Simulated effects of ASR injection and withdrawals in the Floridian aquifer system near Lake Okeechobee Florida - a first approximation

The primary objective of this study is to assess the potential effects of injecting and then withdrawing 1.0 bgd of water into the Upper Floridan aquifer near Lake Okeechobee on changes in Floridan aquifer system heads near the well sites and north of the lake. The project objectives will be met by simulating the effects of injecting, and then withdrawing, water near Lake Okeechobee on the Floridan aquifer flow system using the USGS ‘Mega’ model (Sepulveda, 2002). The Mega model extends from the Suwannee River south to the middle of Lake Okeechobee, and from coast to coast. The model has a constant surficial aquifer fixed head array and is vertically discretized to simulate ground-water flow in both the Upper and Lower Floridan aquifers. The model input files, which are presently formatted for inclusion in the original version of the MODFLOW model, will need to be converted to the newer USGS MODFLOW-2000 format (Harbaugh and others, 2000). The locations of injection/withdrawal sites and accompanying rates will be provided by project development team members. Because of the close proximity of the model’s southern boundary to the proposed injection sites, the model cannot be used to evaluate the possible effects of injections and withdrawals to areas south of the lake.

The Comprehensive Everglades Restoration Plan's (CERP) Aquifer Storage and Recovery (ASR) project proposes to inject and then withdraw up to 1.0 billion gallons per day (bgd) of surface water into and out of the Upper Floridan aquifer during wet and dry weather conditions at selected sites near Lake Okeechobee. Concern exists, however, that the injection and withdrawal of such large volumes of water may affect water levels in the Floridan aquifer system for considerable distances away from the proposed well sites in South Florida. As a result, project team scientists agree that a regional GW-flow model will eventually be needed to evaluate the magnitudes and extents of head changes resulting from ASR operations. At this time, however, uncertainty exists in just how far the northern boundary for such a model would need to be extended to avoid the undesirable effects caused by assignment of other than natural aquifer boundaries (i.e., constant head or flux boundaries). Also, because water-use data collection represents such a large investment of manpower in the construction of any regional model, it is desired not to extend the boundary any further north than necessary.