Altitude of the base of the surficial aquifer system in eastern Palm Beach, Martin, and St. Lucie counties, Florida, USGS WRIR 99-4214, fig. 1

The surficial aquifer system underlies Palm Beach, Martin, and St. Lucie Counties and primarily consists of sand, clay, silt, shell, and limestone of Holocene, Pleistocene, and Pliocene age. Its thickness is variable (decreasing westward and northward) and is estimated to be as much as 400 feet in Palm Beach County (Miller, 1987; Shine and others, 1989), 210 feet in Martin County (Miller, 1980), and 180 feet in St. Lucie County (Miller, 1980).The surficial aquifer system is composed of several stratigraphic units. The maps shows the altitude of the base of the surficial aquifer system in Palm Beach, Martin, and St. Lucie counties in 1997-1998. The contour interval is 40 feet.

Urban development in Palm Beach, Martin, and St. Lucie Counties, Florida, has expanded rapidly in recent decades, resulting in a need for additional freshwater withdrawals from the surficial aquifer system - the primary source of drinking water for this tri-county area. Potable-water demand for urban users is projected to increase 115 percent in Palm Beach County and 89 percent each in Martin and St. Lucie Counties from 1990 to 2010 (South Florida Water Management District, 1998).The increased demand on the coastal well fields, which draw water from the surficial aquifer system, may contribute to saltwater intrusion. There are limited data as to the location or movement of the saltwater interface in the tri-county area, with the exception of previously collected data in the immediate vicinity of the existing coastal well fields. It is possible that the combination of pumpage from the well fields and drainage caused by rivers and canals has a regional effect on the saltwater interface. In October 1996, the U.S.Geological Survey (USGS) entered into a cooperative study with the South Florida Water Management District to determine the present location of the interface between freshwater and oceanic saltwater in the surficial aquifer system along the coast of southeastern Florida. This map report documents the position of the saltwater interface in the surficial aquifer system in 1997- 98 through the evaluation of chloride and geophysical data. This map was developed to delineate the base of the surficial aquifer system in the study area.