TRACE METAL MOBILIZATION DURING AQUIFER STORAGE AND RECOVERY CYCLING IN THE LOWER HAWTHORN AQUIFER, COLLIER COUNTY, FLORIDA

Aquifer storage and recovery (ASR) is a proposed major component of the Comprehensive Everglades Restoration Project, as there are plans for the installation of approximately 300 ASR storage facilities in the Lake Okeechobee region within the next 20 years. The need to address the drinking and irrigation water shortages in Florida and the scale of the Everglades Restoration Project exemplify the need to understand the possible geochemical interactions that occur due to the injection of surface/drinking water into the aquifer system. Current studies of ASR wells injecting surface water into the Floridan aquifer in Hillsborough and Charlotte counties have indicated that trace metals such as arsenic and uranium are chemically leached from the aquifer system matrix during the injection of oxygen-rich surface waters. No previous studies of trace metal mobilization have been undertaken at the Collier County Manatee Road ASR, Collier County, Florida. This facility utilizes the Lower Hawthorn (Zone 2) Aquifer to store drinking water primarily during the winter dry season.

This study was initiated to examine the concentrations of Fe, SO42-, and trace metals As, Co, Ni and Zn during the injection, storage and recovery of drinking water at the Collier County Manatee Road ASR facility. Increased concentrations of trace metals have been detected primarily during ASR recovery periods at other ASR locations in Florida. A release of SO42- along with an increase in the concentration of Fe and trace metals As, Co, Ni, and Zn would suggest that the oxidation of pyrite is occurring during the cycling of the ASR well. Preliminary analysis of Fe, As, Zn and SO42- concentrations in groundwater samples during this yearlong study reveals elevated concentrations of Fe, but no significant increase in As, Zn and SO42- as compared to native groundwater concentrations. Petrographic, whole-rock chemistry and XRD analysis of rock samples from the Miocene-aged Hawthorn Group reveal no evidence to demonstrate that the oxidation of pyrite is the source of trace metal mobilization in the Lower Hawthorn aquifer. These results warrant further investigation to determine the possibility that Fe-hydroxide grain-coating desorption/dissolution is a mechanism for trace metal mobility in this aquifer.