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<br /> <br />Chemical Interactions of <br />Ground Water and Surface Water <br /> <br />EVOLUTION OF WATER CHEMISTRY <br />IN DRAINAGE BASINS <br /> <br />Two of the fundamental controls on water <br />chemistry in drainage basins are the type of <br />geologic materials that are present and the <br />length of time that water is in contact with <br />those materials, Chemical reactions that affect <br />the biological and geochemical characteristics of <br />a basin include (1) acid-base reactions, (2) precipi- <br />tation and dissolution of minerals, (3) sorption and <br />ion exchange, (4) oxidation-reduction reactions, <br />(5) biodegradation, and (6) dissolution and exsolu- <br />tion of gases (see Box 0), When water first infil- <br />trates the land surface, microorganisms in the soil <br />have a significant effect on the evolution of water <br />chemistry. Organic matter in soils is degraded by <br /> <br />microbes, producing high concentrations of <br />dissolved carbon dioxide (C02), This process <br />lowers the pH by increasing the carbonic acid <br />(H2C03) concentration in the soil water, The <br />production of carbonic acid starts a number of <br />mineral-weathering reactions, which result in <br />bicarbonate (HC03 -) commonly being the most <br />abundant anion in the water, Where contact times <br />between water and minerals in shallow ground- <br />water flow paths are short, the dissolved-solids <br />concentration in the water generally is low, In <br />such settings, limited chemical changes take place <br />before ground water is discharged to surface <br />water, <br /> <br />"Two of the fundamental controls <br />on water chemistry in drainage <br />basins are the type of geologic <br />materials that are present and the <br />length of time that water is in <br />contact with those materials" <br /> <br />22 <br />