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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 ).