Objectives
Understanding how land-use change influences the water cycle is of critical importance to land and water management. Several physical and hydrochemical/isotope techniques were used to estimate groundwater recharge rates and identify key controlling factors in a rapidly urbanising catchment in southeast Australia (Western Port catchment).
Design and methodology
Multiple lines of qualitative and quantitative evidence regarding groundwater recharge were collected: environmental tracers (radiocarbon, tritium, chloride), time-series soil moisture profiles, high frequency water level and electrical conductivity measurements and lithological mapping.
Original data and results
The presence of significant tritium (>1.0 TU), along with radiocarbon activities >85 pMC and low chloride concentrations (<150 mg/L), allowed identification of areas with significant recent recharge. These were strongly associated with elevated topography and the absence of volcanic clay (the dominant near surface lithology) along the catchment margin. This finding was supported by time-series analysis of soil moisture profiles, indicating minimal vertical propagation of precipitation below 1.5 m depth in volcanic soils. Chloride mass balance and water table fluctuation-based recharge rates mostly ranged between 1.5 and 50 mm/yr; however, rates exceeding 100 mm/year were identified in a spatially restricted zone. In this area, the volcanics are absent and Quaternary sand directly overlies the lower Cainozoic aquifer. This zone comprises a small percentage of the study area (approximately 15%) but is estimated to contribute approximately half of the recharge. This new understanding of the recharge resulted in a significant revision of the conceptual hydrogeological model for the region.
Conclusions
The findings underscore the importance of characterising recharge locations and processes based on field data, to support protection of groundwater quality and quantity. For example, careful land-use planning could be carried, to strategically protect areas of high recharge rates and quality within catchments experiencing rapid urban growth.