N in Stream Biota in Areas with Septic Tank Systems in an Urban Watershed, Chester County, Pa

Anthropogenic inputs of nitrogen from human sewage are a central concern in urban watershed management. However, identifying the locations of these inputs, whether from improperly functioning or ill-maintained septic tanks or from leaking sewer lines, is difficult. We used nitrogen stable isotope analysis of aquatic food webs as indicators of sewage-derived nitrogen in Valley Creek watershed. Stable nitrogen isotope analysis revealed elevated δ N in all trophic levels at stations located downstream of the divide between sewered and non-sewered (septic tanks) neighborhoods in the watershed. Stations not located in the septic system area or on the other branch of the creek did not show this elevated level of δ N. Allochthonus inputs, that do not derive nitrogen from aquatic sources, such as detrital leaf material, showed no difference in δ N between Valley Creek and Little Valley Creek. Particular fish species that are found throughout both branches, such as blacknose dace and creek chub, had as much as a 10 ‰ enrichment in δ N in septic versus sewered areas. These results indicate that stable isotope analysis can be a useful tool for determining the location of inputs of human sewage and may have broad applications in watershed planning and management, especially in urban systems.