Hydrochemical processes in the Lower Murrumbidgee Area, NSW : the influence of weathering reactions, evaporation, and salt dissolution on groundwater quality
Creators: name: McDonald, John Gilbert Walton
Publisher: name: The Australian National University
Year: 2014
Subjects: subject: Groundwater Quality Australia Murray-Darling Basin (N.S.W.)
subject: Aquifers Australia Murray-Darling Basin (N.S.W.)
subject: Water quality management Australia Murray-Darling Basin (N.S.W.)
subject: Salinization Control Australia Murray-Darling Basin (N.S.W.)

description: The Lower Murrumbidgee area is a major agricultural region located on the Eastern margin of the sedimentary Murray Basin. A large low-salinity groundwater body extends from the Eastern margin in the semi-confined Calivil and Renmark Formation aquifers, and is overlain by higher salinity water in the surficial Shepparton Formation. Deep groundwater extraction has increased to ~300 GL in last decade, leading to substantial pressure declines, and the resulting increased vertical hydraulic gradient increases the potential for downward migration of saline water into productive aquifers. This study is aimed at determining the processes that form the wide range of groundwater compositions in the Lower Murrumbidgee area, and using groundwater chemistry to constrain flow paths. Groundwater and surface water were collected from bores and rivers and analysed for major and minor element concentrations, 87Sr/86Sr, and deuterium/oxygen isotope ratios. Total dissolved solids in groundwater samples span 3 orders of magnitude. Low-salinity groundwaters have similar major ion ratios to the surface waters, with high proportions of HCO3, Na, and Ca, whereas high-salinity waters are Na-Cl dominated with ionic ratios and concentrations similar to seawater. The location of the study area and the groundwater 87Sr/86Sr ratios preclude mixing with connate seawater; groundwaters instead evolve from low-salinity recharge sources (surface water and rain). Major ion trends indicate that high-salinity groundwaters are formed through evaporation/transpiration of rainfall during recharge, and that concentration of dilute solutes initially delivered in rainfall is the dominant process that forms chloride-rich saline groundwaters. The high potential evapotranspiration and flat topography (low runoff) result in removal of a large fraction (&gt;99%) of the water and retention of the most soluble solutes (i.e., Na, Cl, Br). Dissolution of aeolian halite-containing dust is a secondary source of chloride to the system, contributing up to 30% of the total in the east of the study area and 50% in the west, but can not alone account for the formation of the saline groundwater. The shift to Na-Cl dominated compositions during concentration is instead caused by precipitation of calcite and high-Mg calcite; these less soluble components are then recycled back into the atmosphere and influence the rainfall compositions. Cation exchange of dissolved Na for Ca appears to occur during concentration, allowing continued precipitation of calcite without depletion of Ca/accumulation of HCO3. The lowest salinity groundwaters are clustered around the Murrumbidgee River at the eastern margin, and have similar major ion ratios to river water but higher Si concentrations and lower 87Sr/86Sr, indicating that weathering reactions occur during/shortly after recharge. Plagioclase and carbonate fractions of the sediments appear to weather preferentially, because groundwater 87Sr/86Sr ratios are significantly lower than bulk ratios of Murray Basin sediments and their source rocks. The more saline compositions of the deep groundwaters in the west of the study area are consistent with mixing with shallow saline water through downward leakage. The salinity increase is most marked 150-200 km west of the basin margin, and is likely related to increased sediment thickness and a resulting decrease in horizontal flow rates.