Groundwater nitrate reductions within upstream and downstream sections of a riparian buffer

Abstract

The objective of this study was to evaluate the water quality benefits provided by a buffer enrolled in the North Carolina Conservation Reserve Enhancement Program (NC CREP). A 5-year study was conducted on two distinct buffer sections along the same stream to evaluate the hydrology and attenuation of groundwater nitrate (NO3−-N) entering from nearby agricultural fields. The average buffer widths were 60m (Section 1, upstream) and 45m (Section 2, downstream). Three transects of groundwater monitoring well nests within each buffer zone were installed to monitor water quality and water table depths for 5years. Mean groundwater NO3−-N concentrations at the 1.5m depth decreased from 4.5mgL−1 to 1.7mgL−1 and from 12.9mgL−1 to 1.4mgL−1 in buffer Sections 1 and 2 respectively. These differences were significant in both buffer sections (α=0.05), but the wider Section 1 received significantly less NO3−-N than did Section 2 (P<0.0001). Groundwater NO3−-N loads were reduced by 0.003kgm−2yr−1 (76% reduction) at the 1.5m depth, while in Section 2 these loads were reduced by 0.02kgm−2yr−1 (94% reduction) and 0.04kgm−2yr−1 (86% reduction) at the 1.5m and 3m depths, respectively. Topography, water table and redox measurements, nitrate to chloride ratios, and deep groundwater cation analyses, indicated both sections were suitable for denitrification to proceed. However, the position of the wider Section 1 buffer in the landscape limited the amount of NO3−-N contaminated groundwater that entered from the agricultural fields, and thus could have been designed to be narrower. The effectiveness of NO3−-N reduction in riparian buffer systems is dependent on multiple landscape and biogeochemical factors and not buffer width alone. Findings provide design guidance for conservation buffer program managers as related to the influence of buffer landscape position on groundwater nitrate reduction.