Processes controlling groundwater chemistry and identification of water resource vulnerability: Yarra catchment, Victoria

Full text: Assessment of processes controlling groundwater chemistry is essential for water resource management and identification of water resource vulnerability. The Yarra Catchment (4045 km2) extends 120 km east of Melbourne and is considered a significant area for Melbourne's water resources. This catchment is characterised by fractured rock aquifers, a high baseflow component, high, localised recharge, and diverse land use. Therefore, identification of processes controlling groundwater chemistry is important for determining areas where groundwater, and inevitably surface water, are most vulnerable to contamination. The regional groundwater flow system is affected by high, localised recharge in areas of high surface elevations. Hence, a local groundwater flow system is investigated in detail to aid regional interpretation. Processes causing spatial heterogeneity of groundwater chemistry include the extent of recharge, water-rock interaction and land use processes. At high surface elevations Na:CI = 1, and direct recharge of precipitation into sedimentary or granitic aquifers results in groundwater with lower TDS values (∼ 50 to 100 mg/L), compared to groundwater where precipitation directly recharges younger basaltic aquifers (TDS ∼ 100 to 150 mg/L). At lower surface elevations Na:CI ratios decrease, and major ion concentrations, pH and TDS are significantly higher (TDS ∼ 400 to 500 mg/L) reflecting cumulative effects of water-rock interaction and agricultural land use lower in the catchment. Groundwater samples were also analysed for stable isotopes, the 180 and 2H values (-6.2 to 5.50, and -38.1 to 32.20 VSMOW respectively) lie on the Melbourne Meteoric Water Line, indicating that groundwater has not been subjected to evaporation or high-temperature water rock interaction. Slightly lower 180 and 2H values for groundwater compared to nearby surface water indicates that recharge of colder rains occurs preferentially, both on local and more regional scales. Copyright (1999) Geological Society of Australia