Influence of Riparian Seepage Zones on Nitrate Variability in Two Agricultural Headwater Streams

Riparian seeps have been recognized for their contributions to stream flow in headwater catchments, but there is limited data on how seeps affect stream water quality. The objective of this study was to examine the effect of seeps on the variability of stream NO₃-N concentrations in FD36 and RS, two agricultural catchments in Pennsylvania. Stream samples were collected at 10-m intervals over reaches of 550 (FD36) and 490 m (RS) on 21 occasions between April 2009 and January 2012. Semi-variogram analysis was used to quantify longitudinal patterns in stream NO₃-N concentration. Seep water was collected at 14 sites in FD36 and 7 in RS, but the number of flowing seeps depended on antecedent conditions. Seep NO₃-N concentrations were variable (0.1-29.5 mg/l) and were often greater downslope of cropped fields compared to other land uses. During base flow, longitudinal variability in stream NO₃-N concentrations increased as the number of flowing seeps increased. The influence of seeps on the variability of stream NO₃-N concentrations was less during storm flow compared to the variability of base flow NO₃-N concentrations. However, 24 h after a storm in FD36, an increase in the number of flowing seeps and decreasing streamflow resulted in the greatest longitudinal variability in stream NO₃-N concentrations recorded. Results indicate seeps are important areas of NO₃-N delivery to streams where targeted adoption of mitigation measures may substantially improve stream water quality.