The regional aquifer beneath most of U.S. Air Force Plant 44 is divided into three main hydrogeologic units including the upper zone, a middle aquitard, and the lower zone. The middle aquitard pinches out several miles north of the site, and the upper and lower zones merge into one regional undivided aquifer. The upper zone (where the majority of the groundwater contamination is present) is composed of layers of gravelly sand interbedded with clayey sand and sandy clay. A low permeability layer of clay and silt occurs near the potentiometric surface within the upper zone which, due to its hydraulic properties, retains high concentrations of VOCs.