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Elevated δ 15N in Stream Biota in Areas with Septic Tank Systems in an Urban Watershed

Luanne Y. Steffy and Susan S. Kilham
Ecological Applications
Vol. 14, No. 3 (Jun., 2004), pp. 637-641
Published by: Wiley
Stable URL: http://www.jstor.org/stable/4493568
Page Count: 5
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Elevated δ 15N in Stream Biota in Areas with Septic Tank Systems in an Urban Watershed
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Abstract

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 15N in all trophic levels at stations located downstream of the divide between sewered and nonsewered (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 15N. Allochthonus inputs, which do not derive nitrogen from aquatic sources, such as detrital leaf material, showed no difference in δ 15N 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 $10\textperthousand$ enrichment in δ 15N in septic vs. 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.

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