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<br />PART V <br /> <br />HISTORICAL AND PRESENT SALINITY CONDITIONS <br /> <br />e <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 <br />composed of waters whose subsurface flow paths are often deep, and move- <br />ment of the water is relatively slow. TDS concentration can, therefore, <br />be very high, often exceeding 10,000 mg/L. Such spring waters vary in <br />composition in the basin. The waters of highest TDS concentrations are <br />of sodium chloride character due to highly soluble halite. Other springs <br />are high in concentrations of calcium and sulfate due to contact with <br />gypsum (hydrated calcium sulfate). <br /> <br />The water quality of many seeps throughout the Colorado River <br />Basin often reflects relatively shallow geology and mineralogy. Sodium, <br />calcium, and sulfate concentrations can be fairly high (4,000 to 10,000 <br />mg/LTDS). The chemlcal makeup is due to a variety of reactions, in- <br />cluding dissolution of gypsum, partial reprecipitation of carbonate <br />minerals, and adsorption of calcium onto clays that have high amounts of <br />exchangeable sodium and magnesium. <br /> <br />e <br /> <br />Due to the extremely hot and arid conditions throughout the <br />basin, extensive evaporation can cause TDS of the surface waters to in- <br />crease greatly. Under such conditions, carbonate and hydrated sulfate <br />minerals can precipitate out along the streambeds. These characteris- <br />tically white and often fluffy minerals are highly soluble. A snowmelt <br />or rainstorm event can quickly flush these minerals back into the water, <br />causing a temporary but large increase in TDS concentration. <br /> <br />e <br /> <br />29 <br />