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Show * 22 Water Quality in the Great Salt Lake Basins, Utah, Idaho, and Wyoming, 1998- 2001 are on the western side of the valley, was 7.3 \ igfL, with 43 percent of the samples exceeding the USEPA drinking- water standard of 10 | ig/ L. Concentrations of arsenic were generally lower in water from public- supply wells completed in the basin- fill aquifer underlying the valley ( median of 1.1 \ ig/ L with 16 percent exceeding the standard) because many of these wells are on the eastern side of the valley. About 20 percent of the wells sampled in the recharge areas of the basin- fill aquifers throughout the Study Unit exceeded the drinking- water standard. Water from most wells sampled is used for domestic supply ( provides drinking water to a small number of families). The Safe Drinking Water Act does not regulate domestic wells and therefore homeowners may not be aware of possible risks associated with elevated arsenic concentrations in their wells. Radon occurs naturally as a gas that is soluble in ground water and is released through radioactive decay from rocks containing uranium. Higher amounts of radon occur in areas with uranium- rich sources such as granite, metamorphic rocks, and basin- fill deposits weathered from these rocks. Because of a short half- life of 3.8 days, radon is detected only near its source. Radon moves easily through highly permeable material, such as sand, gravel, and fractures; and readily degasses from water exposed to air. Radon occurred commonly at elevated concentrations in shallow and basin- fill aquifers underlying the Great Salt Lake Basins Study Unit, with a median of 667 pCi/ L ( picocuries per liter). Most samples ( 95 percent) Breathing radon :" '••-"- -'-'- '"- leading cause of lung cancer and is a greater health concern than drinking water that contains radon. Most of the risk from radon in water is from breathing radon released to indoor air from household water uses. The USEPA proposed rule for radon allows States and community water systems to use a higher alternative drinking- water standard if they implement a program to address radon risks in indoor air ( U. S. Environmental Protection Aaencv 1999) exceeded the USEPA proposed drinking- water standard of 300 pCi/ L, but none exceeded the alternative standard of 4,000 pCi/ L. Radon concentrations greater than 2,000 pCi/ L measured in three water- supply wells in Davis County are likely the result of their proximity to metamorphic rocks in the Wasatch Mountains. The mountain block and basin- fill deposits in the southeastern part of Salt Lake Valley are composed primarily of uranium- bearing rocks, resulting in radon concentrations greater than 1,000 pCi/ L in water from monitoring and public- supply wells in the area. jiii& k - *^-*"^.** *\ jgi » Past mining activities contribute to elevated levels of trace elements in sediment and water Trace elements are elevated in sediment and water in Silver Creek, Weber River below Silver Creek, and Echo Reservoir, largely because of historical mining for silver and lead ores in the headwaters of the Silver Creek drainage through the 1970s ( fig. 37). Specifically, concentrations of arsenic and lead in bed sediment were more than 10 times higher in the Weber River below Silver Creek than in the Weber River above the creek ( fig. 38). Enrichment in Echo Reservoir as well as at sites on the Weber River below Silver Creek is largely due to inflow from the creek. Concentrations of cadmium, copper, selenium, silver, and zinc also were elevated, but concentrations of chromium and nickel in bed s_ x- x. N m J UTAH B P: * LJI Great Salt Laki X •* v L East ( an von i Rt- servoi Salt • ' Lake Octty J ? •"^ 1 > * 1 Cj Rockmat- 1 V « f>, Ti. iir Riv* S V b4 3 it' .. ' / V EXPLANATION • Historical mining site • Historical smelter site 3 O Sampling site and number 15 KILOMETERS Figure 37. Mining in the Silver Creek and Little Cottonwood Creek watersheds has increased concentrations of arsenic and other trace elements in the water and sediment of these streams. |