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Hydrology of a Clay Settling Area



An intensive data collection and modeling study was undertaken to better understand the hydrologic function and impact of clay settling areas (CSAs) associated with phosphate mining on surrounding water bodies and aquifers. Data were collected and a surface (HSPF) and groundwater (MODFLOW) model was calibrated for a four-year period from 2006 through 2009. The calibrated model was then applied to a 10-year period to better quantify the water-budget of a CSA compared to surrounding landforms. Results indicate that, depending on the degree (condition and thickness) or absence of capping sands used in reclamation, the CSA has one of two types of complex and integrated surface and shallow subsurface water flow systems. Where thicker capping sands exist there will be a perched and possibly rapidly responding water table. With minimal or non-existing capping there will be an interflow type shallow subsurface flow system in relic desiccation cracks. The CSA studied was shown to restrict recharge to and ET from the underlying aquifer but exhibits higher surface flows and slightly lower surface ET than surrounding areas. Modeling results indicate CSAs with higher percentages of exposed overburden deposits will exhibit higher recharge and lower runoff response than those with uniform clay placement. Also, given the high ET rates of open water and wetland landforms on the clays, CSAs with high percentages of wetlands will discharge less surface and subsurface fluxes to surrounding surface water bodies and aquifers than average unmined upland landforms.