Geohydrology and simulation of ground-water flow for the Ohio River alluvial aquifer near Owensboro, northwestern Kentucky
The Ohio River alluvial aquifer is the primary source of drinking water for the residents of Owensboro and Daviess County and adjacent counties in Kentucky. The aquifer consists of sand and gravel deposits that partly fill a bedrock-valley system consisting of shales of Pennsylvanian age. The valley is a result of dissection by the Ohio River during the Pleistocene epoch. The sand and gravel deposits in the bedrock valley are glacial-outwash deposits of Illinoian and Wisconsin age. The thickness of the alluvium ranges from just a few feet near the bedrock-valley walls to nearly 150 feet in the Bon Harbor Hills area west of Owensboro. Estimates of transmissivity of the alluvium near the Ohio River are in excess of 50,000 gallons per day per foot. A two-dimensional, steady-state ground-water-flow model was developed to estimate the hydraulic properties, the rate of recharge, and the contributing areas to discharge boundaries for the Ohio River alluvial aquifer near Owensboro. Results from previous studies, available geohydrologic data, and observations of water levels from area ground-water wells were compiled to conceptualize the ground-water-flow system and construct the numerical model. Ground water enters the modeled area primarily by infiltration from precipitation and river leakage towards nearby wells and exits the modeled area primarily by withdrawal wells, flow through the valley across model boundaries, and discharge to the Ohio River. A sensitivity analysis of the model indicates the model is most sensitive to changes in horizontal hydraulic conductivity, especially near the Ohio River boundary. Particle tracking was used to compute the contributing areas to discharge boundaries. Contributing areas for withdrawal wells at Owensboro Municipal Utilities extended south and east toward the valley walls and model boundaries and toward the Ohio River, where most of the water withdrawn by the wells is from induced flow from the river.