Watershed nitrogen retention is an important mechanism to decrease nitrogen loading to downstream water bodies. Generally, in the northeastern U.S., the amount of nitrogen exported from a forested watershed via stream discharge is less than 40% of the atmospheric input, suggesting significant watershed retention. Past work suggests that vegetation and soils in the upland portions of forested watersheds regulate the concentrations of nitrogen in streams. However, recent work shows that low-lying riparian areas (the zone adjacent to the stream) and the stream channel itself may also play important roles in regulating stream nitrogen concentrations.
Groundwater seeps (locations where upwelling groundwater saturates the surface) that feed mountain streams are common, but their influence on stream nitrogen concentrations has not yet been established.
Scientists at East Carolina University and Penn State University investigated the effects of groundwater seeps on stream nitrogen export in a forested watershed in the Appalachian Plateau region of southwestern Pennsylvania. Specifically, dissolved nitrogen concentrations and stream discharge were monitored monthly for a year along 15 individual seeps and 7 stations along the main stream channel. The study was funded by the USEPA from May 2002–2003, and results are published in the January–February 2010 issue of the Journal of Environmental Quality.
The study revealed that groundwater seeps had a strong influence on stream nitrogen concentrations. Nitrate was the dominant form of dissolved nitrogen in the surface waters. Along seeps, the nitrate concentrations declined by more than 30% as water cycled down the seeps and discharged to the main stream channel. During dry and warm periods, when biological activity was high, seeps behaved as nitrogen sinks. During wet and cold periods, they behaved as nitrogen sources to the stream channel. Seasonal variations in stream nitrate concentration have been attributed to upland soil and vegetation processes in numerous watersheds. In this study, seep nitrate processing regulated the seasonal variability of stream nitrate concentrations. These results suggest that seeps can modulate the effects of elevated regional nitrogen deposition in Appalachian catchments. From an ecological perspective, it has been shown that seep areas are important habitats for salamanders and other species, and the conservation of seeps can protect these fragile habitats.
These new findings suggest that seep zones are also important for their water quality functions. Because seep nitrogen cycling is linked to near-surface processes, land disturbance in seep zones may alter watershed nitrogen cycling. In settings where mining or other land-use changes have altered seeps, the restoration of seep areas can help re-establish a watershed’s ability to retain nitrogen.
“The impacts of groundwater seeps on stream water quality have not been thoroughly addressed in many Appalachian mountain streams where seeps may contribute a large proportion of the streamflow and have the potential to exert a major influence on stream chemistry,” says Michael O’Driscoll, co-author of the study.
Because of the complexities of seep hydrology and seasonal variability of seep–river and seep–groundwater interactions, seeps may not always be considered jurisdictional (protected by the Clean Water Act). The authors conclude that more research is needed to improve the understanding of seep hydrology and to develop a broader model for how surface and subsurface flowpaths along seeps influence stream water quality during dry and wet weather conditions.