aquifer is greater than 300 days to conservatively allow for pathogen attenuation to occur giving some allowance for possible preferential flow in the aquifer.

Simulations with the FEFLOW groundwater simulation package were performed to explore alternate well-field designs involving arrangements of two-, four- and six-wells. A six-well system of four outer injection wells and two inner recovery wells, all equi-spaced and contained within a rhombic domain, was found to meet the constraints over the long term at full-scale with inter-well separation distances of 75 and 100 m (values of 50 and 150 m were also tested). The 75 m separation met the criteria above with the lowest annual volume of injectant for the assumed aquifer parameters at the site. These parameters require confirmation after drilling and testing of wells. The analysis also took into account:

• the volume of water available from the wetland • the timing of the demand for recovered water • an injection sequence to maximise flushing of the ‘transfer zone’ in the first stage of

operation • uncertainties in aquifer properties (notably porosity and dispersivity) • the effect of operational scheduling on travel time • the orientation of the well field with respect to regional groundwater flow • the effect of the ambient hydraulic gradient • the ratio of recovered to injected volumes • the relative efficiency of an alternative six-well arrangement

Simulations of measured solute breakthrough data from an observation well at the Parafield ASR site, situated one kilometre NE of the proposed ASTR site, suggest the target aquifer is heterogeneous, and that the effective porosity may be lower than the base case used to model the ASTR well-field. If so, this would imply the reduction in travel time to recovery wells and higher than anticipated volumes of water to flush ambient groundwater from the transfer zone.

A semi-analytical model was developed based on the Theiss solution for well drawdown and tracking of moving fronts of injected and recovered water. Comparisons between the numerical and semi-analytical methods demonstrate that the FEFLOW results are accurate in defining fronts at all stages of injection and recovery. The semi-analytical method is a useful and robust tool for assessing the distribution of injected water bodies in confined aquifers for a range of well configurations.


8 RECOMMENDATIONS 1. Well-field Design and Layout

It is recommended that drilling for the upcoming ASTR trial at the Greenfield Railway site follow a six-well rhombic arrangement as defined in this report, subject to re-evaluation once aquifer characterisation has been undertaken. A separation distance of 75 m is recommended ahead of a 100 m separation (which also meets the constraints). The major axis that runs in-line with four of the six wells should be aligned as close as possible to the direction of local ambient groundwater flow to minimise losses due to downstream drift. At the Greenfield Railway Station site the gradient imposed by Parafield ASR operations is five times the ambient gradient (to the west), suggesting that aligning the major axis with the channel (ie. pointing towards Parafield ASR) may be the optimal configuration. In translating this idealised layout to an on-the-ground layout, the mixture of public and private space at the site presents physical constraints and challenges. Therefore adjustment of the layout within a range of ±5 m would be considered acceptable.

The proposed layout for the ASTR well-field is shown in Figure 25. Five new production wells would be constructed in addition to the one existing T2 well.

Figure 25. Proposed layout of ASTR well-field at the Greenfields Railway Station site

2. Hydraulic Characterisation of Aquifer

In order to deal with the issue of aquifer heterogeneity, it is strongly recommended that for all new wells drilled at the site the vertical distribution of aquifer hydraulic conductivity be

Inj. WellRec. Well

75m

Inj. WellRec. Well

75m

Inj. WellRec. Well

75m

Inj. WellRec. Well

75m


quantified using electromagnetic flowmeter and other downhole geophysical logging techniques, and that aquifer pump testing be performed once the well-field is established.

3. Monitoring Wells

Observation wells are critical for demonstrating the viability of ASTR and are required to:

• characterise subsurface processes and verify rates of attenuation and travel times • define the distribution of the injected water to assess efficiency of flushing efforts prior to

commencing routine operations • provide regular monitoring data that would offer early warning in the event of polluted

water being recharged

Observation wells would be constructed within the well-field to serve the following functions:

1) to give a clear representation of breakthrough and allow early warning of fate of any contaminants that may be injected – located between adjacent injection and recovery wells

2) to detect the extent of aquifer flushing within the transfer zone – wells located where pockets of brackish water may be present

3) to better predict volumes that can be recovered at acceptable concentrations – wells located at the margins of the injected water plume

4) to detect the lateral extent of the plume of fresh water to determine the opportunities for improved management of the banked water – wells located at and beyond the margins of the injected water plume

5) to characterise contaminant attenuation in relation to travel length and travel time within discreet flow paths between two pairs of injection and recovery wells – wells constructed with short open intervals (ie. piezometers) at depths identified after downhole flowmeter logging of production wells performed

Approximately 15 wells are anticipated, although this depends on financial constraints. An indication of the prospective locations for the observation wells in relation to the five functions listed above is shown in Figure 26. Their position would be fine-tuned by drilling, geophysical testing and the next stage of groundwater modelling, as detailed below.

At least two of the observation wells identified in Figure 26 as having functions ‘1 & 5’ should be completed as a nest of piezometers to allow anticipated stratification in the aquifer to be interpreted. This will give a clearer definition of minimum travel time and the vertical distribution of injectant in the aquifer. This was found to be vital at the Bolivar ASR site for interpreting subsurface behaviour.

Piezometers are also needed in low permeability layers to determine the extent of aquifer flushing, to improve predictions of salinity of recovered water and develop long term management strategies for banked water.


Figure 26. Proposed location of observation wells and their functions in relation to the injection and recovery wells

4. Further Modelling

Further groundwater modelling work will be required to refine well field design and layout and deal with a range of operational issues that may arise before and during the trial. The ‘What If’ scenarios that would be handled by such modelling include:

• accounting for known hydrogeological conditions (not just assumptions) • effectiveness and duration of flushing operations • alternative operational scheduling • handling of operational malfunctions and breakdowns • effect of volume available for recharge on impacts on recovered water quality and

quantity • strategies to mitigate impacts if polluted water is inadvertently injected into the aquifer

Modelling is also likely to be fundamental to the interpretation of data from the trial. The travel times to observation and recovery wells are clearly dependent on the operational regime, and this is likely to vary from year to year. The development of a reliable calibrated model based on revised modelling taking into account aquifer characterisation data, solute breakthroughs and applied tracer tests would provide a useful evaluation tool and assist with operational management over the longer-term.

Obs. well

Inj. WellRec. Well

Injectant

AmbientMixture

Obs. wellfunctions(1-5)(3,4)

(3,4)

(3,4)

(3,4)

(2) (2)

(4)

(4)

(1,5)

(1,5)

(2)

Obs. well

Inj. WellRec. Well

Injectant

AmbientMixture

Obs. wellfunctions(1-5)(3,4)

(3,4)

(3,4)

(3,4)

(2) (2)

(4)

(4)

(1,5)

(1,5)

(2)


9 REFERENCES Bear, J. (1979) Hydraulics of Groundwater, McGraw-Hill Publishers, ISBN. 00 7004 1709.

Bear, J. and Jacobs, M. (1965) On the movement of water bodies injected into aquifers. Journal of Hydrology 3:37-57.

Clark, Richard and Associates (2001) Parafield Stormwater Management and Supply / Parafield Drain Scheme: the estimation of the catchment yield and the simulation of the operation of the scheme using the WaterCress model. Final Report to the City of Salisbury, May 2001.

Diersch, H.-J. (2004) FEFLOW: Interactive, graphics-based finite-element simulation system for modelling groundwater flow, contaminant mass and heat transport processes. Getting Started; User’s Manual; Reference Manual, Version 5.1. WASY, Institute for Water Resources Planning and System Research Ltd, Berlin, Germany.

Dillon, P.J., Miller, M., Fallowfield, H. and Hutson, J. (2002) The potential of riverbank filtration for drinking water supplies in relation to microsystin removal in brackish aquifers. Journal of Hydrology 266(3-4):209-221.

Dillon, P., Barry, K., Pavelic, P., Sovich, T., Hutchinson, A. and Woodside, G. (in press) Talbert Gap Saline Intrusion Barrier, Orange County, California. In: Final Report to AwwaRF, Project No. 2618 (Volume 2, Chapter 3).

Fildebrandt, S., Pavelic, P., Dillon, P. and Prawoto, N. (2003). Recharge enhancement using single or dual well systems for improved groundwater management in the Bandung Basin, Indonesia. CSIRO Technical Report 29/03, May 2003.

Gelhar, L.W. and Collins, M.A. (1971) General analysis of longitudinal dispersion in nonuniform flow. Water Resources Research 7(6):1511-1521.

Gerges, N.Z. (1999) The geology and hydrogeology of the Adelaide metropolitan area. PhD Thesis, Flinders University of South Australia.

Harpaz, Y. and Bear, J. (1963) Investigations on mixing of waters in underground storage operations. Int. Assoc. Hydrogeol. Sciences, Publication No. 64, pp.132-153.

Hodgkin, T. (pers. comm.) Hydrogeologist. Department of Land, Water and Biodiversity Conservation (DWLBC), June 2004.

Hoopes, J.A. and Harleman, D.R.F. (1967) Wastewater recharge and dispersion in porous media. ASCE Journal of the Hydraulics Division 93(HY5):51-71.

McCarthy, J.F., Gu, B., Liang, L., Mas-Pla, J., Williams, T.M. and Yeh, T.-C. J. (1996) Field tracer tests on the mobility of natural organic matter in a sandy aquifer. Water Resources Research 32(5):1223-1238.

Merritt, M.L. (1986) Recovering fresh water stored in saline limestone aquifers. Ground Water 24(4):516-529.

Miller R., Corell, R., Dillon P. and Kookana R. (2002) ASRRI: A predictive index of contaminant attenuation during aquifer storage and recovery. In: Management of Aquifer Recharge for Sustainability, P.J. Dillon (Ed.) Proceedings of the 4th International Symposium on Artificial Recharge (ISAR4), Adelaide, Sept. 22-26, 2002, Swets & Zeitlinger, Lisse, ISBN. 90 5809 527 4, pp.69-74.

Molz, F.J., Guven, O., Melville, J.G., Crocker, R.D. and Matteson, K.T. (1986) Performance, analysis and simulation of a two-well tracer test at the Mobile site. Water Resources Research 22(7):1031-1037.

Pavelic, P., Dillon, P.J., Ragusa, S.R. and Toze, S. (1996) The fate and transport of microorganisms introduced to groundwater through wastewater reclamation. Centre for Groundwater Studies Report No. 69.

Pavelic, P., Dillon, P.J. and Gerges, N.Z. (2000) Challenges in evaluating solute transport from a long-term ASR trial in a heterogeneous carbonate aquifer. Proc. IAH Congress, Groundwater: Past Achievements and Future Challenges, (Eds. Sillio et al.), Balkema, Rotterdam, ISBN. 90 5809 159 7, p.1005-1010.


Pavelic, P., Dillon, P., Martin, R., Traegar, B. and Simmons C. (2001) Multi-scale permeability characterisation of a confined carbonate aquifer targeted for aquifer storage and recovery. Proc IAH XXXI Congress:“New Approaches to Characterising Groundwater Flow”, (Eds. K.-P. Seiler & S. Wohnlich), Swets & Zeitlinger Lisse, ISBN 902 651 848 X, pp.859-862.

Pavelic, P., Dillon, P.J. and Simmons, C.T. (2002) Lumped parameter estimation of initial recovery efficiency during aquifer storage and recovery. In: Management of Aquifer Recharge for Sustainability, P.J. Dillon (Ed.) Proceedings of the 4th International Symposium on Artificial Recharge (ISAR4), Adelaide, Sept. 22-26, 2002, Swets & Zeitlinger, Lisse, ISBN. 90 5809 527 4, pp.285-290.

Rinck-Pfeiffer, S., Dillon, P., Sibenaler, Z. and Gerges, N. (2004) Stormwater aquifer storage transfer and recovery (ASTR) for a potable water supply. Confidential Draft Proposal, February 2004.

Rinck-Pfeiffer, S. (2004) Future research directions in stormwater ASR. Water: Journal of the Australian Water Association, 31(5):4.

Stuyfzand, P.J., Vogelaar, A.J. and Wakker, J. (2002) Hydrogeochemistry of prolonged deep well injection and subsequent aquifer storage in pyritiferous sands; DIZON pilot, Netherlands. In: Management of Aquifer Recharge for Sustainability, P.J. Dillon (Ed.) Proceedings of the 4th International Symposium on Artificial Recharge (ISAR4), Adelaide, Sept. 22-26, 2002, Swets & Zeitlinger, Lisse, ISBN. 90 5809 527 4, pp.107-110.

Swierc, J., Van Leeuwen, J. and Dillon, P. (in prep.) Preparation for a Hazard Analysis and Critical Control Points Plan (HACCP): City of Salisbury Stormwater to Drinking Water Aquifer Storage Transfer and Recovery (ASTR) Project. CSIRO Land and Water Report. Toze, S. and Hanna J. (2002) The survival potential of enteric microbial pathogens in a treated effluent ASR project. In: Management of Aquifer Recharge for Sustainability, P.J. Dillon (Ed.) Proceedings of the 4th International Symposium on Artificial Recharge (ISAR4), Adelaide, Sept. 22-26, 2002, Swets & Zeitlinger, Lisse, ISBN. 90 5809 527 4, pp.139-142.

Trefry, M.G. and Johnston, C.D. (1996) Hydrologic modelling and design of emplacement strategies for amendment solutions. CSIRO Division of Water Resources Technical Memorandum 96.30.

Waegeningh van, H.G. (1985) Overview of the protection of groundwater quality. In: Theoretical Background, Hydrology and Practice of Groundwater Protection Zones, Chapter 6. International Contributions to Hydrogeology, Vol. 6, International Association of Hydrogeologists.

Wescombe, K. and Furness, B. (2004) Parafield - Mawson Lakes Pipeline hydraulic modelling report, United Water, April 2004.

Zulfic, H. (2002) Northern Adelaide Plains Prescribed Wells Area groundwater monitoring status report 2002. South Australia. Department of Water, Land and Biodiversity Conservation. Report DWLBC 2002/14.


10 APPENDIX: Verification of FEFLOW FEFLOW has been successfully benchmarked against a number of standard test problems and solutions (Diersch, 2004), including for the analytical solution of Hoopes and Harleman, (1967) for the two-well ASTR system operated under a continuous injection/recovery regime. Verification was performed as part of this study to ensure that the discretisation of grids and time-steps was sufficiently accurate and that boundary conditions were not perturbing conditions within the area of interest, so that the solute concentrations were of sufficient accuracy. FEFLOW was first tested against two analytical solutions; one pertaining to solute breakthrough at observation wells, the other to solute changes in water recovered from ASR well, before being tested against the semi-analytical procedure.

10.1 Breakthrough of injected water at observation wells The approximate analytical solution for the breakthrough of injected solute from a fully penetrating well in a confined homogeneous and isotropic aquifer, and also accounts for the effects of well radius and molecular diffusion is given by Gelhar and Collins, (1971; Eqn. 38) as:

][ )4

)'(3

)'((16/)'(

21 4433

22 RwRA

DmRwRRwRerfcf

−+

−−=

αα (5)

where:

bnQA i

π2=

f = mixing fraction (defined in text) erfc = complimentary error function Rw = radius of the ASR well (m) Ro = radial distance between the ASR and observation well of interest (m) R' = radius of the advective front (m) α = dispersivity (m) Dm = molecular diffusivity (m2 s-1)

Figure 27 shows an excellent agreement between the analytical and the numerical modelling for the breakthrough response at the observation well.

Figure 27. Comparison of analytically and numerically determined breakthrough curves at a distance of 50 m from the ASR well

0

0.2

0.4

0.6

0.8

1

0 50 100 150 200Time (days)

Mix

ing

frac

tion,

f

FEFLOWAnalytical

0

0.2

0.4

0.6

0.8

1

0 50 100 150 200Time (days)

Mix

ing

frac

tion,

f

FEFLOWAnalytical


10.2 Recovered water quality changes with time The approximate analytical solution for the temporal change in the quality of the water pumped during the recovery phase for a homogeneous and isotropic aquifer, assuming there is no period of subsurface storage and no regional hydraulic gradient is given by Gelhar and Collins, (1971; Eqn. 42) as:

][ ))1(12('3

16)1(21

max/

ViVr

ViVr

RVVerfcf

i

r−−−−=

α (6)

where:

Vi = total volume of water injected (m3) Vr = total volume of water recovered (m3) R'max = radius of the advective front at the end of the injection phase

The scenario considered here for a single-well ASR system consists of a 394.2 ML injection (182.5 days @ 25L/s), followed by a 1182.6 ML recovery (547.5 days @ 25L/s) – three times the volume of injection. The aquifer is 52 m thick homogeneous and isotropic aquifer with porosity of 0.25 and dispersivity of 5 m.

Figure 28 shows an acceptable agreement between the analytical and numerical recovered water quality response, although slightly more dispersion is apparent overall for the numerical solution in the middle ranges of the recovery. Mass balance differences between the outputs are acceptable. Previous experience with similar problems has demonstrated that even with further refinements in the mesh, lowering of the maximum permissible time-steps, and expansion of the model domain, they fail to make a significant improvement to the goodness-of-fit.

It should be recognised that both analytical solutions are approximate, and therefore variations of the kind evident here need not be taken to be significant.

Figure 28. Comparison of analytically and numerically determined solute changes in the recovered water

0

0.2

0.4

0.6

0.8

1

0 1 2 3Ratio of the volume extracted to that injected

Mix

ing

frac

tion,

f

FEFLOWAnalytical

0

0.2

0.4

0.6

0.8

1

0 1 2 3Ratio of the volume extracted to that injected

Mix

ing

frac

tion,

f

FEFLOWAnalytical


10.3 Comparison with semi-analytic method

A FEFLOW simulation was performed for the six-well previously discussed case illustrated in Figures 13-15. The case refers to a 100 m separation with 1000 ML of flushing in year one and 400 ML injected - 320 ML recovered in years two to ten. Semi-analytical (SA) fronts were calculated and overlain on the FEFLOW-generated isofringe contours of solute distribution at four stages during the simulation – two during flushing mode and two during operational mode (125, 365, 1095 and 3650 days). Since the SA method only considers advective transport, the longitudinal dispersivity for the FEFLOW simulations was reduced by an order of magnitude to 0.5 m (as in section 6.4.1) in order to make the two outputs more comparable.

The results, presented in Figure 29, show excellent agreement between the isofringes and fronts at all stages of the simulation. The SA method demonstrates it can capture all the distortions to the fronts that arise due to interactions between the six wells. The method also clearly delineates between the major front arising from the current year of injection from the residual front which that was not previously recovered. Residual fronts lie amid the primary fronts for the 1095 and 3650 day results. Numerical simulations performed with the base-case dispersivity of 5 m do not distinguish these secondary features due to higher levels of mixing, as can be seen by comparing Figures 13 and 29. Future work will compare time-series recovered water qualities from the SA approach with those of the numerical approach, as previously reported. A future report will focus specifically on the semi-analytical approach.

Figure 29. Comparison between FEFLOW solute isofringes and semi-analytical fronts

for 6-well system at 125, 365, 1095 and 3650 days

Documents

Groundwater Modelling to Assist Well-Field Design and Operation … › publications › technical2004 › tr27-04.pdf · 2005-10-14 · Groundwater Modelling to Assist Well-Field