Salt-water encroachment, geology, and ground-water resources of Savannah area, Georgia and South Carolina

Abstract

The Savannah area consists of about 2,300 square miles of the Coastal Plain along the coast of eastern Georgia and southeastern South Carolina. Savannah is near the center of the area. Most of the large ground-water developments are in or near Savannah. About 98 percent of the approximately 60 mgd of ground water used is pumped from the principal artesian aquifer, which is composed of about 600 feet of limestone of middle Eocene, Oligocene, and early Miocene ages. Industrial and other wells of large diameter yield as much as 4,200 gpm from the principal artesian aquifer. Pumping tests and flow-net analyses show that the coefficient of transmissibility averages about 200,000 gpd per ft in the immediate Savannah area. The specific capacity of wells in the principal artesian aquifer generally is about 50 gpm per ft of drawdown. The coefficient of storage of the principal artesian aquifer is about 0.0003 in the Savannah area. Underlying the Savannah area are a series of unconsolidated and semiconsolidated sediments ranging in age from Late Cretaceous to Recent. The Upper Cretaceous, Paleocene, and lower Eocene sediments supply readily available and usable water in other parts of the Coastal Plain, but although the character and physical properties of these formations are similar in the Savannah area to the same properties in other areas, the hydraulic and structural conditions appear to be different. Deep test wells are needed to evaluate the ground-water potential of these rocks. The lower part of the sediments of middle Eocene age acts as a confining layer to the vertical movement of water into or out of the principal artesian aquifer. Depending on the location and depth, the principal artesian aquifer consists of from one to five geologic units. The lower boundary of the aquifer is determined by a reduction in permeability and an increase in salt-water content. Although the entire limestone section is considered water bearing, most of the ground water used in the area comes from the upper part of the Ocala limestone of late Eocene age and the limestones of Oligocene age. The greatest volume of water comes from the upper part of the Ocala limestone, but the greatest number of wells are supplied from the rocks of Oligocene age. The Tampa limestone and Hawthorn formation of early Miocene age are generally water bearing; the amount and quality of the water depends on the location. The water from some wells in the Tampa and most of the water from the Hawthorn is high in hydrogen sulfide. In the northeastern part of the area the principal artesian aquifer is close to the land surface. Here the confining layer is thin and in some of the estauaries it may be completely cut through by the scouring action of the streams during tidal fluctuations. In this part of the area artesian groundwater at one time discharged from the aquifer as submarine springs. Now a reverse effect may be occurring; ocean and river water may be entering the aquifer. The silts, clays, and very fine sands of the upper Miocene and Pliocene ( ?) series generally have low permeabilities and form the upper confining layer for the principal artesian aquifer. Although all the sediments overlying the principal artesian aquifer are considered to be part of the confining layer, locally some of the upper units are water bearing. The uppermost geologic units in the Savannah area are sediments of Pliocene ( ?) to Recent age and consist of sands, silts, and clays with shell and gravel beds which are a source of water for shallow wells. The first large ground-water supply from the principal artesian aquifer was developed in 1886 by the city of Savannah. Additional municipal and industrial supplies have been developed since that time. Pumpage progressively increased to a peak of 62 mgd in 1957. Outside of the city and industrial area the 1957 pumpage was about 9 mgd. In 1958 the total pumpage in the Savannah area was about 68 mgd or about 3 mgd less th