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

• Published in: Water resources research 2019-07, Vol.55 (7), p.5468-5486
• Main Authors: Cook, P.G., Dogramaci, S.
• Format: Article
• Language: English
• Subjects: Aquifers; Chlorofluorocarbons; Coastal inlets; Composition; Creeks; Dissolved gases; Gases; geochemistry; Groundwater; Groundwater recharge; Hydrologic analysis; Hydrology; Infiltration; mine dewatering; Mine drainage; mine hydrology; Mine waters; Open pit mining; recirculation; River flow; Rivers; Spatial variations; Surface water; Tracers; Uncertainty; Water discharge
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2019WR025012

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), 14C, and 3H 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 14C to be accurately determined. Since CFC‐12 reequilibrates more rapidly than 14C, recirculating groundwater had a CFC‐12 concentration that was close to modern, but a 14C activity that was higher than the original groundwater, but less than modern recharge. As 3H 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. Key Points Recirculation of water between the land surface and the aquifer can be difficult to identify using ionic tracers The use of dissolved gases allows quantification of groundwater recirculation due to gas reequilibration on exposure to the atmosphere Infiltration of mine water discharged to creeks is differentiated from natural creek flow using CFC‐12, 14C, and 3H