Identifying modern and historic recharge events from tracer-derived groundwater age distributions

Understanding groundwater ages offers insight into the time scales of recharge, aquifer storage turnover times, and contaminant protection time frames. The ability to quantify groundwater age distributions heavily depends on the choice of the interpretive model, and often important features of the age distribution cannot be identified with the subset of available models. In this paper, we implemented a multiple tracer method using a technique that assumes limited details regarding the shape of the age distribution and applied it to dewatering wells at a mine site in the Pilbara region of north-western Australia. Using our method, we were able to identify distinct age components in the groundwater. We calculated the presence of four distinct age groups in the samples. All wells contained water aged between zero and 20 years. However, the rest of the samples were composed of water between 50 and 100 years, 100 and 600 years, or water approximately 1000 years old. These were consistent with local recharge sources (50-100 years) and knowledge of paleoclimate from lake sediment records. We found that although the age components were well constrained, the relative proportions of each component were highly sensitive to errors of environmental tracer data. Our results show that our method can identify distinct age groups in groundwater samples without prior knowledge of the age distribution. The presence of distinct recharge times gives insight into groundwater flow conditions over long periods of time.