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The Tertiary limestone aquifer of the southeastern United States is a sequence of carbonate rocks that underlies all of Florida, south Georgia, and adjacent parts of Alabama and South Carolina. It is the principal source of municipal, industrial, and agricultural water supply in south Georgia and most of Florida. The aquifer, known as the Floridan aquifer in Florida and the principal artesian aquifer in Georgia, Alabama, and South Carolina, includes various carbonate units of Paleocene to early Miocene age that are hydraulically connected in varying degrees. Very locally, in the Brunswick, Ga., area, a thin sequence of rocks of Late Cretaceous age is part of the system. In general the aquifer consists of either one vertically continuous permeable zone or two major permeable zones separated by a less permeable unit of highly variable water-transmitting characteristics. Aquifer conditions range from unconfined to confined depending upon whether the clayey Miocene and younger rocks that form the upper confining unit have been removed by erosion. Digital model simulation shows that prior to development, most flow in the aquifer occurred in the unconfined and thinly confined areas of northwest and central Florida and southwest Georgia. Springs in these areas are visible evidence of major flow activity. Spring discharge to streams accounted for about 90 percent of the average predevelopment discharge from the regional aquifer. About 18,100 cubic feet per second left the limestone aquifer as spring flow, and 2,500 cubic feet per second discharged as diffuse upward leakage from the confined areas where the vertical head gradient was upward. Most of the 20,600 cubic feet per second recharge necessary to balance total discharge entered the limestone aquifer in the unconfined and thinly confined areas. Because the areas of greatest recharge before development were near the areas of highest discharge, flow paths were generally short. Much water went into and out of the limestone quickly. A very active shallow flow system at the expense of deep circulation has evolved in unconfined and slightly confined spring areas. Transmissivities commonly exceed 1,000,000 feet squared per day. In contrast, predevelopment flow in the aquifer in the tightly confined areas of southeast and coastal Georgia, far west Florida, and in south Florida was sluggish. In these areas the aquifer is overlain by several hundred feet of sand and clay, except for the outcrop areas along the updip limit of the aquifer. This thick overburden severely retards discharge from the aquifer, causing lethargic flow. Large-discharge springs are nonexistent. The south Florida and southeast Georgia segments of the flow system, which taken together occupy about 50 percent of the regional system, only accounted for slightly more than 3 percent of the predevelopment regional limestone discharge. Transmissivities are on the average lower (generally less than 250,000 feet squared per day) than those in areas of high-flow activity. INTRODUCTION The ground-water hydrology section of this report describes the predevelopment flow system of the highly productive limestones of Tertiary age that underlie all of Florida, south Georgia, and adjacent parts of Alabama and South Carolina. The primary tool used to develop a regional understanding of this flow system is the U.S. Geological Survey's three-dimensional digital ground-water model (Trescott, 1975; Trescott and Larson, 1976). The modeling section of this report documents the use of that model to simulate the regional flow system as it is conceptualized to have been before ground-water development. The study that generated this report is one of a series that comprise the U.S. Geological Survey's Regional Aquifer Systems Analysis program (RASA), a Federally funded nationwide effort to provide comprehensive descriptions of major aquifer systems in the United States. Specifically, the RASA projects are intended to define (1) the hydrology of the regional aquifers before significant development of ground water occurred; (2) the changes or stresses on the aquifers caused by man; and (3) the hydrology of the aquifers as they exist today including the effects of development. This is one of several interim reports summarizing "first phase" work on the 4-year Southeast Carbonates RASA project, which began in October 1978. Other reports include a series of interpretive maps depicting the hydrogeologic framework of the aquifer (Miller, 1982a, b, c, d, e) and the water chemistry of the aquifer (Sprinkle, 1982a, b, c, d). Regional potentiometric surface maps for predevelopment conditions and for 1980 have been prepared (Johnston and others, 1980; Johnston and others, 1981). In addition, other reports describe in more detail the subregional predevelopment flow systems in west-central Florida (Ryder, 1982), east-central Florida (Tibbals, 1981), and southeast Georgia (Krause, 1982). Each of the subregional reports is based on the results of separate digital model analysis of the respective subregional areas. The relation of these three subregional flow models to the regional flow model that is the basis for this report is discussed in the modeling section. GROUND-WATER HYDROLOGY Ground-water flow in the Tertiary limestones of the southeastern United States is controlled by (1) the hydrogeologic framework, that is, the extent, nature, and permeability of the rocks that are in and adjacent to the aquifer; and (2) the areal distribution and rate of recharge to and discharge from the aquifer. These are the topics of this section. Emphasis will be on the factors that most strongly influence the flow system: the areal variation of transmissivity, and recharge and discharge. Hydrogeologic Framework The Tertiary limestone aquifer consists of various carbonate units (permeable zones of different geologic formations and time-stratigraphic units) , in different combinations, hydraulically connected in varying degrees, in different places. The rock units, which generally range in age from Paleocene to early Miocene, are collectively known as the Floridan aquifer in Florida and the principal artesian aquifer in Georgia, Alabama, and South Carolina. The relation of the term Tertiary limestone aquifer to the terms Floridan aquifer (as defined by Parker and others, 1955) and principal artesian aquifer (as redefined by Stringfield, 1966) is shown in table 1. Study of the geology and permeability distribution within the limestone aquifer (Miller, 1982a) suggests that the aquifer in general consists of either one vertically continuous permeable zone, or two major permeable zones separated by a less-permeable unit of highly variable water-transmitting characteristics. Table 1 shows how the two permeable zones and the intraaquifer low-permeability zone are related to various geologic units. The estimated lateral extent of the predevelopment freshwater flow system of the two major permeable zones is shown in figure 1 (in pocket). In coastal areas, permeable limestone is presumed to extend seaward of the limits of freshwater flow in the upper and lower permeable zones shown in figure 1. A basic and major premise about the predevelopment limestone-aquifer flow system in and near coastal areas is that a stable freshwater-saltwater interface exists at depth, rising seaward toward and ultimately intersecting the top of the aquifer as distance from inland areas increases seaward. Beneath the interface static saltwater exists; flowing freshwater occurs above the interface. Thus the predevelopment flow system in the Tertiary limestone aquifer is conceptualized as an exclusively freshwater flow system with a sharp interface between the freshwater and static saltwater. In theory the equilibrium position of a sharp freshwater-saltwater interface relative to a datum is directly proportional to the freshwater head on the interface. In reality the nature and position of the interface may be different. In some areas, zones of relatively low permeability, and (or) very sluggish flow through the system over geologic time, have resulted in transition zones from freshwater to saltwater that may or may not be abrupt; and the locations of which may be related to sea levels that existed in earlier gelogic periods. Determination of the seaward extent of the interface in each of the two major permeable zones is explained in the modeling section. (Note: The boundary of the freshwater flow system is placed at the estimated midpoint of the transition zone between freshwater and seawater, which would correspond to a chloride concentration of about 10,000 mg/L. Thus all water of less than 10,000 mg/L chloride is considered part of the freshwater flow system, even though for water-supply purposes freshwater is generally defined as water containing less than 250 mg/L chloride.) The northern limit of freshwater flow in the limestone aquifer coincides with the updip limit of the aquifer. In updip areas, the limestone is thin and interbedded with calcareous sands and clays, ultimately grading into fully clastic units stratigraphically equivalent to the limestone units. Miller

Predevelopment flow in the Tertiary limestone aquifer, southeastern United States: A regional analysis from digital modeling