SOURCES AND QUALITY ISSUES

Ideally the historical record of stream water quality would extend back to a time when human activities in the drainage basin had no significant effects. This pristine condition had probably already passed in most U.S. rivers before any oiganized water quaUty studies were made, as concern about apparent stream pollution was commonly a motivating factor in starting such studies (see Water, pollution). 

Oxygen is by far the most abundant element in cmstal rocks, composing 46.6% of the lithosphere (4). In rock mineral stmctures, the predominant anion is 0, and water (H2O) itself is almost 90% oxygen by weight. The nonmetaUic elements fluorine, sulfur, carbon, nitrogen, chlorine, and phosphoms are present in lesser amounts in the lithosphere. These elements aU play essential roles in hfe processes of plants and animals, and except for phosphoms and fluorine, they commonly occur in earth surface environments in gaseous form or as dissolved anions. 

Chlorine plays a less significant role in chemical weathering processes than do sulfur and carbon. Most geochemists believe that much, or most, of the chloride in stream water in coastal areas is derived from sea salt that is carried landward or deposited by rainfall. Farther inland, however, a major part of the chloride loads in streams is the result of human activities. 

The final composition of stream water is the product of the weathering reactions and related processes outlined above. However, the chemical processes are influenced and controlled by an intricate combination of environmental factors that are characteristic for each drainage system. Therefore, the composition of the bedrock in an area and the residual material left at the surface as soil and subsoil exert a strong influence on the chemical composition of mnoff from the area. The reactions of water with this material are the ultimate geological control and are the source of soluble weathering products. 

Most igneous and metamorphic rocks are composed predominantiy of aluminosilicate minerals, including feldspar such as albite (NaAlSi Og) or anorthite (CaAl2Si20g) and crystaUine forms of silica such as quartz (Si02). Various mixed metal-plus-silicon oxides such as oHvine [(Mg,Fe)2(SiO ] and 

Chemical analyses of stream water that have been pubhshed since the early years of this century generally include deterrninations for four positively charged ions (cations)—calcium (Ca ), magnesium (Mg ), sodium (Na ), and potassium (K )—and five negatively charged ions 

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