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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) are derived from ~ <br />other sources. Most of the ground waters in the Seneca II-W area are calcium and sodium <br />bicarbonate types (Brogden and Ciles, 1977). Calcium and sodium are common products of <br />weathered granitic rocks which conipri se the basement complex and bordering mountains of <br />the area. Carbonate and bicarbonate ions are usually derived from the dissolved carbon ~, <br />dioxide of naturally circulating water. Calci urn sulfate type waters are found where water <br />has been in contact with clay, organic material, or coal. <br />Calcium bicarbonate, sodium bicarbc na te, and calcium sulfate water types occur in alluvial <br />aquifers. Alluvial water quality is affected by the flow of water from surface streams <br />and underlying aquifers, the type of alluvial material, and phreatophyte activity. <br />Dissolved-solids concentrations of alluvial waters range from 82 to 2970 mg/1 and average <br />724 mg/1 (Brogden and Giles, 1977). Concentrations of arsenic, iron, manganese, nitrate, <br />selenium, sulfate, and dissolved solids may locally exceed U.S. Public Health Service <br />(1962) standards for drinking water, although alluvial water is normally suitable for <br />agricultural, municipal, and industrial uses. <br />C ~ <br />J <br />Waters from the Lewis and Mancos shales are 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 iri these units. Shale aquifers in the area have <br />dissolved soli ds concentrations of 272 to 4230 mg/l, with an average of 1113 mg/1. <br />Chloride, fluoride, sulfate, iron, manganese, nitrate, selenium, and 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 are too low for uses other <br />than stock watering. <br />The Mesaverde Group (Iles and Williams Fork Formations) contains sodium bicarbonate or <br />calcium magnesium, bicarbonate sulfate type waters (Steele et al., 1979). The Mesaverde <br />Croup contains few sulfur coals; the presence of bicarbonate sulfate type water is <br />probably due to the upward flow of water from the Mancos shale rather than the reduction <br />of sulfide minerals. Dissolved solids concentrations range from 334 to 1460 mg/1, with an <br />average of 746 mg/1 (Brogden and Giles, 1977). Fluoride, iron, manganese, selenium, <br />s u]fate, and dissolved solids concentrations may locally exceed recommended drinking water <br />standards, but water from this group normally is suitable for agricultural and domestic • <br />uses. <br />4 <br />