Abstract Introduction Inventory of Data Case Studies Summary References Tables PDF Version

Detailed information regarding four sites is presented in this section. For the most part, the sites were selected on the basis of the number of cycles that were conducted. Two sites are located in southeastern Florida, and two are in southwestern Florida. The selected sites illustrate the contrast in hydrogeology between the coastal areas. Each case study includes a graphical representation of the hydrogeology at the site and well construction information.

Figure 10. Ambient water salinity of storage zones in the Floridan aquifer system at aquifer storage and recovery sites in southern Florida. [larger image]

The Boynton Beach East WTP ASR site located on the east coast in Palm Beach County is operated by Boynton Beach Utilities. The source of water for recharge is treated drinking water from the WTP. The location of the storage zone in relation to lithology, geophysical log signatures, and hydrogeologic units at the Boynton Beach site is shown in figure 11. Also shown are the location of flow zones as determined by flowmeter, fluid resistivity, and caliper logs for the interval extending from a depth of 804 to 1,200 ft below land surface. The flow zones in this interval primarily occur in the basal Hawthorn unit (Reese and Memberg, 2000) or near its base. The flow zones are thin, they tend to coincide with formation resistivity peaks possibly indicating cementation and secondary porosity, and they occur just below intervals of higher gamma-ray response. These intervals of higher gamma-ray response indicate beds high in phosphate sand content. The base of the Upper Floridan aquifer was not penetrated in the ASR well but is estimated to be at a depth of at least 1,500 ft below land surface.

The thickness of the storage zone open interval at the Boynton Beach site is 105 ft (fig. 9); transmissivity is reported to be about 9,400 ft²/d (fig. 9; CH₂M Hill 1993), and ambient water had a chloride concentration of 1,900 mg/L (fig. 10). The site is located in a structurally high area along the east coast where the altitude of the Hawthorn Group basal contact is 930 ft below sea level (Reese and Memberg, 2000).

Figure 11. Location of the storage zone in relation to geophysical logs, lithology, flow zones, and geologic and hydrogeologic units for aquifer storage and recovery well PB-1194 at the Boynton Beach East Water Treatment Plant site in Palm Beach County. [larger image]

Cycle testing at the Boynton Beach site began in late 1992, and by early 2000, 16 recharge-recovery cycles had been conducted for an average of about 2 cycles per year (fig. 12). Potable recovery efficiency increased rapidly during the first four cycles to 90 percent per cycle; however, as noted previously, the 90 percent recovery for cycle 4 is questionable. During the next three cycles, recovery efficiency decreased to less than 30 percent, possibly because of longer storage periods. Recovery efficiency for cycles 8 to 16 generally increased to greater than 80 percent.

Figure 12. 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 an ending chloride concentration of 146 milligrams per liter. [larger image]

Percent recovery is plotted against the chloride concentration of recovered water during each cycle in figure 13. For most cycles, water was recovered until the chloride concentration in the recovered water slightly exceeded 300 mg/L (also see table 5). During cycle 14, however, recovery continued until chloride concentration increased to about 1,000 mg/L, contributing to a lower recovery rate for cycle 15. The data points for cycle 15 are shifted to substantially lower recovery percentages than for cycle 14 (fig. 13). The recovery efficiency for cycle 16 is the best obtained, with the exception of cycle 4, which has a recharge volume that could be higher than reported. However, the storage period for cycle 16 was only 4 days, and the recovery efficiency for this cycle could have benefited from the large recharge volume (111 Mgal) and incomplete recovery (recovery up to a chloride concentration of only 146 mg/L) for cycle 15.

Potable water recovery efficiencies for test and operational cycles at the Boynton Beach site appear to be greater than for all other Floridan aquifer system ASR sites in southern Florida. However, the number of cycles conducted at most other sites are limited, and the chloride concentration of the recharge water used at the Boynton Beach site is only about 50 mg/L (table 5). Several hydrogeologic, and design and management factors are favorable at this site that may explain the higher recovery efficiencies. The storage zone is located at the top of the Upper Floridan aquifer and is thin in comparison to the average storage zone thickness (about 180 ft) for wells in the Floridan aquifer system (fig. 8). Transmissivity and ambient salinity of the storage zone are moderate, being less than 30,000 ft²/d and 3,000 mg/L of chloride concentration, respectively, and the site is located in a structurally high area.

Figure 13. Percent recovery of recharged water during operational cycles in relation to chloride concentration of recovered water at the Boynton Beach East Water Treatment Plant site in Palm Beach County. Recovery for cycle 14 was continued until reaching a chloride concentration of about 1,000 milligrams per liter. [larger image]

Figure 14. Location of the storage zone in relation to gamma-ray geophysical log and geologic and hydrogeologic units for aquifer storage and recovery well G-2914 at the Springtree Water Treatment Plant site in Broward County. [larger image]

The Springtree WTP ASR site located in Broward County is operated by the City of Sunrise. Treated drinking water is used for recharge. The location of the storage zone in relation to lithology, geophysical log signatures, and hydrogeologic units at the Springtree site is shown in figure 14. Geophysical logs, such as the flowmeter, used to identify flow zones in the well were not run. Unlike the Boynton Beach site, the storage zone is not located at the top of the Upper Floridan aquifer. Casing was set through virtually all of the basal Hawthorn unit. The thickness of the storage zone open interval at the Springtree site is 160 ft (fig. 8). A photograph of the Springtree site wellhead site is shown in figure 15.

Storage zone transmissivity at the Springtree site is reported to be about 5,700 ft²/d (table 3 at end of report; Montgomery Watson, 1998a). This value is lower than at surrounding sites (fig. 9), perhaps because only a small part of the basal Hawthorn unit is included in the open interval. The chloride concentration of ambient water in the storage zone is 3,600 mg/L (fig. 10). The altitude of the Hawthorn Group basal contact is 1,105 ft below sea level, and the site is located at the edge of a structurally low area (Reese, 1994).

Cycle testing at the Springtree site began at the end of July 1999, and five recharge-recovery cycles had been completed by the end of November 2000 (table 5). The ending chloride concentration for all cycles was 225 mg/L or less. The increase in recovery efficiency during the first four cycles was not as great as the Boynton Beach East WTP site, increasing from 20 percent for the first cycle to 37.5 for the fourth cycle. Although the volume recharged for the fifth cycle (120 Mgal) was at least three times that recharged in each of the first four cycles, recovery efficiency diminished to 27.5 percent. The lower recovery efficiencies at the Springtree site relative to the Boynton Beach East WTP site could be explained by high storage zone ambient water salinity and the storage zone position relative to the top of the aquifer.

Figure 15. Wellhead piping, valves, and control system for the aquifer storage and recovery well at the Springtree Water Treatment Plant site in Broward County. Storage well on left side of concrete pad as shown by arrow. [larger image]

The Marco Lakes ASR site located on the west coast in Collier County is operated by Florida Water Services for the City of Marco Island. The source of recharge water is partially treated surface water. The storage zone at the site straddles the contact between the basal Hawthorn unit and the Suwannee Limestone (fig. 16). The Suwannee Limestone is thick and well developed in the area, unlike southeastern Florida where the formation is thin or absent. The thickness and diameter of the open interval for the storage zone in well ASR-1 at the Marco Lakes site are 45 ft and 10 in., respectively (fig. 8; table 2 at end of report). By comparison, these dimensions are 44 ft and 12.25 in., respectively, for well ASR-2 (fig. 16; table 2 at end of report).

Transmissivity of the storage zone is reported to be about 9,100 ft²/d (fig. 9; ViroGroup, Inc., 1998b). Storage zone ambient water is brackish; the reported chloride concentration is about 2,600 mg/L (fig. 10; well DZMW, table 4). Chloride concentration ranged from about 2,500 to about 3,700 mg/L in other wells completed in the storage zone at the Marco Lakes site (table 4). The site is located in a structurally high area where the altitude of the Hawthorn Group basal contact is 742 ft below sea level (Reese, 2000).

Figure 16. Location of storage zone in relation to gamma-ray geophysical log, flow zones, and geologic and hydrogeologic units for aquifer storage and recovery well C-1208 (ASR-2) at the Marco Lakes site in Collier County [larger image]

Five recharge-recovery cycles were conducted at the Marco Lakes site in ASR-1 between June 1997 and July 2000 (table 5). The ending chloride concentration for the recovery period used for comparison of cycles was 350 mg/L (Water Resources Solutions, Inc., 2000d), and the recovery efficiencies at this chloride concentration level increased from 31 percent for the first cycle to about 51 percent for the fifth cycle. The total volume of water recharged per cycle increased from about 20 to 132 Mgal, respectively. The potable water recovery efficiency increased from 22 to almost 36 percent for the same two cycles.

Percent recovery was compared with the chloride concentration of recovered water during each cycle at the Marco Lakes site (fig. 17). On the basis of numerical simulation, the erratic recovery curve and poor recovery efficiency for cycle 2 is attributed to preferential well plugging during recharge of one of two receiving intervals (flow zones) in the storage zone (Water Resources Solutions, Inc., 1999c). Calcium carbonate is the likely precipitate causing plugging, and acidification of the recharge water prior to injection has reduced or eliminated the problem in later cycles.

The Marco Lakes recovery efficiencies at an ending chloride concentration of 350 mg/L rather than those at 250 mg/L concentration could serve as a better comparison with the Boynton Beach East WTP site potable recovery efficiencies. The chloride concentration of the recharge water at Marco Lakes averages 120 mg/L (table 5), whereas the concentration at Boynton Beach averages about 50 mg/L. Perhaps calculations of recovery efficiencies based on a mass-balance approach would provide a better means of comparison between these two sites. These calculations would include the chloride concentrations of both the ambient and recharged water. Although the Marco Lakes site is in early phases of testing and operation, several factors could explain the moderate to good recovery efficiencies. The storage zone is thin and located near the top of the Upper Floridan aquifer (fig. 16). The storage zone has moderate transmissivity, and the site is located on a structural geologic high.

Figure 17. Percent recovery of recharged water during operational cycles in relation to chloride concentration of recovered water at the Marco Lakes site in Collier County. [larger image]

Figure 18. Location of storage zone in relation to gamma-ray geophysical log, flow zones, and geologic and hydrogeologic units for aquifer storage and recovery well L-5812 at the San Carlos Estates site in Lee County. [larger image]

The San Carlos Estates ASR site located near the west coast in Lee County is operated by Bonita Springs Utilities. Treated drinking water is used as the recharge water source. The storage zone at the site is located within the basal Hawthorn unit (fig. 18). The top of the Suwannee Limestone was not reached in any of the wells at the site. The thickness of the ASR well storage zone is only 51 ft (fig. 8).

Compared to the Marco Lakes site, transmissivity of the storage zone at the San Carlos Estates site is high; it is reported to be about 70,000 ft²/d (fig. 9; CH₂M Hill, 1999b). The chloride concentration of ambient water in the storage zone is only 1,100 mg/L (fig. 10). The site may be located in a slightly low area structurally (Reese, 2000); however, additional wells that intersect the basal contact of the Hawthorn Group are required to confirm this setting.

Two cycles, one a short test cycle, were conducted at the San Carlos Estates ASR site (table 5). Despite a second cycle recharge volume of 138 Mgal, potable recovery efficiency has been no greater than about 3 percent. High transmissivity of the storage zone and the distribution of permeability within it may explain the poor recovery efficiency obtained thus far. Flowmeter log data indicate that most flow in the storage zone occurs within a 4-ft-thick interval between 698 and 702 ft below land surface (fig. 18). The high permeability of this thin flow zone may cause high dispersive mixing within the storage zone resulting in the poor recovery.