Evidence suggests that potable groundwater occurs within the fractured crystalline rock of the Upper Murrumbidgee catchment, with anecdotal reports that bore yields and fracture networks could form aquifers capable of sustaining regional population growth and agricultural development. However, little knowledge relating to groundwater flow or recharge and discharge mechanisms exists. Hydrogeochemical methods are particularly useful in studying groundwater in complex geological environments. Unfortunately, limited information regarding groundwater geochemistry exists in this catchment. Surface water surveys provide an opportunity to quickly obtain preliminary information regarding groundwater-surface water interactions and likely groundwater geochemistry.
Four surface water surveys were conducted at up to 285 sites during wet and dry conditions from September 2017 to April 2019. Physicochemical parameters were collected at each site where water was present, along with samples for stable isotopes (δ18O and δ2H) and dissolved ion analysis.
Surface water geochemistry is influenced by interactions between altitude, rainfall, and outcropping geology. Stream electrical conductivity (EC) is positively correlated with altitude and negatively correlated with rainfall in all sub-catchments except the Yass River catchment. Outcrop geology also influenced stream EC, with the EC lowest over Paleozoic granite and granodiorites; moderate over Paleozoic felsic volcanic, Cenozoic mafic volcanic, and Cenozoic alluviums; and highest over Ordovician metasediments. Dissolved ion chemistry is similarly influenced by outcrop geology, and is reflected in preliminary groundwater surveys. The cations Na+ and K+ dominate Paleozoic granite and granodiorite waters while Ca2+ and Mg2+ dominate Cenozoic mafic volcanic waters. Bicarbonate (as HCO3-) is the dominant anion, with elevated HCO3-/Cl- associated with Cenozoic mafic volcanics. This suggests that mineral weathering dominates hydrogeochemical processes. Spatial distributions of stream water δ18O and δ2H highlight potential baseflow areas. Results suggest that potable water is highly probable within Paleozoic granite and granodiorites, with water Na+-K+-HCO3- dominated. Potable water is possible within Paleozoic felsic volcanic, Cenozoic mafic volcanic, and Cenozoic alluviums, with water potentially more suitable for agricultural use due to the higher Ca2+, Mg2+, and HCO3- content. Ordovician metasediment water is more suitable for agricultural use.
Surface water survey data will be used in this study to optimise groundwater survey design and develop improved conceptual models to describe groundwater flow and recharge and discharge mechanisms in fractured rock environments in the Upper Murrumbidgee catchment. This will increase our understanding on how water resources may be utilised to support regional population growth and agricultural development, providing future water security for the region.