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Show 16 Water Quality in the Great Salt Lake Basins, Utah, Idaho, and Wyoming, 1998- 2001 7rT <*• Salt = | Lake • fl Valtey H A 0 1 2 3 KILOMETERS EXPLANATION Discharge area Primary recharge area Secondary recharge area Recent residential/ commercial land use . Approximate limit of basin- fill material Atrazine concentration, in micrograms per liter Less than 0.001 0.001 to 0.020 0.020 to 0.100 0.100 to 1.00 1.00 to 1.58 Figure 24. Atrazine was detected more frequently and at higher concentrations in water sampled from monitoring wells on the west side of Salt Lake Valley, Utah. four wells with the highest concentrations of atrazine and its degradation products also had minimal concentrations of chloroform, which likely comes from watering lawns with chlorinated water. This correlation suggests that the recharge area for these wells includes more agricultural or nonirrigated industrial and vacant land than residential land. The compounds detected in shallow ground water and used at the land surface have the potential to move to the deeper basin- fill aquifer in Salt Lake Valley, which is used for public supply. VOCs and ( or) pesticides were detected, mostly at low concentrations, in water from 23 of 31 public- supply wells sampled in the valley. Although the concentration of these compounds measured in ground water used for public supply is not a known health concern according to current standards, their occurrence in the deeper ground water presents the possibility that water with higher concentrations may enter the aquifer in the future. Most ground water used for public supply in Salt Lake Valley was recharged in the past 50 years and contains manmade compounds Elevated nitrate concentrations and the occurrence of pesticides. VOCs, and other manmade compounds in the deeper basin- fill aquifer underlying Salt Lake Valley indicate relatively recent ( within the past 50 years) recharge of water to the ground- water system ( Thiros and Manning. 2004). The apparent aye for water sampled from public- supply wells in the valley ranges from 3 to more than 50 years. 1 Ground water is generally older with distance from the mountain front, the oldest water being in the discharge area toward the center of the valley dig. 25). Concentrations of manmade compounds generally decreased with the age of water and were not detected in water older than 50 years from many of the public- supply HHIH; 100 10 I 1 0.1 0.01 0.001 • Discharge • Recharge area area Water from 6 wells did not contain detectable concentrations of VOCs or pesticides and had apparent ages of greater than 50 years „ No VOC or pesticide detected i i i i I i i i i I • • P3 rr cc 6 - 5 4 - 3 - 2 " 1 ' i ] ' i i i i I i i i i I i • • Cf » r © © CO • l 0 0 10 20 30 40 > 50 APPARENT TRITIUM/ HELIUM- 3 AGE, IN YEARS Figure 26. The concentration of man- made compounds and nitrate in water from 31 public- supply wells in Salt Lake Valley generally increased as the apparent tritiiim/ helium- 3 age of the water decreased. wells. For example, summed concentration of VOCs and pesticides generally was in the range of 0.1 to 10 iig/ L in water younger than 20 years of age and was less than 0.01 ttg/ L in water about 50 years or more in age dig. 26). Similarly, nitrate concentrations were lower in the older ground water. Manmade compounds were detected or chlorofluorocarbon ( CFC) contamination occurred in water from five wells with shallow, open intervals in groundwater discharge areas within Salt Lake Valley. This indicates that the aquifer in the discharge area can receive modern water recharged in the \ alley despite the natural upward gradient. 0 3 6 KILOMETERS EXPLANATION Apparent 3H/ 3He age. in years CD Less than 10 H10 to 20 « 20 to 30 LZ330to40 a 40 to 50 H Greater than 50 Approximate boundary of basin- fill deposits in Salt Lake Valley 18* Public- supply well - Number is apparent age in years Figure 25. Apparent ages for water sampled from public- supply wells in the basin- fill aquifer range from 3 to more than 50 years. ' Ground- water ages determined from en\ ironmental tracers such as tritium, helium- 3. and CFCs arc interpreted from measured concentrations and can be affected b\ mixing between waters with different ages, dispersion, and other factors and. therefore, are usually qualified as " apparent ages." |
