LINKAGES BETWEEN NITRATE RETENTION IN HEADWATER SEEPAGE ZONES AND SEASONAL NITRATE EXPORT FROM AN APPALACHIAN CATCHMENT

Riparian zones which contain extensive seeps may allow short-circuiting of groundwater to stream channels and modify retention processes and pathways that control delivery of nitrogen to streams. In this study seep nitrogen export to an Appalachian headwater catchment (535 ha) was examined at Baldwin Creek, southwestern PA. Baseflow to Baldwin Creek is derived from multiple surface seepage zones. NO3 inputs from near-stream seepage zones and NO3 transformations along these zones were monitored over a one year period (May 2002-2003) using a nested sampling approach. 264 seep samples were collected at the top and the bottom of each of 15 seeps. Seep samples were analyzed for NO3-N, NH4-N, TDN, TN, DOC, and DON along with streamwater and stream discharge at 7 stream stations on a monthly basis and SiO2 and complete cation/anion chemistry were analyzed on two sampling dates (fall and spring). Stream nitrogen chemistry at the watershed outlet was dominated by NO3 (81.5 % of the dissolved nitrogen). Seep NO3 concentrations typically decreased downseep, 25.5% on an average annual basis, with peak losses at specific seeps as great as 90 %. Seep NO3 losses increased with temperature and decreased with baseflow rates, resulting in the maximum seep losses due to assimilation and denitrification occurring during summer and fall months. Mean NO3 concentrations at the bottom of seeps were remarkably similar to streamwater NO3 concentrations at the watershed outlet. Because seep flows provide a substantial portion of streamflow for this catchment, seeps exert a major control on watershed NO3 export and seasonal variations in NO3 concentrations. These data suggest that best management practices for headwater streams with dominant seep contributions should consider protection of near seep zones and seep recharge areas.