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Show Major Findings 15 Trihalomethanes ( THMs, see box below) were the most frequently detected group of compounds in the Jordan River. Treated wastewater was a large component of streamflow at the Jordan River site, which most likely explains why THMs were detected more frequently in water samples from the Jordan River than in samples from Little Cottonwood Creek. Concentrations of most VOCs in water samples were less than 1 \ xg/ L. Eleven percent of the VOC detections were greater than 1 ug/ L, with a maximum concentration of 20 [ xg/ L ( acetone). Chloroform, bromodichloromethane, dibro- mochloromethane, and bromoform form a group of compounds known as trihalomethanes, or THMs. THMs are formed when chlorine, used to disinfect drinking water, reacts with naturally occurring organic materials. THMs aie carcinogenic and have a USEPA cumulative drinking- water standard of 80 pg/ L Pesticides and VOCs are detected in basin- fill aquifers underlying all types of land use Pesticides in water from the basin- fill aquifers throughout the Study Unit were detected more often and at higher concentrations in samples collected from wells in agricultural areas than in areas dominated by rangeland or urban land uses ( fig. 23). Detections of pesticides in water from the aquifers consisted predominantly of the herbicides atrazine and its degradation product deethylatrazine ( DEA), prometon, and simazine. Atrazine and simazine are selective herbicides commonly used on crops and orchards in agricultural areas and to control vegetation in industrial areas. Prometon is used primarily along road rights- of- way and urban corridors. Atrazine and DEA were detected in 23 and 31 percent, respectively, of samples collected from wells in the basin- fill aquifers. Many of the samples with detections were collected from wells in primary recharge areas that historically have been undeveloped or used for agricultural purposes but have become increasingly urbanized. VOCs were detected in 63 percent of the ground- water samples collected from the basin- fill aquifers throughout the Study Unit. Wells surrounded by predominantly urban and agricultural land had a slightly higher frequency of detection, 66 and 63 percent respectively, than wells located in rangeland ( 54 percent) areas ( fig. 23). The two most frequently detected VOCs in the basin- fill aquifers were the THMs chloroform and bromodichloromethane, which were detected in 37 and 19 percent of samples, respectively. The detected pesticide and VOC concentrations were well within USEPA drinking- water standards ( U. S. Environmental Protection Agency, 2002b) and are not a known health concern; however, their widespread occurrence in the aquifers indicates the presence of water recharged recently enough to be affected by human activities. I I Pesticides VOCs Agricultural Rangeland Urban DOMINANT LAND- USE SURROUNDING SAMPLED WELLS Figure 23. Pesticides were detected more frequently in ground- water samples from the basin- fill aquifers in agricultural areas than in rangeland and urban areas, and VOCs were detected at a higher frequency in agricultural and urban areas than in rangeland areas. VOCs and pesticides commonly are detected in shallow ground water and in the basin- fill aquifer used for public supply in Salt Lake Valley At least 1 VOC was detected in water from 27 of 30 monitoring wells completed in the shallow aquifer beneath recently developed residential and commercial areas of Salt Lake Valley ( Thiros, 2003b; see inset on page 14). The most frequently detected VOCs were THMs ( chloroform and bromodichloromethane) and those used as solvents ( PCE and 1,1,1- trichloroethane). The widespread occurrence of THMs at low concentrations in shallow ground water is likely a result of chlorinated public- supply water used to irrigate lawns and gardens in residential areas of Salt Lake Valley that has recharged the aquifer. Water disinfected for public supply also can enter the ground- water system through leaking water lines, sewer lines, and swimming pools. PCE was detected in water from 16 monitoring wells completed in the shallow aquifer at concentrations up to 7.8 [ xg/ L, exceeding the USEPA drinking- water standard of 5 u, g/ L. PCE is used primarily as a dry- cleaning agent and a solvent. The herbicides atrazine and its degradation product DEA were the most frequently detected pesticides in water from the shallow aquifer in Salt Lake Valley and were found in 77 and 70 percent of the samples, respectively. Atrazine concentrations in water from the monitoring wells ranged from less than 0.001 to 1.58 u. g/ L ( fig. 24), less than the USEPA drinking- water standard of 3 ug/ L. All but one of the water samples from the western side of the valley contained detectable concentrations of atrazine. The high detection frequency of atrazine in shallow ground water underlying primarily residential areas on the western side of the valley may be the result of past application in agricultural or industrial areas that have been converted to residential uses. Also, atrazine applied in nonurban areas may have been transported to urban areas in the Study Unit by ground water. The |