Distinguishing groundwater flow paths in different fractured-rock aquifers using groundwater chemistry: Dandenong Ranges, southeast Australia

Abstract

Major ion geochemistry is used to qualitatively interpret groundwater residence times within an aquifer, and the extent of mixing between aquifers with distinctive mineralogy. In conjunction with hydraulic heads and stable isotope geochemistry, flow paths and inter-aquifer exchange are defined in a fractured-rock aquifer system in the Dandenong Ranges, southeast Australia. Stable isotopes indicate modern seasonal recharge throughout the system. At high elevations in the sub-catchment, which includes both marine Silurian-Devonian sedimentary and Tertiary basalt aquifers, Cl is derived primarily from cyclic salts, and differences in mineralogy result in groundwater from the basalt aquifer having higher TDS contents (123–262 mg/L) and (Ca+Mg)/Na ratios (0.9–1.3) than groundwater from the sedimentary aquifer (TDS: 55–79 mg/L; (Ca+Mg)/Na: 0.1–0.2). At low elevations, in areas of local groundwater discharge, the more regional flow system in the Silurian-Devonian sediments contains additional Cl from water–rock interaction and has distinctly higher TDS contents (517–537 mg/L). Differences in groundwater chemistry between the aquifers and between shallower and deeper flow systems highlights areas of inter-aquifer mixing. This is particularly important for aquifer vulnerability where groundwater quality in the deeper aquifer may be impacted by surface activities.