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Sediments of the northern Atlantic Coastal Plain compose a complex multiaquifer flow system. On a large scale (greater than 500 square miles), ground water in this system evolves from predominantly calcium magnesium bicarbonate water with low dissolved-solids content and low pH, near outcrop-recharge areas, to predominantly sodium bicarbonate water with high dissolved-solids content and high pH, downgradient. This sodium bicarbonate water grades into a sodium chloride water. This large-scale, predictable progression of hydrochemical fades results from the summation of many smaller scale geochemical processes that depend chiefly on the sedimentary depositional environments of the aquifers. Nonmarine sediments contain predominantly quartz, feldspars, clay minerals, and grain-coating ferric oxyhydroxides deposited in freshwater, fluvial environments. The evolution of water chemistry in these sediments is characterized by dissolution of silicate minerals and by redistribution of iron by oxidation-reduction processes. Marine sediments contain carbonate shell material and authigenic clay minerals deposited in estuarine or marginal marine environments. The evolution of water chemistry in these sediments is dominated by carbonate dissolution-precipitation reactions and by cation exchange on the surfaces of marine clay. In both types of sediments, these reactions appear to be driven by local production of carbon dioxide. The metabolism of viable heterotrophic bacteria, which are present in numbers ranging up to 106 cells per gram, is a likely source of carbon dioxide production (10"6 to 10~3 millimoles per liter per year) in deep aquifers. Remarkably similar patterns of hydrochemical faeies are found in both nonmarine and marine deposits. This similarity implies that, although the chemical processes that take place in nonmarine aquifers (silicate hydrolysis) and marine aquifers (carbonate-mineral reactions) are dissimilar, when coupled with ion exchange, these processes can result in similar hydrochemical faeies. The surficial aquifer that mantles most of the study area is characterized by high rates of recharge and discharge and by relatively short flow paths. Many small-scale flow systems have developed in which water chemistry is controlled mainly by the marine or nonmarine nature of the sediments. Because of its proximity to land surface, water from the surficial aquifer locally has been affected by human activities. Nitrate contamination of this aquifer by animal wastes, effluent from septic systems, and land application of inorganic fertilizers is present in some areas. A number of chemical processes occur in the saltwater-freshwater transition zone, including reverse ion exchange (sodium replacing calcium on exchange sites) and caleite precipitation. These processes progressively enrich ground water in calcium relative to seawater and deplete water in dissolved inorganic carbon. INTRODUCTION The northern Atlantic Coastal Plain aquifer system is a significant ground-water resource. In 1980, approximately 17.6 million people used about 1.4 billion gallons of ground water per day from this system for agricultural, industrial, and public supply purposes. As part of the U.S. Geological Survey's (USGS) Regional Aquifer-System Analysis Program (see Foreword), a comprehensive study of the geology, hydrology, and geochemistry of this aquifer system was initiated in 1979 (Meisler, 1980). The findings of this study are presented in USGS Professional Paper 1404, chapters A-M. This report (chapter L) describes the geochemistry of the northern Atlantic Coastal Plain aquifers. Other chapters in this professional paper series describe (1) the regional hydrogeologic framework of the aquifer system (Trapp, 1992); (2) the digital computer simulation of regional ground-water flow (Leahy and Martin, 1993); (3) finer scale digital computer simulations of ground-water flow and more detailed hydrogeologic frameworks for New Jersey, Delaware, Maryland, "Virginia, and North Carolina (Meng and Harsh, 1988; Zapecza, 1989; Harsh and Laczniak, 1990; Vroblesky and Fleck, 1991; Fleck and Vroblesky, 1996; Winner and Coble, 1996; Giese and others, in press; Martin, in press); and (4) the occurrence and geochemistry of salty ground water (Meisler, 1989). Chapter A of Professional Paper 1404 by Trapp and Meisler (1992) summarizes the overall results and conclusions of the regional study. The chemical quality of ground water is an important consideration in any assessment of an aquifer system. For an aquifer system to be a reliable economic resource, not only must predictable quantities of water be available, but the water must also be of predictable and suitable chemical composition. To predict the chemical quality of water in the aquifer system requires (1) sufficient water-chemistry data to be able to accurately

Geochemistry of the northern Atlantic Coastal Plain aquifer system