The Florida Everglades, a unique aquatic ecosystem that supports a diversity of habitats and 67 threatened and endangered species, has been greatly altered over the past century by water management to support agricultural and urban development. The Comprehensive Everglades Restoration Plan (CERP), launched in 2000, is a joint effort led by the state and federal government to reverse the decline of the ecosystem. Increasing water storage is a critical component of the restoration, and the CERP included projects that would drill over 330 aquifer storage and recovery (ASR) wells to store up to 1.7 billion gallons per day in porous and permeable units in the Floridan aquifer system during wet periods for recovery during seasonal or longer-term dry periods.
To address uncertainties regarding regional effects of large-scale ASR implementation in the Everglades, the U.S. Army Corps of Engineers (USACE) and the South Florida Water Management District conducted an 11-year ASR Regional Study, focusing on the hydrogeology of the Floridan aquifer system, water quality changes during aquifer storage, possible ecological risks posed by recovered water, and the regional capacity for ASR implementation. At the request of the USACE, the National Research Council’s Water Science and Technology Board convened a committee of experts to review the ASR Regional Study Technical Data Report (TDR) and assess progress reducing uncertainties related to full-scale CERP ASR implementation (see Box S-1). The committee was not asked to provide recommendations on the appropriate role of ASR in the CERP.
The ASR Regional Study, which included literature syntheses, laboratory testing, and field-scale experimentation primarily at two pilot ASR sites, represents a significant advancement in the understanding of large-scale ASR implementation in South Florida. The Regional Study improved the understanding of the hydrogeology of the Floridan aquifer system in southern Florida and used this information to develop a regional groundwater model. Through modeling simulations, the study concluded that the number of ASR wells that can be operated without significant regional effects on well pressure and groundwater heads is much lower (approximately 131 wells) than originally envisioned in the CERP. The study also examined subsurface geochemical changes and associated mechanisms that are likely to occur through water-rock interactions. The Regional Study TDR reported reductions in mercury and phosphorus in recovered water and initially elevated arsenic levels that become substantially reduced with additional cycles and storage. The Regional Study also examined pressures that could cause hydraulic fracturing and ranked components of ecological risk.
Statement of Task
The National Research Council was asked to convene a committee to review and evaluate the scientific methods, principles, and data that form the basis for the U.S. Army Corps of Engineers' ASR Regional Study Technical Data Report. The TDR is divided generally into the following four focus areas: (1) regional hydrogeological framework, (2) water quality changes during cycle testing, (3) groundwater flow simulations, and (4) ecotoxicology. The committee was tasked to assess progress toward the ASR Regional Study’s stated goals “to reduce uncertainties related to full-scale CERP ASR implementation by conducting studies based on existing and newly acquired data, develop a regional groundwater model of the Floridan Aquifer System [FAS], and identify an appropriate magnitude of ASR capacity with minimal impact to the environment and existing users of the FAS.” The committee was specifically tasked to review and comment on the following aspects:
- Validity of the data collection and interpretation methods,
- Integration of studies comprising the four focus areas mentioned above,
- Evaluation of scaling from pilot- to regional-scale application of ASR, and
- The adequacy and reliability of the study as a basis for future applications of ASR.
The committee also was asked to assess the scientific assumptions and logic on which the report’s conclusions are based.
The committee agrees with the Regional Study findings that no “fatal flaws” have been discovered, but many uncertainties remain (see Chapter 2) that merit additional study before large-scale ASR should be implemented. The following represent the highest-priority uncertainties, considering their implications to CERP decision making.
- Operations to Maximize Recovery and Reduce Water Quality Impacts. More research is needed to assess improvements in recovery efficiency and recovered water quality by establishing a freshwater buffer zone and maintaining it throughout subsequent cycle testing (termed a target storage volume approach). This approach could have major implications for the ecotoxicity of the recovered water if the proportion of native groundwater is substantially reduced. However, a larger buffer zone could create an expanded zone of near-term arsenic mobilization that is anticipated to attenuate over time. The use of well pairs or clusters should also be examined to improve recovery efficiencies and performance.
- Ecotoxicology and Ecological Risk Assessment. Some of the largest uncertainties remaining after the ASR Regional Study are associated with the ecological risks of ASR in the Everglades. The results of chronic toxicity testing and regional water quality modeling suggest some cause for concern and a need for further analysis considering longer storage times and greater recharge volumes, use of a target storage volume approach to improve recovered water quality, and more ASR sites. Ecotoxicological testing should be
designed in light of the fact that water from ASR operations will primarily be recovered during dry, low-flow conditions. Research should also examine the impacts of calcium and hardness on soft-water areas of the Everglades. The ecological risk assessment should be probabilistic in nature and can be improved using advancements in quantitative methods drawn from other successful regional-scale assessments.
- Understanding Phosphorus Reduction Potential. Removal of phosphorus represents a key unexplored benefit of ASR, and more research is needed to examine the long-term rates and extents of subsurface phosphorus removal under various aquifer conditions.
- Disinfection. Disinfection permitting requirements were not uniformly achieved during the pilot studies due to high organic matter in the recharge water. Additional work is needed to develop appropriate pretreatment strategies without hindering subsurface biogeochemical processes that attenuate dissolved arsenic. Research on pathogen survival in groundwater has demonstrated inactivation in flow-through chambers at varying rates. However, substantial additional research is needed on a wider suite of pathogens under groundwater conditions, and this information needs to be coupled with an understanding of groundwater travel times and the locations of potential human exposures to determine the level of disinfection necessary to protect human health.
- Cost and Performance of ASR Compared to Alternatives. Decision makers are unlikely to support continued research on ASR without clear documentation of the potential benefits of ASR relative to other possible water storage alternatives. Thus, a comparative cost-benefit assessment for water storage alternatives, including integrated operation of ASR wells and surface storage reservoirs, is an important next step. Benefits should be assessed in terms of new water delivered to the Everglades, flood flow prevention, or water quality improvements. Such an analysis should document performance uncertainties, which may help prioritize research to inform future decision making.
These high-priority uncertainties can be resolved through research at a range of scales, from computer modeling and laboratory testing to continued pilot testing with expanded ecotoxicological testing to expansion of the current pilot sites. Although current uncertainties are too great to justify near-term implementation of ASR at a large scale in the Everglades, opportunities exist to target future phased implementation of ASR in a way that addresses critical uncertainties while providing some early restoration benefits. Until the uncertainties related to ecological effects are substantially resolved, any new ASR wells to be drilled should be sited adjacent to large water bodies with adequate mixing zones to minimize adverse ecological impacts.