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The city of Natchez, located in Adams County, Mississippi, relies on ground water for public supply and industrial needs. Most public supply and industrial wells are developed in Catahoula Formation sands of Miocene age. In 1991, an investigation began to describe the hydrogeology, analyze the effects of ground-water withdrawal from currently pumped wells, and project the possible effects of increased ground-water withdrawals on water levels in the Catahoula aquifer system within the Natchez area. The study area covers about 80 square miles in Adams County, southwestern Mississippi. The study area contains several aquifers; however, the most important aquifers in terms of water supply are the Mississippi River alluvial aquifer and the Catahoula aquifer system. In the Natchez area, the Catahoula aquifer system consists of three main sand intervals that form the upper, middle, and lower Catahoula aquifers. Ground-water withdrawal from the Catahoula aquifer system in the study area currently (March 1995) is from 24 wells screened in the three aquifers. The current daily rate of withdrawal is about 9.2 million gallons of water per day. Analysis of the effect of ground-water withdrawal from these wells was made using the Theis nonequilibrium equation and applying the principle of superposition. The calculated drawdown surfaces under current conditions indicate cones of depression surrounding the principal wells. In the upper Catahoula sand, most of the drawdown is concentrated about 1 mile east of the downtown Natchez area, where a maximum drawdown of 95 feet was calculated. Most of the drawdown in the middle Catahoula sand occurred in the same general vicinity as in the upper sand, with a maximum calculated drawdown of about 113 feet. Drawdown in the lower Catahoula sand was concentrated about 4 miles northeast of downtown Natchez, with a maximum calculated drawdown of about 31 feet. Drawdown-surface maps were made using calculations based on current pumping rates for 10 years and 20 years beyond March 1995. Planned changes in the pumping configuration were incorporated into these analyses. The drawdown surface calculated for 10 years beyond March 1995 indicates an average total increase in drawdown of about 7.3 feet for the upper Catahoula sand, with a maximum increase of about 28 feet. An average total increase in drawdown of only 1.2 feet was calculated for the middle Catahoula sand due to the planned discontinued pumping of many of the wells. An average total increase in drawdown of about 19 feet was calculated for the lower Catahoula sand, with a maximum increase of about 41 feet. The drawdown surface calculated for 20 years beyond March 1995 indicates an average total additional increase in drawdown over the 10 year drawdown surface of about 1.9,0.6, and 2.7 feet for the upper, middle, and lower Catahoula sands, respectively. INTRODUCTION The city of Natchez, located in Adams County, Mississippi, relies on ground water for public supply and industrial needs. Most public supply and industrial wells are developed in Catahoula Formation sands of Miocene age. In 1982, about 7.4 million gallons of water per day was withdrawn from the Catahoula aquifer system in Adams County (Boswell and Bednar, 1985). Currently (March 1995), more than 9 million gallons of water per day is withdrawn from the Catahoula aquifer system in the Natchez area alone. Increased ground-water withdrawals from a growing population and the development of new industries may increase water-level declines and affect water quality. In 1991, the U.S. Geological Survey, in cooperation with the Mississippi Department of Environmental Quality, Office of Land and Water Resources (OLWR), began an investigation to describe the hydrogeology, to analyze the effects of groundwater withdrawal from currently pumped wells, and to project the possible effects of increased ground-water withdrawals on water levels in the Catahoula aquifer system within the Natchez area. This report presents the results of that study. General Setting of the Study Area The study area covers about 80 square miles in Adams County, southwestern Mississippi (fig. 1). The area is located in the Mississippi Alluvial Plain and the Bluff Hills of the Gulf Coastal Plain physiographic province (Fenneman, 1938). Most of the study area is hilly due to the erosion of loess deposits; however, a broad flat alluvial plain formed by the Mississippi River is located in the western part of the study area. The mean annual temperature at Natchez is about 64.4 degrees Fahrenheit; annual precipitation is about 53.4 inches (National Weather Service, oral commun., 1995). Previous Investigations Previous investigations that include all or part of the study area have been published by Stephenson and others, (1928); Vestal (1942); Callahan and others (1963,1964); Childress and others (1976); Boswell and Bednar (1985); and Martin and Whiteman (1989).

Hydrogeology and analysis of ground-water withdrawal from the Catahoula aquifer system in the Natchez area, Adams County, Mississippi