as seasonal variations. Because of the 10-year sampling period, trends in concentration and aquatic life over time were detected and correlated to pesticide use.

The Volatile Organic Compounds National Synthesis Project5 and corresponding national synthesis report, The Quality of Our Nation’s Waters—Volatile Organic Compounds in the Nation’s Ground Water and Drinking-Water Supply Wells, presents information about the concentrations of 55 VOCs in aquifers, considering factors such as geography, aquifer characteristics, VOC type, detection frequency, and well type (Zogorski et al., 2006). This information was used to examine associations between natural and anthropogenic factors and the 10 most frequently detected VOCs. Many of these VOCs are solvents and industrial chemicals that are of concern for aquatic and human health in drinking water sources. These associations should help federal, state, and local agencies design sampling programs to detect contamination.

The Nutrients National Synthesis Project6 and corresponding national synthesis report, Nutrients in the Nation’s Streams and Groundwater, describes nutrient occurrence, source, effects on humans and aquatic ecosystems, and trends in concentration between 1992 and 2004 (Dubrovsky et al., 2010). Median concentrations of total phosphorus and nitrogen in agricultural streams were six times greater than background levels. However, exceedence of the federal drinking water standard for nitrate as N (10 mg/L) is uncommon in streams used for drinking water and deep aquifers; this standard was exceeded in more than 20 percent of shallow7 domestic wells in agricultural areas. Data for nitrogen and phosphorus show minimal changes in concentration in the majority of streams over the time frame studied, but more upward than downward trends occurred in those streams that did change in a statistically significant manner.

CONTINUITY AND CONSISTENCY IN STUDY METHODS AND DESIGN

In the late 1980s when discussions about a national water-quality assessment were gathering momentum, federal agencies could not answer the question of whether the 1972 Clean Water Act was producing the intended improvements in water quality nationwide (Knopman and Smith, 1993). A national-level water-quality assessment was not possible because of analytical inconsistencies and a multitude of sampling networks designed for other purposes and ultimately unsuitable for spatial or temporal compari-

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5 See http://water.usgs.gov/nawqa/vocs/.

6 See http://water.usgs.gov/nawqa/nutrients/.

7 Less than 100 feet below the water table.



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