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3

Water Quality Issues

Motivation

Although aquifer storage and recovery (ASR) has been used as a means of supplementing water supplies in Florida for over a decade, the unprecedented scale of the ASR program proposed in the CERP raises a wide range of water quality issues that go beyond the concerns associated with local ASR projects. In part, these issues arise because of the chemical diversity of the water bodies and aquifers involved in ASR across the large geographic range of the CERP. The issues also arise because ASR water may be used for agriculture, for augmenting water inflows to natural ecosystems in the Everglades, and, indirectly, to supplement municipal (drinking water) supplies. Different regulations and different concerns about water quality arise in connection with these different intended uses.

In practical terms, answers to a range of water quality questions are critical to the regulatory process for permitting the ASR wells and the release of recovered water to Lake Okeechobee and various canals. The ASR pilot program thus has placed an appropriately heavy emphasis on collecting an extensive array of water quality measurements. However, chemical analyses on water recharged to and recovered from pilot wells cannot answer many questions about potential biological impacts of ASR water on the Everglades ecosystem. Such analyses by themselves also cannot provide the mechanistic information needed to develop geochemical models to predict how water chemistry/water quality will change in a full-scale ASR program. Consequently, the Committee on Restoration of the Greater Everglades Ecosystem (CROGEE) concludes that the water quality measurements proposed for the ASR pilot program should be supplemented by laboratory and field-scale investigations.

Issues Discussed

Workshop discussions related to water quality issues included the following topics. The list is not intended to suggest an order of importance, nor does it reflect the amounts of time spent discussing topics during the formal session on water quality. Some topics on the list represent an integration of several related themes or issues raised during the workshop.

    1. Adequacy of existing regulatory standards for water quality to assure that the extensive use of ASR in the Everglades restoration program has minimal negative impacts on ecosystems.

    2. The need for ecotoxicological studies and bioassays at laboratory and field scales to elucidate potential impacts of water quality changes including not only concentrations of nutrients and trace metals, but also concentrations of sulfate and chloride and properties such as pH and alkalinity.

    3. Need for and nature of studies on microbial (pathogen) survival to be conducted during the pilot projects. Particular concern was expressed about use of coliform bacteria to represent the pathogens present in surface water sources used for aquifer recharge.



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Page 15 3 Water Quality Issues Motivation Although aquifer storage and recovery (ASR) has been used as a means of supplementing water supplies in Florida for over a decade, the unprecedented scale of the ASR program proposed in the CERP raises a wide range of water quality issues that go beyond the concerns associated with local ASR projects. In part, these issues arise because of the chemical diversity of the water bodies and aquifers involved in ASR across the large geographic range of the CERP. The issues also arise because ASR water may be used for agriculture, for augmenting water inflows to natural ecosystems in the Everglades, and, indirectly, to supplement municipal (drinking water) supplies. Different regulations and different concerns about water quality arise in connection with these different intended uses. In practical terms, answers to a range of water quality questions are critical to the regulatory process for permitting the ASR wells and the release of recovered water to Lake Okeechobee and various canals. The ASR pilot program thus has placed an appropriately heavy emphasis on collecting an extensive array of water quality measurements. However, chemical analyses on water recharged to and recovered from pilot wells cannot answer many questions about potential biological impacts of ASR water on the Everglades ecosystem. Such analyses by themselves also cannot provide the mechanistic information needed to develop geochemical models to predict how water chemistry/water quality will change in a full-scale ASR program. Consequently, the Committee on Restoration of the Greater Everglades Ecosystem (CROGEE) concludes that the water quality measurements proposed for the ASR pilot program should be supplemented by laboratory and field-scale investigations. Issues Discussed Workshop discussions related to water quality issues included the following topics. The list is not intended to suggest an order of importance, nor does it reflect the amounts of time spent discussing topics during the formal session on water quality. Some topics on the list represent an integration of several related themes or issues raised during the workshop. 1. Adequacy of existing regulatory standards for water quality to assure that the extensive use of ASR in the Everglades restoration program has minimal negative impacts on ecosystems. 2. The need for ecotoxicological studies and bioassays at laboratory and field scales to elucidate potential impacts of water quality changes including not only concentrations of nutrients and trace metals, but also concentrations of sulfate and chloride and properties such as pH and alkalinity. 3. Need for and nature of studies on microbial (pathogen) survival to be conducted during the pilot projects. Particular concern was expressed about use of coliform bacteria to represent the pathogens present in surface water sources used for aquifer recharge.

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Page 16 4. Effects of dissolved and particulate organic matter in recharge water on biogeochemical processes in the subsurface environment and the potentially adverse effects of these processes on ASR system functioning. 5. Rates of mineral dissolution reactions in the aquifer storage zone that could release heavy metals, arsenic, radionuclides, and major ions from the geological matrix into water that is to be recovered for use as drinking water or to supplement flows to the Everglades. 6. Mechanisms of mixing of relatively dilute recharge water with more saline pore fluids in the storage zone, which will affect the extent to which water quality is changed during the aquifer storage and recovery process. Conclusions and Recommendations The CROGEE concludes that ASR water used in the Everglades restoration will probably need to achieve concentrations for some variables that are lower than existing Florida numerical water quality criteria for drinking water. Likewise, Florida's existing Class III water quality criteria (i.e., criteria for protection of fish and wildlife) may not be sufficient to ensure that use of ASR water does not have negative ecological impacts. This is partly due to the great diversity in chemical composition among surface waters within South Florida. For example, surface waters in the southern part of the Everglades have low concentrations of ions, whereas waters in Lake Okeechobee and waters just to the south of the lake have relatively high ionic content. The species composition of plant life growing in these waters reflects the chemical composition of the water. Specifically, native flora of the soft water in the southern Everglades are adapted to those conditions. If the use of recovered ASR water during periods of low rainfall increases the ionic strength of Everglades water (see Mirecki et al., 1998, for example), the composition of the plant community may change significantly even though the water may meet all existing criteria. The likelihood of such changes cannot be determined merely by conducting chemical analyses as part of the pilot program nor even by short-term laboratory-scale bioassays. The CROGEE recommends that ecotoxicological studies, including long-term bioassays conducted at the field scale, be undertaken during the period of the pilot program to evaluate the ecological impacts of water quality changes resulting from broad use of ASR water to supplement inflows to the Everglades during droughts. Water recharged to ASR wells will come primarily from Lake Okeechobee and other impoundments. The source waters and surrounding lands can and should be managed to minimize the occurrence of pathogens in the water being pumped down the wells. However, because these surface water bodies may contain viruses and bacteria that are not naturally present in the aquifer system, the CROGEE recognizes that pathogen die-off studies need to be performed as part of the permitting process. It recommends that those studies be done under conditions as realistic of the full-scale ASR process as possible. For example, aquifer materials and their resident microbial communities can be returned to the laboratory for experimentation. In instances in which it is impossible to obtain core segments, drill cuttings can be substituted, although this may result in a substantial loss in the quantitative value of the results. In particular, die-off studies should focus on the pathogenic species of concern, especially viruses, rather than on model microorganisms, whose survival rates may not necessarily mimic those of actual pathogenic species. The source water for recharge will be relatively high in dissolved organic matter, and, depending on the degree of filtration, may contain some particulate carbon as well. The biodegradation of this material within the aquifer could have major impacts on aquifer chemistry in the

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Page 17 recharge zone. Increased oxygen demand imposed by this organic matter could stimulate nitrate and sulfate reduction, generating low redox potentials and high sulfide levels. In turn, this could stimulate methylation of mercury and reductive dissolution of mineral phases, possibly increasing the dissolved concentrations of heavy metals, arsenic and radionuclides in the water being stored for recovery. Although the alkalinity of the source water is likely to be moderately high, and the waters thus should be well-buffered with respect to pH, production of carbon dioxide from the biodegradation of organic matter could lower the pH of the stored water. This could affect a variety of chemical processes and promote heavy metal dissolution. Microbial growth stimulated by the dissolved organic matter could form biofilms on mineral phases in the aquifer. While this process could protect some phases from dissolution, biofilms could possibly accelerate other dissolution reactions. Thus, the characterization of organic carbon in the source water should be a priority, as should studies designed to anticipate the effects of this material on biogeochemical processes in the subsurface. In addition, the effects of chlorination as a proposed pre-treatment process for recharge water in the Western Hillsboro ASR need to be evaluated carefully with respect to potential toxic halogenated organic compounds (e.g., Thomas et al., 2000; Landmeyer et al., 2000). The usefulness of other treatment options, such as ultraviolet radiation, should be studied during the pilot project. The potential effects of these changes on the chemical quality of recovered water need to be examined during the pilot study. While routine water quality monitoring conducted as part of the pilot well studies will provide important information, such studies are not sufficient to answer questions about the mechanisms that cause observed changes in water quality. Consequently, monitoring alone will not provide the required basis to develop predictive models of important redox and dissolution/precipitation reactions under full-scale, long-term implementation of ASR. The CROGEE thus recommends that additional laboratory experiments and chemical modeling be undertaken during the pilot phase to address these issues in a scientifically defensible way. Process studies can be conducted under controlled conditions if aquifer materials and their resident microbial communities are returned to the laboratory for experimentation. Incremental core segments can be used to examine the rates of dissolution and precipitation, biological oxidation and reduction reactions and the movement and reaction of the products to determine both rates and extent of kinetically controlled phenomena. As with pathogen studies, core segments are superior to drill cuttings for this work, because the drilling process destroys the rock fabric and creates uncertainty with respect to the depth of origin of a given cutting. Finally, a better understanding of the mechanisms responsible for mixing relatively dilute recharged water with more saline pore fluids in the storage zone is essential for anticipating changes in dissolved solids during ASR. The transport and mixing processes in the Upper Floridian aquifer may not conform to the “equivalent porous medium” model commonly used in groundwater flow models. Elucidating these processes should be an objective of the pilot studies (see chapter 4 ).