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<br />
<br />
<br />For example, when iron dissolved in water that does not
<br />contain dissolved oxygen mixes with water that does contain
<br />dissolved oxygen, the iron and oxygen interact by oxidation
<br />and reduction reactions, The result of the reactions is that
<br />the dissolved iron loses electrons (the iron is oxidized) and
<br />oxygen gains electrons (the oxygen is reduced), 1J:' this case,
<br />the iron is an electron donor and the oxygen is an electron
<br />acceptor, Bacteria can use energy gained from oxidation-
<br />reduction reactions as they decompose organic material.
<br />To accomplish this, bacterially mediated oxidation-reduction
<br />reactions use a sequence of electron acceptors, including
<br />oxygen, nitrate, iron, sulfate, and carbon dioxide, The pres-
<br />ence of the products of these reactions in ground water and
<br />surlace watel can be used to identify the dominant oxidation-
<br />reduction reactions that have taken place in Ihose waters, For
<br />example, the bacterial reduction of sulfate (SO;-) to sulfide
<br />(HS-) can result when organic matter is oxidized to CO2,
<br />
<br />BIODEGRADATION
<br />
<br />Biodegradation is the decomposition of organic
<br />chemicals by living organisms using enzymes. Enzymes
<br />are specialized organic compounds made by living
<br />organisms that speed up reactions with other organic
<br />compounds, Microorganisms degrade (transform) organic
<br />chemicals as a source of energy and carbon for growth,
<br />Microbial processes are important in the fate and transport
<br />of many organic compounds, Some compounds, such as
<br />
<br />petroleum hydrocarbons, can be used dilectly by microorgan-
<br />isms as food sources and are rapidly degraded in many situa-
<br />tions, Other compounds, such as chlorinated solvents, are not
<br />as easily assimilated, The rate of biodegradation of an organic
<br />chemical is dependent on its chemical structure, the environ-
<br />mental conditions, and the types of microorganisms that are
<br />present. Although biodegradation commonly can result in
<br />complete degradation of organic chemicals to carbon dioxide,
<br />water, and other simple products, it also can lead to interme-
<br />diate products that are of environmental concern, For
<br />example, deethylalrazine, an intermediate degradation
<br />product of the pesticide atrazine (see Box P), commonly is
<br />detected in water throughout the corn-growing areas of the
<br />United States,
<br />
<br />DISSOLUTION AND EXSOLUTION
<br />OF GASES
<br />
<br />Gases are directly involved in many geochemical
<br />reactions. One of the more common gases is carbon dioxide
<br />(C02), For example, stalactites can form in caves when
<br />dissolved CO2 exsolves (degasses) from dripping ground
<br />water, causing pH to rise and calcium carbonate to precipi-
<br />tate, In soils, the microbial production of CO2 increases the
<br />concentration of carbonic acid (H2C03), which has a major
<br />control on the solubility of aquifer materials, Other gases
<br />commonly involved in chemical reactions are oxygen,
<br />nitrogen, hydrogen sulfide (H2S), and methane (CH4), Gases
<br />such as chlorofluorocarbons (CFCs) and radon are useful as
<br />tracers to determine the sources and rates of ground-water
<br />movement (see Box G),
<br />
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