Reflux Brines and Saline Groundwater, Murray Basin, Australia
Abstract
Groundwater in the Murray Basin typically becomes more saline along its flowpaths; however, geochemical data, particularly Br/Cl ratios, indicate limited dissolution of salt from the aquifer is occurring. In the southern Murray Basin, recharge of groundwater to the deeper aquifers (Renmark Formation) is generally considered to occur at the highlands at the south basin margin while recharge to the shallow unconfined Parilla Sands aquifer occurs across much of the region. Regionally, discharge of groundwater occurs within the centre of the basin in zones of salt lakes. Groundwater in the regional recharge area of the southern Murray Basin shows dramatic variations in salinity (TDS contents ranging from 650 to >100,000 mg/L) over distances of a few kilometres in both shallow and deep aquifers. While the variation in topography is low (<70 m over 15,000 km2), local recharge and discharge processes control groundwater composition. Fresher groundwater underlies sand ridges that contain freshwater lakes located above the water table. The high salinity areas underlie a major palaeochannel, the Douglas Depression, which forms a topographic low. This depression contains abundant salt lakes and playas that represent local discharge sites for shallow groundwater. Stable isotope data show that the water in the high salinity zones underwent evaporation. Major element data (particularly Mg/Ca/SO4 ratios) indicate that the saline groundwaters have precipitated gypsum. Together the data indicate that brines produced in these saline lakes reflux into the underlying aquifers to depths of up to 180 m. In the semi-arid environment of SE Australia, reflux brines in both local and regional discharge areas are important in controlling the distribution of salinity in the Murray Basin as a whole. The observation that both shallow and deep aquifers show similar chemical trends implies that there is significant vertical interconnection throughout the basin. The confining layers are thin, locally discontinuous, contain lenses of sand, and form relatively poor aquitards. This is in keeping with observations that hydrographs from adjacent deep and shallow wells are generally in phase and that the chemistry of shallow and deeper wells is often similar. Overall, recharge to both shallow and deep aquifer systems may locally occur throughout the basin. The results of this study show the control that surface processes have on the deeper groundwater in this basin. Recent land clearing has increased the salinity of shallow groundwater across the basin, which may begin to impact the deeper groundwater.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.H51C0326C
- Keywords:
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- 1829 Groundwater hydrology;
- 1831 Groundwater quality;
- 1832 Groundwater transport