Hydrogeology of a landfill, Pinellas County, Florida

The Pinellas County landfill site is on a flat, coastal area characterized by a high water table is subject to tidal flooding. Altitudes within the study area range from 8 to 12 feet above sea level. Three geohydrologic units underlie the landfill site: a surficial aquifer about 19 feet thick composed of sand and shells; a confining bed about 35 feet thick composed of marl and clay; and the Floridan aquifer composed of limestone. The rate of lateral movement of ground water away from the site is about 1.2 feet per year; however, the rate of movement along the boundary of the landfill cells is about 20 feet per year. Vertical movement through the confining layer is about 0.005 foot per year. Landfill operations have not altered surface-water quality. Leachate migration downward into the Floridan aquifer is not indicated, but data do indicate leachate is migrating from the oldest section of the landfill site through the surficial aquifer. Peaks in concentration of selected chemical parameters and flow-rate analysis of water from trenches indicate the possibility of slug-flow leachate. (USGS)