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The Mississippi embayment aquifer system study is a subproject of the U.S. Geological Survey's Gulf Coast Regional Aquifer-System Analysis project. Within the Mississippi embayment aquifer system, five major aquifers and two confining units in the Wilcox and Claiborne Groups in the Tertiary System were identified in the 160,000-square-mile subproject area. The major aquifers and confining units identified are: (1) upper Claiborne aquifer, (2) middle Claiborne confining unit, (3) middle Claiborne aquifer, (4) lower Claiborne confining unit, (5) lower Claiborne-upper Wilcox aquifer, (6) middle Wilcox aquifer and, (7) lower Wilcox aquifer. The digital ground-water flow model developed to represent the aquifer system has five layers representing the five major aquifers. In 1980, pumpage from the aquifers within the system ranged from 67 million cubic feet per day (501 million gallons per day) in the middle Claiborne aquifer to 3 million cubic feet per day (22 million gallons per day) in the middle Wilcox aquifer. Mean horizontal hydraulic conductivity values from aquifer tests range from 11 feet per day in the lower Claiborneupper Wilcox aquifer in Louisiana to 172 feet per day in the middle Claiborne aquifer in Arkansas. Vertical hydraulic conductivity values used in the model for confining units range from IxlO"3 to IxlO" 5 feet per day. Steady-state predevelopment model simulation results indicate that flow from the subcropping Mississippi embayment aquifer system to the overlying Mississippi River Valley alluvial aquifer ranges from less than Vio million cubic feet per day (% million gallons per day) in the lower Wilcox aquifer to 25 million cubic feet per day (187 million gallons per day) in the upper Claiborne aquifer. Under stressed conditions (1980 pumping rates) flow to the alluvial aquifer is significantly reduced and the net flow is reversed in several of the aquifers. Flow rates range from 2 % million cubic feet per day (21 million gallons per day) from the alluvial aquifer into the middle Claiborne aquifer to 10 million cubic feet per day (75 million gallons per day) from the upper Claiborne aquifer into the alluvial aquifer. Model results indicate that & million cubic feet per day (1 % million gallons per day) of water moved upward from the Mississippi embayment aquifer system into the Coastal lowlands aquifer system under predevelopment conditions. With 1980 pumping rates applied to the Mississippi embayment aquifer system, the net flow is reversed and about Ys million cubic feet per day (2/2 million gallons per day) moves into the Mississippi embayment aquifer system from the Coastal lowlands aquifer system. INTRODUCTION Description of Area The Gulf Coast Regional AquiferSystem Analysis (GCRASA) project is part of the Survey's regional aquifer-system analysis program (fig. 1). The program, which began in 1979, will describe the aquifers in the Nation that are regional in extent and are major sources of freshwater for municipal, industrial, and agricultural use. The GCRASA is a study of the Upper Cretaceous and younger aquifers that underlie about 230,000 mi2 (square miles) in all or parts of Alabama, Arkansas, Florida, Illinois, Kentucky, Louisiana, Mississippi, Missouri, Tennessee, and Texas (fig. 2). The objectives of the GCRASA project are to define the geohydrologic framework in which the regional aquifers exist, to describe the chemical and physical characteristics of the ground water, and to analyze the flow patterns within the regional ground-water system. The three regional aquifer systems defined in the GCRASA project are the Mississippi embayment, the Texas Coastal uplands, and the Coastal lowlands (Grubb, 1984). Each of the regional aquifer systems will be studied in more detail by several subregional projects under the larger GCRASA project. This report presents the preliminary results of the subregional project study involving several of the major aquifers in the Mississippi embayment aquifer system. Purpose and Scope The purpose of this report is to describe the geohydrologic framework of the Mississippi embayment aquifer system and to give a preliminary description of the ground-water flow system. The study includes all of the aquifers in the Mississippi embayment aquifer system as defined by Grubb (1984, table 1, p. 11) except the Mississippi River Valley alluvial aquifer of Holocene and Pleistocene age and the Ripley Formation in sediments of late Cretaceous age, which are studied in other GCRASA subregional projects. The study area for this report includes about 160,000 mi2 in parts of Alabama, Arkansas, Illinois, Kentucky, Louisiana, Mississippi, Missouri and Tennessee (fig. 3). The area roughly bisected by the Mississippi River, extends from about the confluence of the Mississippi and Ohio Rivers to the Gulf of Mexico, and from about the Texas-Louisiana line to the Mobile River in western Alabama. Topography The area lies mainly in the Gulf Coastal Plain. A large part (about 35 percent) of the area is in the Mississippi River alluvial plain, a flat to slightly undulating surface with about a Yz ft/mi (foot per mile) Gulfward slope. The remainder of the area is in the Gulf Coastal Plain uplands and is characterized by a gently rolling terrain. A major interruption in the alluvial plain is Crowleys Ridge, a narrow segmented ridge about 200 miles long in northeastern Arkansas and southern Missouri (fig. 4). The ridge, which is as much as 250 feet higher than the alluvial plain, is an erosional remnant of the Gulf Coastal Plain uplands. Upland areas on the eastern side of the study area are significantly higher in altitude than those on the western side. Climate The climate of the study area is basically humid subtropical in the southern part of the area to temperate in the northern part. Normal annual temperature ranges from 60 to 70 °F. Precipitation throughout the area generally is abundant and well distributed areally. Average annual precipitation ranges from about 48 inches in the northern part of the area to 68 inches in the southeastern part (fig. 5). Most of the precipitation occurs in the winter and spring throughout the entire study area. Drainage Drainage from roughly one-third of the study area flows into the Mississippi River from its major tributary streams. Major tributary streams to the Mississippi River are the St. Francis River in Arkansas and Missouri, the White and Arkansas Rivers in Arkansas, and the Yazoo and Big Black Rivers in Mississippi. Other major streams in the area that are not tributaries to the Mississippi River are the Ouachita-Black River in Arkansas and Louisiana, the Atchafalaya and Calcasieu Rivers in Louisiana, and the Pearl and Pascagoula Rivers in Mississippi (fig. 6). The remainder of the area is drained by streams in southern Louisiana and southern Mississippi that flow directly into the Gulf of Mexico. Average annual runoff in the area ranges from about 12 inches in southern Arkansas to about 32 inches in southeastern Mississippi (fig. 7). GEOHYDROLOGIC FRAMEWORK Defining the geohydrologic framework is one of the objectives of the GCRASA study on a regional scale and subsequently of the subregional projects on a more detailed scale. The following overview describes the general setting of the geohydrologic environment in the Mississippi embayment aquifer system. General Geology The Mississippi embayment area has experienced a gentle downwarping accompanied by cyclic invasion and regression of the sea for the last 225 million years. Sediment deposition and subsequent subsidence resulted in the formation of the Mississippi embayment syncline, a structural trough now filled with sedimentary deposits (fig. 3). During the Tertiary Period, each marine transgression stopped successively farther to the south. The outcrops of the older units, the Wilcox and Claiborne Groups of Paleocene and Eocene age, roughly parallel the edge of the Mississippi embayment (fig. 4). Outcrop areas of these units on the eastern edge of the embayment have significantly higher land altitudes and water-table altitudes than the corresponding outcrop areas on the western side (fig. 8). Miocene and younger units roughly parallel the present Gulf of Mexico coastline. In the northern part of the area, beds generally dip toward the axis of the embayment, which generally coincides with the present Mississippi River. In the central part of the area the dip of the beds changes gradually as a result of regional structure and in southern Mississippi and Louisiana the dip is toward the axis of the Gulf Coast geosyncline (fig. 3). Structural features such as the Desha basin, Jackson dome, Monroe uplift, and the Sabine uplift affect the thickness and dip of the beds. Generally the beds thicken downdip. Mississippi embayment deposits are composed predominately of unconsolidated to slightly consolidated beds of sand and clay with some interbedded gravel, silt, lignite, chalk, and limestone. The Midway Group, a thick sequence of marine clays of Paleocene age, is the basal confining unit for the Mississippi embayment aquifer system. Where present, the Jackson and Vicksburg Groups of Eocene and Oligocene age, respectively, which include thick marine clays, overlie the Mississippi embayment aquifer system. These confining groups virtually isolate hydraulically the aquifers of this study by restricting the movement of water to and from the underlying and overlying geologic units. Relation of previously mapped regional aquifers and confining units in the Mississippi embayment aquifer system has been reported by Grubb (1984, p.ll). Major Water-bearing Units The Mississippi embayment aquifer system in the study area comprises five major aquifers of Paleocene and Eocene age. Within the system, two confining units separate the upper three aquifers, while the lower two aquifers are separated by discontinuous clay beds in the Wilcox Group. Because equivalent aquifers have different names in adjacen

water-resources investigations report

Definition of the geohydrologic framework and preliminary simulation of ground-water flow in the Mississippi embayment aquifer system, gulf coastal plain, United States