National Water-Quality Assessment (NAWQA) Program
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Probability of Nitrate Contamination of Recently Recharged Ground Waters in the Conterminous United States
By Bernard T. Nolan, Kerie J. Hitt, and Barbara C. Ruddy
[Environmental Science and Technology, v. 36, no. 10, p. 2138-2145.]This article supersedes ES&T vol. 31, no. 8; WCP vol. 39, no. 12; and FS-092-96.
Abstract
A new logistic regression (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
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
less than or equal to 0.17 to > 0.83.