F W R J
Challenges and Solutions to Developing Alternative Water Supplies in Central Florida: Polk Regional Water Cooperative Experiences Robert G. Maliva, Scott Manahan, Mary Thomas, Stephen James, and Ryan Taylor
C
entral Florida is projected to experience continued rapid population growth and concomitant increases in water demands. Groundwater modeling performed for the Central Florida Water Initiative (CFWI) indicates that the region is approaching the sustainable limit for Upper Floridan aquifer (UFA) groundwater withdrawals, its predominant water source, and projects a shortfall of 95 mil gal per day (mgd) by 2040 that will have to be met through expanded water conservation and other alternative sources. Communities will therefore be forced to find alternative water sources (AWS) to meet future demands. Fragmentation of regional water supply systems into numerous local utilities increases the cost of developing AWS due to poor economies of scale. A solution to fragmentation is joint development of large AWS projects, which were adopted by the Polk Regional Water Cooperative (PRWC), a consortium of 16 Polk County governments. The PRWC is in the design stage for four AWS projects, two brackish groundwater desalination facilities, and surface water facilities withdrawing from the Peace Creek and Peace River. Brackish groundwater desalination is widely adopted as AWS in Florida because much
of the state is underlain by brackish groundwater and it’s a reliable year-round supply. A technical challenge in Polk County is that its brackish groundwater source, the Lower Floridan aquifer (LFA), contains calcium sulfate-type water (rather than the sodium chloride-type water of coastal brackish aquifers). The reverse osmosis (RO) desalination process will produce a concentrate that is highly supersaturated with respect to gypsum and thus poses a high threat of scale formation in injection wells used for its disposal. The solution adopted is pretreatment using a precipitation process to decrease calcium and sulfate concentrations. Potential increases in the salinity of the raw water over time were addressed by rigorous solute-transport modeling and a robust treatment process design. Desalination concentrate disposal in central Florida is particularly challenging because of the absence of a high-transmissivity injection zone below the regulatory base of the underground source of drinking water (USDW). Injection of concentrate into a basal LFA zone could result in upward migration into a USDW, which would be a regulatory violation, but would not impair actual potable water supplies as the migrating water would
Table 1. Bureau of Economic and Business Research Projected Future Increases in Central Florida Population
County
2020 Medium Estimate
2045 Medium Estimate
Increase
Lake
360,700
493,600
36.8%
Orange
1,415,500
1,891,800
33.6%
Osceola
380,700
591,000
55.2%
Polk
699,600
884,700
26.5%
Seminole
477,800
573,700
20.1%
(source: Rayer and Wang, 2019)
64 February 2022 • Florida Water Resources Journal
Robert G. Maliva is principal hydrogeologist and Scott Manahan is senior engineering manager with WSP USA Inc. in Fort Myers. Mary Thomas is associate vice president with Carollo Engineers in Orlando. Stephen James is administrator and Ryan Taylor is executive director of Polk Regional Water Cooperative in Bartow.
be captured by the shallower LFA production wells. The PRWC is investigating both avenues of regulatory relief for LFA injection wells and use of a deeper Upper Cretaceous injection zone. Upper Cretaceous injection wells are more expensive and may operate at higher pressures than LFA injection wells, but the greater cost per well would be at least partially offset by an expected greater capacity per well. The Peace Creek and Peace River projects would involve capturing and treating excess available surface water during the wet season for aquifer recharge. The goal is to increase aquifer heads so that additional UFA groundwater can be extracted without impacting minimum flows and levels of nearby lakes. Key technical issues are developing the most-cost-effective water treatment and storage options and identifying design options through detailed groundwater modeling that maximize the permittable additional UFA withdrawals. An overarching lesson of the PRWC experience is that there are no simple solutions for AWS and a wide range of options need to be rigorously considered.
Background The strong economic and population growth of central Florida is projected to continue in coming decades. According to the Florida Bureau of Economic and Business Research (BEBR), the population of the counties in the region is projected to increase by 20.1 to 55.2 percent between 2020 and 2040 (Table 1). The primary water source in central
