Aquifer storage and recovery (ASR) in southern Florida has been proposed on an unprecedented scale as part of the Comprehensive Everglades Restoration Plan (CERP). ASR wells were constructed or are under construction at 27 sites in southern Florida, mostly by local municipalities or counties in coastal areas. The Upper Floridan aquifer, the principal storage zone of interest to the Restoration Plan, is the aquifer being used at 22 of the sites. The aquifer is brackish to saline in southern Florida, which can greatly affect the recovery of the freshwater recharged and stored.

The purpose of this study is to inventory and compile data for existing ASR sites in southern Florida and identify various hydrogeologic, design, and management factors that control the recovery of freshwater injected (recharged) into ASR wells. Data for all wells at most of the 27 sites were compiled into four main categories: (1) well identification, location, and construction data; (2) hydraulic test data; (3) ambient formation water-quality data; and (4) cycle testing data. Each cycle during testing or operation includes periods of recharge of freshwater, storage, and recovery that each last days or months. Cycle testing data include calculations of recovery efficiency, which is the percentage of potable recharged water recovered for each cycle.

Potable water recovery efficiencies for 16 of the 27 sites were calculated and, generally, recovery efficiency improves with the number of cycles. Except for two sites, the highest number of cycles was five. Only nine sites had a recovery efficiency above 10 percent for the first cycle or two. However, at two out of the other seven sites, low recharge volumes per cycle of less than 10 million gallons (Mgal) could explain the poor recovery. Ten sites achieved a recovery efficiency above 30 percent during at least one cycle. The highest recovery efficiency achieved per cycle was 84 percent for cycle 16 at the Boynton Beach site (fig. 1). Recharge volume per cycle averaged 95 Mgal, ranging from 0.6 to 714 Mgal, for 55 cycles at 14 Upper Floridan aquifer sites. Recharge volume for cycle 3 at the West Well Field site in Miami-Dade County was 714 Mgal and included simultaneous recharge into 3 ASR wells. All cycles were conducted in a single well, except for two cycles at the West Well Field site.

Figure 1. Operational cycles at the Boynton Beach East Water Treatment Plant site in Palm Beach County and relations of volumes recharged and recovered, time, and percent recovery for each cycle. Recovery for cycle 15 was 34 percent for and ending chloride concentration of 146 milligrams per liter. [larger image]

Factors that could affect recovery of freshwater varied widely among sites. The thickness of the open storage zone at all sites ranged from 45 to 452 feet. For Upper Floridan aquifer sites, transmissivity based on tests of the storage zones ranged from 800 to 108,000 ft²/d (feet squared per day), chloride concentration of ambient water ranged from 500 to 11,000 mg/L (milligrams per liter), and leakance values indicated that confinement between the storage zone and lower zones may be limited in some areas. High transmissivity can adversely affect recovery, because it may equate to high dispersive mixing in a limestone aquifer. Additionally, depending on the ambient salinity of the storage zone, the probability of buoyancy stratification increases as transmissivity increases. At three sites that have transmissivities above 70,000 ft²/d with 3 to 5 cycles at each, recovery efficiencies over 10 percent per cycle were not obtained.

Based on review of four case studies and data from other sites, several hydrogeologic and design factors appear to be important to the performance of ASR in the Floridan aquifer system. Performance is maximized when the storage zone is thin and located at the top of the Upper Floridan aquifer (fig. 2), and transmissivity and ambient salinity of the storage zone are moderate (less than 30,000 ft²/d and 3,000 mg/L of chloride concentration, respectively). The structural setting at a site could also be important because of the potential for updip migration of a recharged freshwater bubble due to density contrast or loss of overlying confinement due to deformation.

Figure 2. Location of storage zone in relation to geophysical logs, lithology, flow zones, and geologic and hydrogeologic units for the aquifer storage and recovery well at the Boynton Beach East Water Treatment Plant site in Palm Beach County. [larger image]

This five-year study is divided into two phases, the first of which lasted two years. The first phase laid the groundwork for data inventory, review, and analysis. The second phase will allow for collection of additional data as it becomes available, expand the hydrogeologic framework, and perform a more complete comparative analysis of ASR sites. Results from the first phase are provided in Reese (2001); results from the current phase also will be published.

REFERENCES

Reese, R.S., 2001, Inventory and review of aquifer storage and recovery in southern Florida: U.S. Geological Survey Water Resources Investigation Report 02-4036, 56 p.

Contact: Ronald S. Reese, U.S. Geological Survey, Center for Water and Restoration Studies, 9100 NW 36 Street, Suite 107, Miami, FL, 33178, Phone: (305) 717-5821, Fax: (305) 717-5801, rsreese@usgs.gov

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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