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Scientific Investigations Report 2007–5179

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5179

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Table 7. Nitrate occurrence in shallow ground water beneath agricultural and urban areas within regional National Water-Quality Assessment Program study units in the arid to semiarid Western United States, 1993–2004.

[Abbreviations: LRL, laboratory reporting limit; MCL, maximum contaminant level; USEPA, U.S. Environmental Protection Agency; mg/L, milligram per liter. Symbol: <, less than]

Land use Number
of wells
Range in nitrate
concentration
(mg/L)
Median nitrate
concentration
(mg/L)
Percent of wells
exceeding USEPA MCL
(10 mg/L)
Important factors relating to
exceedence of USEPA MCL
Agricultural 272 <LRL to 74.6 0.27 25 Irrigation practices, fertilizer application rate, general soil permeability characteristics, aquifer-redox condition.
Urban 179 <LRL to 53.3 0.22 10 General soil permeability characteristics.1

1A sufficiently strong predictive model could not be developed for the exceedence of the nitrate MCL in urban areas. The only variable that provided any significant contribution was soil hydrologic group (p = 0.044).

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