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Regional Science Issues

Motivation

The Upper Floridan aquifer (UFA) extends across all of southern Florida and it is likely that ASR can be accomplished in this aquifer at some scale nearly anywhere in South Florida. However, assessing the impacts of ASR at the scale proposed in the CERP – 1.7 billion gpd (6.3 million m3/day) – requires a regional, fairly detailed characterization of the hydrogeologic framework. The hydrogeologic framework information would form the basis for development of a numerical model capable of simulating the cumulative hydraulic effects of the proposed total number of ASR wells and their combined recharge and withdrawal rates. Without such a numerical simulation, the aggregate hydraulic impact of ASR wells on the existing UFA flow system cannot be quantified, and the feasibility of ASR at the scale proposed cannot be assessed reliably. The proposed ASR scale of 1.7 billion gpd (6.3 million m3/day) is to be concentrated in a small area compared with the current total withdrawal of about 3 billion gpd (11 million m3/day) that is pumped from the UFA throughout its areal extent of all of Florida, and parts of Alabama, Georgia, and South Carolina (Johnston and Bush, 1988). The project delivery team should be commended for recognizing the need for a regional study and for planning to pursue funding beyond the constraints of the currently funded pilot projects.

Issues Discussed

Workshop discussions included the items briefly listed below and expanded upon in the paragraphs that follow.

    1. Identification of the information needed for adequately characterizing the hydrogeologic framework.

    2. Compilation of existing information from reports on the hydrogeology of the UFA and from file data on existing wells and boreholes.

    3. Assessment of how well the identified information needs are met by existing data and development of a test drilling program to fill in data gaps.

    4. Coordination of surface geophysical surveys to augment drill-hole data.

    5. Development of a conceptual model of the hydrogeology as a basis for construction of a numerical flow model.

    6. Construction of an appropriate numerical flow model.

    7. Application of the numerical model to predict head changes and provide the basis for assessing overall feasibility of proposed ASR scale and well deployment.



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Page 12 2 Regional Science Issues Motivation The Upper Floridan aquifer (UFA) extends across all of southern Florida and it is likely that ASR can be accomplished in this aquifer at some scale nearly anywhere in South Florida. However, assessing the impacts of ASR at the scale proposed in the CERP – 1.7 billion gpd (6.3 million m3/day) – requires a regional, fairly detailed characterization of the hydrogeologic framework. The hydrogeologic framework information would form the basis for development of a numerical model capable of simulating the cumulative hydraulic effects of the proposed total number of ASR wells and their combined recharge and withdrawal rates. Without such a numerical simulation, the aggregate hydraulic impact of ASR wells on the existing UFA flow system cannot be quantified, and the feasibility of ASR at the scale proposed cannot be assessed reliably. The proposed ASR scale of 1.7 billion gpd (6.3 million m3/day) is to be concentrated in a small area compared with the current total withdrawal of about 3 billion gpd (11 million m3/day) that is pumped from the UFA throughout its areal extent of all of Florida, and parts of Alabama, Georgia, and South Carolina (Johnston and Bush, 1988). The project delivery team should be commended for recognizing the need for a regional study and for planning to pursue funding beyond the constraints of the currently funded pilot projects. Issues Discussed Workshop discussions included the items briefly listed below and expanded upon in the paragraphs that follow. 1. Identification of the information needed for adequately characterizing the hydrogeologic framework. 2. Compilation of existing information from reports on the hydrogeology of the UFA and from file data on existing wells and boreholes. 3. Assessment of how well the identified information needs are met by existing data and development of a test drilling program to fill in data gaps. 4. Coordination of surface geophysical surveys to augment drill-hole data. 5. Development of a conceptual model of the hydrogeology as a basis for construction of a numerical flow model. 6. Construction of an appropriate numerical flow model. 7. Application of the numerical model to predict head changes and provide the basis for assessing overall feasibility of proposed ASR scale and well deployment.

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Page 13 Conclusions and Recommendations Based on the discussions at the workshop and also taking into account written material submitted prior to and after the workshop by the Project Development Team and others, the Committee on Restoration of the Greater Everglades Ecosystem (CROGEE) formulated the following recommendations. Development of a preliminary list of information needs and compilation of available data should be undertaken as soon as possible. This exercise can be done for relatively low cost and within a short time can reveal where existing data are insufficient to meet identified needs. Categories of information needs include, but are not limited to, vertical distribution of potential recharge zones and their lateral extent, hydraulic properties, ambient water quality, and degree of confinement. Information on location, depth, and use of existing wells completed in the Floridan aquifer system is needed to assess potential impacts of proposed ASR wells on existing uses. Most of the existing well information is clustered in population centers along the coast. Little information is thought to be available inland where most of the ASR installations are proposed to be located. A regional-scale three-dimensional numerical flow model will be required to assess impacts of proposed recharge and recovery volumes and rates on the UFA and adjacent hydrostratigraphic units. The model may need to account for variable density flow since the lower part of the UFA contains water that is somewhat saline, with several thousand or more mg/l of dissolved solids. Existing model codes based on an “equivalent porous medium” approach, such as HST3D (Kipp, 1997), are appropriate at a regional scale for estimation of head changes (Anderson and Woessner, 1992). Telescoped models of sub-regions, extracted from the regional model, also may be useful in assessing pressure buildup or drawdown at an intermediate scale. This regional and sub-regional modeling effort should be considered distinct from more detailed modeling that may be required to assess local scale feasibility questions related to recharge “bubble” growth and migration. Because of the solution conduit and fracture flow nature of the UFA, conventional models that employ an equivalent porous medium approach are likely to be poor simulators of solute transport in the vicinity of recharge wells (Long and Billeaux, 1987; Cacas et al., 1990). Development of suitable models for bubble growth and migration is considered in Chapter 4 of this report. Development of the regional scale numerical flow model should proceed in parallel with initial conceptualization of the hydrogeologic framework. Sensitivity studies employing preliminary versions of the model can be used to identify data needs and gaps and thus help guide the planning of the test-drilling program. The regional study should have a budget adequate for the necessary drilling, core sampling, downhole geophysical logging, hydraulic testing and water quality sampling of these test wells. The tests and data collection should include all items planned for the exploratory ASR wells as specified in the Project Management Plan for the Lake Okeechobee Aquifer Storage and Recovery Pilot Project. The CROGEE concurs with existing plans to instrument observation wells with pressure recorders early in the regional study to obtain data with which to calibrate the numerical model. Surface geophysical techniques, such as seismic reflection surveys, should be a component of the regional study. Such surveys can help to constrain the three-dimensional geometry and continuity of hydrostratigraphic units (Kindinger et al., 1999), especially when used in conjunction with results from a relatively limited number of new exploratory wells. The surveys would provide a means for extrapolating detailed information collected at test well sites across broad areas where well control is sparse, thereby improving the quality of input to the regional-scale flow model. Surveys that use land-based techniques, as well as those that employ marine techniques (across Lake Okeechobee or along canals), should be considered. Without the geophysical surveys, a much more extensive and more costly drilling program may be required to adequately characterize the hydrogeology. Once constructed, the regional flow model should be used to assess potential impacts related to full-scale implementation of ASR. Aggregate head buildup or drawdown estimated from the

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Page 14 model can be used to predict changes in direction and velocity of flow in the UFA, and to evaluate the possible consequences of these changes on existing wells (see Bradbury and Krohelski, 1999 for a similar approach). The aggregate head buildup is needed also to predict areas where more detailed studies of the potential for hydraulic fracturing (see chapter 4) should be done. An elevated potentiometric surface that could induce fracturing and upward migration of recharged water could result from the superposition of head buildups of multiple ASR wells. The regional or telescoped models of sub-regions can be used to compare and evaluate alternative ASR system designs. For example, regional effects of an ASR system that is concentrated in a small geographical area, such as the planned deployment of 100 ASR wells along the northern half of Lake Okeechobee, could be compared to a system of similar volume that is more geographically dispersed. Various recharge and withdrawal scenarios can be easily simulated with the regional model, and the model results can be used to optimize the ASR system while minimizing deleterious impacts (e.g., using the approach described by Wagner, 1995). The regional study should include development of a formal procedure for the siting of ASR wells. For the few exploratory ASR wells included in the pilot project, preliminary siting was based solely on location of a water source for recharge and proximity of a water body for receipt and conveyance of drilling and testing fluids. However, for full implementation of the ASR proposal, well siting should include evaluation of other factors (e.g., anticipated well capacity, proximity to other users, etc.) as well. It is important that the site selection process be explicit, repeatable, and based on best available data and selection methods. The proposed Phase 1 siting analyses include simple overlay of GIS data to evaluate site suitability. This approach may be adequate for the local pilots, but a more formal, multi-criteria siting approach should be designed and tested during the pilot studies that would facilitate the much larger and more complex problem of siting many wells over the region. Promising approaches range from structured scoring methods such as Analytical Hierarchy Process and Knowledge Bases (Reynolds et al., 1999) to more complex optimization models using mathematical programming or heuristic search procedures (e.g. Haight et al., 2000; Matthews et al., 1998; Miller, 1996; Murray et al., 1998). In conclusion, the pilot projects provide a valuable means for acquiring detailed information on ASR performance at a few specific sites. However, even if all the sites tested prove successful, they will not by themselves demonstrate the feasibility of ASR implementation regionally at the scale of 1.7 billion gpd (6.3 million m3/day). A regional study involving construction of a regional flow model is an invaluable and indispensable tool to assess feasibility of ASR at the proposed scale.