Groundwater Processes at the Tamala Park Landfill Facility · Groundwater Processes at the Tamala Park Landfill Facility . Data integration and numerical simulations . M. G. Trefry,

Groundwater Processes at the Tamala Park Landfill Facility Data integration and numerical simulations M. G. Trefry, G. B. Davis and R. J. Woodbury 8 February 2008 Report to Mindarie Regional Council

Water for a Healthy Country report series ISSN: 1835-095X

Enquiries should be addressed to: Dr Mike Trefry CSIRO Land and Water, Private Bag 5, Wembley WA 6913, Australia Email: [email protected]

Copyright and Disclaimer © 2008 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.

Important Disclaimer CSIRO 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 (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.

Contents Executive Summary.....................................................................................................iv

1. Introduction..........................................................................................................1

2. Summary of Recent Investigations....................................................................1

3. Development of a Groundwater Model ..............................................................5 3.1 Model Grid and Boundary Conditions ...................................................................... 5 3.2 Groundwater Abstraction and Local Regime............................................................ 7 3.3 Precipitation and Recharge ...................................................................................... 8 3.4 Fluxmeter Experiments........................................................................................... 13 3.5 Steady Relationships Between Hydraulic Conductivity and Recharge .................. 13

3.5.1 An Upper Bound on Hydraulic Conductivity ........................................................ 15

4. Numerical Simulations of Flow and Solute Transport ...................................15 4.1 Flow Boundary Conditions...................................................................................... 16 4.2 Flow Calibration ...................................................................................................... 16 4.3 Leachate Production and Migration........................................................................ 21 4.4 Discussion and Further Work ................................................................................. 31

5. Conclusions .......................................................................................................32

References...................................................................................................................34

Appendix A – Head Observations .............................................................................36

Appendix B – Ammonia Observations......................................................................38

i

List of Figures Figure 2.1.1: QuickBird digital satellite image of the Tamala Park Landfill Facility and environs.

Image taken in June 2007 in 3-band multispectral mode at 0.6 m spatial resolution; resampled to 300 dpi for reproduction here. ..........................................................................2

Figure 2.1.2: QuickBird digital satellite image of the Tamala Park Landfill Facility and environs, showing groundwater bores and sampling locations of interest to the superficial aquifer system. ...................................................................................................................................3

Figure 2.1.3: Zoomed image of the Tamala Park Landfill Facility, showing groundwater bores and sampling locations of interest to the superficial aquifer system......................................4

Figure 3.1.1: Boundary conditions and mesh detail for the TPLF site. .........................................5

Figure 3.1.2: Triangular finite-element mesh developed for the TPLF groundwater model. Local mesh refinement is visible at the abstraction locations “Neerabup Q40 (QL10)” and “Wash Bay”, and also at a potential future Tamala Park Regional Council redevelopment location “TPRC”. ..................................................................................................................................6

Figure 3.2.1: Observed temporal variation of heads at the upgradient bores BB3, BB5 and TPL2. The blue line charts the mean of the elevations at the three bores, used as the model upgradient boundary condition, while the error bars indicate the spread of the bore heads. The vertical grey line shows the time of the UF fluxmeter experiment in 2006. ........7

Figure 3.2.2: Abstraction data for the four nearby Neerabup production bores QL-10, QO-10, QS-10 and QX-10...................................................................................................................8

Figure 3.3.1: Monthly rainfall totals since 1990 for the three nearby meteorological recording stations. ..................................................................................................................................9

Figure 3.3.2: Mean monthly rainfall trace (black) for Tamala Park Landfill Facility estimated from rainfalls at the three nearby meteorological recording stations (Yanchep, Wanneroo and Beenyup). Site rainfall measurements (pink) only commenced recently...............................9

Figure 3.3.3: Mean annual rainfall since 1990 averaged over the three nearby meteorological recording stations. ................................................................................................................10

Figure 3.3.4: Locations of the six landfill stages for TPLF. Stage 2 Phase 3 has not yet commenced operation..........................................................................................................11

Figure 3.4.1: A typical parabolic head profile (solid line) between a groundwater divide (x = L) and a discharge point (x = 0) for the case of positive recharge (R > 0). If recharge was zero (R = 0), the head profile would be linear (dashed line) for a specified head hmax at x = L...14

Figure 4.1.1: Historical abstraction volumes for the TPLF Wash Bay Bore (data supplied by MRC). ...................................................................................................................................16

Figure 4.2.1: Simulated heads (blue) versus measured heads (red squares) for the calibrated groundwater model. The vertical blue line at the right hand end of the plots indicates the date interval during which the University of Florida flux meter tests were carried out.........19

Figure 4.2.2: Flow paths (red) integrated forward in time for 100 years using the final time step (late March 2007) of the calibrated groundwater model. The paths start on the westward boundary of Stage 1 and move westwards. The Wash Bay Bore eventually captures a significant proportion of that flow..........................................................................................20

ii

Figure 4.2.3: Flow paths (red) integrated forward in time for 100 years using a steady flow solution with abstraction rates as at late March 2007. The paths start on the westward boundary of Stage 1 and move westwards. The Wash Bay Bore is switched off and an equivalent abstraction is located in the vicinity of BB22. ..................................................... 20

Figure 4.2.4: Flow paths (red) integrated backwards in time using a steady flow solution with three abstraction wells each at nominal abstraction rates of 80 m3 d-1. The Wash Bay Bore is switched off....................................................................................................................... 21

Figure 4.3.1: Simulated solute breakthrough curves (blue) compared to observed ammonia concentrations (red). The simulated curves are generated using a nominal Cmax = 100 mg/L NH3 with a PL+PC scenario. ................................................................................................ 23

Figure 4.3.2: Simulated solute breakthrough curves (blue) compared to observed ammonia concentrations (red). The simulated curves are generated using a nominal Cmax = 100 mg/L NH3 with a delayed leachate production scenario. .............................................................. 24

Figure 4.3.3: Simulated solute breakthrough curves (blue) compared to observed ammonia concentrations (red). The simulated curves are generated using a least-squares fitted Cmax = 40 mg/L NH3 value with a delayed and ramped leachate production scenario............... 25

Figure 4.3.4: Simulated solute breakthrough curves (blue) compared to observed ammonia concentrations (red) for bore BB18. The simulated curves are generated using a least-squares fitted Cmax = 40 mg/L NH3 value with a delayed and ramped leachate production scenario................................................................................................................................ 26

Figure 4.3.5: Measured head profiles at three bores. Data collected in April 2005.................... 27

Figure 4.3.6: Simulated solute distribution for early 2002, for Cmax = 100 mg/L. Both cells in Stage 1 are in full leachate production. ............................................................................... 28

Figure 4.3.7: Simulated solute distribution for early 2005, for Cmax = 100 mg/L. Stage 1 North is in full leachate production, while Stage 1 South has ceased leachate production.............. 29

Figure 4.3.8: Simulated solute distribution for late March 2007, for Cmax = 100 mg/L. Both cells in Stage 1 have ceased leachate production. Capture of dilute solute by the Wash Bay Bore is visible along the south-western edge of the plume. ................................................ 30

List of Tables Table 3.3.1: Dates of major landfill staging operations............................................................... 10

Table 3.3.2: Recharge function timings for PL+PC base scenario. Base recharge rate is R = 25% of rainfall. ..................................................................................................................... 12

Table 3.4.1: Estimated groundwater speeds from BB20 and BB21 from fluxmeter data collected in September 2006. A matrix porosity of 0.3 was assumed. ............................................... 13

Table 4.2.1: Residual values Ω and groundwater speeds as a function of hydraulic conductivity K. Speeds are calculated at the phreatic surface. ............................................................... 17

Table 4.3.1: Leachate production timings for PL+PC base scenario. Cmax indicates the presence of leachate concentration at the water table. ....................................................................... 22

iii

Executive Summary

An extensive data set of hydrological and water quality parameters has been collected at the Tamala Park Landfill Facility (TPLF) over the past 17 years. This data set covers much of the major operational stages of the Facility to date. Integration of the data permits the construction of a quantitative groundwater model for the site which can be used to assess the impact of the Facility on the subsurface environment and to gauge the potential effects of management interventions.

This report describes the development of a transient and three-dimensional groundwater model for TPLF, based on the measured data set. The model incorporates:

• Spatial zonation and gridding aligned with GIS-based maps of site activities

• High-resolution, geo-referenced aerial photography of the site and surrounds

• Long-term historical rainfall and pumping records

• Distributed recharge zones based on vegetation cover

• Sixty-five head and solute monitoring locations with individual screen depths

A key feature of the site data was a sustained 0.5 m drop in groundwater heads along the eastern boundary just after the year 2000. The model boundary conditions were adjusted to reflect this drop. The model was calibrated against 139 head values measured at the site from 1990-2007. Best agreement with measured heads was found when the western half of the model domain was assigned a lower hydraulic conductivity than the eastern half, i.e. the conductivity distribution was non-uniform.

Simulations of leachate generation based on the calibrated flow model were able to reproduce observed ammonia breakthrough curves at most wells, except for those wells directly underneath the body of the landfill. In this case, breakthrough curves were variable, perhaps reflecting the effects of preferential flows and high-permeability zones. Further downgradient the agreement between observation and simulation improved as spatial averaging of the solute plume took hold.

Conclusions of the modelling study are:

• Local rainfall is well described by a simple average of rainfall traces from three surrounding weather stations.

• There is no need to describe the local aquifer using a spatially random conductivity field – uniform or composite aquifer structures are sufficient to reproduce the observed heads.

iv

• It is unlikely that operation of the Neerabup Bore Field (for potable supply) has mobilized leachate plumes towards the east or north. In fact, the Neerabup Bore Field may have reduced the mobility of leachate plumes under the site.

• The leachate plume appears to be located within the site boundaries. This is in contrast to an earlier modelling study which raised the possibility of rapid off-site movement to the west.

• Further monitoring is warranted to remain vigilant against rapid changes in site flow conditions (caused by external influences) which may potentially mobilize the leachate plume.

• Simulations indicate that migrating leachate may potentially be intercepted by the use of several abstraction wells along the western boundary.

v

1. INTRODUCTION

The Tamala Park Landfill Facility (TPLF) is WA’s biggest landfill facility. It is managed by Mindarie Regional Council (MRC) and provides centralized waste disposal services for the MRC member local government councils. At present the following councils are members of MRC:

• Town of Cambridge

• City of Joondalup

• City of Perth

• City of Stirling

• Town of Victoria Park

• Town of Vincent

• City of Wanneroo

Because of the expected long duration of the TPLF (projected to last 30 years from 1990) and the large annual load of waste disposed to landfill (340 000 tonnes in 2006), there is potential for the facility to affect the surrounding environment. MRC has commissioned a series of investigations to elucidate and monitor environmental and health impacts arising from the TPLF operations. The subject of the present report is the study of impacts on groundwater beneath the TPLF. The issue of potential impacts to local groundwater systems was recognized early in the lifetime of the facility and a comprehensive and regular set of investigations was set in train (see next section). The present report summarises data collected over the previous 17 years and integrates the data sets into a groundwater flow and transport model suitable for simulating macroscale groundwater and contaminant dynamics for the TPLF site and locations west to the coastline. The report is commissioned by MRC and was researched and compiled by CSIRO Land and Water at the Floreat Centre for Environment and Life Sciences in 2007.

2. SUMMARY OF RECENT INVESTIGATIONS

The waste disposal and landfill activities at TPLF engender risk of contamination and other impacts to the local groundwater resource and to nearby population centres and special use areas. As a general rule, the groundwater maintains a westerly to south-westerly flow toward the beachfront at Mindarie (under the Tamala Park dunes to the west), so that risks of contamination to the near-shore marine environment also need to be assessed and managed. For these reasons, TPLF has been under constant monitoring and review since excavation operations commenced in 1989. Figure 2.1.1 shows an aerial view of TPLF and the population centres to the north (Quinns Rocks) and south (Kinross). Marmion Avenue is a main arterial road running parallel to the coast on the western boundary of TPLF, with Connolly Drive forming the eastern boundary.

Groundwater Processes at the Tamala Park Landfill Facility • 8 February 2007 1

Figure 2.1.1: QuickBird digital satellite image of the Tamala Park Landfill Facility and environs. Image taken in June 2007 in 3-band multispectral mode at 0.6 m spatial resolution; resampled to 300 dpi for reproduction here.

Environmental Monitoring

Significant investment in site monitoring infrastructure has been made over the last 20 years (Figures 2.1.2 and 2.1.3). CSIRO has played a regular role in the environmental monitoring and review activities over that time, particularly with respect to the underlying groundwater system. Barber et al. (1990) discussed a methodology for performing consistent groundwater monitoring at the facility, which was further refined through reference to numerical simulations of groundwater flow (Davis and Laslett, 1991). A summary of outcomes from the first five years of monitoring was produced by Davis et al. (1993), and then several summary reports were produced (Height et al., 1994; Davis et al., 1996; Davis and Briegel, 1997). This monitoring reporting was continued on an annual basis (Davis and Briegel, 1998; Davis et al., 1999; Davis


et al., 2000a,b; Davis et al., 2001; Davis et al., 2002; Davis et al., 2003; Davis et al., 2004; Davis et al., 2005; Davis et al., 2006).

Figure 2.1.2: QuickBird digital satellite image of the Tamala Park Landfill Facility and environs, showing groundwater bores and sampling locations of interest to the superficial aquifer system.

Planning for Staging Operations

Around 1996, work commenced on planning the commissioning of future landfill cells and the decommissioning of filled cells. Dames and Moore (1996) produced an initial management plan, which was further elaborated to a technical staging plan by BSD Consultants (1999). As part of the scoping of staging scenarios for TPLF, CSIRO was asked to assimilate existing hydrological data and produce estimates of leachate migration to the groundwater system and resulting migration to the Mindarie beachfront. Davis (1999) reported on lag times for leachate


development and mobilization, while Trefry and Davis (1999) employed a conservative approach to estimate travel times of leachate plumes from various staging and capping scenarios. The latter report emphasized the need for measurements of hydrological properties at the site. Groundwater monitoring continued at the site (Davis et al., 2006), but attempts to measure head gradients at TPLF met with limited success, essentially because the gradients were low and comparable with survey errors.

Figure 2.1.3: Zoomed image of the Tamala Park Landfill Facility, showing groundwater bores and sampling locations of interest to the superficial aquifer system.

Hydrological Investigations

In 2007, researchers from CSIRO, University of Florida, and Purdue University performed a series of groundwater tests designed at measuring groundwater flow velocities directly in situ using proprietary fluxmeter technology. Results of beachfront spear probing for groundwater contamination signatures, direct groundwater velocity measurements, general groundwater quality monitoring and extra bore installations were presented (Davis et al., 2007).


3. DEVELOPMENT OF A GROUNDWATER MODEL

Construction of a groundwater model represents a formal means by which site data can be compiled and integrated into a rigorous quantitative framework. This allows diverse data sets, including climatic, geological and hydrological data, to be compared, tested and incorporated into the site conceptual and hydrological models. In principal, as more data is collected, the groundwater model can evolve to greater levels of detail and sophistication, potentially permitting more accurate predictions of hydrological processes, including fluid levels and leachate plume migration. In this report, the first groundwater modelling exercise in eight years, substantially more data is available with which to constrain the groundwater model properties and behaviour.

3.1 Model Grid and Boundary Conditions

Numerical groundwater simulations rely on the use of finite spatial grids. In order to represent the site hydrology, boundary conditions are employed. Choice of grid boundary location and boundary condition type are critical in model development. The previous modelling study employed a rectangular (in plan) grid with a fixed head condition at the beach line, and another fixed head condition at the north-eastern edge of the grid, along a notional groundwater head contour of 1.1 m AHD (Trefry and Davis, 1999). Edges of the grid perpendicular to the beach were set to no-flow conditions, as they lie approximately along streamlines of flow to the beach.

Figure 3.1.1: Boundary conditions and mesh detail for the TPLF site.


In the present report, this general approach is employed again with some extra enhancements. First, the curvature of the beach line is incorporated to improve the fidelity of streamlines at the coast. A constant fixed head of 0.04 m AHD (mean sea level) is used for the ocean elevation boundary condition at the coast line. For the upstream boundary, the available data for the site shows that three wells (BB3, BB5 and TPL2) lie along the western edge of Connolly Drive. These wells are approximately equidistant from the coast and exhibit similar groundwater head variations in time (see Figure 3.2.1). The model grid is depicted in isolation in Figure 3.1.1, and superimposed on the site photograph in Figure 3.1.2. Mesh refinement is visible at key locations of abstraction wells.

Figure 3.1.2: Triangular finite-element mesh developed for the TPLF groundwater model. Local mesh refinement is visible at the abstraction locations “Neerabup Q40 (QL10)” and “Wash Bay”, and also at a potential future Tamala Park Regional Council redevelopment location “TPRC”.


3.2 Groundwater Abstraction and Local Regime

Heads measured at the upgradient bores BB3, BB5 and TPL2 are presented in Figure 3.2.1. It is not clear why the mean upgradient head dropped by approximately 0.5 m during 2001 and 2002. There has been no rebound of the upgradient heads in the five years since. This unusual change seems to be consistent with observed trends elsewhere in the North Metropolitan coastal corridor (J. Miotti, Water Corporation, personal communication, 3 Sep 2007). It is possible that there may have been a change in survey datum, but no evidence of this has been found in site literature and, failing that, the change appears to be too large and consistent to be explained by surveying errors. Another possibility is that significant pumping abstraction commenced in the vicinity of the site around that time. Site records show that a site bore, located at the Wash Bay, was activated in July 2004 with a licensed annual abstraction of 25 ML, increased to 45 ML in 2006. This is incorporated in subsequent modelling (see later sections) and is shown to be too small to influence water levels changes of the magnitudes observed. The commencement of operations of the Neerabup production bore field in 2001 correlates well with the abrupt drop in water levels, although this does not explain the simultaneous drop of levels over a much wider areas mentioned earlier. Data provided by Water Corporation shows that there are four active production bores located in the vicinity of TPLF. These bores are QL-10, QO-10, QS-10 and QX-10 (see Figure 2.1.2). Records of monthly abstractions for these bores are shown in Figure 3.2.2. The data in Figure 3.2.2 show average annual abstractions of 1.08 GL, 1.15 GL, 0.92 GL, 0.56 GL for the bores QL-10, QO-10, QS-10, QX-10, respectively, where the averaging period is the six calendar years between January 2001 and January 2007.

Figure 3.2.1: Observed temporal variation of heads at the upgradient bores BB3, BB5 and TPL2. The blue line charts the mean of the elevations at the three bores, used as the model upgradient boundary condition, while the error bars indicate the spread of the bore heads. The vertical grey line shows the time of the UF fluxmeter experiment in 2006.


Figure 3.2.2: Abstraction data for the four nearby Neerabup production bores QL-10, QO-10, QS-10 and QX-10.

3.3 Precipitation and Recharge

Precipitation provides a major component of the water balance in the Mindarie coastal catchment. Presently, approximately 700 mm of rain falls on the catchment each year. Due to the sandy nature of the local soils, rainfall tends to seep into the soils readily. However, the presence of native vegetation over much of the area surrounding TPLF means that the potential for evapotransporation of rainfall is high, as is the case elsewhere on the Swan Coastal Plain. This means that the net recharge of infiltrating rainfall to the water table is likely to be low, except where vegetation is absent, e.g. cleared landfill cells at TPLF.

In the first set of groundwater simulations for the site, Davis and Laslett (1991) used an approximate recharge rate of 20% of annual rainfall over areas covered by native banksia woodland (Farrington and Bartle, 1991), a value of 30% for areas of native kwongan vegetation, and 40% of rainfall over cleared areas, based on lysimeter data collected at TPLF by Davis et al. (1991). Trefry and Davis (1999) used a cyclic annual recharge signal that was zero during summer and constant during winter, summing to a net value of 200 mm per year. This was equivalent to approximately 25-30% of annual rainfall.

As there is only limited rainfall data collected at TPLF which could be used to estimate recharge variations over recent years, data was sourced from the nearby Beenyup, Wanneroo and Yanchep meteorological stations to construct an estimated rainfall trace for TPLF. Figure 3.3.1 shows the three rainfall traces from the three stations observed from 1990 onwards. Correlation between the three traces is high. Figure 3.3.2 shows the mean of the three traces; this mean trace


is henceforth used as a surrogate for the actual rainfall trace at TPLF. Figure 3.3.3 shows the annual rainfall totals constructed from the mean rainfall trace of Figure 3.3.2.

Figure 3.3.1: Monthly rainfall totals since 1990 for the three nearby meteorological recording stations.

Figure 3.3.2: Mean monthly rainfall trace (black) for Tamala Park Landfill Facility estimated from rainfalls at the three nearby meteorological recording stations (Yanchep, Wanneroo and Beenyup). Site rainfall measurements (pink) only commenced recently.


In view of the significant fluctuations of the upgradient heads, modelling of the recharge signal becomes more important. In this report, the simulations will involve recharge signals constructed from the estimated TPLF rainfall trace presented in Figure 3.3.2. This approach will automatically scale recharge inputs to the TPLF water table by the actual rainfall trace operating at the site. Figure 3.3.3 shows that the annual rainfalls are diminishing slightly in recent years.

Figure 3.3.3: Mean annual rainfall since 1990 averaged over the three nearby meteorological recording stations.

Table 3.3.1: Dates of major landfill staging operations.

Stage and Phase Design Notes Start of Fill End of Fill Capping

Stage 1 North unlined, permanent cap 28 February 1991 15 November 2004 22 February 2005

Stage 1 South unlined, permanent cap 28 February 1991 6 January 2003 26 July 2003

Stage 2 Phase 1 lined, permanent cap 15 July 2004 15 September 2006 15 September 2006

Stage 2 Phase 2 East lined, still active 1 September 2006 - -

Stage 2 Phase 2 West lined, still active 29 May 2007 - -

Stage 2 Phase 3 not commenced - - -

Operations at TPLF were scoped in 1999 as involving individual staging activities for each of the landfill cells (Trefry and Davis, 1999). Predictive simulations presented in that report included scenarios for lined and unlined cells, with and without capping. In the present case we are able to incorporate actual critical dates for cell operations based on site records provided by MRC. The spatial resolution of the historical operations data is somewhat coarser than the actual cell dimensions. Table 3.3.1 shows the critical operation dates for six major landfill phases at TPLF. In simple terms, landfill activities commenced at the eastern end of the landfill


area, then slowly progressed west as cells were filled and capped. Figure 3.3.4 shows the locations and extents of the six landfill phases.

Figure 3.3.4: Locations of the six landfill stages for TPLF. Stage 2 Phase 3 has not yet commenced operation.

Recent estimates of recharge rates for TPLF are lacking, so we take guidance from previous studies. Following Trefry and Davis (1999), we assume that the base recharge rate over the undisturbed land surrounding TPLF equals 25% of annual rainfall. In a recent study in north-east USA, Loehr and Haikola (2003) noted that effective leachate production rates were


approximately two-thirds of net precipitation over uncovered landfill cells. This is much higher than the rates used for TPLF, however the net evaporative potential at TPLF is likely to be much higher than applies in north-eastern USA. Within the facility, we employ a “perfect lining and perfect capping” (PL+PC) assumption for the operations of the different staging activities. The PL+PC assumption is used as a base case against which other scenarios can be compared. In the PL+PC case, recharge to the water table beneath active lined cells is assumed to be zero, as is recharge beneath capped cells. This is an idealization, since no capping or liner is perfectly water tight, and there may be some continuation of recharge after lining/capping as soil moisture contents stabilise in the vadose zone beneath landfill cells. Nevertheless, it is held that the approximations involved in the idealization are small and unlikely to affect the important outputs of the model (i.e. solute plume migration and location) at the scales of interest.

Table 3.3.2 shows the set of recharge functions used to model the base PL+PC scenario. In constructing this table a further assumption was made about the effective recharge rate for unlined cells during filling activities. It was assumed that recharge through unlined cells was reduced to 60% of the base recharge rate. This reduction is intended to account for a “wetting up” phase of the waste material placed in the cells. The time taken for uncovered waste material to reach equilibrium saturation depends on the type of waste material and the precipitation rate. Once wetted up, the waste material will produce leachate which will move under gravity to the bottom of the cell. If the cell is unlined, the leachate will migrate to the water table and will cause a plume of dissolved contamination. If the cell is lined, the leachate will accumulate unless drained via a primary leachate drainage system. Loehr and Haikola (2003) also show that leachate production declines rapidly after cell capping is installed, with even high leachate production rates dropping to minor levels after 1-2 years. In this report we assume that effective recharge (i.e. leachate production) stops immediately that capping is applied.

Table 3.3.2: Recharge function timings for PL+PC base scenario. Base recharge rate is R = 25% of rainfall.

Stage and Phase Prior to Fill During Fill After Fill

Stage 1 North R 3R/5; 28 Feb 91 to 22 Feb 05 0

Stage 1 South R 3R/5; 28 Feb 91 to 26 Jul 03 0

Stage 2 Phase 1 R 0; 15 Jul 04 – 15 Sep 06 0

Stage 2 Phase 2 East R 0; 1 Sep 06 - now -

Stage 2 Phase 2 West R 0; 29 May 07 - now -

Stage 2 Phase 3 R - -


3.4 Fluxmeter Experiments

The fluxmeter experiments carried out by University of Florida and CSIRO teams in 2006 (Davis et al., 2007) provide valuable information on groundwater flow speeds at TPLF. These experiments use synthetic materials containing compounds that desorb in water to assess effective groundwater speeds. The central idea is that a material of known concentration of desorbant is placed in the groundwater flow field using an in situ bore. The material is left for a period of time, during which desorption occurs at a rate dependent on the local groundwater flow speed. The material is recovered and the amount of desorbant remaining is measured. Table 3.4.1 shows estimated groundwater flow speeds from the University of Florida -CSIRO experiments at bore locations BB20 and BB21 (Davis et al, 2007).

Table 3.4.1: Estimated groundwater speeds from BB20 and BB21 from fluxmeter data collected in September 2006. A matrix porosity of 0.3 was assumed.

Bore Min Speed (m d-1)

Mean Speed (m d-1)

Max Speed (m d-1)

BB20 0.003 0.06 0.13

BB21 0.07 0.27 0.7

Table 3.4.1 shows that there is significant variability in the groundwater speeds between the two studied bores. Disparities in speeds between the bores range approximately over a factor of 5, whilst vertical variations in speeds within a single bore study are of a similar order (Davis et al., 2007). This gives us partial information on spatial scales of heterogeneities at TPLF. Bores BB20 and BB21 are 315 m apart horizontally, while the scale of vertical variation is as low as 1-2 m. Based on this data, it is likely that the aquifer materials display strong stratigraphic bedding with vertical correlation scales of the order of 1 m or less (see core photos in Davis et al., 2007). There is very sparse evidence of at least one long-range horizontal correlation at a length scale less than the order of hundreds of metres. In order to define better the scales of heterogeneity more field experimentation would be required. Henceforth in this study the hydraulic conductivity field is assumed to be spatially homogeneous.

3.5 Steady Relationships Between Hydraulic Conductivity and Recharge

The process of developing a detailed computational model of groundwater flow and solute transport processes is aided by gaining a preliminary understanding of key hydrological parameters operating at the site. Apart from boundary conditions, the two most important hydrological parameters are the effective recharge, R (units of metres per day, m d-1), and the effective saturated hydraulic conductivity, K (units of metres per day, m d-1). This can easily be seen from the exact solution to the one-dimensional, steady, confined groundwater flow equation


0=+⎟⎠⎞

⎜⎝⎛

∂∂

∂∂ R

xhKB

x (3.1)

which describes flow in a homogeneous and uniform aquifer subject to uniform recharge in the absence of pumping. Here h is the piezometric head (units of metres above Australian Height Datum, m AHD), B is the effective thickness of the aquifer (units of metres, m) and x represents a horizontal spatial coordinate (units of metres, m). The use of eqn (3.1) is a simplification in that it ignores nonlinear effects due to the phreatic surface (water table) and capillarity, but it is a reasonable approximation at the regional scale. In order to represent the regional context of the TPLF, we apply boundary conditions suitable to describe a reach of aquifer extending from a regional groundwater mound (i.e. the Gnangara Mound) to a distant discharge point (the beachline). These boundary conditions are:

)dividergroundwateupgradient(0)(')boundarybeach()0( 0

==

Lhhh

(3.2)

Solving eqns (3.1) and (3.2) simultaneously yields the exact parabolic head profile of eqn (3.3):

( )BK

xLxRhxh

22

)( 0−

+= (3.3)

The maximum head at the top of the distant groundwater mound is

KBRLhLhh 2/)( 20max +=≡ (3.4)

Clearly, ignoring the discharge datum h0, the head profile h(x) is scaled by the ratio R/K so that, in the absence of independent constraints on the values of R and K, heads may effectively be calibrated to the ratio of R and K. Figure 3.4.1 shows a parabolic head profile extending from a groundwater divide at x = L to the ocean discharge boundary at x = 0. For a transect extending perpendicularly from the Mindarie beachline through TPLF to the top of the Gnangara Mound, the appropriate spatial scale is L ≈ 12 000 m.

Figure 3.4.1: A typical parabolic head profile (solid line) between a groundwater divide (x = L) and a discharge point (x = 0) for the case of positive recharge (R > 0). If recharge was zero (R = 0), the head profile would be linear (dashed line) for a specified head hmax at x = L.


3.5.1 An Upper Bound on Hydraulic Conductivity

From Table 3.4.1 we see that the fluid velocity magnitudes measured during the University of Florida tests in September 2006 were clustered around the range 0.1-0.27 m d-1. If we ignore the variance of the velocity distribution and consider only the highest representative value of 0.27 m d-1, then we may constrain the mean hydraulic conductivity as follows.

The fluid velocity v is given by

hnKv ∇= (3.5)

The head gradient h∇ may be approximated crudely by the slope of the line between the beachline head, h = 0.04 m AHD, and the head observed at TPLF eastern boundary (2800 m from the beach) during the University of Florida tests, h = 0.54 m AHD. This linearized slope is

( ) 00018.02800/04.054.0linear =−−≈∇h (3.6)

Since the TPLF is closer to the beach than it is to the groundwater divide, i.e. x < L/2, the estimated slope magnitude is likely to underestimate the true slope magnitude induced by recharge effects. Re-arranging eqn (3.5) and applying eqn (3.6) shows that

1linear dm450)00018.0/()3.0)(25.0(// −=≈∇≤∇= hnvhnvK (3.7)

Thus, based on the University of Florida tests, we are able to provide an upper bound to the mean hydraulic conductivity of 450 m d-1 in agreement with the estimate of Davidson (1995), but somewhat lower than the value of 518.4 m d-1 assumed by Trefry and Davis (1999) to estimate worst-case plume migration scenarios. Interestingly, the indicated K value is much too high to support the observed hmax of approximately 60 m AHD at the groundwater divide, even for relatively high recharge rates. A hydraulic conductivity of 50 m d-1 together with recharge of 500 mm yr-1 (i.e. greater than 50% of annual rainfall) would be required to yield a 60 m mound at a distance of L = 12000 m inland. This indicates that it is possible that the groundwater flow conditions at TPLF are controlled by local highly conductive features that do not extend throughout the body of the Gnangara mound; these features may be responsible for promoting rapid flows and low head gradients.

4. NUMERICAL SIMULATIONS OF FLOW AND SOLUTE TRANSPORT


4.1 Flow Boundary Conditions

The finite element mesh shown in Figure 3.1.1 was expanded into a three dimensional grid consisting of seven layers (eight slices), totalling 61 568 nodes and 106 876 triangular prism elements. Spatially dependent recharge functions were applied to the upper surface of the model grid, representing specific recharge behaviours for each of the 6 landfill stage zones, plus a default recharge behaviour for the land surface outside the TPLF stages. Each of these seven behaviours were related to the mean rainfall trace discussed in Figure 3.3.2, using a recharge rate of 25% of the rainfall time series, and subject to capping and lining interventions where appropriate. The upgradient head was set to mimic the estimated boundary head of Figure 3.2.1, and the Wash Bay Bore abstraction trace was set to the historical data shown in Figure 4.1.1. Note that some of the water extracted through the Wash Bay was applied directly to the landfill area, either to suppress dust or to extinguish occasional fires. It is believed that the majority of this application would be lost to evaporation since it typically occurs in dry and/or windy conditions.

The three-dimensional model was then run in calibration mode to allow an optimal value of the uniform hydraulic conductivity K to be estimated against measured heads at the site.

Figure 4.1.1: Historical abstraction volumes for the TPLF Wash Bay Bore (data supplied by MRC).

4.2 Flow Calibration

Flow calibration proceeded by making an initial estimate of the uniform K value, running the flow model for 6300 days commencing on 1 January 1990 (finishing in early 2007) and saving predicted heads at locations where head measurements were collected as part of the TPLF field program. A sum of squares residual between the predicted heads (hpred) and the observed heads (h obs) was then calculated and used as an estimator of goodness of fit between the flow model and measured heads. The residual measure was then minimized by choosing a new K value and repeating the process. Appendix A shows the 72 measured head values used in the calibration process.


The residual measure, Ω, was defined as

( ) nhhi

predi

obsi /

2

⎟⎟⎠

⎞⎜⎜⎝

⎛−=Ω ∑ (4.1)

where n is the total number of observations calibrated against, and the units of Ω are metres; Ω corresponds to the root-mean-square error between the observed and predicted heads. Table 4.2.1 shows the variation of Ω as the uniform K value is varied.

Table 4.2.1: Residual values Ω and groundwater speeds as a function of hydraulic conductivity K. Speeds are calculated at the phreatic surface.

Model K (m/d) Ω (m) Speed (m/d) BB20 BB21

1 400 0.168 0.268 0.250 2 200 0.147 0.127 0.106 3 100 0.111 0.059 0.041 4 50 0.085 0.025 0.011

5 (composite) 200/100 0.089 0.075 0.049

A declining trend in residual is indicated as K gets smaller, whilst most simulated groundwater speeds are within the ranges of variation reported from the fluxmeter experiments (Table 3.4.1), although the fluxmeter estimates for Bore BB21 tend to be higher than predicted by the simulations. Further comparison of the predicted and observed head values indicates that the uniform K model underestimates the heads at the downstream bores BB19 and BB20. A further reduction in Ω can be gained by defining a zone of low-K value to the west of Marmion Avenue. Setting the K value to 100 m/d inside this zone and retaining the 200 m/d value outside (see the last record in Table 4.2.1) yields an Ω value comparable to the value gained for the uniform 50 m/d simulation, i.e. a mean error of 8 cm across all head observations. Whilst this is a useful result, it is not proof that the coastal margins of the aquifer indeed consist of lower conductivity aquifer materials than to the east of Marmion Avenue. Rather, this calculation shows that there is considerable scope for matching well the observed heads using a variety of different structural models for the local hydraulic conductivity distribution. A simple quantity like Ω does not incorporate all the characteristics of a suitable solution. For example, simulations for low values of K across the domain (e.g. Model 4 of Table 4.2.1) are contraindicated, as they tend to display significant reverse flows to the east and large drawdowns near the abstraction wells. These effects are not thought to be present at the site, thus there does seem to be some evidence that the aquifer is not uniform which, in turn, implies that the simple estimates of K discussed in Section 3.5 may need to be revised downwards to take account this macroscale heterogeneity into account. The situation would be clarified by a more comprehensive groundwater monitoring program, i.e. continuous water level monitoring at a range of bores across the site and further downgradient. In the absence of such comprehensive data for model calibration, we will assume that the aquifer does not have a uniform hydraulic conductivity distribution, and we will pursue the composite aquifer model (Model 5 of Table 4.2.1).


Figure 4.2.1 shows a set of plots comparing the calibrated head simulations over a 17 year period from 1990 with heads measured at the site and time-referenced appropriately. Agreement is generally good, although calibration data is lacking for the part of the grid near the shoreline. The yearly oscillations in heads are driven by the assumed recharge signal from Figure 3.3.2. The upgradient reduction in heads of approximately 0.5 m shortly after the year 2000 is reflected through a similar step change in heads in Figure 4.2.1. Noting that the western boundary head (sea level) has remained unchanged, this 50% reduction in heads means that mean groundwater fluxes to the west reduced by approximately 50% as well, profoundly affecting the rate of movement of any dissolved solutes under TPLF.

In contrast to the 1999 modelling scoping study (Trefry and Davis, 1999), which predicted potential breakthrough of solutes to the beach shortly after the year 2000, the present study uses a significantly lower K value (hence reducing flow speeds according to equation (3.5)) and a regime post-2000 of reduced head gradient (thereby further reducing flow speeds). The net result is that solutes are now far less likely to migrate significant distances downgradient than was originally thought in 1999.

In an attempt to gauge the relative effect of the recent hydrological changes, we consider fluid flow paths calculated 100 years forward in time from locations on the western boundary of Stage 1, based on the final time step of the present calibrated flow solution. This is an artificial example that is not meant to convey an accurate prediction of leachate migration at the site, since it is impossible to extrapolate current hydrological conditions forward with any degree of certainty. The purpose of the 100 year interval is to place the present hydrological conditions in context with the prior conditions that led to predictions of rapid solute migration.

Figure 4.2.2 shows simple flow paths calculated forward in time by 100 years, assuming that the flow regime of the final time step (late March 2007) of the calibrated run extends forward as a steady state flow. Clearly the flow velocities at the end of summer 2006/07 are very low, and there is significant potential for leachate solutes to reside under TPLF for extended periods (decades) if the present flow conditions are maintained. It may be that the present conditions will prove to be a short-lived anomaly, so no special significance should be placed on the flowpaths depicted in Figure 4.2.2, other than as an indication of the presently low rate of fluid migration under TPLF. Based on the width of the capture zone of the Wash Bay Bore in Figure 4.2.2, it is conceivable that an abstraction bore could be used to capture significant portions of the dissolved leachate plume. Figure 4.2.3 shows the potential capture zone of a similar abstraction well located near BB22 (in the absence of abstraction from the Wash Bay Bore). The capture zone of the well covers approximately 1/3 of the landfill area.


Figure 4.2.1: Simulated heads (blue) versus measured heads (red squares) for the calibrated groundwater model. The vertical blue line at the right hand end of the plots indicates the date interval during which the University of Florida flux meter tests were carried out.


Figure 4.2.2: Flow paths (red) integrated forward in time for 100 years using the final time step (late March 2007) of the calibrated groundwater model. The paths start on the westward boundary of Stage 1 and move westwards. The Wash Bay Bore eventually captures a significant proportion of that flow.

Figure 4.2.3: Flow paths (red) integrated forward in time for 100 years using a steady flow solution with abstraction rates as at late March 2007. The paths start on the westward boundary of Stage 1 and move westwards. The Wash Bay Bore is switched off and an equivalent abstraction is located in the vicinity of BB22.


Figure 4.2.4: Flow paths (red) integrated backwards in time using a steady flow solution with three abstraction wells each at nominal abstraction rates of 80 m3 d-1. The Wash Bay Bore is switched off.

Based on this recovery concept, Figure 4.2.4 shows an indicative simulation for a three-well recovery scenario. In Figure 4.2.4, three wells are arranged along the western boundary of the landfill cells. Each well is assumed to abstract water from the aquifer at a constant rate of 80 m3 d-1, with regional hydrological conditions assumed to be maintained at the March 2007 state (Wash Bay Bore disabled). Backward-in-time flow paths are calculated from the three abstraction wells, showing that the majority of leachate reaching the water table underneath the landfill cells is likely to be captured by one of the three wells. This simulation shows that a simple leachate interception scheme may be possible for TPLF, although clearly the present simple simulations would need to be replaced by a properly engineered interception scheme design for the site.

4.3 Leachate Production and Migration

Having established a groundwater recharge and flow model that correlates well with observed heads and inferred fluxes at the site, we are in a position to consider generation and migration of leachate solutes. Landfill leachates are commonly separated into primary leachates and secondary leachates, according to the following definitions. Primary leachate is that leachate which is collected between the landfill material and an underlying liner. TPLF Stages 2 and 3 are lined, whilst Stage 1 is not. Secondary leachate is that leachate that penetrates any underlying liner and may subsequently migrate to the water table. In principle, any competent liner will prevent secondary leachate production, however it is common for liners to eventually degrade or suffer breaches due to differential subsidence or other geotechnical stresses, thereby


permitting secondary leachate production. There is no direct evidence of secondary leachate production under Stages 2 and 3 at TPLF, hence in the remainder of this report we will focus on the migration of leachate under the unlined Stage 1 cells.

r PL+PC base scenario. Cmax indicates the presence of leachate concentration at the water table.

Stage and Phase Prior to Fill During Fill After Fill

Table 4.3.1: Leachate production timings fo

Stage 1 North No leachate flux Cmax; 28 Feb 91 to 22 Feb 05 No leachate flux

Stage 1 South No leachate flux Cmax; 28 Feb 91 to 26 Jul 03 No leachate flux

Stage 2 Phase 1 No leachate flux No f No leachate flux lux; 15 Jul 04 – 15 Sep 06

Stage 2 Phase 2 East No leachate flux No flux; 1 Sep 06 - now -

Stage 2 Phase 2 West No flux; 29 May 07 - now No leachate flux -

Stage 2 Phase 3 No leachate flux - -

g

d on

f

2

ther inates the possibility of upwards flux due to

diffusive gradients for times of low recharge.

le. The

to

during the initial leachate migration simulations performed under the PL+PC assumptions.

and 0.5 m, respectively. The grid resolution is not fine enough to support lower dispersivities.

The major contaminant of interest at TPLF is ammonia dissolved in the leachate and migratinunder gravity to the water table, where it enters the saturated zone and moves with the local groundwater gradients. As discussed in Section 3.3, the rates of leachate production depenthe local rainfall signal and the staging activities at the site. For an uncapped and unlined staging cell, when rainfall is high there is potential for more rapid downwards migration oleachate than when rainfall is low. This correlation is addressed in the leachate migration simulations by imposing a fixed leachate concentration condition at the water table under the relevant staging cells. The passage of the time-varying recharge fluxes defined by Figure 3.3.and Table 4.3.1 through the fixed leachate concentration condition produces a (downwards) advective leachate flux into the saturated zone that is proportional to the recharge flux. A furconstraint condition on the leachate fluxes elim

The leachate simulations are performed using a surrogate solute species, which may be identified with dissolved ammonia. Ammonia is thought to be conservative and unretarded by the Tamala Limestone and Safety Bay Sands typical of the coastal margins of the Swan CoastalPlain, so the use of a simple conservative tracer as a surrogate for ammonia is reasonablack of reaction terms and the use of a linear dispersion model means that the absolute concentration of the solute specified in the staging cell boundary condition acts as a free scale the results, i.e. the solute concentrations are effectively dimensionless. Table 4.3.1 shows the solute concentration boundary conditions employed under Stage 1

Apart from the imposition of the leachate solute boundary condition in the Stage 1 area, the simulations were identical in every respect to the calibrated recharge+flow model of Section 4.1. Longitudinal and transverse dispersivities were set to 5.0


Breakthrough curves from the initial leachate simulation are presented in Figure 4.3.1. Two properties of the plots in Figure 4.3.1 are notable. First, the vertical scale of the simulated breakthrough curves seems somewhat high. This is simply a function of the arbitrary choice of solute units via the value of Cmax. As discussed earlier, the linearity of the transport problem means that Cmax can be chosen to fit the observed ammonia concentrations without affecting the simulations. In other words, we have no measure of the effective ammonia concentration at the water table on site, so we may estimate this value by calibration of the solute transport simulations. A second important feature is the clear time delay between the simulated breakthrough and the measured presence of ammonia in TPLF bores. As gauged from Figure 4.3.1, this delay is of the order of 1800 days (5 years). This is certainly much longer than reported by Loehr and Haikola (2003), but not implausible for a windy site subject to intense heat in summer. We may seek to improve the agreement between the simulated and measured breakthroughs by introducing a time delay in the action of the leachate boundary condition (see Figure 4.3.2). This provides better temporal agreement, but with an unphysical step change in leachate Cmax value from 0 mg/L to 100 mg/L. We may further adjust this by defining a gradual increase and decrease of Cmax over time to yield a more realistic presence of leachate at the water table under Stage 1. Furthermore, we may adjust Cmax to provide the best fit with the measured ammonia concentrations. Results of this “ramped” leachate process are exhibited in Figure 4.3.3.

The correspondence between simulated and measured ammonia concentrations at bores BB14 and BB15 is encouraging. Both these bores are immediately under Stage 1 cells and are subject to potential issues of preferential leachate flow and production in the heterogeneous cells; such processes may explain the recent elevation in ammonia levels in BB15. Comparison with breakthrough data for bores further downgradient shows similar or better agreement, noting that significant ammonia traces have not yet been recorded at BB20 and BB21 on the western site boundary. An important point of difference between the simulated and measured breakthrough curves is apparent in the lowest two plots, which shows data taken from the bottom screens in BB14 and BB15. Here the simulation underestimates the breakthrough times in each screen by at least five years (BB15C). This may indicate the presence of significant vertical anisotropy and/or bedding stratigraphy underneath TPLF, or it may indicate the need for substantially smaller dispersivities, i.e. lower transverse/vertical dispersion.

Figure 4.3.1: Simulated solute breakthrough curves (blue) compared to observed ammonia concentrations (red). The simulated curves are generated using a nominal Cmax = 100 mg/L NH3 with a PL+PC scenario.


Figure 4.3.2: Simulated solute breakthrough curves (blue) compared to observed ammonia concentrations (red). The simulated curves are generated using a nominal Cmax = 100 mg/L NH3 with a delayed leachate production scenario.

BB18 is the only bore that is not directly under Stage 1 that is showing appreciable ammonia contamination. As such, it provides a view of how even a relatively small distance of lateral migration can contribute to averaging in the dissolved plume profile. Figure 4.3.4 shows the measured ammonia profile at BB18 over time. Again, the measurements show little ammonia penetration to the depth of the lowest screen, BB18C at around -24 mAHD. Higher in the profile, at screens BB18A and BB18B, the simulated breakthrough curves provide under-estimates of the measured data. This strengthens the evidence for a mechanism of vertical stratification at TPLF. In some way, the dissolved leachate plume is prevented from penetrating to the lower regions of the aquifer, whilst promoting horizontal migration at higher elevations in the water column. This effect may be achieved in the simulations by defining a less-conductive layer below the B screens, thereby inhibiting mass transport to lower elevations. If such a layer was extensive across the site, it may also induce significant vertical head gradients. Figure 4.3.5 shows heads measured simultaneously in the three verticals screens of bores BB3, BB20, BB21 in April 2005. It is seen from the Figure that no consistent head gradient, either upwards or downwards, is measured at bores BB3 and BB20. Given that the screen midpoints are located approximately 5-10 m apart in the vertical, the largest vertical gradients deduced from the Figure are in the order of 0.0014 m/m (between BB21A and BB21B) which is approximately 10 times the horizontal gradient at that time. However the gradient defined between BB21A and BB21C is 0.00075 m/m. If these data are to be believed, there is some evidence of vertical layering in one of the head profiles, with vertical gradients significantly larger than the horizontal gradient. This would appear to be consistent with the stratified ammonia concentration seen elsewhere at the site, i.e. in bores BB14, BB15, BB18 to the west of BB21. It may be that the vertical gradients at BB21 are the result of local stratigraphic barriers to vertical flow that may also be present further west under TPLF. Unfortunately, depth-resolved head measurements are lacking for BB14, BB15, BB16 which prevents inclusion of these locations in Figure 4.3.5. Continued monitoring of heads and ammonia concentrations at BB21 may throw light on the vertical stratification effect as the plume eventually migrates westwards past BB21.


Figure 4.3.3: Simulated solute breakthrough curves (blue) compared to observed ammonia concentrations (red). The simulated curves are generated using a least-squares fitted Cmax = 40 mg/L NH3 value with a delayed and ramped leachate production scenario.


Figure 4.3.4: Simulated solute breakthrough curves (blue) compared to observed ammonia concentrations (red) for bore BB18. The simulated curves are generated using a least-squares fitted Cmax = 40 mg/L NH3 value with a delayed and ramped leachate production scenario.


Figure 4.3.5: Measured head profiles at three bores. Data collected in April 2005.

Figures 4.3.6-8 show distributions of solute for the delayed, ramped leachate production model, for simulation dates: early 2002 (Figure 4.3.6), early 2005 (Figure 4.3.7), late March 2007 (Figure 4.3.8). The plume is almost immobile, moving only slowly in a south-west direction towards the beach. This is consistent with the slow flow speeds noted in Figures 4.2.2 and 4.2.3. The Wash Bay Bore, although it only has a small annual abstraction rate, still manages to capture the dilute fringe of the plume. Estimates of the average solute concentration in the Wash Bay Bore abstraction are of the order of 4% of Cmax by March 2007.

Based on these results, and assuming that the present set of hydrological conditions persists at TPLF, it appears that the potential for off-site migration of the plume in the near future is low.


Figure 4.3.6: Simulated solute distribution for early 2002, for Cmax = 100 mg/L. Both cells in Stage 1 are in full leachate production.


Figure 4.3.7: Simulated solute distribution for early 2005, for Cmax = 100 mg/L. Stage 1 North is in full leachate production, while Stage 1 South has ceased leachate production.


Figure 4.3.8: Simulated solute distribution for late March 2007, for Cmax = 100 mg/L. Both cells in Stage 1 have ceased leachate production. Capture of dilute solute by the Wash Bay Bore is visible along the south-western edge of the plume.


4.4 Discussion and Further Work

This modelling study was intended to encompass an integration of existing groundwater flow and quality data for TPLF. As in any groundwater data integration effort, gaps in understanding have been identified. As TPLF moves towards capacity of cells and ultimate closure, it is likely that more certainty will be required regarding various processes at the site and its immediate environs. These processes include:

• Regional scale hydrologic conditions

More work needs to be done to understand the drivers of the sudden loss of regional heads around the years 2000-2001. This head loss has greatly influenced flow conditions at TPLF, reducing migration speeds of leachate under the landfill stages. There is no guarantee that these conditions will persist, so the mechanisms for the head loss must be understood in order to inform future projections of leachate migration. Potential impacts of nearby land use changes and land releases should also be monitored. Methods of recharge estimation might also be considered as relevant data is scarce for the site. This work to provide a clearer understanding of regional hydrological drivers could be performed as a desktop study.

• Stratigraphy and Spatial Heterogeneity

The measured data show signatures of spatial heterogeneity, especially in the breakthrough curves for bores sited within the landfill cells, in the variability of the Passive Flux Meter test results, and for vertical and horizontal head gradients outside the landfill facility. This heterogeneity is not unexpected, given past experience with cavernous and dual-porosity features of the Tamala Limestone formation elsewhere on the Swan Coastal Plain. However, the heterogeneity of the TPLF formations is not well characterized, either in a spatial sense or in a stratigraphic sense. This may potentially affect the interpretation of leachate migration data and consequent risk assessments for off-site impacts. It may be hoped that as the leachate plume migrates further westward, the plume will sample sufficient scales of heterogeneity to behave in a conventional sense, thereby allowing simple models of dilution to be applied. However, if the stratigraphic units that seem to prevent vertical mixing actually persist over large areas of the domain, the plume may remain confined to the upper regions of the saturated aquifer thickness. If this is the case, the simple calibration for the net maximum leachate concentration employed here will have to be adjusted to cater for a modified flow field. Continued tracking of plume breakthrough at BB19, BB20, BB21, BB22 will throw light on this important aspect of the flow dynamics at TPLF.

• Leachate Production and Composition Data

For the simulations included in this report, leachate production data was estimated indirectly from measured breakthrough curves at a small number of locations. No direct information on ammonia concentration either in the leachate or immediately at the water table under Stage 1 was available. Similarly, no data is available that directly


quantifies the presence or absence of secondary leachate production (or leachate quality) underneath Stage 2. Whilst the present simulations provide reasonable fits to the observed ammonia breakthrough curves, the simulations make assumptions about the production and location of leachate migrating to the water table beneath TPLF that cannot be checked by independent observations. It is recommended that the primary leachate stream collected under Stage 2 be measured for production rate and tested for indicator species (e.g. ammonia, nitrate/nitrite, TDS, pH, VOC, metals) on a regular basis, in order to provide greater clarity on the quality and volume of leachate production in TPLF. A similar system should be established for Stage 3 prior to commencement of filling operations there.

5. CONCLUSIONS

Based on an extensive history of measurement and monitoring at the Tamala Park Landfill Facility (TPLF), a data set of hydrological, climatic and groundwater quality has been assembled. In this report, the data was integrated into a quantitative groundwater flow and transport model for the site and surrounds. Gaps in the data exist with respect to flow characterization (e.g. effects of heterogeneity and effective boundary conditions) and with respect to leachate production and leachate quality. Nevertheless, a three-dimensional groundwater flow model was established and calibrated to give reasonable agreement with heads measured on site over the last 17 years. Detailed recharge and groundwater pumping signals were incorporated into the model, although comprehensive information on magnitudes and rates of recharge to the water table is lacking.

The model employed GIS data provided by MRC and other stakeholders to produce a georeferenced tool that contained spatial representations of TPLF landfill stages and Water Corporation production bore locations. This may permit efficient scenario modelling if required for the assessment of impacts from planned land use changes in the surrounding areas.

By assuming a spatial zonation of leachate production correlated with the use of liners in staging cells, the model produced estimates of ammonia breakthrough curves at local bores that were able to be calibrated against measured breakthrough curves. The major discrepancy was in the vertical ammonia concentration profiles – the numerical model overpredicts concentrations at the bottom of the aquifer. This may be due to the presence of significant bedding and stratigraphic barriers to vertical dispersion. There is some independent evidence to support this claim, although the supporting data is not comprehensive. Further investigation may be warranted.

Overall, the simulations show that over the 17 years since landfill operations commenced, the plume has migrated slowly to the west, and is not likely to have move beyond the Marmion Avenue boundary. The dominant factor for this slow rate of movement is a recent and persistent drop in regional groundwater levels to approximately 0.5 m AHD starting from the year 2000. If this regional groundwater level is maintained then it is likely that plume movement will remain slow. Previous modelling studies showing rapid movement to the coast were performed without the benefit of long term groundwater head data and without measurements of local groundwater


flow velocity. The availability of this data for the present study has resulted in a much more refined groundwater model for the site which matches the measured data well, although there is room for improvement in the description of heterogeneity of the Tamala Limestone formation underlying the site, and in the characterization of leachate sources and production rates within the landfill. Even so, simple models of aquifer structure are sufficient to reproduce most of the measured heads and ammonia breakthrough data. If further breakthrough and head data are collected it may be feasible to focus on improving the aquifer stratigraphic model and hence the agreement with measured fluxes. In any event, there is at least some evidence to suggest that it may also be feasible to consider the development of a leachate interception strategy along the western site boundary, based on the use of abstraction wells intercepting westward flow from underneath the landfill cells.


References

Barber, C., Davis, G. B. and Buselli, G., 1990. Development of procedures for more efficient monitoring and assessment of groundwater contamination from point sources of pollution. Final CSIRO Division of Water Resources report to AWRAC and other funding partners. CSIRO Division of Water Resources, Perth, Western Australia.

Davidson, W. A., 1995. Hydrogeology and groundwater resources of the Perth Region, Western Australia, Western Australia Geological Survey, Bulletin 142, pp.56-57.

Davis, G. B., 1999. Assessment of leaching potential from lined and unlined portions of the Tamala Park Landfill. Report to the Mindarie Regional Council. Consultancy Report 99-63, CSIRO Land and Water, Perth, Western Australia.

Davis, G. B., Bastow, T. P. and Innes, N. 2004. Long-term trends in groundwater quality at the Tamala Park Landfill: March 2003 to February 2004. A report to the Mindarie Regional Council. CSIRO Land and Water, Perth, Western Australia.

Davis, G. B. and Briegel, D., 1997. Monitoring the Tamala Park Landfill: July 1996 to June 1997. Report No. 97-33, CSIRO Land and Water, Perth, Western Australia.

Davis, G. B. and Briegel, D. 1998. Monitoring the Tamala Park Landfill: July 1997 to February 1998. Summary report to the Mindarie Regional Council. Consultancy Report No. 98-16, CSIRO Land and Water, Perth, Western Australia.

Davis, G. B., Briegel, D. and Fisher, S. J. 2000a. Monitoring the Tamala Park Landfill: March 1999 to February 2000. A summary report to the Mindarie Regional Council. Report No. 00-17, CSIRO Land and Water, Perth, Western Australia.

Davis, G. B., Briegel, D. and Hosking, J. 1996. Monitoring the Tamala Park Landfill: July 1994 to June 1996. CSIRO Division of Water Resources report to the Mindarie Regional Council. Report No. 96-29, 55 pp.

Davis, G. B., Briegel, D. and Howe, A. 1999. Monitoring the Tamala Park Landfill: March 1998 to February 1999. A summary report to the Mindarie Regional Council. Report No. 99-13, CSIRO Land and Water, Perth, Western Australia.

Davis, G. B., Fisher, S. J. and Innes, N. 2002. Groundwater monitoring at the Tamala Park Landfill: March 2001 to February 2002. A report to the Mindarie Regional Council. CSIRO Land and Water, Perth, Western Australia.

Davis, G. B., Height, M. I., Buselli, G., Howard, D., Briegel, D. and Hosking, J., 1993. Monitoring the Tamala Park Landfill: 1988 to 1993. Report and compilation of data for the Mindarie Regional Council. Report No. 93/29. CSIRO Land and Water, Perth, Western Australia.

Davis, G. B., Innes, N. and Woodbury, R. 2005. Long-term trends in groundwater quality at the Tamala Park Landfill: March 2004 to February 2005. Report to the Mindarie Regional Council. CSIRO Land and Water, Perth, Western Australia.

Davis, G. B., Innes, N., Woodbury, R. J. and Märki, A., 2006. Towards a groundwater management operating strategy for the Tamala Park Landfill: Progress Report, Report to the Mindarie Regional Council, CSIRO Land and Water Report, March 2006. CSIRO Land and Water, Perth, Western Australia.


Davis, G. B. and Laslett, D., 1991. Modelling assessment of the impact of the Tamala Park Landfill on groundwater quality and borehole placement. Report to the Water Authority of Western Australia. Report No. 91/26, CSIRO Division of Water Resources, Perth, Western Australia.

Davis, G. B., Marinovich, M. and Innes, N. 2003. Groundwater monitoring at the Tamala Park Landfill: March 2002 to February 2003. A report to the Mindarie Regional Council. CSIRO Land and Water, Perth, Western Australia.

Davis, G. B., Rayner, J. L. and Fisher, S. J. 2000b. Additional drilling and groundwater monitoring at the Tamala Park Landfill: April to October 2000. A summary report to the Mindarie Regional Council. CSIRO Land and Water, Perth, Western Australia.

Davis, G. B., Rayner, J. L. and Fisher, S. J. 2001. Groundwater monitoring at the Tamala Park Landfill: Including recommendations on future monitoring needs: March 2000 to February 2001. A report to the Mindarie Regional Council. CSIRO Land and Water, Perth, Western Australia.

Davis, G. B., Woodbury, R. J., Bastow, T., Annable, M. D., Hatfield, K., Rao, P. S. C. and Innes, N., 2007. Towards a groundwater management operating strategy for the Tamala park Landfill: 2007 Progress Report, Report to the Mindarie Regional Council, Report No. 5/07, CSIRO Land and Water, Perth, Western Australia.

Diersch, H.-J., 2007. Feflow 5.3 – Finite Element Subsurface Flow and Transport Simulation System. WASY, Berlin.

Farrington, P. and Bartle, G., 1991. Recharge beneath a Banksia woodland and a Pinus pinaster platation on coastal deep sands in south Western Australia. Forest Ecology and Management 40, 101-118.

Height, M. I., Davis, G. B., Briegel, D. and Hosking, J. 1994. Monitoring of the Tamala Park Landfill: July 1993 to July 1994. CSIRO Division of Water Resources report to the Mindarie Regional Council. Report No. 94/31, CSIRO Division of Water Resources, Perth, Western Australia.

Loehr, R. C. and Haikola, B. M., 2003. Long term landfill primary and secondary leachate production. Journal of Geotechnical and Geoenvironmental Engineering 129(11), 1063-1067.

Trefry, M. G. and Davis, G. B., 1999. Simulations of staging scenarios and leachate production at the Tamala Park Landfill, Report to Mindarie Regional Council, Consultancy Report 99-79, CSIRO Land and Water, Perth, Western Australia.


APPENDIX A – HEAD OBSERVATIONS

The head observations used in the flow model calibration are presented in the following table.

Bore Name Date of Observation Head (m AHD) BB14A Thursday 07 September 2000 1.129 BB14A Thursday 19 October 2000 1.017 BB14A Wednesday 02 May 2001 0.753 BB14A Wednesday 17 April 2002 0.521 BB14A Tuesday 01 April 2003 0.474 BB14A Tuesday 23 September 2003 0.699 BB15A Thursday 07 September 2000 1.15 BB15A Thursday 19 October 2000 1.01 BB15A Wednesday 02 May 2001 0.745 BB15A Wednesday 17 April 2002 0.507 BB15A Wednesday 02 April 2003 0.45 BB15A Tuesday 23 September 2003 0.682 BB15A Tuesday 26 October 2004 0.656 BB16A Thursday 07 September 2000 1.138 BB16A Thursday 19 October 2000 1.03 BB16A Wednesday 02 May 2001 0.756 BB16A Wednesday 17 April 2002 0.512 BB16A Wednesday 02 April 2003 0.456 BB16A Tuesday 23 September 2003 0.692 BB17A Thursday 07 September 2000 1.178 BB17A Thursday 19 October 2000 1.075 BB17A Wednesday 02 May 2001 0.814 BB18A Thursday 07 September 2000 1.065 BB18A Thursday 19 October 2000 0.937 BB18A Wednesday 02 May 2001 0.713 BB18A Wednesday 17 April 2002 0.49 BB18A Wednesday 02 April 2003 0.434 BB18A Tuesday 23 September 2003 0.646 BB18A Tuesday 09 November 2004 0.577 BB19A Thursday 07 September 2000 1.01 BB19A Thursday 19 October 2000 0.876 BB19A Wednesday 17 April 2002 0.478 BB19A Wednesday 02 April 2003 0.426 BB19A Tuesday 23 September 2003 0.624 BB19A Tuesday 26 October 2004 0.557 BB19A Tuesday 05 April 2005 0.395 BB19A Thursday 27 October 2005 0.686


BB19A Wednesday 03 May 2006 0.501 BB19A Wednesday 07 June 2006 0.575 BB19A Wednesday 25 October 2006 0.54 BB19A Thursday 15 March 2007 0.355 BB19A Thursday 29 March 2007 0.327 BB19B Wednesday 07 June 2006 0.578 BB19C Wednesday 07 June 2006 0.57 BB20A Thursday 07 September 2000 1.028 BB20A Thursday 19 October 2000 0.893 BB20A Wednesday 02 May 2001 0.693 BB20A Wednesday 17 April 2002 0.979 BB20A Wednesday 02 April 2003 0.4 BB20A Tuesday 23 September 2003 0.628 BB20A Tuesday 26 October 2004 0.566 BB20A Tuesday 05 April 2005 0.398 BB20A Thursday 27 October 2005 0.698 BB20A Wednesday 03 May 2006 0.508 BB20A Wednesday 07 June 2006 0.578 BB20A Wednesday 25 October 2006 0.549 BB20B Wednesday 07 June 2006 0.578 BB20C Wednesday 07 June 2006 0.571 BB21A Wednesday 03 May 2006 0.461 BB21A Wednesday 07 June 2006 0.535 BB21A Wednesday 25 October 2006 0.519 BB21A Thursday 15 March 2007 0.306 BB21A Thursday 29 March 2007 0.286 BB21B Wednesday 07 June 2006 0.512 BB21C Wednesday 07 June 2006 0.52 BB22A Wednesday 03 May 2006 0.427 BB22A Wednesday 07 June 2006 0.5 BB22A Wednesday 25 October 2006 0.478 BB22A Thursday 15 March 2007 0.27 BB22A Thursday 29 March 2007 0.248 BB22B Wednesday 07 June 2006 0.519 BB22C Wednesday 07 June 2006 0.242


APPENDIX B – AMMONIA OBSERVATIONS

The ammonia observations (mg/L) used in the leachate model calibration are presented in the following tables. The data have been compiled during the Tamala park sampling program since 1996.

BB14 and BB15 SampleDate BB14A BB14B BB14C BB15A BB15B BB15C

6/05/1996 0.042 0.007 0 0.533 2.11 0 6/08/1996 0.121 0.108 0 0.717 0.718 0.039

15/12/1996 0.177 0.011 0 1.47 4.82 0.008 5/06/1997 0.57 0.39 0.04 13 5.6 0.02 6/01/1998 1.5 1.1 0.02 12 8 0.02

28/04/1998 3.03 1.77 0.027 14.5 10.5 0.055 27/08/1998 4.1 2.6 0.04 17 12 0.04 11/12/1998 6.8 3.3 0 16 11 0.02 12/05/1999 9.6 3.8 0 19 11 0

9/09/1999 8.2 2.9 0.04 18 11 0.04 29/02/2000 12 4.4 0 31 13 0.09 30/06/2000 30 9.2 0

7/09/2000 9.5 3.2 0 25 14 0 2/05/2001 21 12 0 36 11 0

10/10/2001 18 12 0.05 37 11 0.02 17/04/2002 20 9.1 0.02 31 4 0.05

7/10/2002 32 16 0.01 63 13 0.05 1/04/2003 39 12 0.02 86 7.1 0.12

22/09/2003 24 7.7 0.01 64 18 0.17 1/04/2004 65 15 0 78 58 0.54

26/10/2004 67 19 0.03 52 75 1.5 7/04/2005 22 0 110 130 1.4

26/10/2005 35 17 0.01 48 44 1.6 7/06/2006

25/10/2006 15/03/2007


6/05/1996 0.333 0 0 6/08/1996 0.559 0 0

15/12/1996 1.28 0.005 0.041 5/06/1997 2 0.11 0.03 6/01/1998 5 0.02 0

28/04/1998 7.83 0.006 0 27/08/1998 12 0.02 0.03


11/12/1998 12 0.02 0.02 12/05/1999 11 0.02 0

9/09/1999 11 0.16 0.03 29/02/2000 18 0.24 0 30/06/2000 0 2.2* 0.31*

7/09/2000 14 0.37 0 0.02 0.22 0.01 2/05/2001 28 0.41 0 0.06 0.02 0

10/10/2001 26 0.59 0 0.17 0.02 0 17/04/2002 21 0.26 0 0.22 0 0

7/10/2002 28 0.43 0 0.57 0 0 1/04/2003 16 0.02 0 0.21 0.01 0

22/09/2003 12 0.12 0 0.01 0.04 0 1/04/2004 0 0.13 0

26/10/2004 0.11 0.15 0.01 7/04/2005 0 0.24 0

26/10/2005 0 0.16 0 7/06/2006 0 0.22 0

25/10/2006 0 0.21 0 15/03/2007 0 0.22 0


6/05/1996 6/08/1996

15/12/1996 5/06/1997 6/01/1998

28/04/1998 27/08/1998 11/12/1998 12/05/1999

9/09/1999 29/02/2000 30/06/2000 0 0.2 0.26

7/09/2000 0 0.14 0.27 2/05/2001 0 0.04 0.41

10/10/2001 0 0.18 0.29 17/04/2002 0 0.14 0.36

7/10/2002 0 0.09 0.45 1/04/2003 0.16 0.08 0.34

22/09/2003 0.19 0.06 0.35 1/04/2004 0.43 0.09 0.42

26/10/2004 0.13 0.05 0.35 7/04/2005 0.31 0.05 0.42

26/10/2005 0 0.05 0.31 7/06/2006 0 0.06 0.36 2.4 0.26 0

25/10/2006 0 0.43 0.09 1.6 0.47 0 15/03/2007 0.1 0.4 3.9 0.55 0.12


Documents

Groundwater Processes at the Tamala Park Landfill Facility · Groundwater Processes at the Tamala Park Landfill Facility . Data integration and numerical simulations . M. G. Trefry,