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Show 14 Water Quality in the Great Salt Lake Basins, Utah, Idaho, and Wyoming, 1998- 2001 Pesticides and VOCs were detected more frequently in water from streams than from wells The number and frequency of pesticides and VOCs detected in water from streams was greater than those detected in water from wells ( fig. 22). Ground water has a lower incidence of contamination than streams because water infiltrating the land surface moves slowly through soil and rock formations. This contact, along with the slow rate of flow, allows greater opportunity for sorption, retardation, degradation, and dispersion of contaminants that can reduce the concentration or remove the compounds from ground water. In addition, because of the slow rate of flow, many compounds in recharged water have not yet reached the sampled wells, but may in the future. Pesticides that are generally chemically stable and mobile in ground water, including prometon, atrazine, deethylatrazine, and simazine, commonly were detected in streams and wells. Chemically less stable pesticides, including insecticides that are generally applied in lesser quantities, commonly were detected in water from streams but not from wells. VOCs generally had a greater frequency of detection in streams, with the exception of the chlorinated drinking water byproducts, chloroform and bromodichloro- methane, which were detected at similar frequencies in surface water and ground water. Figure 22. The frequency of detection and number of pesticides and VOCs in water samples were greater for streams than for wells. Pesticides and VOCs that occurred in less than 5 percent of both streams and well samples are not shown. Benzene 1,2,4- Trimethylbenzene p- lsopropyltoluene o- Ethyl toluene m- and p- Xylene o- Xylene Ethylbenzene Styrene Methyl te/ t- butyl ether 1,2,3- Trimethylbenzene n- Butylbenzene Isopropylbenzene Tetrachloroethene Acetone cis- 1,2- Dichloroethylene Trichloroethene Carbon disulfide Methylene chloride 1,3,5- Trimethylbenzene Methylchloride n- Propyl- benzene Carbon tetrachloride Isodurene Chlorobenzene 1,1,1- Trichloroethane ,3,4- Tetramethylbenzene Dichloromethane Methylisobutylketone 1,4 Dichlorobenzene Methylethylketone Ethyl- ether o- Dichlorobenzene BTEX and fuel- related compounds Other VOCs Ground water Surface water 50 40 30 20 10 0 10 20 30 40 50 60 70 8090 DETECTION FREQUENCY, IN PERCENTAGE OF SITES VOCs are more common in shallow ground water underlying Salt Lake Valley than in most other urban areas sampled across the Nation VOCs were commonly detected in shallow ground water underlying residential and commercial areas in Salt Lake Valley. Seventeen of 30 wells ( almost 60 percent) contained water with at least 1 VOC measured at concentrations equal to or greater than a common national assessment level of 0.2 [ ig/ L. The percentage of VOC detections in Salt Lake Valley ranks 9th out of 33 urban areas assessed by the NAWQA Program. The types of VOCs detected were similar. For example, chloroform, a disinfection byproduct and solvent, was the most frequently detected VOC nationally and in shallow ground water underlying Salt Lake Valley. Tetrachloroethene ( PCE), a solvent used for degreasing and dry cleaning, also was common nationally ( found at concentrations greater than 0.2 ng/ L in about 10 percent of wells), and was even more common in Salt Lake Valley ( approximately 25 percent of the wells). The similarity does not extend to methyl teTt- butyl ether ( MTBE), a gasoline additive used to reduce air pollution. MTBE was the second most common VOC nationally, but was not detected above 0.2 | ig/ L in any well in Salt Lake Valley. MTBE is not commonly used in the area. Top five most frequently detected VOCs in urban land- use areas Chloroform Methyl ferf- butyl ether Tetrachloroethene Trichloroethene H i1 1,1,1- Trichloroethane ( assessment level of 0.2 ^ g/ L) Urban land- use studies CZI Across the nation « Salt Lake Valley Disinfection byproduct Fuel oxygenate Solvent Solvent Solvent 0 10 20 30 40 50 60 DETECTION FREQUENCY, IN PERCENTAGE OF SITES |