Scientific Investigations Report 2008–5025
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5025
Estimating the Probability of Elevated Nitrate Concentrations in Ground Water in Washington State
Prepared in cooperation with the Washington State Department of Health
By Lonna M. Frans
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Figures
Figure 1. Location of counties and major cities in Washington.
Figure 2. Hydrogeomorphic regions in Washington.
Figure 3. Nitrate concentrations in water from wells in Washington from the Washington State Department of Health database (1995–2006).
Figure 4. Nitrate concentrations in ground-water samples collected in hydrogeomorphic regions in Washington from the Washington State Department of Health database (1995–2006).
Figure 5. Nitrate concentrations in water from wells in Washington from the U.S. Geological Survey and Washington State Department of Ecology databases (1995–2006).
Figure 6. McFadden’s rho squared from relations of amount of nitrogen application and percentage of agricultural, urban, and undeveloped lands within circular buffers around wells with nitrate concentrations greater than 2 milligrams per liter in ground water in Washington.
Figure 7. Percentage of actual nitrate concentrations greater than 2 milligrams per liter (mg/L) and the estimated probability of nitrate concentrations greater than 2 mg/L in the calibration dataset.
Figure 8. Percentage of actual nitrate concentrations greater than 2 milligrams per liter (mg/L) and the estimated probability of nitrate concentrations greater than 2 mg/L in the validation dataset.
Figure 9. Probability of detecting nitrate concentrations greater than 2 milligrams per liter in ground water at a depth of 145 feet below land surface for the two logistic regression models with and without hydrogeomorphic regions, Washington.
Figure 10. Probability of detecting nitrate concentrations greater than 2 milligrams per liter in ground water at a depth of 145 feet below land surface for the two logistic regression models with and without hydrogeomorphic regions, Puget Sound region, Washington.
Figure 11. Depth to which wells need to be drilled for at least a 90-percent probability of detecting a nitrate concentration less than 2 milligrams per liter in ground water for the logistic regression model without hydrogeomorphic regions, State of Washington and Puget lowlands.
Tables
Table 1. Summary of anthropogenic and natural explanatory variables used for model development.
Table 2. Regression coefficients and individual p-values of independent variables for the two logistic regression models with and without hydrogeomorphic regions that were significantly related with nitrate concentrations greater than 2 milligrams per liter in ground water in Washington.
Table 3. Statistical results from models that estimate the probability of nitrate concentrations exceeding 2 milligrams per liter in ground water in Washington.
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Send questions or comments about this report to the author, L.M. Frans, (253) 552-1694.
For more information about USGS activities in Washington, visit the USGS Washington Water Science Center home page.
U.S. Department of the Interior | U.S. Geological Survey