The Aquifer Storage and Recovery (ASR) Regional Study resolved many uncertainties about large-scale application of ASR in South Florida and developed tools and an impressive knowledge base to support future ASR projects, if implemented, but the research identified several new questions, and some key uncertainties remain. This chapter presents the committee’s assessment of the highest-priority uncertainties that need to be resolved before large-scale implementation of ASR is considered. These uncertainties could be addressed across a range of scales, from continued operation and cycle testing of existing pilot project wells to an incremental adaptive management approach (NRC, 2007, 2008), where a few ASR well clusters are constructed and operated to provide some restoration benefits while building knowledge and resolving critical uncertainties to inform future project implementation.
Chapter 2 identified many remaining uncertainties. This section reflects the committee’s judgment on the highest-priority uncertainties, considering their implications to Comprehensive Everglades Restoration Plan decision making. These uncertainties are discussed in more detail in Chapter 2, but summarized here. The following are not listed in rank order but are organized by topic.
Operations to Maximize Recovery and Reduce Water Quality Impacts
As discussed in Chapter 2, the ASR pilot operations were appropriate to examine possible geochemical changes occurring in groundwater during storage and to limit the spatial extent of arsenic transport, but they were inappropriate to explore maximum well recoveries. Most or all of the injected water was removed without initial formation of a buffer zone, thereby lowering the recovery efficiencies compared with what could likely be achieved under a target storage volume approach. More research is needed at the Hillsboro site, where only 30 percent recovery was documented, to determine whether improvements in recovery can be achieved when a buffer zone is established prior to cycle testing and maintained throughout the subsequent cycle testing. The use of well clusters should also be examined to improve recovery efficiencies and performance. This approach will have major implications for the ecotoxicity of the recovered water, because the proportion of native groundwater will
be substantially reduced. However, a larger buffer zone could create an expanded zone of near-term arsenic mobilization that is anticipated to attenuate over time.
Ecotoxicology and Ecological Risk Assessment
Some of the largest uncertainties remaining after the ASR Regional Study are associated with the ecological risks of using recovered ASR water in the Everglades. The results of chronic toxicity testing to date suggest some cause for concern and a need for further analysis considering longer storage times and greater recharge volumes, as well as more sites. Current in situ bioconcentration tests and assessment of community composition are not adequate to draw conclusions or for informing a robust and probabilistic risk assessment. Lack of a fully developed buffer zone presents a key limitation in the interpretation of the existing results. Recovered water quality and the resulting ecotoxicological impacts are likely to be quite different under an ASR operational strategy that builds and maintains a buffer zone within the aquifer (a target storage volume approach). For mercury methylation in the Everglades, the impacts are likely to decrease because groundwater contributions of sulfate would be reduced. Toxicity and bioconcentration of arsenic and trace metals would likely differ with a target storage volume approach, and more study is needed on the water quality and ecotoxicological effects under these conditions, including rigorous bench-scale chronic toxicity tests and in situ testing over extended time periods. Planners could also consider ways to manage the risks of mercury methylation through sulfate concentration limits during ASR recovery.
Future ecotoxicological testing should be designed in light of the fact that water from ASR operations will primarily be recovered during dry, low-flow conditions. If toxicity or bioconcentration is determined, more work is needed to understand the underlying causes so that strategies can be developed to mitigate these effects, if feasible. Additionally, researchers should examine the availability of mixing zones under a range of hydrologic conditions at potential ASR sites to reduce near-field ecological effects. As additional toxicological uncertainties are evaluated, results should be incorporated into a modernized probabilistic regional assessment of relative risk of adverse effects to receptor populations. In addition, a formal sensitivity analysis will help guide the prioritization of what additional toxicological data should be generated to inform future 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. Laboratory experiments could be developed to better understand these processes, with subsequent field testing to examine phosphorus removal at larger scales. If ASR proves to substantially remove phosphorus over long-term operations, additional work is warranted to examine
whether ASR wells could also be sited south of Lake Okeechobee in addition to the originally planned sites north of Lake Okeechobee (see Figure 1-1). The benefits of near-term water storage and water quality improvements to the remnant Everglades ecosystem would need to be weighed against potential ecological risks at these sites.
Disinfection permitting requirements were not uniformly achieved during the pilot studies due to high organic matter in the recharge water. To meet regulatory requirements, additional work is needed to determine appropriate pretreatment strategies—ones that would not hinder subsurface arsenic attenuation processes, which are thought to be particularly dependent on organic carbon and iron in recharge water. Some have suggested that a regulatory exemption may be possible for ASR with respect to microbial contaminants in recharge water. Laboratory research on pathogen survival in groundwater has demonstrated inactivation in flow-through chambers at varying rates, but substantial additional research is needed on a wider suite of pathogens under groundwater conditions before such an exemption is considered. Also, this information needs to be coupled with an understanding of groundwater travel times and flow patterns and the locations of potential human exposure to determine the level of disinfection necessary to protect human health and meet regulatory requirements.
Cost and Performance of ASR Compared to Alternatives
The Regional Study examined an array of configurations to provide 1.7 billion gallons per day storage capacity for Everglades restoration, but the Technical Data Report lacks a comprehensive comparison of the costs and benefits of ASR technology with other storage alternatives. Regional modeling suggests that the number of 5-million-gallon-per-day wells feasible on existing state-owned land without exceeding well-pressure constraints is less than half of the number originally envisioned (131 versus 333 wells) and many of those have low assumed recoveries. The benefits provided by these wells are not clearly described in terms of flood prevention or water supplied during drought years, and the capacity for ASR to address the timing of storage and water supply demands remains poorly understood. Decision makers are unlikely to support continued research on ASR without clear documentation of the potential benefits of ASR relative to other possible alternatives. Thus, a comparative cost-benefit assessment for water storage alternatives 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, and opportunities to enhance these benefits through integrated operations of ASR wells with storage reservoirs should be examined. If a target storage volume approach is judged to be potentially beneficial, the analysis of costs and benefits should also consider the volume of water necessary to form and maintain the freshwater buffer zone. Cost and benefit analyses should
also document performance uncertainties, which may help prioritize research needs to inform future decision making.
The high-priority uncertainties can be resolved through research at a range of scales, from laboratory experiments and computer modeling to continued cycle testing at existing sites with expanded ecotoxicological testing to phased expansion of ASR. 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 and enhances future implementation. The National Research Council (NRC, 2007) advocated an “incremental adaptive restoration” (IAR) approach to expedite restoration in light of disagreements over scientific uncertainties that were holding back decisions on how to move forward. In contrast to pilot studies, which rarely provide restoration benefits, an IAR approach allows increments of proposed projects to move forward, providing tangible restoration benefits while resolving key uncertainties. Within an IAR strategy, decision-critical uncertainties that could be addressed by the project increment are clearly identified up front, so that subsequent implementation can be improved by knowledge gained. An IAR approach for ASR could involve one or more clusters of three to five ASR wells, perhaps including wells in both the Upper Floridan aquifer and the Avon Park permeable zone, to address critical uncertainties such as recovery efficiencies, performance, long-term water quality, and ecological effects. Given the existing uncertainties, these initial wells should be sited adjacent to large surface water mixing zones to reduce adverse ecological impacts. The downside of an IAR approach is that operations that aim to provide restoration benefits may come at the detriment of ideal experimental design, and, initially, resolving critical uncertainties may need to be the primary objective of near-term ASR testing. Additionally, several key uncertainties may be most cost-effectively addressed through continued cycle testing of existing wells. However, phased installation of ASR wells to resolve key uncertainties could provide earlier restoration benefits compared to continued operations of the single-well ASR pilots.