Estimating Recharge From Recirculated Groundwater With Dissolved Gases: An End‐Member Mixing Analysis

Abstract

Abstract In many hydrological systems, groundwater is pumped from the aquifer onto the land surface, and a fraction of this water subsequently infiltrates to recharge the groundwater system (recirculated groundwater). Tracers that undergo different degrees of reequilibration with the atmosphere during this recirculation process can enable ambient groundwater and recirculated groundwater to be differentiated. In this paper, the recirculated groundwater has been pumped to dewater open pit mines and discharged into ephemeral creeks. Some of this water subsequently recharged back into the aquifer. Chlorofluorocarbon‐12 (CFC‐12), 14 C, and 3 H are used in a four end‐member mixing analysis to differentiate between (1) ambient groundwater, (2) recirculated groundwater, (3) river recharge from natural flows prior to commencement of mining operations (in 2007), and (4) natural river recharge post‐2007. Sampling of the surface water when discharge of mine water was the only source of river flow enabled the extent of reequilibration of both CFC‐12 and 14 C to be accurately determined. Since CFC‐12 reequilibrates more rapidly than 14 C, recirculating groundwater had a CFC‐12 concentration that was close to modern, but a 14 C activity that was higher than the original groundwater, but less than modern recharge. As 3 H does not reequilibrate, it enabled easy differentiation between recirculated groundwater and infiltration of natural creek flows. Uncertainty of end‐member compositions is due to changes in the end‐member concentrations over time in the case of natural river flows, uncertainty in the extent of tracer reequilibration for the groundwater recirculation end‐member, and spatial variations in the composition of the ambient groundwater end‐member.