Modeling sulfate transport and distribution and methylmercury production associated with Aquifer Storage and Recovery implementation in the Everglades Protection Area

Aquifer Storage and Recovery (ASR) technology has been proposed to meet the competing ecological and water-supply needs of south Florida and the Everglades Protection Area (EPA). The water recovered from ASR, however, may have altered water quality. Of particular concern is the enrichment of ASR recovery water in sulfate, which can stimulate microbial sulfate reduction and methylmercury (MeHg) production within the EPA. MeHg is already a serious issue with regard to wildlife and human health, and there is concern that ASR might exacerbate the problem. In order to address these concerns, the Lake Okeechobee Environmental Model (LOEM) and the Everglades Landscape Model (ELM) were adapted with sulfur modules to predict concentrations and distributions of sulfate within Lake Okeechobee and the EPA resulting from the release of ASR water. In addition, equations were developed relating the biogeochemistry of sulfate and MeHg production to produce a MeHg production risk assessment from the modeled ASR sulfate loading. Baseline runs (no ASR discharge water), and three different ASR release scenarios with varying sulfate loading were evaluated. Results show that ASR release will temporarily elevate sulfate concentrations in Lake Okeechobee from the present level of about 30 mg/L to as high as 50 mg/L in a worst case scenario, but that this will have little impact on MeHg production in the lake. The model indicates that ASR release will have minimal impacts on sulfate loading to the EPA, primarily because of the already large sulfate loading from other sources within the Everglades Agricultural Area (EAA). Maps of sulfate distributions show that certain locations in the EPA, especially those near canal or Stormwater Treatment Area (STA) discharges, may experience significantly higher sulfate loading from ASR. Overall impacts with regard to increased MeHg production risk are predicted to be low based on this model, although sites with increased ASR sulfate loading (e.g. canal and STA discharge sites) may experience some change in MeHg risk.