Groundwater geochemistry of aquifer thermal energy storage: Long‐term test cycle

Groundwater chemistry was monitored during a test of the feasibility of long‐term (180‐day) aquifer thermal energy storage. From a source well 92,100 m 3 of groundwater was pumped, heated (110°C), and injected into the aquifer consisting primarily of quartz sandstone, with lesser amounts of dolomite, feldspar, and clay minerals. Softening the water prior to heating effectively prevented mineral precipitation in the heat exchanger and injection well. Recovered water was saturated with respect to quartz, dolomite, and calcite, which indicates that quartz, dolomite, and calcite dissolved during aquifer storage. Loss of sodium in the aquifer is thought to result from mixing of ambient groundwater (up to 27%) with heated, injection water. Upon subsequent cycles, softening requirements are expected to decrease, and less mineral dissolution is expected to occur during aquifer storage, resulting in negligible changes in the hydraulic characteristics of the aquifer.