Geohydrologic units of the coastal lowlands aquifer system, south-central United States

Abstract

The coastal lowlands aquifer system is one of the three regional aquifer systems studied as part of the Gulf Coast Regional AquiferSystem Analysis (RASA). The coastal lowlands aquifer system underlies about 160,000 square miles of the coastal areas of Texas, Louisiana, Mississippi, Alabama, and westernmost Florida, and nearby offshore areas; the aquifer system is composed of sediments of Oligocene age and younger. The sediments consist predominantly of interbedded sand, silt, and clay with minor amounts of lignite and limestone. The average thickness of the sediments is about 6,000 feet, with a maximum thickness of more than 18,000 feet occurring offshore from southern Louisiana. The base of the coastal lowlands aquifer system is the top of the Vicksburg-Jackson confining unit, which is a massive clay that represents the last major transgression of the sea. A zone of abnormally high fluid pressure (geopressured zone) is present above the top of the Vicksburg-Jackson confining unit onshore in a narrow band along the coast of Texas and Louisiana and on the Continental Shelf. Where the geopressured zone is present, it is considered to be the base of the coastal lowlands aquifer system. The sediments in the coastal lowlands aquifer system are divided into five permeable zones and two confining units. The permeable zones are not separated by intervening, regionally mappable confining units in about 64 percent of the study area. In much of the area boundaries between permeable zones were extended, as a constant proportion of the total aquifer system thickness, from areas with hydraulic-head data to areas without such data. Average sand percentage of the permeable zones ranges from about 40 percent to more than 60 percent. However, the areal distribution of sand is variable within and among permeable zones. A lobate pattern of greater sand percentages is typical of the permeable zones, and all zones except one have at least one area with sand percentage greater than 80 percent. Data that are useful for quantitative analysis of regional groundwater flow in the coastal lowlands aquifer system are presented in map format. Included for each of the five permeable zones are maps of altitude of the top, thickness, sand percentage, and aggregate thickness of sand. Included for each of the two confining units are maps showing altitude of the top and thickness of the unit. INTRODUCTION A major objective of the Regional Aquifer-System Analysis (RASA) program is to provide an understanding of ground-water flow systems on a regional scale (Bennett, 1979). Natural hydrologic boundaries, rather than political boundaries, have been used to determine the various areas to be studied. The Gulf Coast RASA study encompasses about 230,000 mi2 of onshore area in parts of Alabama, Arkansas, Florida, Illinois, Kentucky, Mississippi, Missouri, Tennessee, Texas, and all of Louisiana. The study area also includes 60,000 mi2 of offshore area on the Continental Shelf where the permeable strata extend beyond the coastline beneath the Gulf of Mexico. The Gulf Coast RASA study and its relation to adjacent RASA studies is shown in figure 1. This report describes the geohydrologic framework of the coastal lowlands aquifer system, one of the three regional aquifer systems delineated in the Gulf Coast RASA study area (fig. 2 and Grubb, 1984). The coastal lowlands aquifer system is composed predominantly of Oligocene and younger sediments, whereas the Mississippi embayment aquifer system and the Texas coastal uplands aquifer system are composed predominantly of Eocene sediments. Each of the three aquifer systems is composed of thousands of feet of deposits, which contain numerous aquifers, permeable zones, and confining units. Some of these geohydrologic units are regionally extensive, whereas others are of local importance. The geohydrologic framework of two aquifer systems, the Mississippi embayment and the Texas coastal uplands, is described in chapter B of this Professional Paper (Hosman and Weiss, 1991). The