EVALUATION OF DENITRIFICATION IN AN URBAN STREAM RECEIVING WASTEWATER EFFLUENT

Abstract

Urban streams often contain elevated concentrations of nitrogen (N) which can be amplified in systems receiving effluent from wastewater treatment plants (WWTP). In this study, we evaluated the importance of denitrification in a stream draining urban Greensboro, NC, USA, using two approaches: (1) natural abundance of 15N–NO 3 − in conjunction with background NO 3 − –N concentrations along a 7 km transect downstream of a WWTP; and (2) C2H2 block experiments at three sites and at three habitat types within each site. Overall lack of a longitudinal pattern of δ15N–NO 3 − and NO 3 − –N, combined with high concentrations of NO 3 − –N suggested that other factors were controlling NO 3 − –N flux in the study transect. However, denitrification did appear to be significant along one portion of the transect. C2H2 block experiments showed that denitrification rates were much higher downstream of the WWTP compared to upstream, and showed that denitrification rates were highest in erosional and depositional areas downstream of the WWTP and in erosional areas upstream of the plant. Thus, the combination of the two methods for evaluating denitrification provided more insight into the spatial dynamics of denitrification activity than either approach alone. Denitrification appeared to be a significant sink for NO 3 − –N upstream of the WWTP, but not downstream. Approximately 46% of the total NO 3 − –N load was removed via denitrification in the upstream, urban section of the stream, while only 2.3% of NO 3 − –N was lost downstream of the plant. This result suggests that controlling NO 3 − –N loading from the plant could result in considerable improvement of downstream water quality.