Central Avra Valley storage and recovery project geochemical modeling study

Abstract

The chemical and isotopic characteristics of the groundwater in A vra Valley, Arizona, were investigated, and existing hydrologic information was compiled to determine processes affecting groundwater chemistry in this area. The characteristics of Central Arizona Project (CAP) water, the surface water source for recharge operations in the Central Avra Valley Storage and Recovery Project (CAVSARP), were investigated for comparison. The changes that occurred over time in the chemistry of the groundwater were evaluated with respect to vadose zone processes and mixing with CAP water during recharge operations. Two major vadose zone processes were identified: flushing of perched water remaining from historical agricultural irrigation, and cation exchange as recharged CAP water infiltrated through clay layers at the site. Geochemical modeling using PHREEQC was conducted for groundwater, CAP water and mixtures of the two waters to determine processes that could affect recharge rates and the chemical composition of groundwater that will be recovered for delivery to customers. The results of the modeling were used to infer the likelihood that mineral precipitation and ion exchange occurred during the study period of January 1998 to December 1999. Based on the modeling results, significant mineral precipitation and subsequent decreases in recharge rates seems unlikely. Cation exchange processes will continue to alter the chemical composition of the CAP water between the recharge basin and its arrival at the water table.