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<br /> <br />o <br /> <br />Some Common Types of <br />Biogeochemical Reactions <br />Affecting Transport of Chemicals <br />in Ground Water and Surface Water <br /> <br />ACID-BASE REACTIONS <br /> <br />Acid-base reactions involve the transfer of hydrogen <br />ions (W) among solutes dissolved in water, and they affect <br />the effective concentrations of dissolved chemicals through <br />changes in the H+ concentration in water. A brief notation for <br />W concentration (activity) is pH, which represents a negative <br />logarithmic scale of the W concentration, Smaller values of <br />pH represent larger concentrations of W, and larger values of <br />pH represent smaller concentrations of W, Many metals stay <br />dissolved when pH values are small; increased pH causes <br />these metals to precipitate from solution, <br /> <br />PRECIPITATION AND DISSOLUTION <br />OF MINERALS <br /> <br />Precipitation reactions result in minerals being <br />formed (precipitated) from ions that are dissolved in water, <br />An example of this type of reaction is the precipitation of <br />iron, which is common in areas of ground-water seeps and <br />springs, At these locations, the solid material iron hydroxide <br />is formed when iron dissolved in ground water comes in <br />contact with oxygen dissolved in suriace water, The reverse, <br />or dissolution reactions, result in ions being released into <br />water by dissolving minerals, An example is the release of <br />calcium ions (CaH) and bicarbonate ions (HC03 -) when <br />calcite (CaC03) in limestone is dissolved, <br /> <br />SORPTION AND ION EXCHANGE <br /> <br />Sorption is a process in which ions or molecules <br />dissolved in water (solutes) become attached to the suriaces <br />(or near-suriace parts) of solid materials, either temporarily or <br />permanently. Thus, solutes in ground water and surface water <br />can be sorbed either to the solid materials that comprise <br />an aquifer or streambed or to particles suspended in ground <br />water or suriace water, The attachments of positively charged <br />ions to clays and of pesticides to solid suriaces are examples <br />of sorption, Release of sorbed chemicals to water is termed <br />desorption, <br />When ions attached to the suriace of a solid are <br />replaced by ions that were in water, the process is known <br />as ion exchange, Ion exchange is the process that takes <br />place in water softeners; ions that contribute to water hard- <br />ness-calcium and magnesium-are exchanged for sodium <br />on the suriace of the solid, The result of this process is that <br />the amount of calcium and magnesium in the water declines <br />and the amount of sodium increases, The opposite takes <br />place when saltwater enters an aquifer; some of the sodium <br />in the saltwater is exchanged for calcium sorbed to the solid <br />material of the aquifer, <br /> <br />OXIDATION-REDUCTION REACTIONS <br /> <br />Oxidation-reduction (redox) reactions take place when <br />electrons are exchanged among solutes. In these reactions, <br />oxidation (loss of electrons) of certain elements is accompa- <br />nied by the reduction (gain of electrons) of other elements, <br /> <br />24 <br />