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Page 18 4 Local Performance/FeasibilityIssues Motivation The feasibility of injecting, storing, and recovering specified volumes of water at individual ASR wells and in local clusters of ASR wells is the issue to which the authorized pilot projects at Lake Okeechobee and Western Hillsboro are most specifically directed. The draft project management plans that were available for review prior to the Committee on Restoration of the Greater Everglades Ecosystem (CROGEE) workshop indicated that the general locations of exploratory wells that will be drilled as part of these pilot studies have already been selected. These exploratory wells ultimately will be converted to pilot ASR wells for cycle testing. As described in the draft project management plans, the tests at these wells can be expected to provide reliable information on feasibility at the specific locations selected for testing. However, in keeping with the principle of adaptive assessment, the CROGEE judges that the pilot projects should not be considered simply as demonstrations at particular sites. Instead, they should be viewed as an opportunity to develop a better understanding of the hydrogeologic and well construction characteristics that control the relationships between storage intervals, recharge volumes, and recoverability. The discussion during this period of the workshop was intended to highlight design considerations for the pilot projects that could maximize the value of the data obtained for improving understanding of ASR performance at both the specific sites tested and more generally in the Upper Floridan aquifer (UFA). The better understanding that would result from carefully designed pilot studies then could be translated into improved siting and design of additional ASR wells both in the immediate vicinity of the pilot studies and elsewhere in the UFA. Issues Discussed Workshop discussions included questions, comments and responses related to the items listed below. 1. Selection criteria used to determine locations for proposed exploratory wells. 2. Selection criteria to be used for selection of target recharge intervals. 3. Advantages and disadvantages of using short or long open intervals in ASR wells. Short intervals may simplify aquifer management and minimize in-well circulation, but may limit recharge rates. 4. Design of monitoring wells to allow monitoring of “bubble” evolution, to evaluate potential leakage across confining units, and to assess well interference effects for pilot clusters. 5. Availability of models to simulate site-scale processes for comparison with monitoring data collected during cycle testing. 6. Considerations of trade-offs between rates of recharge and recoverability.
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Page 19 7. Quantification of efficiency of recovery, recoverability, or other measures of “success,” particularly when considering water quality standards of the surface water bodies receiving recovered water. 8. Time scales of cycle testing and how these relate to anticipated operational cycles in recharge and recovery. 9. Use of data that may be available from studies of existing ASR systems to address questions related to feasibility. 10. Whether the pilot studies alone or in conjunction with other studies will provide sufficient information to demonstrate feasibility or infeasibility of a regional scale ASR system. Conclusions and Recommendations Based on the discussions at the workshop and also considering written material submitted prior to and after the workshop by the Project Development Team and others, the CROGEE formulated the following recommendations. Test conditions In order to maximize transferability of pilot project results to design and implementation of regional scale ASR systems, the pilot projects should be designed as tests to elucidate processes of “bubble” formation and migration under a variety of conditions. Some of these conditions include long and short open intervals of recharge wells, high and low injection pressures, homogeneous and heterogeneous aquifer properties within the storage zone, presence or absence of highly transmissive beds within the storage zone, greater and lesser contrasts in salinity between injected and native water, and long and short periods of storage prior to recovery. Tests designed to compare effects of short and long open borehole sections in the ASR wells would be particularly useful to improving understanding of factors important to ASR well design at the selected pilot facilities and, more generally, throughout the UFA and in other aquifers with significant vertical variations in hydraulic properties. One possible way to conduct such tests within the constraints of a limited total number of pilot wells would be to construct an ASR well with multiple open sections for testing individually or in combination. Monitoring wells An important component of any test design is the monitoring strategy. Adequate monitoring is essential to obtaining data that can be used to develop conceptual models of bubble development and to test and calibrate numerical models of the relevant processes. Multiple monitoring wells at each pilot ASR site should be a high priority to delineate the geometry and heterogeneity of fresh water migration during recharge. The shape and heterogeneity of the fresh water bubble may ultimately dictate the extent of mixing between injected and ambient pore water during storage. This mixing is one of the important factors in limiting efficiency or recoverability. Multiple monitoring wells also are required to obtain better estimates of magnitudes, spatial variability and possible anisotropy of hydraulic properties of aquifers and confining units. Nested monitoring wells, or monitoring wells that can be packed off and sampled from discrete intervals, should be constructed for purposes of delineating potential preferential flow paths from the ASR wells and for quantifying mixing between relatively dilute recharged water and more saline ambient pore water in the storage zone. The draft project management plans suggest that only a single UFA monitoring well and a single surficial aquifer well are anticipated in the conceptual designs for ASR pilot facilities. The CROGEE recognizes that significant costs would be associated with additional monitoring beyond those included in the initial plans. In case of budget constraints, the CROGEE concludes that it
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Page 20 would be preferable to do more detailed studies, with enhanced monitoring, at a reduced number of pilot ASR sites or with a reduced number of exploratory ASR wells, rather than to do more limited studies using the currently planned number of exploratory wells but with too few monitoring wells. Cycle duration During the workshop discussion it was noted that data are available from a number of existing ASR systems that have operated for over a decade. These existing data can be quite useful in design of the pilot projects and in anticipating the hydraulic performance of ASR wells in general. However, because existing ASR systems that are used for water supply management generally operate on relatively short cycle periods, it may be difficult to use results from those systems to predict the degree of mixing and water-rock interactions during longer-term recharge and storage. Such mixing and water quality changes due to water-rock interactions may be the limiting factors in efficiency or recoverability of the regional scale ASRs proposed as part of the CERP. Design of cycle testing of the pilot wells must take into account the anticipated duration of recharge and recovery cycles that will be employed during the final operation. The hydrologic models used for design of the CERP indicate that continuous recharge is likely to be required in the Lake Okeechobee regional ASR for multi-year periods. This suggests that continued recharge for periods on the order of a year or more prior to recovery should be employed for the Lake Okeechobee pilot ASR cycle testing in order to obtain data that will be relevant to operational performance. Interpretation of results Observations obtained from a suitable network of monitoring wells can provide the basis for development of new or improved conceptual and numerical models to simulate “bubble” development and migration. Additional data may also be available from monitoring of other existing ASR systems, and this information would also be useful in developing and testing such models. Numerical codes used to test conceptual models and to simulate bubble migration may need to account for three-dimensional, density dependent flow and solute transport in multi-permeability media containing fractures and solution conduits. The CROGEE is not aware of any existing codes that have the full capability to accommodate these aquifer characteristics. Thus, construction of models for these local scale processes is likely to require substantial technical effort with a commensurate level of funding. Additional considerations While the pilot projects are likely to be most useful in assessing local scale feasibility issues, they can also contribute data to the assessment of the regional hydrogeologic framework and aquifer characteristics. Use of data collected from the exploratory wells as part of a regional scale aquifer characterization is anticipated in the draft project management plans that were available prior to the workshop. The CROGEE encourages such use of data collected at the pilot sites, noting that careful coordination between the pilot studies and the regional study will maximize the value of these data to understanding the system at a variety of scales. Fracture potential studies, which may be limited to desk-top analyses or expanded to more rigorous modeling approaches, are described briefly in the draft project management plans. The CROGEE recognizes the importance of such studies and commends the project management team for including them in the plans. The pilot studies should provide valuable additional information on the potential for fracture development due to a single ASR well or cluster. Results of fracture potential studies conducted as part of the pilot projects should be combined with results of the regional hydrogeologic models in order to assess the potential for fracturing during full-scale ASR operation.
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