Studies by the USGS's National Water-Quality Assessment program in the last decade describe water quality conditions in numerous watersheds.
* Nitrate is often elevated above background levels in shallow ground water underlying the farmland. Concentrations in about 20 percent of shallow wells sampled in agricultural areas exceeded the EPA drinking water standard. This result is a concern in rural areas where shallow ground water is used for domestic supply. These domestic wells are not regulated and owners often do not know the quality of their well water or whether their wells are vulnerable to contamination. Nitrate is most often elevated in karst (carbonate) areas or where soils and aquifers consist of sand and gravel. These natural features enable rapid infiltration and downward movement of water and chemicals. Some of the more vulnerable areas are the Central Valley of California and parts of the Pacific Northwest, the Great Plains, and the Mid-Atlantic region. In contrast, ground water contaminants underlying farmland in parts of the upper Midwest are barely detectable, despite similar high rates of chemical use. In these areas, ground water contaminants may be limited because of relatively impermeable, poorly drained souls and glacial till that cover much of the region, and because tile drains provide quick pathways for runoff to streams.