Constraining modern and historical recharge from bore hydrographs, 3H, 14C, and chloride concentrations: Applications to dual-porosity aquifers in dryland salinity areas, Murray Basin, Australia

Abstract

Water table fluctuations and renewal rates based on 3H and 14C concentrations in bores screened within 5 m of the water table were used to estimate recharge rates in five dryland salinity areas in northern Victoria, Australia. Given the resolution of each technique both methods yield broadly similar results of 4–90 mm/yr (up to 14% of modern rainfall) that reflect the recharge rates following land clearing over the last 200 years. Deeper bores show mixing of modern recharge with older water flowing laterally through the aquifers. Recharge estimates based on Cl concentrations are up to two orders of magnitude lower (<1 mm/yr), implying that they largely reflect pre-land clearing recharge rates, as is the case elsewhere in the Murray Basin. Recharge rates are similar across the areas from upper slopes to plains, indicating that recharge occurs across substantial areas of each catchment. This is also implied by bore hydrographs in all parts of the flow systems being in phase, the presence of 3H in groundwater from across the area, and seasonal variations in salinity and δ18O values in groundwater from water table bores. Since recharge occurs across broad areas, any attempts to slow the rate of water table rise requires management of the landscape as a whole, not just the uplands. That the increased recharge since land clearing has not lowered groundwater salinity is due to the system having dual porosity, where older saline water trapped in clays within the unsaturated zone is slowly incorporated into the groundwater as the water table rises.