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<br /> <br />E <br /> <br />Evolution of Ground-Water Chemistry <br />from Recharge to Discharge Areas in <br />the Atlantic Coastal Plain <br /> <br />Changes in the chemical composition of ground water <br />in sediments of the Atlantic Coastal Plain (Figure E-1) provide <br />an example of the chemical evolution of ground water in a <br />regional flow system, In the shaliow regime, infiltrating water <br />comes in contact with gases in the unsaturated zone and <br />shaliow ground water, As a result of this contact, localized, <br />short-term, fast reactions take place that dissolve minerals <br />and degrade organic material. In the deep regime, long- <br />term, slower chemical reactions, such as precipitation and <br /> <br />Figure E-l. In a coastal plain, such as <br />along the Atlantic Coast of the United States, <br />the interrelations of different rock types, shallow <br />and deep ground-waler flow systems (regimes), <br />and mixing wilh saline water (A) results in the <br />evolution of a number of different ground-water <br />chemical types (B), (Modified from Back, <br />William, Baedecker, M.J" and Wood, W. w., <br />f993, Scales in chemical hydrogeology- <br />A historical perspective, in Alley, W,M., ed" <br />Regional Ground-Water Quality: New York, <br />van Nostrand Reinhold, p, 111-129.) <br />(Reprinted by permission of John Wiley & <br />Sons, Inc,) <br /> <br />dissolution of minerals and ion-exchange, add or remove <br />solutes. These natural processes and reactions commonly <br />produce a predictable sequence of hydrochemical facies, In <br />the Atlantic Coastal Plain, ground water evolves from water <br />containing abundant bicarbonate ions and small concentra- <br />tions of dissolved solids near the point of recharge to water <br />containing abundant chloride Ions and large concentrations <br />of dissolved solids where it discharges into streams, estu- <br />aries, and the Atlantic Ocean, <br /> <br />A HYPOTHETICAL COASTAL PLAIN <br /> <br />Piedmont <br />/ Fall line <br />J Local flow systems <br /> <br />Estuary <br />\ Ocean <br /> <br /> <br />Crystalline rocks <br /> <br />I <br />o <br /> <br />B <br /> <br />I <br />60 MILES <br /> <br />TYPICAL HYDROCHEMICAL FACIES <br /> <br />Average flow velocity = 5 meters per year <br /> <br />Increasing dissolved solids <br /> <br />Calcite saturation: increasing -1- decreasing -I super- <br />I I saturated <br />Mixed-cation bicar onate I I <br />Calcium bicarbonate <br /> <br />Calcium-sodium bicarbonate <br /> <br /> <br /> <br />SO'dium chloride <br />'.., <br /> <br />Crystalline rocks <br /> <br />, <br />o <br /> <br />,*, <br /> <br />I <br />60 MILES <br /> <br />26 <br />