Probability of Nitrate Contamination of Recently Recharged Groundwaters in the Conterminous United States

Authors

• Bernard Nolan, U.S. Geological Survey
• Kerie Hitt
• Barbara Ruddy

Document Type

Article

Date of this Version

2002

Comments

Published in ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 10, 2002.

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 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 ≤0.17 to >0.83.