Spatial and temporal variability of nitrogen transport in riparian zone seeps

摘要

Excess nitrate-nitrogen (NO3--N) in terrestrial and aquatic ecosystems has resulted in numerous water quality problems throughout the U.S., which emphasizes the need to develop effective management plans to deal with NO3--N pollution, especially in agricultural catchments. Emergent groundwater-fed seeps have been identified as a potentially significant source of water and NO3--N to streams in headwater catchments, yet few studies have evaluated NO3--N transport in seeps and their influence on downstream water quality. To address this need, four studies were undertaken to better understand the role of seeps in FD36 and RS, two small (0.40 and 0.45 ha, respectively) agricultural headwater catchments located in the Ridge and Valley physiographic region of central Pennsylvania.;The first study (Chapter 2) examined spatial and temporal variations in NO3--N concentration and transport potential in shallow groundwater of three seep and adjacent non-seep areas in the riparian zone. Results showed that NO3--N concentrations within the seep areas were significantly greater than NO3--N concentrations in the non-seep areas. Water table depth was also more spatially variable in the seep areas compared to the non-seep areas, which indicated less uniform flow, shorter residence times for groundwater, and greater potential for NO 3--N delivery to the stream from seep areas.;The second study (Chapter 3) used electrical resistivity imaging (ERI) to visualize seep zone formation in order to enhance our understanding of the chemical-hydrologic interactions in the riparian zone. ERI data showed large, localized decreases in resistivity within a seep area following a series of three precipitation events, which suggested that groundwater was upwelling through discontinuities in the fragipan. The non-seep area showed no such response. The changes in resistivity within the seep area coincided with increased NO3--N concentrations. Results showed that seeps were highly dynamic and responsive to precipitation events compared to nonseep areas, and subsurface hydrologic processes in seep zones could significantly affect NO3--N concentrations in the groundwater being delivered to the stream.;The third study (Chapter 4) was intended to improve our understanding of spatial patterns in stream NO3--N concentrations during baseflow and stormflow conditions, and to determine if seeps contributed to the patterns observed in stream chemistry. Semi-variogram analysis showed that stream NO3--N concentrations were spatially dependent during baseflow conditions. Spatial patterns in stream NO3--N concentrations were positively correlated with the number of flowing seeps in the monitored reach. Additionally, stream reaches with seep inputs had increased stream NO3--N concentrations relative to areas with no seep inputs. These results suggest that seeps are an important source of NO3--N in headwater agricultural catchments and play a key role in determining stream NO3--N concentrations.;The final study (Chapter 5) was undertaken to quantify factors that influenced seep NO3-- N concentrations, such as NO3--N retention and/or removal along seep surface flow pathways and N application rates throughout the catchment, and to determine the relationship between seep and stream water quality. Results showed that in FD36, NO3-- N concentrations generally decreased downseep and that NO3-- N retention within these seeps varied seasonally, with most of the retention occuring during in the summer. Only minimal NO3--N retention was observed in seeps within the RS catchment. Seasonal variation in seep NO3--N retention was related to both discharge and air temperature in FD36. However, the effects of N application rates throughout the catchment played a more significant role in determining NO3--N concentrations in seep water in both catchments. In both FD36 and RS, seep NO3-- N concentrations were significantly correlated with stream NO3-- N concentration at the catchment outlet.;The results of this research highlight the need for management practices that decrease NO3--N concentrations in seep discharge (i.e., changes in N application rates) or enhance NO3--N retention and/or removal within seep surface flow paths (i.e., riparian zone wetlands or bioreactors) in order to improve stream water quality in agricultural streams. Future studies are needed to improve our ability to identify seepage inputs to streams, as well as to predict where seepage zones are likely to occur on the landscape. Seeps should be considered critical source areas for NO 3--N delivery to streams since they have been shown to contribute higher fluxes of water and nutrients to streams relative to the surrounding riparian zone. Overall, the four studies in this dissertation demonstrated that seeps exert a strong influence on streamflow generation and NO3-- N fluxes in headwater catchments. Dissertation results also showed that an improved understanding of seep formation, hydrology, and biogeochemistry is necessary for evaluating the potential effectiveness of management practices aimed at improving stream water quality in headwater catchments.