Geochemical processes during five years of aquifer storage recovery

A key factor in the long-term viability of aquifer storage recovery (ASR) is the extent of mineral solution interaction between two dissimilar water types and consequent impact on water quality and aquifer stability. We collected geochemical and isotopic data from three observation wells located 25, 65, and 325 m from an injection well at an experimental ASR site located in a karstic, confined carbonate aquifer in South Australia. The experiment involved five major injection cycles of a total of 2.5 × 10⁵ m³ of storm water (total dissolved solids [TDS] ∼150 mg/L) into the brackish (TDS ∼2400 mg/L) aquifer. ∼60% of the mixture was pumped out during the fifth year of the experiment. The major effect on water quality within a 25 m radius of the injection well following injection of storm water was carbonate dissolution (35 ± 6 g of CaCO₃ dissolved/m³ of aquifer) and sulfide mineral oxidation (50 ± 10 g as FeS ₂/m³ after one injection). <0.005% of the total aquifer carbonate matrix was dissolved during each injection event, and ∼0.2% of the total reduced sulfur. Increasing amounts of ambient ground water was entrained into the injected mixture during each of the storage periods. High ¹⁴C_DIC activities and slightly more negative δ¹³C_DIC values measured immediately after injection events show that substantial CO_2(aq) is produced by oxidation of organic matter associated with injectant. There were no detectable geochemical reactions while pumping during the recovery phase in the fifth year of the experiment.