2008-03-13_PERMIT FILE - C1992081 (8)

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dissolved silica concentrations result from the solution of minerals in rocks, while • anions (mainly bicarbonate, carbonate, chloride, sulfate, and nitrate) are derived from other sources. Most of the ground waters in the Seneca II-W area are calcium and sodium bicarbonate types (Brogden and Ciles, 1977). Calcium and sodium are common products of weathered granitic rocks which comprise the basement complex and horde ri ng mountains of the area. Carbonate and bicarbonate ions are usually derived from the dissolved carbon dioxide of naturally circulating water. Calcium sulfate type waters are found v+here water has been in contact with clay, organic material, or coal. Calcium bicarbonate, sodium bicarbonate, and calcium sulfate water types occur in alluvial aquifers. Alluvial water quality is affected by the ftow of grater from surface streams and underlying aquifers, the type of alluvial material, and phreatoph yte activity. Dissolved solids concentrations of alluvial waters range from 82 to 2970 mg/1 and averaee 724 mg/1 (Brogden and Ciles, 1977). Concentrations of arsenic, iron, manganese, nitrate, selenium, sulfate, and dissolved solids may locally exceed U.S. Public Health Service (1962) standards for drinking water, although alluvial water is normally suitable for agricultural, municipal, and industrial uses. _J Waters from the Lewis and Mancos shales are primarily calcium or sodium bicarbonate types. Sulfate type water, produced from the reduction of sulfide minerals and organic materials in the shales, also occurs in these units. Shale aquifers in the area have dissolved solids concentrations of 272 to 4230 mg/l, with an average of 1113 mg/l. Chloride, fluoride, sulfate, iron, manganese, nitrate, selenium, and dissolved-solids concentrations may exceed U.S. Public Health Service (1962) recommended levels for drinking water (Brogden and Ciles, 1977); however, well yields are too low for uses other than stock watering. The Mesaverde Group (Iles and Williams Fork Formations) contains sodium bicarbonate or calcium magnesium, bicarbonate sulfate type waters (Steele et al., 1979). The Ple sa verde Group contains few sulfur coals; the presence of bicarbonate sulfate type water is probably due to the upward flow of water from the Mancos shale rather than the reduction of sulfide minerals. Dissolved-solids concentrations range from 334 to 1460 mg/1, with an average of 746 mg/1 (Brogden and Giles, I!l77). Fluoride, iron, manganese, selenium, sulfate, and dissolved-solids concentrations may locally exceed recommended drinking ova ter standards, but water from this group normally is suitable for agricultural and domestic uses.