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<br />o <br /> <br />CONCENTRATION, IN MICROGRAMS PER LITER <br />50 100 150 200 <br /> <br />. 1 <br />. . <br />~ 1 I <br />. I I <br />'. I I Concentration <br /> ._9.1-- _"t. MCL or HA <br />9. 1 L Frequency of detection <br /> <br />METHYL TERT-BUTYL ETHER <br /> <br />TETRACHLOROETHENE <br /> <br />CHLOROFORM <br /> <br />1,1,1-TRICHLOROETHANE <br /> <br />1,2,4- TRIMETHYLBENZENE <br /> <br />DICHLOROMETHANE <br /> <br />o <br /> <br />5 10 15 20 <br />FREQUENCY OF VOC DETECTION, IN PERCENT <br /> <br />Figure 10. Volatile organic compounds were detected at low frequencies in <br />monitoring and drinking-water wells sampled in the Southern Rocky Mountains. <br />With a few exceptions (2 samples), concentrations of VOCs were substantially <br />less than water-quality standards. All concentrations shown are greater than <br />o l-lg/L. (MCL, maximum contaminant level; HA, health advisory.) <br /> <br />MTBE was detected in water from <br />one municipal well. MTBE con- <br />centrations greater than the drink- <br />ing-water lifetime health advisory <br />of 20-40 )lg/L (U.S. Environmen- <br />tal Protection Agency, 1997) were <br />not detected. However, once <br />MTBE enters the ground water, it <br />is less vulnerable to decomposition <br />and travels farther than other <br /> <br /> <br />Urban land use in the Southern Rocky Mountains, <br />near Fraser, Colorado. (Photograph by Jeffrey B. <br />Bails, U.S. Geological Survey.) <br /> <br />gasoline compounds (Squillace and <br />others, 1996). <br /> <br />Tetrachloroethene is present in <br />solvents for adhesives and is also a <br />by-product of dry cleaning. Chlo- <br />roform can occur naturally or result <br />from the chlorination of drinking <br />water. I, I , I-Trichloroethane is <br />present in solvents and cleaning <br />agents. 1,2,4- Trimethylbenzene is <br />present in petroleum by-products. <br /> <br />250 <br /> <br />Dichloromethane is predominantly <br />used as a solvent in paint strippers <br />and removers. Concentrations of <br />dichloromethane and tetrachloro- <br />ethene were greater than the <br />USEPA drinking-water standard of <br />5 )lg/L at one site in Silverthorne. <br /> <br />Total coliform bacteria were <br />detected in 21 percent of the <br />ground-water samples collected <br />in urban areas. Total coliform <br />bacteria were detected only in sam- <br />ples collected from the shallow <br />monitoring wells and in none of the <br />water samples from drinking-water <br />wells. Bacteria in ground water <br />may occur naturally in soils or may <br />be related to human or animal <br />waste and, therefore, may be an <br />indication of the sanitary quality of <br />the water (Myers and Sylvester, <br />1997). However, the presence of <br />total coliform bacteria does not <br />necessarily indicate the presence of <br />potentially pathogenic bacteria <br />such as Escherichia coli (E. coli), <br />which was not detected in any of <br />the samples. <br /> <br />25 <br /> <br />Chlorofluorocarbons (CFCs) were used to determine the age <br />of ground water <br />Shallow ground water, which is used for drinking water in part of the <br />Southern Rocky Mountains, is generally young (less than 10 years old) <br />and is more susceptible to contamination as a result of land-use practices. <br />The age of the ground water refers to the time from when the water <br />recharged, or entered, the aquifer to the time it was withdrawn from the <br />aquifer. The presence of contaminants, if any, in ground water can reflect <br />land-use conditions at the time of recharge. The age of the ground water <br />was determined in five urban areas where water-quality samples were <br />collected from shallow alluvial aquifers. The age of ground water at most <br />of the sites sampled ranged from 0 to 10 years old. Ground-water ages of <br />greater than 10 years were found in 7 of 25 wells. <br />In the samples where CFC dates were obtained, four of the five nitrate <br />concentrations greater than 2.0 mg/L were from recently recharged <br />ground water (10 years or younger). This is an indication that current land <br />use may be affecting water quality. Increasing urban development may <br />add nitrate to the ground water from different sources, such as septic sys- <br />tems, application of fertilizers, and domestic animal wastes. Effects of <br />increased urban development on ground-water resources are not extensive <br />· in the study area at present. <br /> <br />10 Water Quality in the Upper Colorado River Basin <br />