HYDRAULIC IMPACT ASSESSMENT OF MINING-RELATED LAKES IN FLORIDA

Sand and rock mining in coastal plain areas of Florida creates numerous large, deep lakes. These lakes impact the local hydrogeological and ecological systems by altering land use and aquifer hydraulics including hydraulic conductivity, storage, and evapotranspiration rates. A primary concern is that mine lakes could result in a lowering of the water-table and potentially have an adverse impact on wetland hydroperiods. Unlike natural lakes, mining pit lakes in Florida receive water mainly from the up-gradient flow direction and discharge in the down-gradient direction. The potential hydraulic impacts associated with mining pit excavations have been analyzed for a number of proposed sites in Florida using groundwater flow models. Proposed mining lakes were simulated by assigning high transmissivity and high specific yield values. Evapotranspiration rates were also changed to reflect the change in land use. Existing drainage features such as canals and pumpage from wells were removed from the simulations in order to evaluate solely the impacts of excavation on the local hydrogeology.

A number of mining lakes proposed for construction in Florida were analyzed and some 2- dimensional cross section MODFLOW models were developed to study their hydraulic impacts. The results of the analyses indicate that the potential hydraulic impacts due to the proposed mining lakes are dependant upon a number of factors including existing land use, size of the lakes and orientation with regard to the local hydraulic gradient, depth of the lakes, and the hydraulic gradient and conductivity of the adjoining aquifer. Mine lakes can cause drawdown in up-gradient and build-up of water levels in down-gradient areas. However, the increased storage provided by mine lakes results in the retention of significant amounts of water from previous wet seasons that may help maintain overall higher local groundwater water levels during the dry season. Groundwater modeling can be used to optimize mining plans so as to maximize resource recovery while minimizing potential adverse impacts to local water resources and sensitive environments.