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Water Quality 13 Turbidity, the cloudiness of water, can foster the growth of microbes and impede the water-disinfection process (Apodaca and Bails, 2000). Turbidity for water samples from all wells except site 1 was less than 5.0 nephelometric turbidity units (NTU), the USEPA treatment technique value for public-supply water (table 2); most were less than or equal to 2.4 NTU. Higher turbidity for water samples from site 1 probably was due to the shallowness of the well and its location in a field that frequently has standing water. The median turbidity for water samples from alluvial aquifer wells was statistically higher than the median turbidity for water samples from wells in the Trouble- some Formation aquifer (table 3). Dissolved Solids, Major Ions, and Trace Elements Dissolved solids, major ions, and trace elements in uncon- taminated ground water are the result of the natural dissolution of materials along the ground-water flow path in soil and rock. Dissolved-solids concentration is used as one indicator of the suitability of water for drinking, residential, and industrial use, and irrigation purposes. The concentrations of dissolved solids in water samples from wells in the Fraser River watershed were low to moderate, ranging from 55 to 269 mg/L (table 2) and indicated that the water is suitable for domestic, municipal, industrial, and agricultural uses. Median dissolved-solids concentrations were lowest in the vicinity of Fraser and Winter Park and were highest around and to the west of Tabernash. Water samples from site 8 had the greatest median dissolved- solids concentration, 252 mg/L. All dissolved-solids concentra- tions were less than the State of Colorado water-quality standard of 400 mg/L (table 2). The dominant water type in the ground-water samples was calcium-bicarbonate type water (fig. 4). Only in samples from site 1 was the water type a mixed-cation (calcium-potassium or calcium-sodium) -bicarbonate type water or a mixed-cation (calcium-potassium or calcium-sodium) -mixed-anion (bicar- bonate-chloride) type water. There were differences among the wells in the concentra- tions of the major ions that formed the calcium-bicarbonate type water. Median concentrations of most major ions were greatest for wells in the Tabernash area, especially site 8. The median chloride concentration for water samples from site 8 was 3 times greater than the median concentrations for samples from the other wells, and all sulfate concentrations in samples from site 8 were greater than 10 mg/L. Site 7, located just east of site 8, was the only other well with a sulfate concentration greater than 10 mg/L. The Tabernash area, including sites 7 and 8, is the most downgradient part of the study area. Because of this, the ground water would have had a longer residence time, which could cause the increase in specific conductance and higher concentrations of dissolved solids and major ions. Chlo- ride and sulfate concentrations in water samples from all wells were substantially less than the USEPA SDWR of 250 mg/L for each constituent (table 2). Concentrations of chloride, magne- sium, and sulfate in water samples from wells in the alluvial aquifer and urban and ISDS areas were statistically higher than those from wells in the Troublesome Formation aquifer and nonurban and non-ISDS areas, as indicated by median concen- trations of the three ions (table 3). Sources of chloride in ground water can include natural processes (evaporative concentrations in precipitation, brines, dissolution of halite, fluid inclusions in rock) and human activ- ities (fertilizers, septic-system effluent, road salt). In other areas of the Southern Rocky Mountains physiographic province, chloride concentrations have been higher in urban than non- urban areas (Apodaca and Bails, 2000). Magnesium can be a major constituent of igneous and metamorphic rocks, depend- ing on the presence or alteration of ferromagnesian minerals in the rocks, and is present in minerals such as dolomite in sedi- mentary rocks. Most sulfate in ground-water samples from the 11 wells probably resulted from the oxidation of sulfide miner- als in igneous and sedimentary rocks in the watershed. Greater magnesium and sulfate concentrations in urban wells as com- pared to the nonurban wells probably resulted from the greater exposure of the ground water to more igneous and metamorphic rocks and sulfide minerals in the more areally extensive water- shed draining to the urban areas. Concentrations of iron in the ground-water samples ranged from less than 10 to 310 µg/L (table 2). Concentrations greater than the reporting level of 10 gg/L only were detected in sam- ples from sites 4, 6, and 7. The highest concentrations occurred in water samples from site 7 (median of 210 µg/L), with one sample exceeding the USEPA SDWR for iron of 300 µg/L (table 2). Manganese concentrations ranged from an estimated 2 µg/L to 510 µg/L (table 2), with water samples from four wells having concentrations greater than the laboratory report- ing levels. Elevated concentrations occurred in samples from site 7 (median of 457 µg/L) and site 9 (median of 73.8 µg/L). All seven samples from site 7 had manganese concentrations greater than the USEPA SDWR for manganese of 50 µg/L (table 2), as did four of seven samples for site 9. As discussed in "Field Properties," the solubility of iron and manganese increases when reducing conditions (characterized by very low dissolved-oxygen concentrations) are present in an aquifer. Median dissolved-oxygen concentrations for water samples from sites 7 and 9 were low, 0.2 and 0.5 mg/L, respectively, and indicated the presence of reducing conditions. Manganese con- centrations in water samples from wells in the alluvial aquifer and ISDS areas were significantly higher than those in water samples from wells in the Troublesome Formation aquifer and non-ISDS areas, as indicated by median manganese concentra- tions (table 3). Nutrients and Dissolved Organic Carbon Nutrients (nitrogen and phosphorus) can occur in ground water as a result of natural and anthropogenic factors. Nitrogen is a naturally occurring element in crustal rocks, plants, soil, and the atmosphere, whereas phosphorus is common in igneous rocks and occurs in mineral form as apatite (Hem, 1992).