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<br />shallow ground-water system have <br />been affected by urban land use, <br />but these effects were not found in <br />the deeper ground water used for <br />drinking water. <br />Generally, low levels of pesti- <br />cides and volatile organic com- <br />pounds were detected in ground <br />water from shallow alluvial aqui- <br />fers in urban areas. Most agricu]- <br />ture (and pesticide use) in the basin <br />occurs in the Colorado Plateau <br />where ground-water studies were <br />not conducted. The pesticides that <br /> <br />were detected in the ground water <br />sampled in the Southern Rocky <br />Mountains were primarily herbi- <br />cides used for controlling grasses <br />and weeds in nonagricultural areas. <br />Concentrations were very low (less <br />than 0.] /lg/L) for most of the pes- <br />ticides detected. Two herbicides, <br />bromaci] and prometon, were <br />detected in urban areas at concen- <br />trations less than the USEPA drink- <br />ing-water guidelines (U.S. <br />Environmenta] Protection Agency, <br />]996). <br /> <br />Volatile organic compounds <br />(VOCs) were detected in ground- <br />water samples at generally low <br />concentrations. The six most fre- <br />quent]y detected VOCs were <br />methyl tert-buty] ether (MTBE), <br />tetrach]oroethene, chloroform, <br />],], I-trichloroethane, 1,2,4-tri- <br />methylbenzene, and dich]oro- <br />methane (fig. 10). MTBE, a <br />gasoline additive, was detected in <br />at least one well in four of the five <br />urban areas where shallow ground <br />water was sampled. In addition, <br /> <br />RADON CONCENTRATIONS IN UCOL GROUND WATER WERE <br />GENERALLY HIGHER THAN OTHER AREAS OF THE NATION <br /> <br />Radon concentrations in alluvia] aquifers of the UCOL are among the highest of all NAWQA ground-water <br />samples nationwide (S.J. Ryker, USGS, written commun., 1999). Radon is a naturally occurring, colorless, odor- <br />less, radioactive gas derived from the decay of uranium. Radon has been identified as a human carcinogen and <br />can enter a building from seepage of air through dirt floors, cracks in concrete floors or walls, floor drains, or <br />through the use of water supplied from wells. A greater risk of exposure to radon exists in urban areas due to the <br />prevalence of enclosed structures, such as basements, which can trap radon gas. <br />Though the USEPA does not currently (2000) regulate radon in drinking water, there is a proposed maximum <br />contaminant ]evel (MCL) of 300 picocuries per liter (pC ilL) for radon in drinking water (U.S. Environmenta] <br />Protection Agency, 1999). Radon concentrations were greater than the proposed MCL for all wells sampled in <br />the Southern Rocky Mountains. Higher radon concentrations were measured in the Winter Park/Fraser and <br />Silverthorne areas. Igneous and metamorphic rocks, which can contain uranium-bearing minerals, are common <br />rock types in these areas. <br /> <br /> <br />Ranking of ground-water radon concentrations <br />relative to all NAWQA ground-water studies <br />(data from multiple wells in indicated areas) <br /> <br />. Greater than the 95th percentile <br /> <br />. Between the 90th and 95th percentile <br />o Between the 75th and 90th percentile <br /> <br />. 1,000 in at least 25 percent of samples <br />m 600 in at least 25 percent of samples <br />a 300 in at least 25 percent of samples <br />o 300 in fewer than 25 percent of samples <br /> <br />Major Findings 9 <br />