Elsevier

Journal of Hydrology

Volume 380, Issues 1–2, 15 January 2010, Pages 203-221
Journal of Hydrology

Physical hydrogeology and environmental isotopes to constrain the age, origins, and stability of a low-salinity groundwater lens formed by periodic river recharge: Murray Basin, Australia

https://doi.org/10.1016/j.jhydrol.2009.11.001Get rights and content

Summary

A low-salinity (total dissolved solids, TDS, <5000 mg/L) groundwater lens underlies the Murray River in the Colignan–Nyah region of northern Victoria, Australia. Hydraulic heads, surface water elevations, δ18O values, major ion geochemistry, 14C activities, and 3H concentrations show that the lens is recharged from the Murray River largely through the riverbank with limited recharge through the floodplain. Recharge of the lens occurs mainly at high river levels and the low-salinity groundwater forms baseflow to some river reaches during times of low river levels. Within the lens, flow through the shallow Channel Sands and deeper Parilla Sands aquifers is sub-horizontal. While the Blanchetown Clay locally separates the Channel Sands and the Parilla Sands, the occurrence of recently recharged low-salinity groundwater below the Blanchetown Clay suggests that there is considerable leakage through this unit, implying that it is not an efficient aquitard. The lateral margin of the lens with the regional groundwater (TDS >25,000 mg/L) is marked by a hectometer to kilometer scale transition in TDS concentrations that is not stratigraphically controlled. Rather this boundary represents a mixing zone with the regional groundwater, the position of which is controlled by the rate of recharge from the river. The lens is part of an active and dynamic hydrogeological system that responds over years to decades to changes in river levels. The lens has shrunk during the drought of the late 1990s to the mid 2000s, and it will continue to shrink unless regular high flows in the Murray River are re-established. Over longer timescales, the rise of the regional water table due to land clearing will increase the hydraulic gradient between the regional groundwater and the groundwater in the lens, which will also cause it to degrade. Replacement of low-salinity groundwater in the lens with saline groundwater will ultimately increase the salinity of the Murray River reducing its utility for water supply and impacting riverine ecosystems.

Introduction

Low-salinity groundwater lenses occur beneath rivers in many arid and semi-arid regions (e.g., Sorman and Abdulrazzak, 1993, Winter et al., 1998, Lange, 2005, Lamontagne et al., 2005, Dahan et al., 2007). They are formed by recharge from rivers that are above the local water table, especially during flood events. If the regional groundwater is saline, these lenses may provide water for irrigation or domestic use. Additionally, if the river systems alternate between losing and gaining (as may happen if there is a large variation in river discharge), the groundwater in the lenses will provide relatively low-salinity baseflow to the rivers during low flow periods. While the origin of these low-salinity groundwater lenses is conceptually straightforward, they are part of dynamic hydrological systems, and the protection and management of groundwater and surface water resources associated with these lenses require that both their long- and short-term behaviour be understood. This is particularly the case where the hydrogeological system is changing due to land clearing, irrigation, river regulation, and/or climate change. This paper discusses the hydrogeology of a low-salinity (total dissolved solids, TDS, <5000 mg/L) groundwater lens beneath the Murray River, Australia in an area where river flow is regulated and where land clearing has caused the regional water table to rise (Allison et al., 1990). It addresses the following issues: (1) the controls on the location of the lens; (2) the timescales over which the water in the lens is replenished; (3) the extent of mixing between the water in the lens and the regional groundwater; (4) whether the lens is recharged directly from the river or through the floodplain; and (5) whether the lens is growing or degrading under current hydrogeological conditions. Resolution of these issues is critical for the management of the groundwater and surface water resources in this area and for understanding the hydrogeology of similar low-salinity groundwater lenses globally.

The Murray Basin (Fig. 1) occupies ∼300,000 km2 of southeast Australia and contains a sequence of late Palaeocene to Recent sediments that overlie a basement of Proterozoic to Mesozoic igneous and metasedimentary rocks (e.g., Lawrence, 1975, Lawrence, 1988, Brown, 1989, Evans and Kellett, 1989). The Murray Basin is up to 600 m thick and comprises three sub-basins or provinces (Riverine, Scotia, and Mallee-Limestone: Fig. 1a) that are separated by basement ridges. Except for a small region in the southwest that discharges to the Southern Ocean, the basin is closed and groundwater discharges to rivers, salt lakes, and playas near its centre. The Murray River, which is part of the much larger (∼1073,000 km3) Murray Darling surface water catchment, is the only major river draining the basin. TDS concentrations of groundwater within the Murray Basin range from <500 mg/L to >100,000 mg/L (e.g., Evans and Kellett, 1989, Macumber, 1991, Herczeg et al., 2001, Cartwright et al., 2006, Cartwright et al., 2008). Groundwater in the centre and north of the basin groundwater generally has TDS concentrations of >25,000 mg/L except close to the major rivers where low-salinity groundwater locally occurs (Fig. 2, Fig. 3).

This study addresses the hydrogeology of a low-salinity groundwater lens along the Murray River between Nyah and Colignan (Fig. 2) at the border between the Riverine and Mallee-Limestone Provinces (Fig. 1a). Average annual rainfall in this area increases eastward from 300 to 350 mm; annual potential evaporation is ∼1150 mm and exceeds rainfall in all but the austral winter months (July–September) (Bureau of Meteorology, 2009). Landuse across the study area includes remnant vegetation (mainly river red gum forests), grain farming (wheat and barley), and intensive horticulture (notably vineyards, citrus fruits, olives and almonds). Most of the area in Fig. 2 is unirrigated. Widespread irrigation occurs northwest of Colignan, south of Nyah, and around Robinvale with only local irrigation elsewhere, mainly for horticulture (Murray Darling Basin Commission, 2008). Irrigation in the Robinvale area occurs on the floodplain; however, elsewhere the floodplain contains remnant native vegetation and locally dryland pastures with irrigation occurring away from the river.

This study concentrates on the shallow (<80 m) hydrogeology of the region, and the main aquifers and aquitards are summarised in Table 1 and Fig. 3 (descriptions from Lawrence, 1975, Lawrence, 1988, Brown, 1989, Evans and Kellett, 1989, Thorne et al., 1990). The Pliocene Parilla Sands comprises a sequence of marine sands and silts that is up to 90 m thick, which forms a regional aquifer. In the east, this unit is locally overlain by the Pliocene to Quaternary Shepparton Formation, which is a heterogeneous series of laterally discontinuous fluvio-lacustrine clays, sands, and silts. The Parilla Sands overlies marginal marine clays and shelly horizons of the Bookpurnong Beds and clays and calcareous silts of the Winnambool Formation, which form regional aquitards (Fig. 1b). The Channel Sands comprises up to 15 m of locally coarse-grained, high-hydraulic conductivity riverine sands intercalated with silty and clay-rich alluvial beds that were formed by the Murray River and its tributaries (Thorne et al., 1990). Hydraulic conductivities estimated from pumping tests are 2–70 m/day in the Parilla Sands and 26–27 m/day in the Channel Sands (Thorne et al., 1990). The Lowan and Woorinen Formations are discontinuous units of aeolian and riverine sands and silts that are locally up to several meters thick and which overlie much of the area forming an unconfined aquifer system. The main shallow aquitard is the Blanchetown Clay that separates the Channel Sands from the underlying Parilla Sands and which comprises up to 40 m of lacustrine clays with minor sand and silt layers. The Blanchetown Clay varies in thickness and is locally discontinuous (Fig. 3). Some of this thickness variation was caused by variable subsidence during deposition of this unit; additionally, uplift subsequent to deposition has caused erosion of some parts of the clay (Lawrence, 1975, Thorne et al., 1990). Aside from the Blanchetown Clay, the clay and silt rich upper part of the Parilla Sands, the Channel Sands and Shepparton Formation where silty, and the clay-rich parts of the Woorinen Formation form local aquitards (Fig. 3). One of the questions that this study addresses is whether the presence or thickness of the Blanchetown Clay has an impact on the location or morphology of the lens. The Murray River and its floodplain are up to 10 m below the regional land surface. The river is incised into stratigraphy locally as deep as the Parilla Sands.

Section snippets

Sampling and methods

Groundwater was sampled from observation bores with screens of typically 0.5–2 m long that were constructed to investigate groundwater-surface water interaction adjacent to the Murray River in Victoria (Thorne et al., 1990). The bores are arranged in a series of transects or lines approximately normal to the river (Fig. 2, Fig. 3) with a greater concentration of bores close to the river (Fig. 3). Nested bores are present at many locations and detailed stratigraphic logs exist for most locations.

Current hydraulic conditions

Hydraulic heads and river elevations may be used to assess whether the Nyah–Colignan low-salinity groundwater lens is being replenished or degraded under current hydrological conditions. Fig. 4, Fig. 5 show hydrographs for a number of representative bores and river sites between 1982 and 2007.

Groundwater chemistry

The Nyah–Colignan low-salinity groundwater lens (defined as groundwater with TDS <5000 mg/L) is present at all locations between lines Z and K (Fig. 2, Fig. 3). The low-salinity groundwater grades into regional groundwater with TDS concentrations >25,000 mg/L over distances of up to 10 km (the intervening groundwater with 5000 < TDS < 25,000 mg/L is termed transitional). Groundwater with lower salinity than the regional groundwater is locally present close to the Murray River north of line K (Thorne et

Stable isotope ratios

Groundwater δ18O and δ2H values define a broad group with a slope of ∼4.5 that intercepts the Global Meteoric Water Line at a δ18O value of between −5‰ and −6‰ (Fig. 7a). There is little difference between the δ18O and δ2H values of groundwater from the different units; however, the groundwater from the lens has a wider range of δ18O and δ2H values than the regional groundwater. The slope of 4.5 suggests that evaporation at a humidity of ∼50%, which is approximately the average annual afternoon

Carbon-14 and tritium

Shallow groundwater close to the Murray River has 14C activities that are typically >100 pMC (Table 4, Fig. 3), implying that it contains a component of modern water recharged following the commencement of atmospheric nuclear testing the early to mid 1950s (c.f., Clark and Fritz, 1997). 14C activities in the shallowest aquifer along lines L, P, S, and T within the lens decline with distance from the river (Fig. 3). Except for groundwater from the Parilla Sands ∼10 km from the river at loc. P5a

Nature of the external boundary of the lens

While mixing within the lens is precluded by the 14C and 3H data, mixing between low-salinity and saline groundwater at the margins of the lens produces the gradient in TDS concentrations (Fig. 3) and the trends in major ion ratios (Fig. 6). As groundwater from the margin of the lens has 3H concentrations that are close to background this mixing is not apparent on Fig. 8. The transition in salinity at the margin of the lens may reflect advection and/or diffusion. An order of magnitude estimate

Discussion

The combination of physical hydrogeology, geochemistry, and environmental isotopes has allowed the hydrogeology of the Nyah–Colignan low-salinity groundwater lens to be understood. Fig. 11 is a conceptual model of the hydrogeology of the lens. Recharge of the Channel Sands or the Parilla Formation where it forms the shallowest aquifer is dominantly via the riverbank during times of high river levels (Fig. 11a). Some recharge through the floodplain may locally occur, but it is not the major

Acknowledgments

We would like to thank Sean Dwyer of Mallee CMA for help in accessing the bores. Field sampling was carried out with the help of Nikki Eldridge, Michael Hoban, Phil Matherson, Heath Pawley, and Eliza Wilkshire. Analyses were made with the help of Massimo Raveggi (stable isotopes and anions), Andy Christie and Linda McMorrow (cations), Stewart Fallon and Fred Leaney (14C). Funding for this project was provided by Land and Water Innovation Grant UME072, ARC Grant DP0984539 and AINSE Grant 07174.

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