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FACTORS IN SAUNITY TRENDS 25 channel are similar to those found in the surrounding watershed soils. Sediment and the resulting salt yield is highly dependent upon landform type. Three major landform types-badlands, pediments, and alluvial valley&-are associated with the Mancos Shale terrain. Badlands are the most erosionally unstable, with sediment yields as high as 15 tons per acre [21]. Rilling accounts for approximately 80 percent of the erosion [20]. Because salt production is closely related to sediment yield and the badland soils have not been leached of their soluble minerals, they produce the greatest amount of salt of the landform types. Pediments are gently inclined planate erosion surface carved in bedrock and generally veneered with fluvial gravels. The surface slopes of pediments are gentle, making them relatively stable. Pediments have deeper soils and higher infiltration rates than badlands, thus they support a greater vegetation cover and are less erosive. i \ Alluvial valleys are formed by a change in gradient and the deposition of sediment. They are stable except along the channel where headcutting and gullying occur. Most of the salts have been leached from the alluvial deposits, thus erosion of their landform type yields less salts per unit volume of sediment than the other two landform types. However, channels incised into alluvium incorporate both sediment and salt from sloughed channel backs and salts from efflorescence at the alluvium-bedrock contacts [19]. The soluble mineral content of saline formations is variable and can be significantly different within one stratigraphic unit. The variability is a result of the parent material, topography, microclimate, and leaching. As a result, the salts contributed from any stratigraphic unit are very S1te specific. The determination of the soluble mineral contact ~l surficial soils is highly dependent upon the sampling and analytical methods used. The effects of contact time and sediment to water ratios on rate and extent of dissolution are extremely important. Since much of the salt is dependent upon sediment load, contact time and sediment to water ratio must be considered. Laronne [22] recommends a sediment to water ratio of 1 to 99. This ratio allows for greater dissolution of salts and a better estimate of salinity being contributed from erosion. Geochemistry Dissolution of efflorescence on the surface or minerals in subsurface formations is a major source of salinity. Runofffrom snowmelt and thunderstorms, which causes alluvial, bank, and gully erosion, suspends solids from barren marine shales. The increased concentrations of calcium, magnesium, and sulfate in these waters are due to dissolution of gypsum (calcium sulfate) and dolomite (calcium or magnesium carbonate). Much of the sodium is contributed by exchange of calcium for sodium on clays found in saline marine shales. Point sources of salinity contribute chemical constituents that reflect the mineralogy and the chemical reactions which occur in the rock formations through which the ground waters flow. Natural springs are composed of waters whose subsurface flow paths are often deep, and movement of the water is relatively slow. Therefore, salinity can be very high, often exceeding 10,000 mg/L. Such spring waters vary in composition in the basin. The waters of highest salinity are of sodium chloride character due to highly soluble halite. Other springs are high in concentrations of calcium and sulfate due to contact with gypsum (hydrated calcium sulfate). The water quality of many seeps throughout the Colorado River Basin often reflects relatively shallow geology and mineralogy. Sodium, calcium, and sulfate concentrations can be fairly high (4,000 to 10,000 mg/L). The chemical makeup is due to a variety of reactions, including dissolution of gypsum, partial precipitation of carbonate minerals, and adsorption of calcium onto clays that have high amounts of exchangeable sodium and magnesium.