Average altitude of the water table (1990-99) and frequency analysis of water levels (1974-99) in the Biscayne aquifer, Miami-Dade County, Florida

Abstract

Miami-Dade County occupies about 2,000 square miles along the southeastern part of peninsular Florida. Five prominent physiographic features are located within the confines of Miami-Dade County and include the Atlantic Coastal Ridge, Big Cypress Swamp, the Sandy Flatlands, coastal marshes and mangrove swamps, and the Everglades (fig. 1). The highest altitudes occur along the Atlantic Coastal Ridge (2 to 10 miles wide), ranging from 8 to 15 feet above sea level but may be as high as 20 feet. The Atlantic Coastal Ridge forms a natural barrier to drainage from the interior. The Sandy Flatlands in northeastern Miami-Dade County ranges from 6 to 18 feet above sea level; this area was poorly drained prior to development. The Everglades forms a natural trough across the county, with altitudes ranging from 9 feet above sea level in the northwestern part to about 3 feet above sea level in the southwestern part. A small section of the Big Cypress Swamp occupies northwestern Miami-Dade County, with altitudes ranging from 7 to 10 feet above sea level. Along the coast, marshes and mangrove swamps have altitudes ranging from 0 to 3 feet above sea level (Fish and Stewart, 1991). The U.S. Geological Survey (USGS), in cooperation with the Miami-Dade County Department of Environmental Resources Management (DERM), conducted a study to determine the average altitude of the water table in the Biscayne aquifer in Miami-Dade County and to perform a water-level frequency analysis for selected time periods. Water-table maps in this report are based on average yearly May, October, high, and low water levels during 1990-94, 1995-99, and 1990-99. Data collected in May and October are used to show the yearly water-level conditions at the end of the dry and wet seasons, respectively. Water-level frequency analysis maps (5-, 10-, and 25-year recurrence water levels) in this report are based on analysis of water-level data from continuous water-level recorders for the 1974-99 period. Water-level data for the maps were collected by Everglades National Park (ENP), the USGS, and the South Florida Water Management District (SFWMD). Data for ENP sites were provided as averages by ENP. Determination of averages for the USGS data was performed using options available in the USGS Automated Data Processing System (ADAPS), and averages for the SFWMD data were determined from options available in the SFWMD Hydrometeorologic and Water-Quality Database (DBHYDRO). The ENP, USGS, and SFWMD data are reported as mean daily water levels and are referenced to sea level. In this report, sea level refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)-a geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929. In this report, horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).