By S.A. Thiros
Product of the National Water-Quality Assessment Program
In 1995, ground water was the source of drinking water to about 52 percent of the population served by public drinking water systems in the Great Salt Lake Basins study unit, which includes parts of Utah, Idaho, and Wyoming. Existing nitrate and volatile organic compound data for ground water collected in the study unit were compiled and summarized as part of the National Water-Quality Assessment Programs objective to describe water-quality conditions in the Nations aquifers. Prerequisites for the inclusion of nitrate and volatile organic compound data into this retrospective analysis are that the data set is available in electronic form, the data were collected during 1980-98, the data set is somewhat regional in coverage, and the locations of the sampled sites are known. Ground-water data stored in the U.S. Geological Surveys National Water Information System and the Idaho and Utah Public Drinking Water Systems databases were reviewed. Only the most recent analysis was included in the data sets if more than one analysis was available for a site.
The National Water Information System data set contained nitrate analyses for
water from 480 wells. The median concentration of nitrate was 1.30 milligrams
per liter for the 388 values above minimum reporting limits. The maximum contaminant
level for nitrate as established by the U.S. Environmental Protection Agency
was exceeded in water from 10 of the 200 wells less than or equal to 150 feet
deep and in water from 3 of 280 wells greater than 150 feet deep. The Public
Drinking Water Systems data set contained nitrate analyses for water from 587
wells. The median concentration of nitrate was 1.12 milligrams per liter for
the 548 values above minimum reporting limits. The maximum contaminant level
for nitrate was exceeded at 1 site and 22 sites had concentrations equal to
or greater than 5 milligrams per liter.
The types of land use surrounding a well and the well depth were related to measured nitrate concentrations in the sampled ground water. Overall, water sampled from wells in rangeland areas had a lower median measured nitrate concentration (0.76 milligrams per liter) than water from areas with an agricultural or urban/residential land use (1.41 and 1.20 milligrams per liter, respectively). In the National Water Information System data set, the median measured nitrate concentration in water from urban/residential areas varied from 1.00 milligrams per liter for wells greater than 150 feet deep to 1.84 milligrams per liter for wells less than or equal to 150 feet deep.
The Public Drinking Water Systems and the National Water Information System data sets contained analyses for most of the State and Federally regulated volatile organic compounds in water from about 368 and 74 wells, respectively. Fifteen different volatile organic compounds were detected at least once in ground water sampled from the Great Salt Lake Basins study unit. Water from 21 wells contained at least 1 volatile organic compound at detectable concentrations. About 68 percent of the volatile organic compounds detected were in water sampled from wells in Salt Lake County, Utah. Tetrachloroethylene was the most commonly detected volatile organic compound in ground water sampled from the study unit, present in 8 out of 442 samples. Maximum contaminant levels for tetrachloroethylene and 1,1-dichloroethylene as established by the U.S. Environmental Protection Agency were exceeded in water from one well each.
This report is contained in the following file:
wrir004043.pdf (2.1 mb)
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Purpose and scope
Description of study unit
Characteristics of nitrate
Characteristics of volatile organic compounds
Sources of available ground-water-quality data
Data screening, comparability, and distribution
Analysis of nitrate data
Analysis of volatile organic compound data
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