Isotopic Apportionment of Atmospheric and Sewage Nitrogen Sources in Two Connecticut Rivers

• Published in: Environmental science & technology 2007-09, Vol.41 (18), p.6363-6369
• Main Authors: Anisfeld, Shimon C, Barnes, Rebecca T, Altabet, Mark A, Wu, Taixing
• Format: Article
• Language: English
• Subjects: Air Pollutants - analysis; Air Pollutants - chemistry; Applied sciences; Atmosphere; Connecticut; Continental surface waters; Denitrification; Earth sciences; Earth, ocean, space; Effluents; Engineering and environment geology. Geothermics; Environmental Monitoring - methods; Exact sciences and technology; Geography; Isotopes; Natural water pollution; Nitrogen; Nitrogen - analysis; Nitrogen - chemistry; Nitrogen Isotopes; Pollution; Pollution, environment geology; Rivers; Sewage - analysis; Sewage - chemistry; Sewage disposal; Water Movements; Water Pollutants, Chemical - analysis; Water Pollutants, Chemical - chemistry; Water treatment and pollution
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es070469v

We used the dual isotope approach to identify sources of nitrate (NO3 -) to two mixed land-use watersheds draining to Long Island Sound. In contrast to previous work, we found that sewage effluent NO3 - was not consistently enriched in 15N. However, these effluents followed a characteristic denitrification line in δ15N−δ18O space, which could be used as a source signature. We used this signature, together with those of atmospheric deposition and microbial nitrification, to calculate ranges of possible contributions from each of these sources. These estimates are unaffected by any denitrification that may have taken place in soils or streams. Our estimates for atmospheric nitrogen only include unprocessed atmospheric deposition, i.e., NO3 - that is not taken up in watershed soils before being delivered to rivers. Using this method, the contribution of atmospheric NO3 - could be assessed with good precision and was found to be very low at all our sampling sites during baseflow. During a moderate storm event, atmospheric deposition contributed up to ∼50% of stream NO3 -, depending on the site, with the sites that experienced more stormflow showing a greater contribution of atmospheric NO3 -. Our estimates of sewage contribution generally had too large a range to be useful.