Applying Geochemistry to Managed Aquifer Recharge Projects
Managed aquifer recharge is being implemented at sites across the southwestern United States as an important water resource management tool, in order to recharge and store surplus water within aquifers, and to recover this water for later use. Compatibility of the recharged water with both groundwater and the aquifer matrix may be predicted using geochemical models and mixing calculations supported by site-specific data. Several examples from New Mexico will be presented, illustrating the use of geochemical techniques to predict the water quality of the recovered water, and to track recharged water in the aquifer.
In the Santa Rosa-Chinle aquifer system, the potential for reactions between treated surface water and the aquifer materials that would mobilize iron (Fe) and manganese (Mn) was evaluated, and the calculated saturation indices (SI) and oxidation reactions indicate that Fe and Mn solubility should be limited during recharge operations.
In the Tesuque Formation, fluoride concentrations were predicted to exceed the U.S. Environmental Protection Agency maximum contaminant level (MCL) in the recovered water based on fluorite solubility and mixing calculations. This information was used to identify treatment requirements for potable water use.
In the Santa Fe Group aquifer system, arsenic concentrations were initially diluted by the injected water but quickly rebounded during recovery, as a result of dissolving iron coatings on aquifer sediments releasing iron and adsorbed arsenic into solution. In another case, sulfur hexafluoride (SF6) was injected into the recharge water as a conservative tracer used to monitor recharge water movement in the aquifer. The SF6 tracer was observed in the monitor well network eight days after injection operations started, at a distance of 75 feet from the injection well.
These geochemistry studies provided critical information about the operation and treatment of waters recovered from these managed aquifer recharge systems.
