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Show Major Findings 11 Median nitrate concentration in Salt Lake Valley shallow ground water is the highest for urban areas in the Nation The median concentration of nitrate in shallow ground water underlying residential and commercial land in Salt Lake Valley ( 6.8 mg/ L) was almost 5 times the national NAWQA median for urban ground- water studies ( 1.4 mg/ L) and was the highest found in 34 NAWQA studies across the Nation. Nitrate concentrations in water from 3 of the 30 monitoring wells in Salt Lake Valley exceeded the USEPA drinking- water standard of 10 mg/ L, which was established because concentrations above this level are known to cause a blood disorder in infants, commonly called blue- baby syndrome. Water from the shallow aquifer underlying Salt Lake Valley is not currently used for drinking, but a longer term concern is the potential for contaminated water that might enter the shallow aquifer to move downward to the underlying basin- fill aquifer that is used for public supply. Such downward movement may already be evident, as water from 12 of 31 public- supply wells contained concentrations of nitrate greater than 2 mg/ L ( the naturally occurring or " background" concentration). The median concentration was 1.3 mg/ L. No value exceeded the drinking- water standard, and public suppliers regularly monitor the source water for nitrate. Although nitrate does occur naturally in ground water, elevated concentrations in urban areas, such as in Salt Lake Valley, are likely caused by human activities related to, for example, fertilizers applied to lawns and gardens and leaking or improperly functioning septic systems and sewer pipes. Urban areas EXPLANATION Median concentration of nitrate- In milligrams per liter. Each circle represents an urban ground- water study O Highest ( greater than 5) © Medium ( 0.4 to 5) O Lowest ( less than 0.4) Background concentration O Bold outline indicates median values greater than background concentration ( 2 milligrams per liter) Average annual total nitrogen input- In pounds per acre, by county, for 1995- 98. Inputs are from fertilizer, manure, and the atmosphere B Greater than 25 [ Zl 6to25 FT! Less than 6 • No data available Although none of the sampled domestic- supply wells in the basin- fill aquifers contained nitrate at concentrations that exceeded the drinking- water standard, the elevated concentrations ( above background) indicate that the basin- fill aquifers are susceptible to human activities at the land surface. A chemical analysis of the water is generally required before a domestic- supply well can be connected to a residence, but subsequent monitoring typically is not required. It may be important for domestic well owners to assess activities around their wells and to monitor nitrate in their wells if concentrations are elevated or changing. Urban storm runoff affects stream chemistry and temperature Storm runoff can carry substantial quantities of nutrients, pesticides, and VOCs to streams. These contaminants accumulate on impervious surfaces in urban areas of the basins between storms and are transported to streams in storm runoff. For example, mean concentrations of total nitrogen and phosphorus in runoff to Little Cottonwood Creek were 9.6 and 1.8 mg/ L, respectively, during a storm in April 2000 ( Gerner and Wad- dell, 2003) ( fig. 18). The storm runoff resulted in elevated concentrations in the creek during the storm ( 6.5 and 1.2 mg/ L of total nitrogen and phosphorus, respectively). The phosphorus concentrations greatly exceeded the USEPA recommended goal of 0.1 mg/ L to prevent nuisance plant growth in streams. Storm runoff also carried pesticides, particularly the insecticide carba- ryl and the herbicide prometon ( which is easily dissolved in and transported by water). Carbaryl can be toxic to fish and aquatic invertebrate animals. In fact, the concentration of carbaryl, in storm runoff and in stream samples collected during rainstorms, exceeded the Canadian water- quality guideline for the protection of aquatic life ( 0.20 / ag/ L)( Canadian Council of Ministers of the Environment, 2002) ( fig. 18). These events are intermittent; however, they are important to consider, particularly when they occur during critical life cycles of aquatic organisms. The concentration of dissolved solids in streams generally is lower during storms because of dilution. Exceptions occur in some study- unit urban streams, such as Little Cottonwood Creek, during and immediately following snowstorms |
