REGIONAL ASSESSMENT OF NLEAP NO3‐N LEACHING INDICES1

Abstract

ABSTRACT: Nonpoint source ground water contamination by nitrate nitrogen (NO3‐N) leached from agricultural lands can be substantial and increase health risks to humans and animals. Accurate and rapid methods are needed to identify and map localities that have a high potential for contamination of shallow aquifers with NO3‐N leached from agriculture. Evaluation of Nitrate Leaching and Economic Analysis Package (NLEAP) indices and input variables across an irrigated agricultural area on an alluvial aquifer in Colorado indicated that all leaching indices tested were more strongly correlated with aquifer NO3‐N concentration than with aquifer N mass. Of the indices and variables tested, the NO3‐N Leached (NL) index was the NLEAP index most strongly associated with groundwater NO3‐N concentration (r2 values from 0.37 to 0.39). NO3‐N concentration of the leachate was less well correlated with ground water NO3‐N concentration (r2 values from 0.21 to 0.22). Stepwise regression analysis indicated that, although inorganic and organic/inorganic fertilizer scenarios had similar r2 values, the Feedlot Indicator (proximity) variable was significant over and above the NO3‐N Leached index for the inorganic scenario. The analysis also showed that combination of either Movement Risk Index (MIRI) or NO3‐N concentration of the leachate with the NO3‐N Leached index leads to an improved regression, which provides insight into area‐wide associations between agricultural activities and ground water NO3‐N concentration.