Geochemical and isotopic constraints on the interaction between saline lakes and groundwater in southeast Australia Ian Cartwright & Sharon Hall & Sarah Tweed & Marc Leblanc
Abstract Major ion and stable isotope geochemistry allow groundwater/surface-water interaction associated with saline to hypersaline lakes from the Willaura region of Australia to be understood. Ephemeral lakes lie above the water table and locally contain saline water (total dissolved solids, TDS, contents up to 119,000mg/L). Saline lakes that lack halite crusts and which have Cl/Br ratios similar to local surface water and groundwater are throughflow lakes with high relative rates of groundwater outflows. Permanent hypersaline lakes contain brines with TDS contents of up to 280,000mg/L and low Cl/Br ratios due to the formation of halite in evaporite crusts. These lakes are throughflow lakes with relatively low throughflow rates relative to evaporation or terminal discharge lakes. Variations in stable isotope and major ion geochemistry show that the hypersaline lakes undergo seasonal cycles of mineral dissolution and precipitation driven by the influx of surface water and evaporation. Despite the generation of highly saline brines in these lakes, leakage from the adjacent ephemeral lakes or saline throughflow lakes that lack evaporite crusts is mainly responsible for the high salinity of shallow groundwater in this region.
Received: 1 December 2008 / Accepted: 28 May 2009 Published online: 21 June 2009 * Springer-Verlag 2009 Electronic supplementary material The online version of this article (doi:10.1007/s10040-009-0492-5) contains supplementary material, which is available to authorized users. I. Cartwright ()) : S. Hall School of Geosciences, Monash University, Clayton, Melbourne, Victoria 3800, Australia e-mail:
[email protected] Tel.: +61-3-99054887 Fax: +61-3-99054903 S. Tweed : M. Leblanc School of Earth and Environmental Sciences, James Cook University, Cairns, Queensland 4870, Australia Present Address: S. Hall Sinclair Knight Merz, 590 Orrong Rd, Armadale, Melbourne, Victoria 3143, Australia Hydrogeol J (2009) 17: 1991–2004
Keywords Groundwater/surface-water relations . Stable isotopes . Hydrochemistry . Salinisation . Australia
Introduction Documenting groundwater/surface-water interaction associated with lakes and wetlands is critical to understanding shallow hydrogeological systems. Lakes vary in their relationship to local groundwater. Recharge lakes are situated above the local water table and recharge the local groundwater via leakage through the lake floor, terminal discharge lakes represent the terminal points of groundwater flow and receive groundwater through the lake base or perimeter, and throughflow lakes cross the water table and both receive and recharge groundwater. In low-relief arid and semi-arid areas recharge and throughflow lakes may provide a significant amount of distributed recharge of evaporated saline water that can increase the salinity of the shallow groundwater (e.g. Macumber 1991; Herczeg et al. 1992; Dutkiewicz et al. 2000; Yechieli and Wood 2002). While it is in theory straightforward to determine the relationship of a lake to the local groundwater using hydraulic heads and lake levels, in practise this can be difficult. Firstly, calculation of equivalent freshwater heads requires that the lake and groundwater salinity is well known and, secondly, in regions of low hydraulic gradients, the number and distribution of groundwater monitoring bores is commonly insufficient. In addition, water balance calculations that rely on comparing changes to lake volume with rainfall, evaporation and surface water fluxes (e.g. Tweed et al. 2009) can only be carried out in regions with adequate monitoring records. This paper discusses the interaction between a series of saline to hypersaline lakes and shallow groundwater in the Willaura region of southeast Australia. This is an important area because groundwater salinities increase in the vicinity of the lakes and there is also a documented increase in the degree of salinisation of groundwater and surface water following land clearing over the last 150 years (Mann and Nolan 1989; Bennetts et al. 2006). However, while there are many lakes in this region, there is insufficient hydrological data such as lake levels or groundwater elevations to define their relationship to the groundwater system, and consequently the role of the lakes in controlling groundwater salinity has hitherto not been explored. Specifically, the study aims to: (1) establish DOI 10.1007/s10040-009-0492-5
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using Cl/Br and stable isotope data whether the lakes are recharge, throughflow, or terminal discharge; (2) determine if the lakes are in steady state or whether there are variations in geochemistry over time that indicate a dynamic system; and (3) determine the role of different types of lakes in controlling the salinity of the shallow groundwater. This study provides an example of how geochemistry may be used to discern the general patterns of groundwater-lake interaction in regions with sparse monitoring data.
Study area The regional geology and hydrogeology of the Willaura region was discussed by Stuart-Smith and Black (1999), Dahlhaus et al. (2000) and Bennetts et al. (2006). The Willaura catchment, shown in Fig. 1a, has an area of ~85,000 ha and is part of the larger (~144,500 ha) regional Glenelg–Hopkins catchment. The region comprises an undulating plain that is bounded to the west by the Grampian Ranges (an inlier of indurated Silurian sandstones that rise some 250 m above the plain), to the east by the Hopkins River and to the south by the low hills of the Staverly Ranges. Willaura has hot dry summers (December–February) and cooler wetter winters (July–August) with an average annual rainfall of ~550 mm (Bureau of Meteorology 2008). Annual potential evaporation is 1350 mm and exceeds precipitation in all months except July and August (Bureau of Meteorology 2008). The period of and immediately prior to sampling was one of abnormally low rainfall (2005 and 2006 rainfall totals were 70–80% of the long-term average), and rainfall since late 1999 has been below long-term averages (Bureau of Meteorology 2008). Average daily temperatures are 12–26°C in summer and 4–12°C in winter and the average humidity (at 3 pm) is ~60% (Bureau of Meteorology 2008). The area has been largely cleared of the original native vegetation cover of eucalypts, casuarinas and deep-rooted grasses, which now exists only as scattered patches. Present-day landuse includes dryland cropping and sheep and cattle grazing. Cambrian marine sandstones with minor siltstones and schists of the Glenthompson Sandstone unit and metamorphosed intermediate to mafic igneous rocks and sediments of the Cambrian Greenstone unit form a lower fractured aquifer that is confined except where it crops out at the margins of the catchment (Stuart-Smith and Black 1999; Dahlhaus et al. 2000; Bennetts et al. 2006: Fig. 1a). Overlying these rocks are the Plio-Pleistocene Newer Volcanics and a series of alluvium and colluvium deposits that form unconfined shallow aquifers. The Newer Volcanics are up to 70 m thick in the centre of the region and include basalt flows, which are individually up to 25 m thick, tuff deposits and scoria. These eruptive deposits commonly filled pre-existing valleys altering drainage patterns. As shown in Fig. 2, the alluvium and colluvium deposits form an unconfined aquifer in the south and east of the study area that is up to 30 m thick. In parts of the north and east of the area, alluvium underlies the Newer Volcanics. Other superficial deposits include: lacustrine greyblack dolomitic clays, silts and sands; clay-rich swamp Hydrogeol J (2009) 17: 1991–2004
sediments; and lunette deposits of clays and sands (StuartSmith and Black 1999; Bennetts et al. 2006: Figs. 1 and 2).
Groundwater flow Groundwater flow in the Willaura area converges on the Cockajemmy Lakes before flowing westwards towards the Hopkins River (Figs. 1 and 2). Horizontal head gradients are between 10-4 and 10-2. Vertical head gradients across the area are generally downward except in the immediate vicinity of saline lakes where they are upward, as illustrated by Fig. 3. Bore hydrographs (e.g. Fig. 3b) show seasonal and decadal variations, which, together with the fact that the shallow aquifers are unconfined, imply that recharge generally occurs across the region. Although the period of and immediately prior to sampling was one of abnormally low rainfall, groundwater flow patterns for wetter periods are substantially similar. Groundwater electrical conductivity (EC) values increase from 5000-10,000μS/cm towards the lakes where EC values locally in excess of 50,000μS/cm are recorded (Fig. 1b). From the relationship between EC and total dissolved solids (TDS) contents in the electronic supplementary material table, ESM Table 1, this corresponds to an increase in TDS contents from 2,500–5,000 mg/L to in excess of 25,000 mg/L
Lake types The Willaura region contains numerous lakes that vary from ephemeral to permanent and from brackish to hypersaline (Figs. 1 and 3, Table 1). The chain of lakes in the centre of the region (the Cockajemmy Lakes) occupies a depression in the landscape resulting from the blocking of the pre-Pliocene drainage by lavas of the Newer Volcanics. Other lakes occupy depressions in the surface of the basaltic lava flows or alluvial deposits. Few of the lakes have significant surface water inflows or outflow and the Willaura area as a whole is an enclosed surface drainage basin with no connection to the Hopkins River (Bennetts et al. 2006). Herein, lakes are defined as ephemeral, saline throughflow or hypersaline (Table 1) as described below. Ephemeral lakes occur at relatively high positions in the landscape. Where a relationship to groundwater could be established (e.g. at Toora, Powells, Lalkaldarno East and the Stavely Lakes), these are demonstrably recharge lakes located above the water table. These lakes are commonly dry in summer and when wet they contain saline water (TDS=11,400–119,000 mg/L: Table 1); however, evaporite crusts are not developed around these lakes. The larger ephemeral lakes, such as Lake Muirhead and Mt William Swamp are also located above the water table (the water table at loc. B (Fig. 1) is estimated to be some 2 m below the base of Lake Muirhead) and are dry except during very wet periods when they contain fresh water. Anecdotal evidence from local landowners is that DOI 10.1007/s10040-009-0492-5
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Fig. 1 a Simplified geological and hydrogeological map of the Willaura region (after Bennetts et al. 2006). Groundwater elevations in m above the Australian Height Datum (AHD) were determined from bores measured as part of this study and additional data from the Victorian Water Resources Data Warehouse (2008); groundwater elevations are poorly defined in the east of the region. Lakes: DT Dead Tree, GC Golf Course, HA Hayden, HS Helens Swamp, LE Lalkaldarno East, LW Lalkaldarno West, JO Johns, MH Muirhead, MWS Mount William Swamp, PO Powells, S1–3 Staverley Lakes, TO Toora, WWE Willaura-Wicliffe East, WWW Willaura-Wicliffe West (Table 1). The red rectangle in inset shows area of Fig. 3. b Salinity of shallow groundwater in the same area (lake types as in Fig. 1a). Data from this study and Victorian Water Resources Data Warehouse (2008)
Lake Muirhead last contained substantial water in the early 1970s. The hypersaline lakes (e.g. Reflection, Johns, Lalkaldarno West, Willaura-Wicliffe West, Willaura-Wicliffe East and Golf Course) occupy lower positions in the landscape, contain brines with TDS contents up to 280,000 mg/L, and have extensive evaporite crusts overlying organic mud (Table 1). These lakes are small (
