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Floridan Aquifer System Groundwater Availability Study

Approach

• Organize and compile geospatial data library and data compilation.

• Assemble a 3-D Hydrogeologic framework.

• Estimate components of the hydrologic budget. A spatially and temporally varying pre- and post-development water budget will be compiled for the Floridan Aquifer System. Spatially varying components of the water budget for different areas will be based on general areas of recharge and discharge, which is closely tied to the confinement of the Floridan Aquifer System, climate zones, and land use/cover. Water budgets will be estimated based on data compilation from previous studies as well as state, local, and federal agencies. Recharge estimates from precipitation and evapotranspiration across the Floridan Aquifer System will be refined using the most recent land use data combined with NEXRAD data and evapotranspiration maps from the evapotranspiration network for Florida. The groundwater model developed for the Floridan Aquifer System will also be used in the calculation of water budgets.
• Assess status and trends in groundwater storage. Pressure and water level data from the Floridan Aquifer System will be compiled from the different states, districts, counties, municipalities and federal agencies. Data will be organized based on depth of casing or open-hole interval, unit being monitored, length of historical record, and overall quality of data. Historical trends in water levels will be compared to changes in precipitation, climate, land use, and groundwater withdrawals to assess changes in groundwater storage. A potentiometric map for the Floridan Aquifer System is planned for FY10, and this map will be compared to the predevelopment map and the 1980 potentiometric map from the previous RASA study. The data will be used to calibrate the regional numerical model and all data to be used in the assessment will be archived and added to NWIS. In the previous RASA study it was concluded that stressed steady-state model runs were adequate for evaluation of pre and post development conditions. However, if climate change is to be evaluated, we conclude that transient calibration is probably required. The evaluation of the trends in historical groundwater level, pumpage, and storage will be required to determine what length of stress periods are reasonable. Thus this task must be accomplished early in the project.
• Develop conceptual model of groundwater system. Numerous studies have been done of the Floridan Aquifer System since the last RASA study. For example, numerical groundwater models have been developed of different areas such as in central Florida (Sepulveda, 2002), Georgia (Provost et al. 2006), and south Florida (Dausman et al. 2009). Previous studies will be used with the compiled data from the previous tasks (the revised hydrogeology and water budgets) to develop an updated representation of the overall groundwater flow system. The revised conceptual model will be a simplified representation of the Floridan Aquifer System; however, it will contain enough detail to assure that the overall system behavior is reproduced. This conceptual model will be used in the numerical model development.
• Construct regional groundwater simulation model. Even though numerous numerical models of varying sizes of the Floridan Aquifer System have been developed, there has yet to be a model developed of the entire Floridan Aquifer System since the previous RASA study 30 years ago. A numerical model of the entire Floridan Aquifer System can be used as a tool to understand the overall groundwater flow system. For example, groundwater divides in the Floridan Aquifer System could potentially be altered by different water use scenarios across the multiple states that contain the Floridan Aquifer System. Smaller scale models are not able to simulate potential changes in the groundwater divides like a large-scale model of the entire aquifer. The previous RASA model was not developed to simulate surface water/groundwater interactions or potential changes in the quality of water (such as the encroachment of seawater), which are important in the availability of fresh groundwater in the Floridan Aquifer System. Technological advances in modeling codes and computing power enable a model to be used as a tool to better address surface water/groundwater interaction, as well as the quantification of the availability of freshwater from the encroachment of seawater. However, 'more complicated' does not necessarily mean 'better model'. Advances in numerical modeling will be used to address some of the questions related to water availability, but complexity to the model will only be increased if it can improve our understanding of the system. The initial numerical model will developed with MODFLOW2005 and have approximately the same horizontal boundaries as the previous RASA study (Bush and Johnston, 1988) and will include parts of the Surficial Aquifer System and the entire Floridan Aquifer System.
• Groundwater availability assessment. A conclusion from the previous RASA study was that the Floridan Aquifer System water was abundant and little was removed from storage with increased groundwater withdrawals. Instead of removing groundwater from storage, groundwater development could actually induce recharge to the aquifer while reducing stream and spring discharge. This has possible environmental implications for sensitive ecosystems supported by the streams and springs. Another conclusion was that vertical and horizontal saltwater encroachment could reduce water quality and freshwater availability in the Floridan Aquifer System. When analyzing groundwater availability for the Floridan Aquifer System today, these concerns and others need to be taken into consideration.