Water Quality Changes during Aquifer Storage Recovery In the Cambrian-Ordovician Aquifer, Oak Creek, WI

Declining water levels and poor water quality in the Cambrian-Ordovician Aquifer led Oak Creek, WI to switch its water supply to Lake Michigan in 1976. As population increases, costly treatment plant expansions are needed to satisfy rapid increases in seasonal water demand. ASR has been sought as an auxiliary water supply, and an economic alternative to plant expansions. An existing municipal well, which is 1800 ft deep and open to 1200 ft of clastic and carbonate strata, was retrofitted for ASR use. From 1999 to 2007, eight injection/recovery test cycles were performed. During a cycle, treated Lake Michigan water is injected into the Cambrian-Ordovician Aquifer in early spring (low daily demand), and recovered during summer (high daily demand). This water has higher levels of dissolved oxygen, dissolved organic carbon and, as a result of disinfection, trihalomethanes. Water chemistry was monitored at the injection well, and at a similar monitoring well located 180 ft away. Concentrations of iron and manganese increased in both wells during the storage period of each cycle, and increased in the ASR well during the recovery period. Following completion of a cycle, they do not return to prior observed “background” levels in either well. Total trihalomethane concentrations in the monitoring well decreased during the storage and recovery phases. In the ASR well, however, they decreased only during recovery (due to addition of chlorinated water during storage to suppress bacteria), and have increased over successive cycles. The long-term feasibility of ASR at this site requires an improved understanding of the processes controlling these concentration increases. A detailed, site-specific flow model will improve our understanding of the hydraulic factors governing the flow of injected water. When combined with geochemical mixing and inverse models, this can constrain the processes affecting water quality, and identify possibilities for mitigating these problems.