Challenges and Solutions to Developing Alternative Water Supplies in Central Florida: Polk Regional Water Cooperative

FWRJ 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

Central 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 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.

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)

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.

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

Florida is fresh groundwater from the UFA, which has the great benefit of relatively low cost, high quality, and widespread availability. Extractions from the UFA in central Florida are considered to be approaching sustainable limits, which are dictated primarily by impacts to minimum flows and levels (MFLs) in lakes and springs. The CFWI is “a collaborative water supply planning effort among the state’s three largest water management districts, Florida Department of Environmental Protection (FDEP), Florida Department of Agriculture and Consumer Services (DACS), water utilities, environmental groups, business organizations, agricultural communities, and other stakeholders” (CFWI, 2018). According to the 2020 CFWI regional water supply plan (RWSP), the sustainable fresh groundwater supply in the CFWI planning area is about 760 mgd and the projected water demand in 2040 is 855 mgd, leaving a projected shortfall of 95 mgd that will have to be met through expanded water conservation and other alternative sources (CFWI, 2020).

The main potable options for AWS in central Florida are desalination of brackish groundwater, fresh surface water from rivers and creeks, and potable reuse (direct and indirect). Utilities may also obtain additional permittable fresh groundwater allocations by purchasing properties with agricultural allocations. Water supply in parts of central Florida is decentralized, with small utilities serving local populations. Decentralization is often preferred by communities because it gives them greater autonomy over their water supplies, but the fragmentation of community water supplies can be an impediment to the development of AWS as small utilities often lack the needed technical and financial resources.

The poor economies of scale of small projects can result in high unit costs for additional water, but collaboration among community water supply systems is a means for smaller utilities to take advantage of the economies of scale of large AWS systems (Mullin, 2020).

The PRWC was established “to proactively identify alternative water resources and projects that ensure the future sustainability of our regional water supply. The PRWC will develop strategies that meet the long-term water demands of Polk County, determine needed infrastructure, and facilitate a regional conservation program, which encourages the responsible use of our water resources” (PRWC, 2020).

The PRWC is overseen by its 16 member governments, which include 15 cities and Polk County. The PRWC, in partnership with the Southwest Florida Water Management District (SWFWMD), investigated various options for AWS in Polk County. The three main projects for the AWS chosen as best meeting the PRWC future requirements are two brackish groundwater desalination facilities, designated the Southeast and West Polk Lower Floridan Aquifer Water Production Facilities (SE LFAWPF and WP LFAWPF,) and surface water aquifer recharge (AR) using the Peace Creek and Peace River as sources (Figure 1). The current design capacities of the SE LFAWPF and WP LFAWPF are 12.5 and 15 mgd. The PRWC projects provide insights into the general opportunities and challenges in the development of AWS in central Florida.

Figure 1. Polk County map showing locations of Polk Regional Water Cooperative alternative water supply projects.

Brackish Groundwater Desalination

The most widely implemented potable option for AWS in Florida is the RO desalination of brackish groundwater. Hydrogeological conditions in southeastern and southwestern Florida are particularly favorable for the implementation of brackish groundwater desalination because abundant brackish groundwater resources are available in the UFA, and, to a lesser degree, in the intermediate aquifer system in southwestern Florida. The

presence of an extraordinarily transmissive injection zone, the so-called “boulder zone” of the LFA, can be used to efficiently and economically dispose of the desalination concentrate. Another high-transmissivity zone, the Avon Park high-permeability zone (APHPZ), is suitable for concentrate disposal in the Tampa Bay region. Brackish groundwater desalination in central Florida is more challenging than in the southeastern and southwestern parts of the state because brackish groundwater is present at greater depths, and in less-transmissive aquifers, there are hydrogeological and regulatory constraints on concentrate disposal via injection wells, and there are unusual groundwater chemistries. The brackish raw water supply for groundwater desalination in central Florida is the uppermost zone of the LFA. Using SWFWMD hydrostratigraphic terminology, the planned production zone at the SE LFAWPF is the LFA below the middle confining unit II-a (LFA II-a; Figure 2); a similarly positioned zone will be used for the WP LFAWPF. The LFA II-a at the SE LFAWPF site occurs from 1,485 to 1,915 ft below land surface (ft bls). The transmissivity of the production zone for the SE LFAWPF obtained from pumping tests of two test production Continued on page 66 Florida Water Resources Journal • February 2022 65

Continued from page 65 wells was 3,810 and 15,300 ft2/d. These low- to moderate-transmissivity values are expected to result in large (> 100 ft) drawdowns in production wells.

The greatest technical and regulatory challenge for brackish groundwater desalination in central Florida is the disposal of generated concentrate. Concentrate disposal in southeastern and southwestern Florida is performed predominantly using Class I injection wells, which by definition inject below the deepest USDW. A USDW is defined as a nonexempt aquifer that contains less than 10,000 mg/L of total dissolved solids (TDS). Both the boulder zone in south Florida and the APHPZ in coastal west-central Florida are located below the base of the USDW. Sufficient confinement must be present at Class I injection well sites to prevent upward migration of the injected liquid wastes into an overlying USDW. The base of the USDW at the SE LFAWPF occurs near the top of the potential LFA injection zone (LFA II-b; Figure 1) and near the base of the injection zone at the WP LFAWPF. Although injection below the base of the USDW may be possible at the SE LFAWPF, upward migration into the overlying USDW would likely occur, which would be a regulatory violation.

The Florida Administrative Code (Section 62-520.500 [1]) allows for a water quality criteria exemption (WQCE), where injection or migration into a USDW would not adversely impact the environment or public health, and granting the exemption would be in the public interest, which is clearly the case for the PRWC projects. The brackish groundwater desalination and injection well systems will merely recirculate salts; the salts present in the upper LFA production zone would be removed by the RO process and be injected into a deeper zone. If upward migration did occur, the salts would be returned to the aquifer from which they were derived.

A WQCE would be required for some primary and secondary drinking water standards. Current FDEP policy does not allow WQCEs for primary drinking water standards (e.g., radionuclides). In the absence of a WQCE or other regulatory relief, concentrate disposal by injection into the basal LFA is not now feasible. The least expensive feasible alternative for the PRWC RO projects is injection into deeper Upper Cretaceous strata.

Upper Cretaceous injection wells are being successfully used at the Tampa Electric Company (TECO) Polk Power Station, located roughly 10 mi west of Fort Meade. The deeper UC injection wells are more expensive to construct and operate at high pressures, but are expected to have higher capacities than LFA injection wells (2 mgd versus 1 mgd) based on operational data from the TECO wells, which would at least partially offset their additional construction costs. Other concentrate disposal options, such as piping the concentrate to a location where the boulder zone is present or there is zero liquid discharge, were found to not be economically viable at the PRWC RO projects.

Key issues for brackish groundwater desalination are predicting future raw water salinity and ionic composition. A number of brackish groundwater desalination plants in Florida experienced much more rapid increases in salinity than expected at the time of their design, which has impacted the performance of treatment systems and necessitated the installation of new production wells in order to reduce the average salinity of the raw water.

Unexpected hydrogeologic conditions were encountered in the wellfields that allowed for greater upconing of more saline waters. To reduce risks associated with unexpected raw water salinity increases at the PRWC facilities, density-dependent solute-transport modeling

was performed using the SEAWAT (a publicdomain computer program) code in which the impacts of a wide variety of plausible adverse hydrogeological conditions were simulated. The model results were used to evaluate worstcase scenarios and guide the design of a robust treatment system (Figure 3).

Brackish groundwater present in southern and west-central Florida is sodium chloride water in which the dissolved solids were ultimately derived from seawater. The major ion concentrations are roughly in proportion to their values in modern seawater. Some changes in calcium, magnesium, and bicarbonate (alkalinity) concentrations may have occurred due to fluid-rock interactions. On the contrary, the LFA groundwater at the SE LFAWPF and WP LFAWPF sites is calcium sulfatetype water in which the dissolved solids were derived mainly from the dissolution of calcium sulfate minerals (gypsum and anhydrite) in the formation (Figure 4).

Geochemical modeling results indicate that the production zone water at the SE LFAWPF and WP LFAWPF sites is close to or at gypsum saturation and that the concentrate at a 75 or 80 percent RO recovery will be significantly supersaturated. The gypsumsupersaturated concentrate would result in the injection wells being susceptible to chemical clogging by gypsum precipitation, which would be very difficult to remediate. The concentrate will therefore require pretreatment via a precipitation process to reduce its calcium and sulfate concentrations. A large amount of gypsum will be generated, which is being investigated as a marketable commodity that could offset the cost of the additional pretreatment.

Peace Creek and Peace River Surface Water Alternative Water Supplies

Florida’s pronounced seasonality in precipitation results in a large intra-annual variability in surface water availability, which is out of sync with the seasonal variation in demand. Surface water availability in Polk County from the Peace Creek and Peace River is now dictated by MFLs. The amount of additional permittable surface water withdrawals varies greatly within and between years; hence, storage is required for a reliable year-round water supply from surface water systems, such as the large reservoir systems constructed by the Peace River Manasota Regional Water Supply Authority (PRMRWSA) and Tampa Bay Water.

Figure 3. Southeast Polk SEAWAT-predicted increases in salinity over time.

Figure 4. Piper diagram of raw water for the Lower Florida aquifer for Polk Regional Water Cooperative reverse osmosis plants and modern seawater.

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The PRWC is investigating three options for AWS using surface water. One option is a conventional surface water treatment system involving withdrawals from the Peace River near Fort Meade, storage in a reservoir, treatment to drinking water standards, and aquifer storage and recovery, similar to the PRMWSA facility located further downstream on the Peace River in DeSoto County. The other two options are recharge of the UFA, with seasonally available water from the Peace Creek (Clear Springs site shown in Figure 5) and the Peace River near Fort Meade. The concept is to use UFA recharge during the wet season to offset impacts to MFL lakes from additional groundwater withdrawals elsewhere using either existing or newly installed wells. The Clear Springs AR project is planned to have a surface water withdrawal capacity of 40 mgd and an average annual recharge rate of 6.8 mgd.

The UFA aquifer recharge wells are categorized as Class V injection wells; injected waters are required to meet Florida drinking water standards at the wellhead. The main parameters of concern with respect to the injection of surface waters are coliform bacteria and other pathogens. There is now abundant data, including from Florida-specific studies (John et al., 2005; John and Rose, 2005; Lisle, 2016), that pathogens do not survive long in the groundwater environment. Natural attenuation processes can be relied upon for the effective removal of pathogens; nevertheless, Florida’s current regulations and FDEP policies do not consider natural contaminant attenuation processes, nor allow for a zone of discharge (ZOD), which is an attenuation zone where the compliance point for water quality standards is at the edge of the ZOD, rather than at the wellhead.

The requirement that water quality standards be met at the wellhead necessitates pretreatment (particularly disinfection) prior to recharge. For the PRWC surface water AR projects, pretreatment options included in the conceptual and preliminary design are wetlands treatment (for the Peace Creek project), followed by slow sand filtration and then disinfection (chloramination).

A key hydrologic issue with the AR projects is converting the wet season recharge into additional fresh groundwater withdrawals during the subsequent dry season. The preference would be to receive a one-to-one credit for the recovery of recharged water; however, the hydrologic reality is that recharge at one geographic location in the UFA will not completely offset the impacts on water levels in MFL lakes from pumping at another location and at another time. Groundwater modeling is therefore a key part of the development of AR schemes in order to identify optimal strategies that allow for the maximum additional groundwater withdrawals.

Other Central Florida Alternative Water Supply Options

Reuse of reclaimed water is a critical AWS that is already being implemented to a large degree in central Florida for nonpotable uses. Reclaimed water is either sent to reuse customers or is used for aquifer recharge using rapid infiltration basins (RIBs). Water Conserv II, developed by Orange County and the City of Orlando, is the largest wastewater reuse project of its kind in the world, combining agricultural irrigation with aquifer recharge using RIBs. The greatest impediment to potable reuse projects has been public acceptance (i.e., the “yuck factor”) but the tide of public opinion on this issue is turning and projects are now under consideration in Florida that would have been unthinkable two or three decades ago. For example, Polk County in 2021 approved a $1.5 million pilot study to investigate whether wastewater can be treated to potable standards at its new Cherry Hill Water Production Facility.

Where reuse of reclaimed water is already high, potable reuse may provide lesser overall water resource benefits if less reclaimed water becomes available for nonpotable uses (forcing users to pump more fresh groundwater) and for aquifer recharge. Some water reclamation facilities in Polk County (e.g., Southwest Regional Wastewater Treatment Facility) already recycle their entire treated wastewater flow (Force, 2020). Potable reuse is not part of the current suite of PRWC projects, but will likely eventually be adopted throughout Florida as public acceptance increases and the technology proves to be less expensive than other options for AWS.

Conclusions

Population and water-demand projections clearly indicate that central Florida will have to develop AWS to meet future needs. The PRWC experience has been that all options for AWS in the region are challenging and will be significantly more expensive than the current