Probability of nitrate contamination of recently recharged ground waters in the conterminous United States

This data set is a national map of predicted probability of
nitrate contamination of shallow ground waters based on a
logistic regression (LR) model. The LR model was used to
predict the probability of nitrate contamination exceeding 4
mg/L in predominantly shallow, recently recharged ground waters
of the United States.

The model contains variables representing (1) nitrogen (N)
fertilizer loading, (2) percent cropland-pasture, (3) natural
log of human population density, (4) percent well-drained soils,
(5) depth to the seasonally high water table, and (6) presence
or absence of unconsolidated sand and gravel aquifers. 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 percent of wells in a validation data set
from the National Water-Quality Assessment (NAWQA) program.
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 less than or equal to 0.17 to greater than 0.83.