Origin of dissolved salts in a large, semi-arid groundwater system: Murray Basin, Australia

Abstract

Hypotheses to explain the source of the 1011 tons of salt in groundwaters of the Murray Basin, south-eastern Australia, are evaluated; these are (a) mixing with original sea water, (b) dissolution of salt deposits, (c) weathering of aquifer minerals and (d) acquisition of solutes via rainfall. The total salinity and chemistry of many groundwater samples are similar to sea-water composition. However, their stable isotopic compositions (δ18O= –6.5 ‰; δ2H = –35) are typical of mean winter rainfall, indicating that all the original sea water has been flushed out of the aquifer. Br/Cl mass ratios are approximately the same as sea water (3.57 x 10-3) indicating that NaCl evaporites (which have Br/Cl<10-4) are not a significant contributor to Cl in the groundwater. Similarly, very low abundances of Cl in aquifer minerals preclude rock weathering as a significant source of Cl. About 1.5 million tons of new salt is deposited in the Murray–Darling Basin each year by rainfall.The groundwater chemistry has evolved by a combination of atmospheric fallout of marine and continentally derived solutes and removal of water by evapo-transpiration over tens of thousands of years of relative aridity. Carbonate dissolution/precipitation, cation exchange and reconstitution of secondary clay minerals in the aquifers results in a groundwater chemistry that retains a ‘sea-water-like’ character.

Using <sup>14</sup>C and <sup>3</sup>H to understand groundwater flow and recharge in an aquifer window

“…Processes that govern the evolution of groundwater geochemistry and sources of solutes in the Eastern View Formation can be determined from the major ion geochemistry. The observation that Cl / Br ratios are between 500 and 1000, which is similar to those expected in rainfall, and do not increase with increased TDS implies that evapotranspiration rather than halite dissolution is the major process controlling groundwater salinity (Herczeg et al, 2001;Cartwright et al, 2006). This conclusion is also consistent with an absence of halite in the aquifer lithologies.…”

“…Together, the major ion geochemistry suggests that water-rock interaction is limited with minimal silicate weathering, negligible dissolution of halite and carbonate minerals and some minor dissolution of gypsum. As is the case elsewhere in southeast Australia, including within the Otway basin, the primary geochemical process is evapotranspiration promoted by the moderate rainfall and water-efficient native vegetation, and groundwater salinity is largely controlled by the degree of evapotranspiration during recharge (Herczeg et al, 2001;Bennetts et al, 2006;Petrides and Cartwright, 2006). Groundwater from the near-river sites 1 to 4 has lower δ 18 O and δ 2 H values relative to that from the floodplain away from the river at site 5.…”

“…Edmunds et al, 2002), while the increase in Na concentrations at the expense of Ca may indicate ion exchange reactions on the surface of clay minerals (e.g. Herczeg et al, 2001). That Ca and mHCO 3 are poorly correlated suggests that negligible dissolution of calcite has occurred.…”

“…Evapotranspiration during recharge is commonly the dominant process in determining the salinity of groundwater in SE Australia (Herczeg et al, 2001). Low recharge rates result in higher degrees of evapotranspiration and higher salinity groundwater, and the resultant correlation between Cl concentrations and 14 C ages has been noted in other catchments (Leaney et al, 2003;Cartwright et al, 2006).…”

Groundwater flow dynamics in the complex aquifer system of Gidabo River Basin (Ethiopian Rift): a multi-proxy approach

“…This includes the interpretation of hydrochemical and isotope data (δ 2 H, δ 18 O, and δ 13 C) to characterize the chemical evolution of groundwater and the mixing of different flow components (e.g. Clark and Fritz 1997;Herczeg et al 2001). These techniques were previously employed for investigations of the hydrogeology of active rift systems such as the East African Rift, Dead Sea Rift and Rio Grande Rift (Salameh and Rimawi 1984;Kebede et al 2008;Chowdhury et al 2008;Bretzler et al 2011).…”