Spatially distributed influence of agro-environmental factors governing nitrate fate and transport in an irrigated stream–aquifer system

Abstract. Elevated levels of nitrate (NO₃) in groundwater systems pose a serious risk to human populations and natural ecosystems. As part of an effort to remediate NO₃ contamination in irrigated stream–aquifer systems, this study elucidates agricultural and environmental parameters and processes that govern NO₃ fate and transport at the regional (500 km²), local (50 km²), and field scales (< 1 km²). Specifically, the revised Morris sensitivity analysis method was applied to a finite-difference nitrogen cycling and reactive transport model of a regional-scale study site in the lower Arkansas River valley in southeastern Colorado. The method was used to rank the influence of anthropogenic activities and natural chemical processes on NO₃ groundwater concentration, NO₃ mass leaching, and NO₃ mass loading to the Arkansas River from the aquifer. Sensitivity indices were computed for the entire study area in aggregate as well as each canal command area, crop type, and individual grid cells. Results suggest that fertilizer loading, crop uptake, and heterotrophic denitrification govern NO₃ fate and transport for the majority of the study area, although their order of influence on NO₃ groundwater concentration and mass leaching varies according to crop type and command area. Canal NO₃ concentration and rates of autotrophic denitrification, nitrification, and humus decomposition also dominate or partially dominate in other locations. Each factor, with the exception of O₂ reduction rate, is the dominating influence on NO₃ groundwater concentration at one or more locations within the study area. Results can be used to determine critical processes and key management actions for future data collection and remediation strategies, with efforts able to be focused on localized areas.