Spatial patterns of NO₃-N concentrations within the delta environment of a large flood-control reservoir, Iowa, USA

Flood control reservoirs in the U.S. Midwest intercept riverine nitrate‑nitrogen (NO₃-N) transport from agricultural watersheds and are known to reduce mass loads as water flows through them. In this study, we conducted six comprehensive boat surveys over a two-year period to evaluate spatial patterns of NO₃-N concentration reductions occurring within the delta environment of Lake Red Rock reservoir in south-central Iowa. Surface water NO₃-N concentrations were mapped over a distance of 26,000 to 36,000 m within the main channel and off-channel delta areas using a boat-mounted Nitratax sensor. Concurrent with the boat surveys, groundwater quality conditions were assessed by installing and sampling five shallow monitoring wells. During the six surveys, background reservoir NO₃-N concentrations ranged from 3.3 to 10.8 mg/L. Higher and more uniform concentrations were observed in the connected flowing channels compared to NO₃-N concentrations in off-channel coves, bays, and side channels, which were approximately 2 to 6 mg/L lower. Shallow groundwater was shown to be highly anoxic and contain nearly non-detectable levels of NO₃-N. Denitrification is hypothesized to be the main process reducing NO₃-N within the off-channel areas and in shallow groundwater, and reductions occurring in these areas are contributing to reservoir-wide N reductions. Overall, the study highlights significant spatial patterns in NO₃-N concentrations that can develop within deltas of typical Midwestern flood-control reservoirs. Future work can focus on evaluating seasonal patterns of reductions in off-channels and groundwater with an eye toward developing strategies for capitalizing on these delta environments to decrease the export of NO₃-N from agricultural watersheds.