Abstract: The Pinellas County landfill site is on a flat, coastal area characterized by a nearsurface water table. Part of the site 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. In descending order, these are: a surficial aquifer about 19 feet thick composed of sand and shell, a confining bed about 35 feet thick composed of marl and clay, and the Floridan aquifer composed of limestone. Landfill operations have not altered surface-water quality. Although leachate movement downward into the Floridan _aquifer is not indicated, vertical movement through the confining bed is about 0.005 foot per year. 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 from the oldest section of the landfill through the surficial aquifer is about 20 feet per year. Peaks in concentration of selected chemical parameters and flow-rate analysis of water from trenches indicate the possibility of intermittent release of leachate from the landfill.

Purpose and scope: This reports presents the results of a 2-year investigation by the U.S. Geological Survey, in cooperation with the city of St. Petersburg, to evaluate geohydrologic conditions at a landfill in a coastal area. The report defines and describes the geologic and geohydrologic units underlying the landfill, and examines the controls these units exert on the flow and quality of surface and ground water in the area. The study increases the understanding of the geohydrologic aspects of landfill operations and will be helpful in the selection of future landfill sites in similar coastal areas.