Aquifer residence times for recycled water estimated using chemical tracers and the propagation of temperature signals at a managed aquifer recharge site in Australia

A prerequisite for minimizing contamination risk whilst conducting managed aquifer recharge (MAR) with recycled water is estimating the residence time in the zone where pathogen inactivation and biodegradation processes occur. MAR in Western Australia's coastal aquifers is a potential major water source. As MAR with recycled water becomes increasingly considered in this region, better knowledge of applied and incidental tracer-based options from case studies is needed. Tracer data were collected at a MAR site in Floreat, Western Australia, under a controlled pumping regime over a distance of 50 m. Travel times for bromide-spiked groundwater were compared with two incidental tracers in recycled water: chloride and water temperature. The average travel time using bromide was 87 +/- 6 days, whereas the estimates were longer based on water temperature (102 +/- 17 days) and chloride (98 +/- 60 days). The estimate of average flow velocity based on water temperature data was identical to the estimate based on bromide within a 25-m section of the aquifer (0.57 +/- 0.04 m day(-1)). This case study offers insights into the advantages, challenges and limitations of using incidental tracers in recycled water as a supplement to a controlled tracer test for estimating aquifer residence times.