Using multiple lines of evidence to map groundwater recharge in a rapidly urbanising catchment: Implications for future land and water management

Understanding how land-use change influences the water cycle is of critical importance to land and water management. Determining the timing, location and rates of groundwater recharge and their relationship to land-use is often challenging, leading to significant uncertainties in water budgets and water cycle planning. In this study, a combination of physical and hydrochemical/isotope techniques were used to estimate and map groundwater recharge rates and identify its key controlling factors in a rapidly urbanising catchment in southeast Australia. The primary objective was to provide qualitative and quantitative information regarding recharge, allowing comparison and monitoring of changes as future urbanisation takes place. The presence of significant tritium in shallow groundwater ( > 1.0 TU), along with radiocarbon activities > 85 pMC and low salinity (e.g. EC < 600 mu s/cm) allowed identification of areas where significant recharge has taken place in recent years. These were strongly associated with elevated topography on the basin margin, and the absence of volcanic clay - the dominant lithology underlying most of the region. This interpretation is supported by time-series analysis of soil moisture profiles, which indicate minimal vertical propagation of precipitation below 1.5 m depth in volcanic clay soils. Estimation of recharge rates was conducted using chloride mass balance and water table fluctuation analysis in water table aquifer monitoring bores. Rates mostly ranged between 1.5 and 50 mm/yr; however, recharge exceeding 100 mm/year was identified in a spatially restricted zone at the edge of the basin. Here, the volcanic lithology is absent and Quaternary sand directly overlies the lower Cainozoic sand aquifer. This area comprises a small percentage of land in the study area (approximately 15%) but is estimated to contribute a large proportion (nearly half) of recharge. The findings underscore the importance of characterising recharge locations and processes to support the protection of groundwater quality and quantity, for example, through careful land-use planning.