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<br />incised into alluvium incorporate both sediment and salt from sloughed channel <br />backs and salts from efflorescence at the alluvium-bedrock contacts [19]. <br /> <br />..... <br />-J <br />0) <br /> <br />The soluble mineral content of saline formations is variable and can <br />be significantly different within one stratigraphic unit. The variability is <br />a result of the parent material, topography, microclimate, and leaching. As a <br />result, the salts being contributed from any stratigraphic unit are very site <br />specific. <br /> <br />c..r. <br /> <br />The determination of the soluble mineral contact of surficial soils <br />is highly dependent upon the sampling and analytical methods used. The <br />effects of contact time and sediment to water ratios on rate and extent of <br />dissolution are extremely important. Since much of the salt is dependent upon <br />sediment load, contact time and sediment to water ratio must be considered. <br />Laronne [22] recommends a sediment to water ratio of 1 to 99. This ratio <br />allows for greater dissolution of salts and a better estimate of salinity <br />being contributed from erosion. <br /> <br />8. Geochemistry <br /> <br />Water quality in the Colorado River Basin varies greatly. Most <br />surface runoff originates from precipitation and is very low in salinity. <br />Salinity steadily increases in its downgradient course due to natural and <br />man-induced activity. <br /> <br />Dissolution of efflorescence on the surface or minerals in <br />subsurface formations is a major source of salinity. Runoff from snowmelt and <br />thunderstorms, which causes alluvial, bank, and gully erosion, suspends solids <br />from barren marine shales. The increased concentrations of calcium, <br />magnesium, and sulfate in these waters are due to dissolution of gypsum <br />(calcium sulfate) and dolomite (calcium or magnesium carbonate). Much of the <br />sodium is contributed by exchange of calcium for sodium on clays found in <br />saline marine shales. <br /> <br />Point sources of salinity contribute chemical constituents that <br />reflect the mineralogy and the chemical reactions which occur in the rock <br />formations through which the ground waters flow. Natural springs are composed <br />of waters whose subsurface flow paths are often deep, and movement of the <br />water is relatively slow. salinity ca~, therefore, be very high, often <br />exceeding 10,000 mg/L. Such spring waters vary in composition in the Basin. <br />The waters of highest salinity are of sodium chloride character due to highly <br />soluble halite. Other springs are high in concentrations of calcium and <br />sulfate due to contact with gypsum (hydrated calcium sulfate). <br /> <br />The water quality of many seeps throughout the Colorado River Basin <br />often reflects relatively shallow geology and mineralogy. Sodium, calcium, <br />and sulfate concentrations can be fairly high (4,000 to 10,000 mg/L). The <br />chemical makeup is due to a variety of reactions, including dissolution of <br />gypsum, partial reprecipitation of carbonate minerals, and adsorption of <br />calcium onto clays that have high amounts of exchangeable sodium and magnesium. <br /> <br />Due to the extremely hot and arid conditions throughout the Basin, <br />extensive evaporation can cause salinity of the surface waters to increase <br />greatly. Under such conditions, carbonate and hydrated sulfate minerals can <br />precipitate out along the streambeds. These characteristically white and <br />often fluffy minerals are highly soluble. A snowmelt or rainstorm event can <br /> <br />v-8 <br />