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dissolved silica concentrations result from the solution of minerals in rocks, while • <br />anions (mainly bicarbonate, carbonate, chloride, sulfate, and nitrate) ere derived from <br />other sources. Most of the ground waters in the region ere calcium and sodium bicarbonate <br />types (Brogden end Giles, 1977). Calcium and sodium ere Common products of weathered <br />granitic rocks which comprise the basement complex end bordering mountains of the area. <br />Carbonate and bicarbonate ions ere usually derived from the dissolved carbon dioxide of <br />naturally circulating water. Calcium sulfate type waters are found where water has been <br />in coot act with clay, organic msterial, or coal. <br />Calcium bicarbonate, sodium bicarbonate, and calcium sulfate water types occur in alluvial <br />aquifers. AlLuviel water quality is affected by the flow of water from surface streams <br />and underlying aquifers, the type of alluvial materiel, end phreetophyte activity. <br />Dissolved-solids concentrations of alluvial waters range from 82 to 2970 mg/l and average <br />724 mg/l (Brogden and Giles, 1977). Concentrations of arsenic, iron, manganese, nitrate, <br />selenium, sulfate, end df ssolved solids may locally exceed U.S. Public Health Service <br />(1962) standards for drinking water, although alluvial water is normally suitable for <br />agricultural, mun(ci pe 1, end industrial uses. <br />Peters from the Lewis and Mancos shales ere primarily calcium or sodium bicarbonate types. • <br />Sulfate type water, produced from the reduction of sulfide minerals and organic materials <br />in the shales, also occurs in' these units. Shale aquifers in the area have <br />dissolved-solids concentrations of 272 to 4230 mg/l, with an average of 1113 mg/l. <br />Chloride, fluoride, sulfate, iron, manganese, nitrate, selenium, end dissolved solids <br />concentrations may exceed U.S. Public Health Service (1962) recommended levels for <br />drinking water (Brogden and Giles, 1977); however, well yields ere too low for uses other <br />then stock watering. <br />The Meseverde Group (Iles and 4illiams Fork Formations) contains sodium bicarbonate or <br />calcium magnesium, bicarbonate sulfate type waters (Steele et al., 1979). The Mesaverde <br />Group contains few sulfur coals; the presence of bicarbonate sulfate type water is <br />probably due to the upward flow of water from [he Mancos shale rather then the reduction <br />of sulfide minerals. Dissolved-solids concentrations range from 334 to 1460 mg/l, with en <br />average of 746 mg/l (Brogden end Giles, 1977). Fluoride, iron, manganese, selenium, <br />sulfate, and dissolved-solids concentrations may locally exceed recommended drinking water <br />standards, but water from this group normally is suitable for agricultural end domestic • <br />uses. <br />4 <br />