Evaluation of groundwater surface water interactions at Bool … · 2017-01-16 · Evaluation of groundwater – surface water interactions at Bool Lagoon and Lake Robe using environmental

Evaluation of groundwater – surface water interactions at Bool Lagoon and Lake Robe using environmental tracers

Smith, SD, Lamontagne, S, Taylor, AR, Cook, PG

Goyder Institute for Water Research

Technical Report Series No. 15/14

www.goyderinstitute.org

Goyder Institute for Water Research Technical Report Series ISSN: 1839-2725

The Goyder Institute for Water Research is a partnership between the South Australian Government through the Department of Environment, Water and Natural Resources, CSIRO, Flinders University, the University of Adelaide and the University of South Australia. The Institute will enhance the South Australian Government’s capacity to develop and deliver science-based policy solutions in water management. It brings together the best scientists and researchers across Australia to provide expert and independent scientific advice to inform good government water policy and identify future threats and opportunities to water security.

The following associate organisation contributed to the report:

Enquires should be addressed to: Goyder Institute for Water Research

Level 4, 33 King William Street Adelaide, SA, 5000

tel: 08-82365200 e-mail: [email protected] Citation Smith , SD, Lamontagne, S, Taylor, AR, Cook, PG, 2015, Evaluation of groundwater – surface water interactions at Bool Lagoon and Lake Robe using environmental tracers, Goyder Institute for Water Research Technical Report Series No. 15/14 Copyright © 2015 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. Disclaimer The Participants advise that the information contained in this publication comprises general statements based on scientific research and does not warrant or represent the completeness of any information or material in this publication.

Evaluation of groundwater – surface water interactions at Bool Lagoon and Lake Robe using environmental tracers | i

Preface: South East Regional Water Balance Project

The South East Regional Water Balance project is a collaboration between Flinders University, CSIRO and the Department of Environment, Water and Natural Resources (DEWNR), funded by the Goyder Institute for Water Research. The project commenced in September 2012, with the objective of developing a regional water balance model for the Lower Limestone Coast Prescribed Wells Area (LLC PWA). The project was initiated following conclusions from the South East Water Science Review (2011) that, due to a number of gaps in understanding of processes that affect the regional water balance, there is uncertainty about the amount of water that can be extracted sustainably from the Lower Limestone Coast region as a whole. The review also concluded that, because of the close link between groundwater and surface water resources in the region, surface water resources and ecosystems are particularly vulnerable to groundwater exploitation.

The South East Regional Water Balance project follows on from the report of Harrington et al. (2011), which recommended that a consistent framework of models is required to support water management in the South East, with the first step being a regional groundwater flow model to:

bring together all existing knowledge,

address regional scale water balance questions

provide boundary conditions for smaller scale models to address local scale questions, including those around “hotspot” areas and significant wetlands.

Harrington et al. (2011) also identified the critical knowledge gaps that limit the outcomes from a regional scale model. These included but were not limited to:

Spatial and temporal variability in groundwater recharge and evapotranspiration.

Interaquifer leakage and the influence of faults on groundwater flow.

The nature of wetland-groundwater interactions

Understanding of processes occurring at the coastal boundary

Surface water-groundwater interactions around the man-made drainage network

The absence of information on historical land use and groundwater extraction

The South East Regional Water Balance project has included numerous tasks that have sought to improve the conceptualisation of the regional water balance, address some of the critical knowledge gaps, incorporate this and existing information into a regional groundwater flow model and understand how this improved understanding can be used in the management of wetland water levels.

An overview of the project and its output can be found in Harrington et al. 2015. South East Regional Water Balance Project – Phase 2. Project Summary Report. Goyder Institute Report 15/39.

ii | Evaluation of groundwater – surface water interactions at Bool Lagoon and Lake Robe using environmental tracers

Associated Reports and Research Papers

Technical Reports:

Harrington, N and Lamontagne, S (eds.), 2013, Framework for a Regional Water Balance Model for the South Australian Limestone Coast Region. Goyder Institute for Water Research Technical Report 13/14.

Morgan, LK, Harrington, N, Werner, AD, Hutson, JL, Woods, J and Knowling, M, 2015, South East Regional Water Balance Project – Phase 2. Development of a Regional Groundwater Flow Model. Goyder Institute for Water Research Technical Report 15/38.

Doble, R, Pickett, T, Crosbie, R, Morgan, L, 2015, A new approach for modelling groundwater recharge in the South East of South Australia using MODFLOW, Goyder Institute for Water Research Technical Report 15/26.

Taylor, AR, Lamontagne S, Turnadge, C, Smith, SD and Davies, P, 2015, Groundwater-surface water interactions at Bool Lagoon, Lake Robe and Deadmans Swamp (Limestone Coast, SA): Data review. Goyder Institute for Water Research Technical Report 15/13.

Turnadge, CJ and Lamontagne, S, 2015, A MODFLOW-based approach to simulating wetland – groundwater interactions in the Lower Limestone Coast Prescribed Wells Area. Goyder Institute for Water Research Technical Report 15/12.

Barnett, S, Lawson, J, Li, C, Morgan, L, Wright, S, Skewes, M, Harrington, N, Woods, J, Werner, A and Plush, B, 2015, A Hydrostratigraphic Model for the Shallow Aquifer Systems of the Gambier Basin and South Western Murray Basin. Goyder Institute for Water Research Technical Report 15/15.

Harrington, N and Li, C, 2015, Development of a Groundwater Extraction Dataset for the South East of South Australia: 1970-2013. Goyder Institute for Water Research Technical Report 15/17.

Harrington, N, Millington, A, Sodahlan, ME and Phillips, D, 2015, Development of Preliminary 1969 and 1983 Land Use Maps for the South East of SA. Goyder Institute for Water Research Technical Report 15/16.

Harrington, N, Lamontagne, S, Crosbie, R, Morgan, LK and Doble, R, 2015, South East Regional Water Balance Project: Project Summary Report. Goyder Institute for Water Research Technical Report 15/39.

Research Papers:

Crosbie R, Davies P, Harrington N and Lamontagne S (2014) Ground truthing groundwater-recharge estimates derived from remotely sensed evapotranspiration: a case in South Australia. Hydrogeology Journal, 1-16. DOI: 10.1007/s10040-014-1200-7

Lamontagne S, Taylor A, Herpich D and Hancock G (2015) Submarine groundwater discharge from the South Australian Limestone Coast region estimated using radium and salinity. Journal of Environmental Radioactivity 140, 30-41.

Evaluation of groundwater – surface water interactions at Bool Lagoon and Lake Robe using environmental tracers | iii

Contents

Preface: South East Regional Water Balance Project ...................................................................................... i

Associated Reports and Research Papers ......................................................................................... ii

Acknowledgments ....................................................................................................................................... vi

Executive Summary .................................................................................................................................... vii

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

1.1 Wetlands in the LLC PWA ....................................................................................................... 1

1.2 Groundwater connections ...................................................................................................... 1

2 Study area........................................................................................................................................ 3

2.1 Bool Lagoon ........................................................................................................................... 3

2.2 Lake Robe .............................................................................................................................. 5

3 Methods .......................................................................................................................................... 7

3.1 Field methods ........................................................................................................................ 7

3.2 Sample analysis ...................................................................................................................... 9

4 Results ........................................................................................................................................... 10

4.1 Bool Lagoon ......................................................................................................................... 10

4.2 Lake Robe ............................................................................................................................ 25

5 Discussion ...................................................................................................................................... 30

5.1 Bool Lagoon ......................................................................................................................... 30

5.2 Lake Robe ............................................................................................................................ 31

5.3 Suggestions for future wetland studies ................................................................................ 31

6 Conclusions .................................................................................................................................... 32

Appendix A Chemistry ............................................................................................................................ 33

References .................................................................................................................................................. 51

iv | Evaluation of groundwater – surface water interactions at Bool Lagoon and Lake Robe using environmental tracers

Figures Figure 2.1 Locations of groundwater and surface water sampling and cross-section (Figure 2.2) at Bool Lagoon .......................................................................................................................................................... 4

Figure 2.2 Piezometric west-east transect through the middle of Bool Lagoon (see Figure 2.1 for location); residual water levels in piezometers are indicated after the piezometer name; (a) Summer 2009/2010 and (b) Summer 2011/2012; from Taylor et al. (2015) ................................................................. 5

Figure 2.3 Location of groundwater and surface water sampling at Lake Robe .............................................. 6

Figure 4.1 Piper plot of Bool Lagoon............................................................................................................ 10

Figure 4.2 Spatial trends of chloride at Bool Lagoon; the general direction of regional groundwater flow is left to right (east to west) and distances are relative to JOA029 ............................................................... 11

Figure 4.3 Mixing model between surface water and halite showing the evolution of the Br/Cl ratio with different initial chloride concentrations ...................................................................................................... 12

Figure 4.4 Spatial distribution of (a) halite added to groundwater and (b) salinity prior to halite dissolution .................................................................................................................................................. 12

Figure 4.5 Time series of chloride concentrations in surface water at Bool Lagoon...................................... 13

Figure 4.6 Time series of EC in wells adjacent to Bool Lagoon ..................................................................... 13

Figure 4.7 Stable isotopes at Bool Lagoon; (a) meteoric and evaporative trends and (b) spatial distribution ................................................................................................................................................. 14

Figure 4.8 Temporal stable isotope in surface water at Bool Lagoon; (a) time series of δ2H and cumulative precipitation (Bool Lagoon (Locksley Farm); (BOM, 2015)) and (b) δ2H vs. δ18O ......................... 15

Figure 4.9 Radon vs. distance at Bool Lagoon; atmospheric equilibration assumes atmospheric 222Rn partial pressure and activity of 6×10-20 atm and 10 Bq/m3, respectively and a solubility of 1.08×10-2 mol/kg/atm. ............................................................................................................................................... 16

Figure 4.10 Spatial trends of tritium at Bool Lagoon .................................................................................... 17

Figure 4.11 Age-corrected groundwater tritium concentrations at Bool Lagoon (compared to expected tritium in precipitation (Kaitoke record) adjusted to modern expected concentrations at Naracoorte) ....... 18

Figure 4.12 Recharge model of Bool Lagoon using tritium with a groundwater flow velocity of 50 m/y; (a) tritium distribution with different input ratios and (b) cumulative surface water contribution resulting from different input ratios .......................................................................................................................... 19

Figure 4.13 Noble gas concentrations at Bool Lagoon; (a) neon vs. argon and (b) helium vs. neon .............. 21

Figure 4.14 Piper plot of Lake Robe ............................................................................................................. 25

Figure 4.15 Stable isotopes at Lake Robe .................................................................................................... 26

Figure 4.16 Age corrected groundwater tritium concentrations at Lake Robe (compared to expected tritium in precipitation (Kaitoke record) adjusted to expected concentrations at Naracoorte)..................... 27

Figure 4.17 Radon vs. chloride at Lake Robe ............................................................................................... 28

Figure 4.18 Noble gas concentrations at Lake Robe; (a) neon vs. argon and (b) helium vs. neon .................. 29

Figure 5.1 Conceptual recharge and discharge processes at Bool Lagoon .................................................... 31

Apx Figure A.1 Bool Lagoon solutes and tracers in groundwater (blue) and surface water (green) versus distance east from JOA029 (a) electrical conductivity, (b) alkalinity, (c) chloride, (d) bromide, (e) sulphate, (f) calcium, (g) magnesium, (h) sodium, (i) Br/Cl ratio, (j) Na/Cl ratio, (k) radon, (l) tritium, (m) deuterium................................................................................................................................................... 35

Evaluation of groundwater – surface water interactions at Bool Lagoon and Lake Robe using environmental tracers | v

Apx Figure A.2 Lake Robe solutes and tracers in groundwater (medium blue), surface water (green) and springs (dark blue) versus distance east from WAT034 (a) electrical conductivity, (b) alkalinity, (c) chloride, (d) bromide, (e) sulphate, (f) calcium, (g) magnesium, (h) sodium, (i) Br/Cl ratio, (j) Na/Cl ratio, (k) radon, (l) tritium, (m) deuterium ............................................................................................................ 38

Apx Figure A.3 Bool Lagoon solutes and tracers in groundwater (blue) and surface water (green) versus chloride; where appropriate lines representing seawater (solid black) and halite (dashed black) are shown , (a) bromide, (b) Br/Cl ratio, (c) sodium, (d) Na/Cl ratio, (e) magnesium, (f) Mg/Cl ratio, (g) calcium, (h) Ca/Cl ratio, (i) bicarbonate, (j) HCO3/Cl ratio, (k) sulphate, (l) SO4/Cl ratio, (m) tritium.............. 41

Apx Figure A.4 Lake Robe solutes and tracers in groundwater (blue) and surface water (green) versus chloride; where appropriate lines representing seawater (solid black) and halite (dashed black) are shown , (a) bromide, (b) Br/Cl ratio, (c) sodium, (d) Na/Cl ratio, (e) magnesium, (f) Mg/Cl ratio, (g) calcium, (h) Ca/Cl ratio, (i) bicarbonate, (j) HCO3/Cl ratio, (k) sulphate, (l) SO4/Cl ratio, (m) tritium.............. 44

Apx Figure A.5 Reaction plots for Bool Lagoon for groundwater (blue) and surface water (green); black lines are dissolution/precipitation ratios; (a) Mg/HCO3: 1:4 ratio is congruent dissolution of dolomite and calcite; 1:2 ratio is incongruent dissolution of dolomite (b) Ca/HCO3: 1:4 ratio is congruent dissolution of dolomite and calcite; 1:2 ratio is dissolution of calcite, (c) Ca/SO4: gypsum dissolution, (d) Mg/SO4: epsomite dissolution, (e) Ca+Mg-0.5HCO3/SO4: gypsum dissolution excluding carbonates, (f), Ca+Mg/SO4+HCO3: carbonate and gypsum dissolution, (g) Ca+Mg/HCO3: dolomite dissolution ................... 46

Apx Figure A.6 Reaction plots for Bool Lagoon for groundwater (blue) and surface water (green); black lines are dissolution/precipitation ratios; (a) Mg/HCO3: 1:4 ratio is congruent dissolution of dolomite and calcite; 1:2 ratio is incongruent dissolution of dolomite (b) Ca/HCO3: 1:4 ratio is congruent dissolution of dolomite and calcite; 1:2 ratio is dissolution of calcite, (c) Ca/SO4: gypsum dissolution, (d) Mg/SO4: epsomite dissolution, (e) Ca+Mg-0.5HCO3/SO4: gypsum dissolution excluding carbonates, (f), Ca+Mg/SO4+HCO3: carbonate and gypsum dissolution, (g) Ca+Mg/HCO3: dolomite dissolution ................... 48

Tables Table 3.1 Sampling site parameters and field measured data ........................................................................ 8

Table 4.1 Isotopic results ............................................................................................................................ 22

Table 4.2 Major ion results ......................................................................................................................... 23

Table 4.3 Temporal surface water chemistry at Bool Lagoon ....................................................................... 24

Apx Table A.1 Minor chemistry from Bool Lagoon and Lake Robe (mg/L) .................................................... 49

vi | Evaluation of groundwater – surface water interactions at Bool Lagoon and Lake Robe using environmental tracers

Acknowledgments

Funding for this project was provided by the Goyder Institute for Water Research including partnership with the CSIRO. The development of the sampling plan was aided by Clare Harding of DEWNR. Monthly surface water sampling was performed by Brian Robins and Abigail Goodman (DEWNR). Additional scientific advice that has shaped the study and reporting was provided by members of the technical working group.

Evaluation of groundwater – surface water interactions at Bool Lagoon and Lake Robe using environmental tracers | vii

Executive Summary

Most wetlands in the Lower Limestone Coast Prescribed Wells Area (LLC PWA) have some degree of groundwater dependency. However the nature of this dependency is not well understood, for example whether the wetlands are primarily part of a local or regional hydrogeological system or primarily recharge or discharge features. To evaluate and better conceptualise surface water – groundwater interactions in LLC PWA wetlands, environmental tracers were measured at two significant wetlands: the Ramsar-listed Bool Lagoon and Lake Robe. Bool Lagoon was expected to be a flow-through wetland where both groundwater discharge and recharge occur simultaneously. Because Lake Robe lies adjacent to the coast at the end of a regional flow path, it was expected that this wetland would be a discharge zone.

Environmental tracers are a useful tool to evaluate the connectivity between groundwater and surface water because they integrate dynamic exchange processes over space and time. This is especially useful in the LLC PWA, where there watertable is shallow and variable at seasonal and decadal scales, resulting in complex groundwater interactions with wetlands.

Measured tracers included tritium, stable isotopes of water, noble gases, radon and major and minor ions. At Bool Lagoon these tracers, specifically tritium, stable isotopes of water and major chemistry, indicate that surface water recharge is occurring. However, overall the groundwater salinity in this area has increased over the past several decades indicating that discharge may be the dominant surface water – groundwater process. Bool Lagoon is a seasonal recharge wetland located in a regional groundwater discharge zone.

At Lake Robe, salinity is very high, which can be expected for a terminal lake. Only slight evidence was found for regional groundwater discharging into this body of water, but significant discharge from the local coastal dune aquifer occurs. The topographically lower Lake Eliza, which sits adjacent to Lake Robe, may be a site of more focused regional discharge. The evidence available indicates that Lake Robe is a discharge for a local coastal dune aquifer, located in a regional groundwater discharge zone.

This study has shown that environmental tracers are a valuable tool to indicate the types of surface water – groundwater interactions in a complex geological setting.

Evaluation of groundwater – surface water interactions at Bool Lagoon and Lake Robe using environmental tracers | 1

1 Introduction

1.1 Wetlands in the LLC PWA

Wetlands were once a prevalent feature of the Lower Limestone Coast Prescribed Wells Area (LLC PWA; also referred to as the South East), but their areal extent has been reduced significantly due to the draining of surface water to increase agricultural productivity – 44% of land cover reduced to approximately 3% (Department for Water, 2010). Furthermore, the majority of these wetlands have been found to be groundwater dependent. The threat to these wetlands continues due to reductions in groundwater level. However, how wetlands are connected to local and regional hydrogeological systems is not well understood. This study aims to improve the conceptualisation of groundwater – surface water interaction regimes for South East wetlands. This was done by collecting and interpreting a suite of environmental tracers in surface water and groundwater at were sampled in two wetlands (Bool Lagoon and Lake Robe) on one occasion (March 2014). Bool Lagoon was hypothesised to be a groundwater flow-through wetland because it is located in a regional groundwater flow-through zone, whereas Lake Robe was hypothesised to be a discharge wetland because it is located in a regional groundwater discharge zone. However, the influence of local hydrogeological systems resulting from local topography variations, aquifer geometry and land use means that wetlands can have a groundwater connection that differ from the connection expected at the regional scale The effects of local conditions are examined in an associated report (Turnadge and Lamontagne, 2015).

1.2 Groundwater connections

1.2.1 IDENTIFYING SURFACE WATER – GROUNDWATER CONNECTIONS WITH ENVIRONMENTAL TRACERS

Geochemical analyses of the surface water and groundwater may be useful for interpreting the magnitude, source, spatial variability, and rate of groundwater flowing to and from a wetland. This information can then be used to parameterise a groundwater flow model. Environmental tracers of interest can be split into two groups: those showing the source and processes that alter the water composition and those showing the age of the water, or how long the water has been isolated from the atmosphere. These tracers are described in detail in various books and publications (e.g. Cook and Herczeg, 2000). Tracers utilised here include tritium, noble gases, radon, major ions, and stable isotopes of water.

Tritium (3H) and radon (222Rn) are radioactive isotopes used to identify young components of groundwater. Tritium has anthropogenic and cosmogenic sources and its activity in groundwater decreases after being isolated from the atmosphere (λ=12.37 years). Radon has a terrigenic source and its activity increases after contact with soils and sediments. However, due to its short half-life (3.82 days) its activity in groundwater quickly reaches secular equilibrium.

Noble gases are found dissolved in almost all natural waters and come from atmospheric sources and terrigenic sources (Ballentine and Burnard, 2002; Kipfer et al., 2002). As solubility of these gases is a function of atomic mass, temperature and water salinity, the concentration in water can be used to determine the conditions during recharge (Aeschbach-Hertig et al., 1999). Furthermore, gas in the water that exceeds atmospheric solubility (known as excess air) can be used to identify rates of water table fluctuation (Heaton and Vogel, 1981). Finally, excess amounts of helium-4 can be used to estimate the age of groundwater and to identify the discharge of regional groundwater into surface water systems (Gardner et al., 2011). However, Turnadge et al. (2013) identified low amounts of helium in regional groundwater in other parts of the South East, so this tracer may have limited use in this study area.

2 | Evaluation of groundwater – surface water interactions at Bool Lagoon and Lake Robe using environmental tracers

Major ions can be used to identify different groups of water and to identify the maturity of water as water-rock interactions alter the solutes present in the water.

Stable isotopes of water (δ2H and δ18O) can be effectively used to identify phase changes of water. This is due to Rayleigh fractionation where water molecules with lighter isotopes evaporate more readily than those with heavier isotopes. Conversely, water molecules with heavier isotopes condense (and fall as precipitation) more readily than those with lighter isotopes. In the wetlands in the South East, this evaporative enrichment will be a useful indicator that groundwater was once surface water from wetlands where evaporation is significant.

Previous groundwater tracer studies in the South East wetlands include Fass and Cook (2007) who determined the degree of groundwater dependence using the mass balance of chloride and radon. The study indicated that the majority of the studied wetlands have a high to moderate dependence on groundwater. Other tracers studies in the South East include Turnadge et al. (2013), Lamontagne et al. (2013) and Love et al. (1993) who studied influence of the drainage network, groundwater flow-across faults, submarine discharge, and regional connectivity of the confined and unconfined aquifers, respectively.


2 Study area

Three study areas were chosen with each wetland representing a different hydraulic scenario. The most upgradient site, Deadmans Swamp was expected to be a recharge zone, Bool Lagoon is located several km down gradient and was expected to represent a flow-through zone where groundwater is both discharging and recharging and Lake Robe, which is located at the coastal boundary, was expected to be a regional discharge zone. The unconfined Tertiary Limestone Aquifer (TLA) underlies much of the South East and is thought to be hydraulically connected to wetlands (Harrington and Lamontagne, 2013). Underlying the TLA is the Tertiary Confined Aquifer (TCA).

Due to a lack of infrastructure at Deadmans Swamp (a single well and no surface water present during a February 2014 reconnaissance), this site was subsequently excluded from the hydrogeochemical study. As a result, more funding and effort was put towards sampling and characterising Bool Lagoon and Lake Robe. A brief description of these two sites is given below and a more in-depth review of the existing data is in a companion report (Taylor et al., 2015).

2.1 Bool Lagoon

Bool Lagoon is located approximately 15 km south of Naracoorte (Figure 2.1). The wetland complex is intermittently wet and includes many adjacent wetland features, including Hack’s Lagoon on the north-east side and Little Bool Lagoon on the north-west side. This complex has been designated a Ramsar Convention site, composed of the Bool Lagoon Game Reserve and the Hacks Lagoon Conservation Park. Surface water enters the complex at Hack’s Lagoon via the channelled Mosquito Creek. This creek originates from the east across the border with Victoria and is ephemeral with episodic flows. These episodic flows generally occur in late winter to early spring. Discharge from Hack’s Lagoon into Bool Lagoon is via excavated channels. The outlet to Bool Lagoon has been connected to Drain M. This outlet is controlled by the Bool Lagoon Outlet Regulator and discharge is controlled in order to regulate water levels in the lagoon which serves the dual purpose of habitat conservation and flood mitigation (Heneker, 2006). Throughout the study period (February 2013 – February 2014) no discharge was reported in Drain M.

The climate in this area has annual precipitation of 552 mm, annual potential evaporation of approximately 1400 mm and mean annual temperatures vary between 8 and 20.7 °C (Struan; minimum and maximum; 1974-1999; BOM, 2015).

Lithological logs indicate that Bool Lagoon is underlain by up to 10 m of clay, followed by limestone with interbedded marl and then limestone (SKM, 2010); the clay layer underlying this wetland may limit groundwater – surface water interactions . An extensive monitoring well network locating in and around the wetland complex indicates that groundwater flow is west to northwest (SKM, 2010). Figure 2.2 shows the surface water and groundwater levels during a dry period (Summer 2009/2010) and a wet period (Summer 2011/2012). This indicates that there are periods when groundwater is discharging into Bool Lagoon and other periods where surface water of Bool Lagoon is recharging to groundwater (see Taylor et al. (2015) for further hydrograph profiles).


Figure 2.1 Locations of groundwater and surface water sampling and cross-section (Figure 2.2) at Bool Lagoon


(a)

(b)

Figure 2.2 Piezometric west-east transect through the middle of Bool Lagoon (see Figure 2.1 for location); residual water levels in piezometers are indicated after the piezometer name; (a) Summer 2009/2010 and (b) Summer 2011/2012; from Taylor et al. (2015)

2.2 Lake Robe

Lake Robe is a saline perennial coastal lake located approximately 6 km southeast of the town of Robe and approximately 1.5 km east of the coastline. The lake has one drain that connects it with the topographically lower Lake Eliza to the east. No other inlets or outlets have been identified. Previous studies at Lake Robe and other coastal lakes of the South East have focussed on water chemistry and biota (Bayly, 1970; Bayly and Williams, 1966), depositional environment analogues (Burne et al., 1980; Burne and Ferguson, 1983) and groundwater levels and lithology (SKM, 2010). In a conceptual diagram, Burne et al., (Fig. 3a; 1980) show Lake Robe receives recharge of seawater as well as ‘continental’ water. SKM (2010) simply identify Lake Robe as a point of groundwater discharge.

Coastal lakes of the South East such as Lake Robe are thought to have been once connected to the ocean before being isolated due to dune formation. The salinity in Lake Robe is variable throughout the year (approximately 45-150 ‰ total dissolved solids (TDS) during March 1964-March 1965 (Bayly and Williams, 1966)), while the ionic composition is relatively constant and similar to seawater (Bayly, 1970; Bayly and Williams, 1966).


The area receives 631 mm precipitation annually with the majority falling in the autumn and winter and the mean annual temperature ranges from 10.9 minimum and 18.1 maximum (Robe Comparison, precipitation mean from 1884-2013; BOM, 2015).

Lithological profiles indicate the lake is underlain by >10 m sand followed by limestone of the unconfined Tertiary aquifer; sandstone is also found under the western portion (SKM, 2010). Few monitoring wells exist in the immediate vicinity of the lake.

Figure 2.3 Location of groundwater and surface water sampling at Lake Robe


3 Methods

3.1 Field methods

The first phase of hydrogeochemical sampling occurred at the beginning of autumn (March 3-7 2014). Sampling at the Bool Lagoon wetlands complex focussed on two east-west transects that aimed to coincide with the regional groundwater flow direction (Figure 2.1). A suite of samples for isotopic and chemical analyses were collected from nine wells and five surface water sites (Table 3.1). Isotope analyses include radon-222 (222Rn), noble gases (4He, 20Ne, and 40Ar), stable isotopes of water (2H and 18O), and tritium (3H). Chemical analyses include major and minor cations and anions. An additional four wells were sampled for only stable isotope and major ion analysis. In the field, groundwater level, water temperature and total dissolved gas pressure (TDGP) were measured (Table 3.1). A more thorough discussion and culmination of groundwater levels can be found in the related report by Taylor et al. (2015).

Subsequent sampling of surface water occurred monthly at several sites for the following eight months (Little Bool, Gunawar, Tee Tree Bird Hide, Drain M; Figure 2.1). This sampling was used to assess the temporal variability of stable isotope and major ion signals in Bool Lagoon surface water.

At Lake Robe, the suite of samples for isotopic and chemical analyses were collected from three wells and one surface water site (Figure 2.3; Table 3.1). Samples were collected from an additional two surface water sites for the analysis of stable isotopes, major ions, and 222Rn. Upon discovering low salinity water flowing into Lake Robe along the southern margin, three additional spring samples were collected for stable isotope and major ion analysis. In the field, water temperature and TDGP were measured (Table 3.1).

Sample collection was done by standard procedures and will be described briefly. Radon samples were collected in 1.25 L bottles and extracted in the field using the PET method (Leaney and Herczeg, 2006). Noble gases were collected using passive headspace diffusion samplers (Gardner and Solomon, 2009) which were left to equilibrate for at least 48 hours. As backup-samples in the case that the diffusion samplers failed, or in locations where it was inconvenient to return after 48 hours, noble gas samples were collected in copper tubes sealed with pinch-off clamps (Weiss, 1968). Water for stable isotope and major ion samples was filtered in the field – stable isotope samples were collected in duplicate in McCartney bottles while major ions samples were collected in duplicate , with the sample for cation analysis being acidified to <2 pH using nitric acid. This acidification occurred several days after collection. Tritium samples were collected in duplicate in 1 L plastic bottles.


Table 3.1 Sampling site parameters and field measured data

SAMPLE UNIT NO LONGITUDE

(°) LATITUDE (°)

SAMPLING DATE

PIEZO. DEPTH

(m)

CASED TO (m)

RSWL (mAHD*)

TEMP (°C)

TDGP (mm Hg)

Bool Lagoon groundwater

ROB005 7023-3403 140.6727 -37.1496 5/03/2014 7.62 ? 47.66 15.4 795

ROB018 7023-6988 140.6507 -37.1192 4/03/2014 12 6 46.47 15.7 779

ROB019 7023-6998 140.6652 -37.1184 4/03/2014 9 4.5 47.33 15.1 -

ROB021 7023-6994 140.6719 -37.1163 4/03/2014 12 6 47.30 16.1 771

ROB022 7023-6981 140.6770 -37.1079 4/03/2014 12 6 46.06 16.5 756

ROB023 7023-6995 140.6920 -37.1155 5/03/2014 12 6 47.18 14.8 779

ROB024 7023-6980 140.6920 -37.1155 5/03/2014 12 6 47.47 15.4 811

ROB025 7023-6997 140.7288 -37.1132 5/03/2014 12 6 47.42 14.9 787

ROB026 7023-6982 140.7443 -37.1172 5/03/2014 16 10 47.56 15.6 785

ROB027 7023-6993 140.7934 -37.1484 5/03/2014 10 4 47.62 15 790

ROB030 7023-6992 140.6727 -37.1496 4/03/2014 12 6 48.00 17.3 774

ROB031 7023-6999 140.6507 -37.1192 3/03/2014 11.4 5.4 49.26 16 777

JOA029 7023-7214 140.6652 -37.1184 5/03/2014 16.1 10.1 50.29 16.3 771

Bool Lagoon surface water

Hacks - 140.7295 -37.1030 3/03/2014 - - - 34.7 835

Bool East - 140.7232 -37.1173 3/03/2014 - - - 30.6

Bool West - 140.6901 -37.1207 5/03/2014 - - - 20.3 741

Bool Outlet - 140.6555 -37.1506 5/03/2014 - - - 20.9 796

Little Bool - 140.6860 -37.0939 4/03/2014 - - - 25 856

Lake Robe groundwater

WAT032 6823-1574 139.7848 -37.2233 6/03/2014 6 6 -0.09 17 752

WAT034 6823-1576 139.8092 -37.2104 6/03/2014 6 6 -1.98 17.4 768.5

WAT037 6823-1599 139.8069 -37.1889 6/03/2014 12.7 1.3 -2.17 17 792.5

Lake Robe surface water

Robe Northeast - 139.8094 -37.2106 6/03/2014 - - - - -

Robe South - 139.8007 -37.2227 6/03/2014 - - - - -

Robe Southeast - 139.8069 -37.2195 6/03/2014 - - - 22.6 786

Spring 1 - 139.7994 -37.2082 6/03/2014 - - - - -

Spring 2 - 139.7991 -37.2083 6/03/2014 - - - - -

Spring 3 - 139.7957 -37.2081 6/03/2014 - - - - -

*mAHD: elevation in metres relative to Australian Height Datum


3.2 Sample analysis

Radon, noble gas and major ion analyses were performed at the Waite campus of the CSIRO Land & Water Flagship. Stable isotopes and tritium samples were analysed at GNS Science, New Zealand.

Radon activity was measured using a LKB Wallac Quantulus counter (Leaney and Herczeg, 2006). Noble gas samples were analysed by quadrupole mass spectrometry after splitting the gas into three aliquots, removing reactive gases and cryogenic removing argon from helium and neon aliquot (Poole et al., 1997). For copper tube samples, dissolved gases were separated from the water before being purified as stated above. Stable isotopes were analysed using an Isoprime mass spectrometer – for δ18O analyses water was equilibrated at 25 °C using an Aquaprep device and for δ2H analyses, water was reduced at 1100 °C using a Eurovector Chrome HD elemental analyser. Chloride was measured by flow injection analysis; F-, Br-, NO3

- and SO4

2- were measured using ion chromatography (Dionex ICS-2500); the remaining major and minor ions were measured using Inductively Coupled Plasma Optical Emission Spectrometry (Spectro ARCOS).


4 Results

Results collected during field campaigns are summarised in Table 3.1. Isotopic, major ion, and temporal data are summarised in Table 4.1, Table 4.2 and Table 4.3, respectively. Complete minor ion results are in Section A.4. All plots showing results versus distance are along an east-west transect that begins at JOA029 (Bool Lagoon) or WAT034 (Lake Robe).

4.1 Bool Lagoon

4.1.1 MAJOR IONS

Ion balance was good (<±5% imbalance) in all samples. Groundwater and surface waters are all NaCl type (Figure 4.1). JOA029 appears to be the least mature (relatively low Na and Cl) groundwater, noted by a lower dominance of Na and Cl. Also shown in Figure 4.1 is data from existing analysed groundwater samples within a 10 km radius of Bool Lagoon (WaterConnect database (DEWNR, 2015)). While there were 98 samples with at least a Cl analysis, only 13 samples (from 10 different wells) have a charge balance within ±5% and have all ions required for a piper plot. These analyses are all NaCl type waters and cluster with the Bool groundwater and surface water samples. Plots of chloride and Br/Cl are presented and discussed below while other plots of other major chemistry are presented in Appendix A .

Figure 4.1 Piper plot of Bool Lagoon

Chloride

Chloride concentrations in groundwater vary between less than 400 mg/L (JOA029) to almost 8000 mg/L (ROB019), whereas surface water concentrations vary over a shorter range of approximately 500-950 mg/L. Chloride concentrations in groundwater tend to increase moving west along the expected flow path, whereas chloride concentrations in surface water decrease slightly along this same flow path (Figure 4.2).


Figure 4.2 Spatial trends of chloride at Bool Lagoon; the general direction of regional groundwater flow is left to right (east to west) and distances are relative to JOA029

Br/Cl

The ratio of bromide (Br) to chloride (Cl) can be used to assess the source of salt. Seawater is known to have a constant Br/Cl molar ratio of 1.55×10-3 (Herczeg and Edmunds, 1999). This ratio is maintained in rainwater derived from the ocean. In comparison, halite has a very low Br/Cl molar ratio of <10-4 as Br is excluded during mineral crystallisation (Herczeg et al., 2001). As a result the Br/Cl chloride ratio can be used to identify the source of salt.

At Bool Lagoon, the majority of groundwater samples have a Br/Cl ratio that is depleted relative to seawater and the surface water samples have a Br/Cl ratio that is enriched (Figure 4.3). This suggests: (1) halite has precipitated at the wetland or in the surrounding land, (2) surface water is being enriched in Br due to Br exclusion during halite precipitation, and (3) halite is being dissolved during recharge. It is unknown if this halite is currently being precipitated or if the halite was precipitated in the past during an arid period.

Using the measured Br/Cl ratios and an assumed halite end member of 10-4 a mixing model of halite and surface water was used to show the Cl concentrations before halite dissolution and to determine the amount of halite dissolved. This was calculated as follows: the Br/Cl ratio measured is the ratio of the sum of the Br and Cl:

[Br

Cl]𝑚𝑒𝑎𝑠

=Br𝑖+Brℎ𝑎𝑙𝑖𝑡𝑒

Cl𝑖+Clℎ𝑎𝑙𝑖𝑡𝑒, (4.1)

where i is the initial concentration before halite dissolution. By rearranging and substituting the known end member ratios gives:

Cl𝑖 =([Br Cl⁄ ]ℎ𝑎𝑙𝑖𝑡𝑒−[Br Cl⁄ ]𝑚𝑒𝑎𝑠)Cl𝑚𝑒𝑎𝑠

[Br Cl⁄ ]𝑚𝑒𝑎𝑠−[Br Cl⁄ ]𝑝𝑟𝑒𝑐𝑖𝑝. (4.2)

This shows that surface water Cl concentrations were between 270 and 2160 mg/L before halite dissolution and between 0.20 g (JOA029) and 10.6 g (ROB019) halite were dissolved per L of water. The spatial trend shows that amounts of halite added are low upgradient of the wetland and increase and become variable within the wetland (Figure 4.4a). Meanwhile, pre-halite dissolution chloride concentrations are variable with no spatial trend (Figure 4.4b).On average 42% of chloride originates as dissolved halite. Some uncertainty remains in the Br/Cl ratio of the endmembers, especially that of halite. However, these findings

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0.0 5.0 10.0 15.0

Ch

lori

de

(mg/

L)

Distance (km)

Groundwater Surface water


illustrate the usefulness of Br/Cl ratios as a tool to identify the sources of chloride. Furthermore, these samples are relatively easy to collect and the cost of analysis is low.

Figure 4.3 Mixing model between surface water and halite showing the evolution of the Br/Cl ratio with different initial chloride concentrations

(a) (b)

Figure 4.4 Spatial distribution of (a) halite added to groundwater and (b) salinity prior to halite dissolution

Time series sampling

Temporal concentrations are similar to the concentrations measured during March 2014 (~400-1100 mg/L), with the exception of lower concentrations (~250 mg/L) at Little Bool during April 2014 and high concentrations at the Tea Tree Bird Hide (~1400-7700 mg/L; Figure 4.5). There is no consistent seasonal trend between sites as some concentrations increase while others decrease. The salinity decrease in Gunawar occurs after rainfall events, suggesting that runoff is diluting chloride in the surface water. Meanwhile the increase in salinity at Drain M suggests that evaporation is increasing the chloride concentration.

0.0E+0

5.0E-4

1.0E-3

1.5E-3

2.0E-3

2.5E-3

3.0E-3

3.5E-3

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

Br/

Cl (

mo

lar)

Chloride (mg/L)

Groundwater Surface water 250 mg/L 500 mg/L 1000 mg/L

2000 mg/L 3000 mg/L Seawater Halite

mixing with Brenriched brines

0

2

4

6

8

10

12

0 5 10 15

Hal

ite

dis

solv

ed (

g/L)

Distance (km)

0

500

1000

1500

2000

2500

0 5 10 15

Init

ial s

alin

ity

(pre

-hal

ite

dis

solu

tio

n (

mg/

L)

Distance (km)


Electrical conductivity has historically been monitored in a few wells adjacent to the wetlands (DEWNR, 2015). These wells generally show increasing EC from the mid-1990s onward (ROB004 and ROB013) and as early as the 1970s at ROB005 (Figure 4.6). This increasing salinity may suggest two scenarios: the wetland is becoming increasingly saline due to (1) groundwater discharge and the ponding of surface water is concentrating salt through evaporation or (2) salinity in regional groundwater is still adjusting to changes to the drainage system and land-use in the region. These two processes are not mutually exclusive.

Figure 4.5 Time series of chloride concentrations in surface water at Bool Lagoon

Figure 4.6 Time series of EC in wells adjacent to Bool Lagoon

4.1.2 STABLE ISOTOPES

March 2014 sampling

All analyses plot to the right of the Adelaide local meteoric water line (LMWL; Crosbie et al., 2012) with the surface water samples having approximate δ18O of 7 to 11 ‰ and δ2H of 37 to 55 ‰. Groundwater samples are more depleted with approximate δ18O of -4 to -2 ‰ and δ2H of -22 to 10 ‰. Furthermore, all analyses plot along a linear trend (local evaporation line; LEL) of δ2H=5.13×δ18O-2.24 (R2 = 0.995; Figure

0

2000

4000

6000

8000

10000

12000

14000

16000

1950 1960 1970 1980 1990 2000 2010

EC

ROB004 ROB005 ROB006 ROB012 ROB013


4.7a). The slope of the LEL indicates evaporative enrichment from a meteoric source with a ratio of δ18O=-6.3 ‰ and δ2H=-34.4 ‰ (mean weighted local precipitation; MWLP) (Gibson et al., 1993).

Stable isotope enrichment in groundwater appears to increase along the expected flow path with the most depleted groundwater sample being the upgradient site JOA027 (δ2H=-22.3 ‰; Figure 4.7b). Groundwater samples from the eastern margin and near the outlet of Bool Lagoon have an average δ2H of -4.1 ‰. This indicates that evaporation-enriched surface water is recharging below the wetland.

(a) (b)

Figure 4.7 Stable isotopes at Bool Lagoon; (a) meteoric and evaporative trends and (b) spatial distribution

Time series sampling

Monthly stable isotope data is given in Table 4.3 and can be seen to vary significantly throughout the sampling period. As stated above, surface stable isotope analyses from the March 2014 sampling were all very enriched with δ2H in the range of 37–55 ‰; in April 2014, all stable isotope ratios decreased with Drain M, Gunawar, Tee Tree Bird Hide and Little Bool having δ2H of -22, -13, 18 and 35 ‰, respectively (Figure 4.8a). As the season progressed, enrichment increased at Drain M and Gunawar, Tee Tree Bird Hide enrichment decreased sharply before increasing sharply, and Little Bool enrichment decreased slightly before increasing slightly. The LEL during this period appears to have a slightly steeper slope, which could result from seasonal decreases in humidity or temperature.

Little Bool has the smallest range in δ2H (excluding Gunawar where data is incomplete because the site became dry during the sampling period), which may suggest that this site receive groundwater discharge consistently throughout the year, attenuating fluctuations in enrichment caused by evaporation. Furthermore, because Little Bool is more of a perennial feature compared with the rest of the Bool Lagoon complex, there is probably more of a steady-state condition where the volume of water is relatively constant and changes in stable isotope ratios resulting from discharge and evaporation are limited. At all of the surface water sampling sites it is unclear whether decreases in enrichment are caused by the addition of precipitation or groundwater discharge.

Whereas the March 2014 data shows a clear separation between groundwater and surface water (Figure 4.7a), the monthly data shows that surface water sometimes has low enrichment, similar to groundwater Figure 4.8b. This could indicate groundwater discharge during these periods. Precipitation could contribute to these decreased levels of enrichment, but there is not a strong trend with the local precipitation.

-40

-30

-20

-10

0

10

20

30

40

50

60

-10 -5 0 5 10 15

δ2

H (

‰)

δ18O (‰)


LEL Adelaide LMWL

-40

-30

-20

-10

0

10

20

30

40

50

60

0 5 10 15

δ2 H

(‰

)

Distance (km)



(a) (b)

Figure 4.8 Temporal stable isotope in surface water at Bool Lagoon; (a) time series of δ2H and cumulative precipitation (Bool Lagoon (Locksley Farm); (BOM, 2015)) and (b) δ2H vs. δ18O

4.1.3 RADON

Radon activities are elevated in the groundwater (5-65 Bq/L) and low in the surface water (0.04-0.21 Bq/L). These low activities of radon in the surface water are higher than expected for equilibrium with the atmosphere (value) and therefore could be an indication of groundwater discharge. But they also could be an indication of diffusive loss of radon emanating from the soil and into the surface water (radon soil emanation rates and radiogenicity were not measured). In groundwater, radon activities increase along the flow path (Figure 4.9) – however, because radon activities in groundwater reach secular equilibrium rapidly (weeks), the variation found in groundwater is probably caused by factors other than from groundwater –surface water interactions. In this environment, low radon activities in surface water do not necessarily indicate low groundwater discharge rates because radon rapidly degasses from surface water to the atmosphere.

-40

-30

-20

-10

0

10

20

30

40

50

60

-10 -5 0 5 10 15

δ2

H (

‰)

δ18O (‰)

Tee Tree Bird Hide Gunawar

Little Bool Drain M

LEL Adelaide LMWL


Figure 4.9 Radon vs. distance at Bool Lagoon; atmospheric equilibration assumes atmospheric 222Rn partial pressure and activity of 6×10-20 atm and 10 Bq/m3, respectively and a solubility of 1.08×10-2 mol/kg/atm.

4.1.4 TRITIUM

Tritium concentrations in all surface water samples have similar concentrations at 2.54±0.05 TU (mean ± standard deviation, n = 5). This is close to (89%) the expected concentration in post-bomb precipitation that has been adjusted for latitude (2.85 TU) (Tadros et al., 2014). Groundwater samples have tritium concentrations between 1.09 and essentially zero TU. While surface water concentrations are similar at each site, the values are statistically different and increase from east to west, the expected groundwater flow direction (Figure 4.10). Groundwater samples have more variable concentrations with a mean and standard deviation of 0.39±0.37 TU (n = 8). The general trend is that tritium concentrations increase along the flow path, which could indicate that recharge of modern water is occurring. With groundwater velocities of 5-50 m/year for the unconfined aquifer (Love et al., 1993) it is expected that the majority of the groundwater was recharged near the wetlands complex as opposed to at the Kanawinka Fault scarp. Using this mean groundwater tritium concentration, it is possible that groundwater is discharging into the surface water, with greater discharge rates being upgradient at Hack’s Lagoon. Using a simple binary mixing model:

𝐶𝑠𝑤 = 𝑅(1 − 𝑥) + 𝐶𝑔𝑤𝑥 (4.3)

where Csw is the tritium concentration in surface water, Cgw is the mean groundwater concentration, R is the tritium concentration in modern precipitation (2.85 TU) and x is the fraction of groundwater. This gives groundwater fractions in surface water at Hack’s Lagoon, Bool Lagoon East, Bool Lagoon West, Bool Lagoon Outlet and Little Bool Lagoon are 17%, 14%, 10%, 9% and 13%, respectively. Of course there is great uncertainty with these values as the tritium concentration in precipitation has not been measured in the LLC PWA and groundwater tritium concentrations vary between essentially zero (ROB030) and 1 TU (ROB022). Furthermore, this model neglects any temporal variations in surface water, where concentrations may vary through different stages of groundwater discharge and precipitation and surface water input.

0.001

0.01

0.1

1

10

100

0.0 5.0 10.0 15.0

Rad

on

(B

q/L

)

Distance (km)


Atm. equilibration


Figure 4.10 Spatial trends of tritium at Bool Lagoon

Recharge age

The tritium precipitation record from Kaitoke, New Zealand can be used to estimate the recharge time of groundwater samples. The concentrations were linearly shifted to match the tritium concentration expected in modern recharge at the latitude of Naracoorte (Tadros et al., 2014). Samples from ROB022, ROB027 and ROB031 have an intermediate concentration of tritium (0.6-1.1 TU); plotting the decay corrected tritium concentrations show that these samples have a wide range of recharge ages because the decay line crosses the precipitation input function several times (Figure 4.11). The remaining groundwater samples do not intersect the tritium precipitation input function, indicating that recharge occurred after 1953 (beginning of thermonuclear weapons testing) but before 1960 where no record has been recorded. These estimates do not consider the mixing of water of different ages, which can be expected in systems with high rates of recharge.

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 5.0 10.0 15.0

Trit

ium

(TU

)

Distance (km)



Figure 4.11 Age-corrected groundwater tritium concentrations at Bool Lagoon (compared to expected tritium in precipitation (Kaitoke record) adjusted to modern expected concentrations at Naracoorte)

Recharge estimates

A more advanced flow model was conceptualised to use tritium to constrain recharge rates across the wetland. The discretised model is a one dimensional cross-section along the expected flow-path. The first cell is at ROB030 where the tritium concentration is essentially zero, suggesting little recharge immediately upgradient of the wetland. In all subsequent cells, groundwater flows from the previous cell at a constant velocity (5-50 m/y) with tritium decaying with a half-life of 12.37 years (λ). At each time-step, surface water is added to each cell and has a historical tritium concentration that is based on the Kaitoke precipitation record and adjusted to the latitude of Naracoorte. Where there is no record of tritium in precipitation, the following assumptions were made: pre-1953 and post-2008, 2.85 TU was used and 1953-1960, a linear interpolation between these dates was used. Because the thickness of the aquifer is unknown, the amount of surface water added is a fraction of the existing amount of groundwater. This model assumes steady-state conditions, neglects the effects of diffusions and dispersion and is mathematically presented as:

𝐶𝑡,𝑙 = 𝐶𝑡−1,𝑙−1exp (ln(0.5)∆𝑡

𝜆) (1 − 𝑥) + 𝑅𝑡𝑥, (4.4)

where C is the tritium concentration, t is the time-step, l is the length-step, x is the fraction of groundwater and R is the tritium concentration in precipitation. The time-step used was 1 year and the length-step was set to be the distance groundwater travelled in 1 year (i.e. 5-50 m).

The results from this model show that a wide range of recharge rates (0.2-2.4%/year) are required to fit all the observed data (Figure 4.12a). This may suggest that recharge is spatially variable and higher rates of recharge may be occurring near the edges of the wetland where the clay thins (SKM, 2010). This model also suggests that the groundwater below Bool Lagoon could be primarily composed of water that was originally surface water in the lagoon. With a flow velocity of 5 m/year, the surface water fraction in groundwater at the sample sites range between 79-100% (Figure 4.12b); with a flow velocity of 50 m/year, the range is 14-96% (Figure 4.12d).

This model may be better constrained by the additional of multiple tracers. Stable isotopes could be added, but the endmembers would be difficult to constrain because of the transient amounts of surface water resulting in a wide range of stable isotope enrichment (Figure 4.8a). Sulphur hexafluoride (SF6) would be a suitable young tracer that could further constrain the age mixture at each site.

0

5

10

15

20

25

30

35

40

1960 1970 1980 1990 2000 2010

Trit

ium

(TU

)

Adjusted to Naracoorte ROB018 ROB019

ROB021 ROB022 ROB023

ROB027 ROB031 JOA029


(a) (b)

(c) (d)

Figure 4.12 Recharge model of Bool Lagoon using tritium with a groundwater flow velocity of 50 m/y; (a) tritium distribution with different input ratios and (b) cumulative surface water contribution resulting from different input ratios

4.1.5 NOBLE GASES

Concentrations of the dissolved noble gases helium, neon and argon are quite variable, indicating a wide range of recharge conditions (Figure 4.13b). Excluding two samples that appear erroneous (JOA029 and ROB031), in groundwater samples, recharge temperature varies between 6 and 15 °C while it appears that recharge salinity could be significant at 15 mg/L, which is consistent with measured salinities. Excess air values range from 0 to approximately 1.5×10-3 cc STP/g (assuming a recharge salinity of 0 mg/L). These relatively high excess air values suggest that groundwater levels change quickly – that is, there is a rapid rise in the watertable resulting in a bubbles being trapped and subsequently dissolved into groundwater (Kipfer et al., 2002)at the time of recharge.

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

5 7 9 11 13

Trit

ium

(TU

)

Distance (km)

groundwater 0.0% 0.1%

0.5% 1.0% 1.5%

2.0% 2.5%

0.0

0.1

0.2

0.3

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0.7

0.8

0.9

1.0

5 7 9 11 13

Surf

ace

wat

er f

ract

ion

Distance (km)

0.0% 0.1% 0.5% 1.0%

1.5% 2.0% 2.5%

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

5 7 9 11 13

Trit

ium

(TU

)

Distance (km)

groundwater 0.0% 0.1%

0.5% 1.0% 1.5%

2.0% 2.5%

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

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1.0

5 7 9 11 13

Surf

ace

wat

er f

ract

ion

Distance (km)

0.0% 0.1% 0.5% 1.0%

1.5% 2.0% 2.5%


Surface water samples indicate higher equilibrium temperature at 15 to 32 °C. These temperatures seem reasonable due to the shallow pool temperatures and surface water temperatures as high as 35 °C were measured (Hacks Lagoon). Furthermore, this suggests that surface water has fully or partially re-equilibrated with the atmosphere, resulting in little to no evidence of a groundwater component – in other words, groundwater may have been present but its former noble gas content has been lost from the surface water.

Groundwater may indicate the presence of terrigenic helium, as helium tends to be higher than expected from the neon vs. argon plot (Figure 4.13b). However, due to the variability in apparent recharge salinity and components of excess air, it is difficult to quantitatively determine the amount of terrigenic helium. However, ROB030 appears to undoubtedly have terrigenic helium, and as this sample also has zero tritium, this sample must be old groundwater. The analysis of carbon-14 at these sites may help constrain the age of the groundwater. However, being a carbonate aquifer, the interpretation of carbon-14 may be difficult due to the addition of ‘dead carbon’ resulting from exchange between the groundwater and the aquifer material.


(a)

(b)

Figure 4.13 Noble gas concentrations at Bool Lagoon; (a) neon vs. argon and (b) helium vs. neon

1.5E-04

2.0E-04

2.5E-04

3.0E-04

3.5E-04

4.0E-04

4.5E-04

5.0E-04

5.5E-04

1.0E-07 1.2E-07 1.4E-07 1.6E-07 1.8E-07 2.0E-07 2.2E-07 2.4E-07 2.6E-07 2.8E-07

40A

r (c

c ST

P/g

)

20Ne (cc STP/g)

Salinity (5 mg/L) Temp (4C) Excess Air (0.0005 cc STP/g) groundwater surface water

40

0

8

16

24

32

0 1E-

3

2E-

3

3E-

3

4E-

3

10

20

30

40

0

1.0E-07

1.2E-07

1.4E-07

1.6E-07

1.8E-07

2.0E-07

2.2E-07

2.4E-07

2.6E-07

2.8E-07

3.0E-08 3.5E-08 4.0E-08 4.5E-08 5.0E-08 5.5E-08 6.0E-08 6.5E-08 7.0E-08

20N

e (c

c ST

P/g

)

4He (cc STP/g)

Salinity (5 mg/L) Temp (4C) Excess Air (0.0005 cc STP/g) Groundwater Surface water

10

20

30

40

0 40

0

8

16

24

32

1E-

3

2E-

3

3E-

3

Terrigenic helium


Table 4.1 Isotopic results

SAMPLE δ2H (‰)

δ18O (‰)

3H (TU)

4He (cc STP/g)

20Ne (cc STP/g)

40Ar (cc STP/g)

222Rn (Bq/L)


ROB005 -18.40 -2.00 - - - - -

ROB018 1.62 0.71 0.238±0.019 4.39E-08 1.71E-07 3.46E-04 65.8±3.1

ROB019 -1.34 -0.23 0.304±0.020 4.97E-08 1.97E-07 - 37.5±1.8

ROB021 -12.09 -2.03 0.122±0.018 4.80E-08 1.94E-07 3.66E-04 12.4±0.6

ROB022 -4.10 -0.67 1.086±0.032 4.75E-08 1.86E-07 3.65E-04 12.0±0.6

ROB023 -4.46 -0.43 0.163±0.015 4.74E-08 1.66E-07 3.50E-04 33.2±1.6

ROB024 4.95 1.67 - - - - -

ROB025 10.11 2.09 - - - - -

ROB026 -17.78 -3.16 - - - - -

ROB027 -10.72 -1.98 0.619±0.020 5.44E-08 1.87E-07 4.13E-04 11.5±0.6

ROB030 -14.57 -2.32 -0.017±0.014 6.11E-08 2.08E-07 4.17E-04 20.6±1.0

ROB031 -16.40 -2.58 0.813±0.024 6.12E-08 3.31E-07 - 7.81±0.40

JOA029 -22.33 -3.96 0.182±0.015 4.90E-08 1.48E-07 3.07E-04 5.61±0.28


Hacks 48.97 10.40 2.434±0.039 4.70E-08 1.61E-07 2.57E-04 0.145±0.014

Bool East 42.30 8.64 2.516±0.052 4.67E-08 1.53E-07 2.46E-04 0.040±0.007

Bool West 38.68 8.05 2.592±0.053 4.57E-08 1.77E-07 - 0.036±0.006

Bool Outlet 55.20 11.19 2.635±0.053 4.55E-08 1.53E-07 2.99E-04 0.205±0.017

Little Bool 36.58 7.09 2.519±0.052 4.57E-08 1.53E-07 2.67E-04 0.092±0.010


WAT032 -28.42 -4.80 1.489±0.036 4.90E-08 1.75E-07 3.56E-04 3.26±0.17

WAT034 -14.57 -2.88 1.169±0.031 4.68E-08 1.78E-07 3.57E-04 1.80±0.10

WAT037 -23.55 -4.31 0.735±0.026 4.90E-08 1.81E-07 3.63E-04 2.21±0.12


Robe Northeast 35.13 6.42 - - - - 0.040±0.006

Robe South 35.32 6.37 - - - - 0.133±0.012

Robe Southeast 33.58 6.40 1.464±0.036 3.35E-08 1.23E-07 2.16E-04 0.076±0.009

Spring 1 -26.10 -4.88 - - - - -

Spring 2 -23.54 -4.55 - - - - -

Spring 3 -16.40 -3.25 - - - - -


Table 4.2 Major ion results

SAMPLE pH

E.C. (dS/m)

TOTAL ALK

(meq/L)

Cl- (mg/L)

F-

(mg/L) Br-

(mg/L)

NO3-

(mg/L) SO4

=

(mg/L) Ca

(mg/L) K

(mg/L) Mg

(mg/L) Na

(mg/L) S

(mg/L)


ROB005 7.89 14.2 15.6 4342 2.4 8.0 1.2 620 61 38.8 284 2740 221

ROB018 7.59 21.1 12.3 6890 1.6 6.5 1.0 1600 215 104 570 3860 505

ROB019 7.59 23.4 12.8 7956 1.8 6.7 <1 1000 179 119 612 4420 425

ROB021 7.69 5.80 6.61 1744 <1 2.9 18 120 152 18.2 153 852 48.9

ROB022 7.67 8.31 9.54 2430 <1 2.9 67 210 138 34.3 196 1340 82.9

ROB023 7.58 13.5 19.2 4176 1.3 4.8 1.6 380 136 46.9 295 2470 160

ROB024 7.64 3.45 11.3 655.0 0.6 2.8 <0.2 250 144 31.0 92.9 507 94.2

ROB025 7.49 8.27 17.9 1966 0.96 1.9 <1 810 206 54.3 214 1400 265

ROB026 7.61 9.47 10.0 2814 1.1 3.8 <1 590 168 31.3 230 1610 188

ROB027 7.39 15.4 11.7 4749 1.7 6.3 <1 1100 294 39.9 386 2650 348

ROB030 7.81 5.54 8.80 1584 0.72 4.0 0.38 180 99.6 12.3 101 986 60.8

ROB031 7.74 6.97 8.94 1993 0.58 5.6 <0.05 430 143 14.4 198 1140 142

JOA029 7.75 1.86 6.68 391 <0.2 0.96 15 45 116 3.88 34.5 214 13.6


Hacks 8.80 3.30 4.53 910.5 0.53 4.1 <0.2 91 74.6 18.8 63.9 525 28.8

Bool East 8.40 4.00 13.1 957.5 0.81 5.2 <0.2 27 123 51.8 86.9 601 11.4

Bool West 8.08 2.67 10.1 518.5 0.76 3.3 <0.2 53 104 52.4 59.4 371 20.3

Bool Outlet 9.23 2.34 8.06 507.5 1.00 3.8 <0.2 27 35.4 34 55.6 407 11.9

Little Bool 8.69 3.08 6.82 726.5 0.37 3.4 <0.2 120 82.5 43 71.9 481 38.3


WAT032 7.91 1.75 5.29 382.7 <0.2 0.86 2.3 69 114 5.14 34.7 189 22.5

WAT034 7.83 16.0 7.77 5098 2.4 16 <1 770 120 127 379 2940 250

WAT037 7.95 1.670 5.05 330.7 1.1 0.95 <0.2 120 84.7 8.5 51.1 180 40.6


Robe Northeast 8.23 102 4.86 46540 <5 140 <5 7300 907 920 3420 25900 2390

Robe South 8.23 100 4.91 45500 <5 140 <5 7100 901 891 3350 25300 2360

Robe Southeast 8.23 101 4.94 46020 <5 140 <5 7300 900 905 3380 25600 2370

Spring 1 - - - 124.7 - - - - - - - - -

Spring 2 - - - 444.1 - - - - - - - - -

Spring 3 - - - 6750 - - - - - - - - -


Table 4.3 Temporal surface water chemistry at Bool Lagoon

SAMPLE Date δ2H (‰)

δ18O (‰)

Cl (mg/L)

Tea Tree Bird Hide (140.7277°, -37.1242°) 02/04/2014 17.52 3.74 1461

30/04/2014 19.34 3.93 1402

30/05/2014 -19.24 -2.69 1014

30/06/2014 -19.02 -2.70 1017

30/07/2014 15.72 3.25 7732

31/08/2014 16.88 3.20 7700

30/09/2014 31.82 5.91 1890

30/10/2014 18.44 3.22 9060

30/11/2014 32.6 5.86 1900

Gunawar (140.7174°,-37.1020°) 02/04/2014 -13.14 -1.98 736

30/04/2014 -15.95 -2.25 1072

30/07/2014 -5.35 -1.65 424

31/08/2014 -4.73 -1.63 431

Little Bool (140.6860°, -37.0939°) 02/04/2014 34.50 6.28 830

30/04/2014 35.02 6.26 805

30/05/2014 23.64 4.13 787

30/06/2014 23.60 4.15 790

02/07/2014 13.70 1.80 739

30/07/2014 13.93 1.80 740

31/08/2014 14.38 1.78 755

30/09/2014 30.28 4.83 1080

30/10/2014 29.86 4.89 1070

30/11/2014 29.33 4.84 1070

Drain M (140.6499°, -37.1507°) 02/04/2014 -21.70 -3.56 240

30/04/2014 -22.44 -3.62 259

02/07/2014 6.50 0.92 968

30/07/2014 5.90 0.90 960

31/08/2014 5.65 0.92 962

30/09/2014 28.32 5.55 869

30/10/2014 27.75 5.58 872

30/11/2014 27.78 5.48 879


4.2 Lake Robe

4.2.1 MAJOR IONS

The analytical ion imbalance in all samples was (<±5%). Groundwater and surface waters are NaCl type to CaMgCl type (Figure 4.14). Also shown in Figure 4.14 is data from existing analysed groundwater samples within a 10 km radius of Lake Robe (WaterConnect database (DEWNR, 2015)). Of 65 samples with at least a Cl analysis, 50 samples (from 11 individual wells) had a charge balance within ±5% and have all necessary ion analyses required to construct a Piper plot. These analyses are predominantly NaCl type waters with a few CaMgCl type waters. The majority of the data is from the TCA and clusters in the lower portion of the NaCl box – because of higher ratios of bicarbonate. Samples from the TLA (and possibly other local unconfined aquifers) plot near the WAT034 and WAT037 samples. Surface water samples from Lake Robe plot close to the standard seawater composition – however, evaporated precipitation also has a similar composition.

Chloride

Chloride concentrations in Lake Robe were approximately 46,000 mg/L, while groundwater was approximately 300-5100 mg/L with the highest salinity at WAT034 (east of Lake Robe). Chloride in the springs were in the range of 100-6800 mg/L with the highest salinity potentially being a mixture of fresh spring water and hypersaline lake water.

Figure 4.14 Piper plot of Lake Robe

4.2.2 STABLE ISOTOPES

All analyses plot to the right of the local meteoric water line. All data plots along the best fit line of δ2H=5.42×δ18O+0.13 (R2 = 0.998). This LEL intersects the LMWL to give a mean weighted local precipitation value of -5.97 ‰ and -32.2 ‰, for δ18O and δ2H, respectively. The LEL here has a steeper slope and higher intercept than the LEL calculated at Bool Lagoon – this indicates relatively lower humidity or temperature.

Groundwater samples show slight evaporative enrichment with WAT034 showing the highest enrichment. Surface water samples plot in two groups: (1) highly enriched samples taken directly from Lake Robe are with δ18O=6.4 ‰ and δ2H=35.1 ‰ and (2) spring water located along the western margin with slight enrichment that plot with the groundwater samples (Figure 4.15).


Figure 4.15 Stable isotopes at Lake Robe

4.2.3 TRITIUM

Tritium concentrations in groundwater at the Lake Robe site vary between 0.74 (WAT037) and 1.49 TU (WAT032). Surface water in Lake Robe has a tritium concentration of 1.46 TU, which is half of the expected rainfall concentration. Assuming no mixing or dilution, these groundwater samples were recharged since 1990.

As explained in Section 4.1.4, the tritium precipitation record from Kaitoke, New Zealand can be used to estimate the recharge time of groundwater samples. After shifting the concentrations to match the tritium concentration expected in modern recharge at the latitude of Naracoorte (Tadros et al., 2014), samples from WAT032, WAT034 and WAT037 have an intermediate concentration of tritium that is plotted against the decay corrected tritium concentrations (Figure 4.11) – this show that these samples have a wide range of recharge ages because the decay line crosses the precipitation input function several times. These estimates do not consider the mixing of water of different ages.

-40

-30

-20

-10

0

10

20

30

40

50

60

-10 -5 0 5 10 15

δ2

H (

‰)

δ18O (‰)


Springs LEL

Adelaide LMWL


Figure 4.16 Age corrected groundwater tritium concentrations at Lake Robe (compared to expected tritium in precipitation (Kaitoke record) adjusted to expected concentrations at Naracoorte)

On the basis of its low tritium concentration surface water in Lake Robe appears to be groundwater dependent. This suggests that the water in Lake Robe is a combination of at least one of the following: (1) regional groundwater containing no tritium, (2) local fresh groundwater that is depleted in tritium and, (3) precipitation with modern concentrations of tritium. As a minimum estimate of groundwater contributions (regional water containing zero tritium), 49% of the water in this lake is derived from groundwater. However, the local dune system appears to contain a local groundwater system (containing young water with elevated tritium concentrations) that discharges into Lake Robe, the groundwater dependence is likely to be much greater.

4.2.4 RADON

Radon activities are elevated in the groundwater at 1.8-3.3 Bq/L – conversely, radon activities in water are 0.04-0.13 Bq/L (Table 4.1; Figure 4.17). These low activities, which are significantly higher than equilibrium atmosphere, in surface water could indicate groundwater discharge, but they could also be the result of radon emanation into the surface water. Because of the long water residence time in a terminal lake, and shallow depth, exchange with the atmosphere is likely to dominate the radon mass-balance in Lake Robe.

0

5

10

15

20

25

30

35

40

1960 1970 1980 1990 2000 2010

Trit

ium

(TU

)

Adjusted to Naracoorte WAT032 WAT034 WAT037


Figure 4.17 Radon vs. chloride at Lake Robe

4.2.5 NOBLE GASES

Helium, neon and argon results from Lake Robe are presented in Table 4.1. Groundwater samples are all similar indicating low salinity during recharge, recharge temperature of ~13.5 °C and excess air that varies between zero and 2×10-4 cc STP/g (Figure 4.18). Using excess air and recharge temperatures derived from neon and argon to constrain the source of helium shows that there is a slight excess of helium that is not attributed to recharge conditions –suggesting a small amount of terrigenic helium (Figure 4.18b). This excess helium may be an indication of regional groundwater discharge.

The noble gas sample from Lake Robe has very low concentrations of all noble gases, which is explained by the high salinity of the water. With a TDS of 87 g/L, this salinity exceeds the experimental data used to calculate noble gas solubilities (40 g/L) (Weiss, 1971). The model of Smith and Kennedy (1983) is constrained by salinities of ~0-5 molarity (~300 mg/L) and gives atmospheric equilibrium temperature and salinity of approximately 20 °C and 80 mg/L, which is relatively consistent with the measured parameters. This suggests that noble gases in Lake Robe show little to no evidence of groundwater discharge that would be evident if noble gas concentrations were more similar (i.e. higher concentrations) to that of groundwater. Furthermore, gas-loss into the atmosphere is expected to be a quick process for such a shallow body of water with persistent winds – so the evidence of groundwater discharge provided by noble gases may have been lost. A mass balance model similar to Fass and Cook (2007) (model not shown) suggests that excess helium would not be preserved in the surface water unless groundwater discharge rates greatly exceeded any realistic estimates.

0.01

0.1

1

10

100 1000 10000 100000

222R

n (

Bq

/L)

Cl- (mg/L)



(a)

(b)

Figure 4.18 Noble gas concentrations at Lake Robe; (a) neon vs. argon and (b) helium vs. neon

1.0E-04

1.5E-04

2.0E-04

2.5E-04

3.0E-04

3.5E-04

4.0E-04

4.5E-04

5.0E-04

5.5E-04

1.0E-07 1.2E-07 1.4E-07 1.6E-07 1.8E-07 2.0E-07 2.2E-07 2.4E-07 2.6E-07 2.8E-07

40A

r (c

c ST

P/g

)

20Ne (cc STP/g)

groundwater surface water Temp (8 °C) Excess Air (0.001 cc STP/g) Salinity (10 mg/L)

0

1E-

3

2E-

3

3E-

3

4E-

3

4032

24

16

8

0

40

30

20

10

0

8.0E-08

1.0E-07

1.2E-07

1.4E-07

1.6E-07

1.8E-07

2.0E-07

2.2E-07

2.4E-07

2.6E-07

2.8E-07

3.0E-08 3.5E-08 4.0E-08 4.5E-08 5.0E-08 5.5E-08 6.0E-08 6.5E-08 7.0E-08

20N

e (c

c ST

P/g

)

4He (cc STP/g)

groundwater surface water Excess Air (0.001 cc STP/g) Salinity (10 mg/L) Temp (8 °C)

10

20

30

0

0

8

16

32

1E-

3 2E-

3

3E-

3

4E-

3

0

40

24

40

Terrigenic helium


5 Discussion

5.1 Bool Lagoon

5.1.1 RECHARGE

Evidence of groundwater recharge at Bool Lagoon comes from tritium and stable isotopes. The increasing tritium concentrations moving along the flow path show that recharge is occurring and is spatially variable. An increase in stable isotope enrichment also gives evidence that evaporated surface water is recharging. This evidence combined with high salinity and high excess air in groundwater suggests the following process: (1) during arid periods (either modern or historical), surface water held in the lagoon evaporates resulting in brackish water in the pore water and halite crystallisation on exposed sediments; (2) the lagoon receives additional surface water via Mosquito Creek during late winter or early spring– the increased pressure head from this surface water causes the brackish soil water and shallow groundwater to recharge, trapping excess air in the process – this process also dissolves halite that had formed in the sediments (Figure 5.1). The occurrence of low Br/Cl ratios in groundwater from Bool Lagoon is intriguing because it suggests halite dissolution as a source of salinity. Halite formation may occur in wetland sediments under current conditions during dry periods. Another possibility is that halite formation occurred under a past climate, when Bool Lagoon would have been a salt lake, and the current Br/Cl ratio in groundwater reflects this past condition (that is salinities have not yet reached a new equilibrium.

5.1.2 DISCHARGE

Evidence of groundwater discharge at Bool Lagoon comes from surface water tritium concentrations. The slight increase in tritium concentrations moving along the flow path suggests that discharge is occurring in the upgradient parts of the wetland. Radon activities also suggest some kind of discharge, but these values could be attributed to emanation. Furthermore, radon does not have a spatial trend in surface water, suggesting emanation rates combined with diffusive loss of the atmosphere exceed the signal created with discharge.

5.1.3 A FLOW-THROUGH WETLAND?

While there is evidence of recharge and discharge occurring at Bool Lagoon, it may not be a flow-through wetland per se because recharge and discharge occur intermittently rather than at the same time (Figure 5.1). Alternatively, Bool Lagoon could be described as a regional discharge wetland that is also an intermittent local recharge feature. Taylor et al. (2015) summarises Bool Lagoon potentiometric surfaces, which also suggest cycles of recharge and discharge over time. A recent examination of the local geology and of modelled regional groundwater flow-paths near Bool Lagoon suggest that it is set in a regional discharge, rather than a regional flow through setting (Harrington et al., 2015).


Figure 5.1 Conceptual recharge and discharge processes at Bool Lagoon

5.2 Lake Robe

The surface water in Lake Robe is characterised by very high salinities with the major ion ratios of seawater. However, there is no indication that seawater is discharging into the wetland. Tritium data shows the water is modern but several years old indicating a water component other than rainfall and runoff. Stable isotopes show that the water originated as rainwater with a depleted signature. Noble gases indicate that groundwater is mixing with the highly saline surface water and has not yet equilibrated with the atmosphere. This information suggests the highly saline coastal lakes of the South East may not be dependent on regional groundwater discharge, but are dependent on local flows from the dunes systems and potentially other local systems.

5.3 Suggestions for future wetland studies

This study has shown that multiple tracers are useful for determining hydrogeological setting in LLC PWA wetlands. All tracers have provided valuable information about recharge conditions and spatial trends. However, further constrains could be made using the following:

Measure tritium in precipitation to give an end member to constrain an upper limit for tritium concentrations in surface water; this could be combined with temporal measurements of tritium in the Bool Lagoon complex and Mosquito Creek

Create a water balance for Bool Lagoon; this should be attempted by combining a water, stable isotope and chloride budget ; this would require the continuous monitoring for water level and solutes in the wetland and a targeted study to measure evaporation rates from the wetland; this data could then be modelled using existing numerical models such as the model of Turner and Townley (2006), but only after modifying the model conceptualisation to account for additional conditions such as transient flow, density-driven flow and evaporation from the water table

Instrument Deadmans Swamp with additional piezometer transects to enable the characterisation of its groundwater – surface water regime

Measure additional environmental tracers at Lake Robe and additional monitoring wells; additional tracers would include SF6 (to obtain a more detailed insight into the age distribution of young groundwater and to better constrain the recharge model of section 4.1.4), carbon-14 (to identify old, regional groundwater) and strontium isotopes (to identify the relative importance of local groundwater sources)

(1)

(2) Recharge phase

Discharge phase

high salinity surface water or shallow groundwater

Fresh discharge

Regional groundwater

Regional groundwater


6 Conclusions

The wetlands in the LLC PWA are thought to be heavily groundwater dependent and these wetlands have significantly reduced in area and quality due to increased drainage as well as other land use changes and management. This study has investigated two wetlands that occupy different parts of the landscape and were expected to have different, yet distinct hydrogeological contexts. These wetlands: Bool Lagoon and Lake Robe were originally thought to be flow-through and discharge features, respectively. This study has shown that Bool Lagoon has separate periods of recharge and discharge, but is in a regional discharge setting. The ponding of surface water coupled with low recharge rates has resulted in salt that is seldom flushed from the wetland. Lake Robe is definitely a site of groundwater discharge, however it appears to be a local discharge feature in a regional discharge setting. There is no definite evidence of regional groundwater, but there is evidence of local discharge of fresh water coming from the adjacent dune system. This demonstrates the power of environmental tracers to help identify the hydrogeological setting of wetlands, which ultimately results in a better management of these resources.


Appendix A Chemistry

A.1 East-west transects

A.1.1 BOOL LAGOON

(a) (b)

(c) (d)

0

5

10

15

20

25

0.0 5.0 10.0 15.0

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. (d

S/m

)

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0.0 5.0 10.0 15.0

Tota

l Alk

alin

ity

(meq

/L)

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3000

4000

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9000

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lori

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(mg/

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4

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9

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mid

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(e) (f)

(g) (h)

(i) (j)

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200

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1800

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hat

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g/L)

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0.0 5.0 10.0 15.0

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ciu

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mg/

L)

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100

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0.0 5.0 10.0 15.0

Mag

nes

ium

(m

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Distance (km)

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4500

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0.0 5.0 10.0 15.0

Sod

ium

(m

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Distance (km)

0.0E+00

5.0E-04

1.0E-03

1.5E-03

2.0E-03

2.5E-03

3.0E-03

3.5E-03

0.0 5.0 10.0 15.0

Br/

Cl (

mo

lar

rati

o)

Distance (km)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 5.0 10.0 15.0

Na/

Cl (

mo

lar

rati

o)

Distance (km)


(k) (l)

(m)

Apx Figure A.1 Bool Lagoon solutes and tracers in groundwater (blue) and surface water (green) versus distance east from JOA029 (a) electrical conductivity, (b) alkalinity, (c) chloride, (d) bromide, (e) sulphate, (f) calcium, (g) magnesium, (h) sodium, (i) Br/Cl ratio, (j) Na/Cl ratio, (k) radon, (l) tritium, (m) deuterium

0.01

0.1

1

10

100

0.0 5.0 10.0 15.0

Rad

on

(B

q/L

)

Distance (km)

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 5.0 10.0 15.0

Trit

ium

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)

Distance (km)

-30.0

-20.0

-10.0

0.0

10.0

20.0

30.0

40.0

50.0

60.0

0.0 5.0 10.0 15.0

δ2H

(‰

VSM

OW

2)

Distance (km)


A.1.2 LAKE ROBE

(a) (b)

(c) (d)

(e) (f)

0

20

40

60

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0.0 0.5 1.0 1.5 2.0 2.5

E.C

. (d

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/L)

Distance (km)

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5000

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15000

20000

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30000

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50000

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del

(m

g/L)

Distance (km)

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hat

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ciu

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(g) (h)

(i) (j)

(k) (l)

0

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3500

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0.0 0.5 1.0 1.5 2.0 2.5

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nes

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Distance (km)

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20000

25000

30000

0.0 0.5 1.0 1.5 2.0 2.5

Sod

ium

(m

g/L)

Distance (km)

0.0E+00

2.0E-04

4.0E-04

6.0E-04

8.0E-04

1.0E-03

1.2E-03

1.4E-03

1.6E-03

0.0 0.5 1.0 1.5 2.0 2.5

Br/

Cl (

mo

lar

rati

o)

Distance (km)

0.74

0.76

0.78

0.8

0.82

0.84

0.86

0.88

0.9

0.0 0.5 1.0 1.5 2.0 2.5

Na/

Cl m

ola

r ra

tio

)

Distance (km)

0.01

0.1

1

10

0.0 0.5 1.0 1.5 2.0 2.5

Rad

on

-22

2 (

Bq

/L)

Distance (km)

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0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0.0 0.5 1.0 1.5 2.0 2.5

Trit

ium

(TU

)

Distance (km)


(m)

Apx Figure A.2 Lake Robe solutes and tracers in groundwater (medium blue), surface water (green) and springs (dark blue) versus distance east from WAT034 (a) electrical conductivity, (b) alkalinity, (c) chloride, (d) bromide, (e) sulphate, (f) calcium, (g) magnesium, (h) sodium, (i) Br/Cl ratio, (j) Na/Cl ratio, (k) radon, (l) tritium, (m) deuterium

A.2 Chloride plots

A.2.1 BOOL LAGOON

(a) (b)

-30

-20

-10

0

10

20

30

40

0.0 0.5 1.0 1.5 2.0 2.5

δ2 H

(‰

VSM

OW

2)

Distance (km)

0.00

0.10

0.20

0.30

0.40

0.50

0 50 100 150 200 250

Br

Cl- (mmol/L)

0.0E+00

5.0E-04

1.0E-03

1.5E-03

2.0E-03

2.5E-03

3.0E-03

3.5E-03

0 50 100 150 200 250

Br/

Cl

Cl- (mmol/L)


(c) (d)

(e) (f)

(g) (h)

0

50

100

150

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250

300

0 50 100 150 200 250

Na+

Cl- (mmol/L)

0.0

0.2

0.4

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1.2

1.4

0 50 100 150 200 250

Na/

Cl

Cl- (mmol/L)

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Mg+

Cl- (mmol/L)

0.00

0.05

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Mg/

Cl

Cl- (mmol/L)

0

1

2

3

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8

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Ca2+

Cl- (mmol/L)

0.00

0.05

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Ca/

Cl

Cl- (mmol/L)


(i) (j)

(k) (l)

0

5

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0 50 100 150 200 250

HC

O3

-

Cl- (mmol/L)

0.0

0.1

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0 50 100 150 200 250

HC

O3 /

Cl

Cl- (mmol/L)

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4

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14

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18

20

0 50 100 150 200 250

SO42+

Cl- (mmol/L)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 50 100 150 200 250

SO4/

Cl

Cl- (mmol/L)


(m)

Apx Figure A.3 Bool Lagoon solutes and tracers in groundwater (blue) and surface water (green) versus chloride; where appropriate lines representing seawater (solid black) and halite (dashed black) are shown , (a) bromide, (b) Br/Cl ratio, (c) sodium, (d) Na/Cl ratio, (e) magnesium, (f) Mg/Cl ratio, (g) calcium, (h) Ca/Cl ratio, (i) bicarbonate, (j) HCO3/Cl ratio, (k) sulphate, (l) SO4/Cl ratio, (m) tritium

A.2.2 LAKE ROBE

(a) (b)

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 50 100 150 200 250

Trit

ium

(TU

)

Cl- (mmol/L)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 500 1000 1500

Br

Cl

0.0E+00

2.0E-04

4.0E-04

6.0E-04

8.0E-04

1.0E-03

1.2E-03

1.4E-03

1.6E-03

1.8E-03

0 500 1000 1500

Br/

Cl

Cl


(c) (d)

(e) (f)

(g) (h)

0

200

400

600

800

1000

1200

0 500 1000 1500

Na

Cl

0.74

0.76

0.78

0.80

0.82

0.84

0.86

0.88

0.90

0 500 1000 1500

Na/

Cl

Cl

0

20

40

60

80

100

120

140

160

0 500 1000 1500

Mg

Cl

0.00

0.05

0.10

0.15

0.20

0.25

0 500 1000 1500

Mg/

Cl

Cl

0

5

10

15

20

25

30

0 500 1000 1500

Ca

Cl

0.00

5.00

10.00

15.00

20.00

25.00

0 500 1000 1500

Ca/

Cl

Cl


(i) (j)

(k) (l)

0

1

2

3

4

5

6

7

8

9

0 500 1000 1500

HC

O3

Cl

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 500 1000 1500

HC

O3

/Cl

Cl

0

10

20

30

40

50

60

70

80

0 500 1000 1500

SO4

Cl

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 500 1000 1500

SO4

/Cl

Cl


(m)

Apx Figure A.4 Lake Robe solutes and tracers in groundwater (blue) and surface water (green) versus chloride; where appropriate lines representing seawater (solid black) and halite (dashed black) are shown , (a) bromide, (b) Br/Cl ratio, (c) sodium, (d) Na/Cl ratio, (e) magnesium, (f) Mg/Cl ratio, (g) calcium, (h) Ca/Cl ratio, (i) bicarbonate, (j) HCO3/Cl ratio, (k) sulphate, (l) SO4/Cl ratio, (m) tritium

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 500 1000 1500

Trit

ium

(TU

)

Cl- (mmol/L)


A.3 Reaction plots

A.3.1 BOOL LAGOON

(a) (b)

(c) (d)

0

5

10

15

20

25

0 5 10 15 20 25

Mg

HCO3

0

5

10

15

20

25

0 5 10 15 20 25

Ca

HCO3

0

5

10

15

20

25

30

0 5 10 15 20 25 30

Ca

SO4

0

5

10

15

20

25

30

0 5 10 15 20 25 30

Mg

SO4


(e) (f)

(g)

Apx Figure A.5 Reaction plots for Bool Lagoon for groundwater (blue) and surface water (green); black lines are dissolution/precipitation ratios; (a) Mg/HCO3: 1:4 ratio is congruent dissolution of dolomite and calcite; 1:2 ratio is incongruent dissolution of dolomite (b) Ca/HCO3: 1:4 ratio is congruent dissolution of dolomite and calcite; 1:2 ratio is dissolution of calcite, (c) Ca/SO4: gypsum dissolution, (d) Mg/SO4: epsomite dissolution, (e) Ca+Mg-0.5HCO3/SO4: gypsum dissolution excluding carbonates, (f), Ca+Mg/SO4+HCO3: carbonate and gypsum dissolution, (g) Ca+Mg/HCO3: dolomite dissolution

0

5

10

15

20

25

30

0 5 10 15 20 25 30

Ca+

Mg-

0.5

HC

O3

SO4

0

5

10

15

20

25

30

0 5 10 15 20 25 30

Ca+

Mg

SO4+0.5HCO3

0

5

10

15

20

25

30

0 5 10 15 20 25 30

Ca+

Mg

HCO3


A.3.2 LAKE ROBE

(a) (b)

(c) (d)

(e) (f)

0

20

40

60

80

100

120

140

0 50 100 150

Mg

HCO3

0

20

40

60

80

100

120

140

0 50 100 150

Ca

HCO3

0

10

20

30

40

50

60

70

80

0 20 40 60 80

Ca

SO4

0

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200

Mg

SO4

0

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200

Ca+

Mg-

0.0

5H

CO

3

SO4

0

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200

Ca+

Mg

SO4+0.5HCO3


(g)

Apx Figure A.6 Reaction plots for Bool Lagoon for groundwater (blue) and surface water (green); black lines are dissolution/precipitation ratios; (a) Mg/HCO3: 1:4 ratio is congruent dissolution of dolomite and calcite; 1:2 ratio is incongruent dissolution of dolomite (b) Ca/HCO3: 1:4 ratio is congruent dissolution of dolomite and calcite; 1:2 ratio is dissolution of calcite, (c) Ca/SO4: gypsum dissolution, (d) Mg/SO4: epsomite dissolution, (e) Ca+Mg-0.5HCO3/SO4: gypsum dissolution excluding carbonates, (f), Ca+Mg/SO4+HCO3: carbonate and gypsum dissolution, (g) Ca+Mg/HCO3: dolomite dissolution

0

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200

Ca+

Mg

HCO3


A.4 Minor chemistry results

Apx Table A.1 Minor chemistry from Bool Lagoon and Lake Robe (mg/L)

SAMPLE Al As B Cd Co Cr Cu Fe Mn Mo Ni P Pb Sb Se Si Sr Zn


ROB005 <0.5 <0.25 1.83 <0.25 <0.25 <0.25 <0.25 <0.5 <0.5 <0.25 <0.25 <1 <0.25 <0.5 <0.25 6.1 6.52 <0.25

ROB018 <0.5 <0.25 1.34 <0.25 <0.25 <0.25 <0.25 <0.5 <0.5 <0.25 <0.25 <1 <0.25 <0.5 <0.25 18 12.3 <0.25

ROB019 <1 <0.5 1.27 <0.5 <0.5 <0.5 <0.5 <1 <1 <0.5 <0.5 <2 <0.5 <1 <0.5 18.2 13 <0.5

ROB021 <0.5 <0.25 <1 <0.25 <0.25 <0.25 <0.25 <0.5 <0.5 <0.25 <0.25 <1 <0.25 <0.5 <0.25 15.2 5.25 <0.25

ROB022 <0.5 <0.25 <1 <0.25 <0.25 <0.25 <0.25 <0.5 <0.5 <0.25 <0.25 <1 <0.25 <0.5 <0.25 14 3.99 <0.25

ROB023 <0.5 <0.25 <1 <0.25 <0.25 <0.25 <0.25 <0.5 <0.5 <0.25 <0.25 <1 <0.25 <0.5 <0.25 11.3 7.4 <0.25

ROB024 <0.5 <0.25 <1 <0.25 <0.25 <0.25 <0.25 <0.5 <0.5 <0.25 <0.25 <1 <0.25 <0.5 <0.25 17.3 2.76 <0.25

ROB025 <0.5 <0.25 <1 <0.25 <0.25 <0.25 <0.25 1.52 <0.5 <0.25 <0.25 <1 <0.25 <0.5 <0.25 27 5.24 <0.25

ROB026 <0.5 <0.25 <1 <0.25 <0.25 <0.25 <0.25 <0.5 <0.5 <0.25 <0.25 <1 <0.25 <0.5 <0.25 15.3 5.39 <0.25

ROB027 <0.5 <0.25 <1 <0.25 <0.25 <0.25 <0.25 <0.5 <0.5 <0.25 <0.25 <1 <0.25 <0.5 <0.25 15.1 10.4 <0.25

ROB030 <0.5 <0.25 <1 <0.25 <0.25 <0.25 <0.25 <0.5 <0.5 <0.25 <0.25 <1 <0.25 <0.5 <0.25 10.9 2.59 <0.25

ROB031 <0.5 <0.25 <1 <0.25 <0.25 <0.25 <0.25 <0.5 <0.5 <0.25 <0.25 <1 <0.25 <0.5 <0.25 11.6 4.83 <0.25

JOA029 <0.1 <0.05 <0.2 <0.05 <0.05 <0.05 <0.05 <0.1 <0.1 <0.05 <0.05 <0.2 <0.05 <0.1 <0.05 10.1 0.757 <0.05


Hacks Lagoon <0.1 <0.05 0.276 <0.05 <0.05 <0.05 <0.05 <0.1 <0.1 <0.05 <0.05 <0.2 <0.05 <0.1 <0.05 0.616 1.66 <0.05

Bool Lagoon East <0.1 <0.05 0.603 <0.05 <0.05 <0.05 <0.05 0.156 <0.1 <0.05 <0.05 <0.2 <0.05 <0.1 <0.05 9.51 2.96 <0.05

Bool Lagoon West <0.1 <0.05 0.506 <0.05 <0.05 <0.05 <0.05 0.298 <0.1 <0.05 <0.05 <0.2 <0.05 <0.1 <0.05 1.83 2.2 <0.05

Bool Lagoon Outlet <0.1 <0.05 0.51 <0.05 <0.05 <0.05 <0.05 <0.1 <0.1 <0.05 <0.05 <0.2 <0.05 <0.1 <0.05 7.89 1.37 <0.05

Little Bool <0.1 <0.05 0.586 <0.05 <0.05 <0.05 <0.05 <0.1 <0.1 <0.05 <0.05 <0.2 <0.05 <0.1 <0.05 12.3 2.48 <0.05


SAMPLE Al As B Cd Co Cr Cu Fe Mn Mo Ni P Pb Sb Se Si Sr Zn


WAT032 <0.1 <0.05 <0.2 <0.05 <0.05 <0.05 <0.05 <0.1 <0.1 <0.05 <0.05 <0.2 <0.05 <0.1 <0.05 2.84 1.31 <0.05

WAT034 <0.5 <0.25 2.56 <0.25 <0.25 <0.25 <0.25 <0.5 <0.5 <0.25 <0.25 <1 <0.25 <0.5 <0.25 5.12 5.11 <0.25

WAT037 <0.1 <0.05 0.213 <0.05 <0.05 <0.05 <0.05 0.625 <0.1 <0.05 <0.05 <0.2 <0.05 <0.1 <0.05 4.96 2 <0.05


Robe East <5 <2.5 13.9 <2.5 <2.5 <2.5 <2.5 <5 <5 <2.5 <2.5 <10 <2.5 <5 <2.5 <25 59 <2.5

Robe Southwest <5 <2.5 13.8 <2.5 <2.5 <2.5 <2.5 <5 <5 <2.5 <2.5 <10 <2.5 <5 <2.5 <25 58.1 <2.5

Robe Southeast <5 <2.5 13.8 <2.5 <2.5 <2.5 <2.5 <5 <5 <2.5 <2.5 <10 <2.5 <5 <2.5 <25 58.4 <2.5


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The Goyder Institute for Water Research is a partnership between the South Australian Government through the Department for Water, CSIRO, Flinders University, the University of Adelaide and the University of South Australia.

Documents

Evaluation of groundwater surface water interactions at Bool … · 2017-01-16 · Evaluation of groundwater – surface water interactions at Bool Lagoon and Lake Robe using environmental