Environmental isotopes meet 3D geological modelling: Conceptualising recharge and structurally-controlled aquifer connectivity in the basalt plains of south-western Victoria, Australia

Abstract

To develop a conceptual model of recharge and inter-aquifer connectivity for a complex aquifer system in southwestern Victoria, Australia, a three-dimensional (3D) geological model was constructed and used to examine the influence of the regional geology on groundwater composition and age, as well as recharge mechanisms. The major aquifers are three basalts, differentiated on age and geomorphological features, and an underlying palaeovalley (deep lead) system. The upper fine-grained sediments of the deep lead form an aquitard that separates the basalt from the permeable sands and gravels beneath. Recharge calculations (chloride mass balance and water-table fluctuation methods), salinity, stable isotope, tritium and radiocarbon data show that volcanoes and the youngest basalts, although volumetrically minor, form preferential recharge areas for all the basalts (up to ∼30 mm/year), and also recharge the deep lead sediments through volcanic necks that penetrate the aquitard. Infiltration through the thick, clay-rich soils on the older basalts is small (typically ⩽ 1–2 mm/year) and saline, resulting in a progressive increase of groundwater salinity along the flow path in these basalts. The comparison of fault location with groundwater age in the deep lead aquifer suggests that in some areas, faults exert significant structural control on groundwater flow. This study demonstrates the value of integrating geological, hydrogeological and hydrochemical data to identify preferential recharge areas, inter-aquifer connectivity and the influence of faults on groundwater flow in a complex aquifer system.