AQUIFER STORAGE AND RECOVERY PERFORMANCE AND HYDROGEOLOGY, UPPER FLORIDAN AQUIFER, SOUTHERN FLORIDA

Interest and activity in aquifer storage and recovery (ASR) in southern Florida has increased greatly during the past 10 to 15 years as a mechanism to store excess freshwater that is available during the wet season in an aquifer and recover it during the dry season when needed to supplement water supplies. The storage zone in ASR wells drilled at 29 of the 30 sites in the carbonate Floridan aquifer system is contained within the brackish to saline Upper Floridan aquifer.

Each ASR cycle is defined by three periods: recharge, storage, and recovery, and the primary measure used to evaluate the performance of ASR sites is the potable water per-cycle recovery efficiency, which is defined as the percentage of the volume of freshwater recharged that is recovered prior to exceeding a recovered water chloride concentration of 250 mg/L. Calculations of the cumulative potable recovery efficiency at ASR sites display substantially less variability than per-cycle efficiencies.

Per-cycle potable water recovery efficiencies vary from 0 to 94 percent. High potable efficiency on a per cycle basis can be related to water banking—an operational approach in which a large volume of water is recharged during an initial cycle. This process can flush the aquifer around the well creating a temporary buffer zone that increases recovery efficiency substantially during subsequent cycles conducted with much lower recharge volumes.

The relative performance for 15 of the 30 sites was determined by arbitrarily grouping performance into “low” (0-20 percent cumulative potable recovery efficiency), “medium” (20-40 percent) and “high” (>40 percent) categories; 3 sites were rated high, 6 were rated medium, and 6 were rated low. Although six sites have a high overall recharge rate that is associated with water banking, three of these are rated low.

Four hydrogeologic and design factors that may affect recovery are the thickness, transmissivity, and ambient salinity of the storage zone, and the thickness of the portion of the aquifer above the top of the storage zone; and threshold values for these factors above which recovery efficiency could be adversely affected were selected. Some general correlation of the site performance ratings with the number of these factors above the threshold value was found.