Probability of nitrate contamination of recently recharged groundwaters in the conterminous United States.
A new logistic regression (LR) model was used to predict the probability of nitrate contamination exceeding 4 mg/L in predominantly shallow, recently recharged groundwaters of the United States. The new model contains variables representing (1) N fertilizer loading (p < 0.001), (2) percent cropland-pasture (p < 0.001), (3) natural log of human population density (p < 0.001), (4) percent well-drained soils (p < 0.001), (5) depth to the seasonally high water table (p < 0.001), and (6) presence or absence of unconsolidated sand and gravel aquifers (p = 0.002). Observed and average predicted probabilities associated with deciles of risk are well correlated (r2 = 0.875), indicating that the LR model fits the data well. The likelihood of nitrate contamination is greater in areas with high N loading and well-drained surficial soils over unconsolidated sand and gravels. The LR model correctly predicted the status of nitrate contamination in 75% of wells in a validation data set. Considering all wells used in both calibration and validation, observed median nitrate concentration increased from 0.24 to 8.30 mg/L as the mapped probability of nitrate exceeding 4 mg/L increased from < or =0.17 to >0.83.
Nolan
Bernard T
BT
US Geological Survey, Reston, Virginia 20192, USA. btnolan@usgs.gov
Hitt
Kerie J
KJ
Ruddy
Barbara C
BC
D005544:Forecasting; D015233:Models, Statistical; D009566:Nitrates / Q000032:analysis*; D012621:Seasons; D012989:Soil Pollutants / Q000032:analysis*; D014481:United States; D014872:Water Movements; D014873:Water Pollutants / Q000032:analysis*; D014881:Water Supply
D009566:Nitrates; D012989:Soil Pollutants; D014873:Water Pollutants