Caitlin Young and Gilbert N. Hanson, Stony Brook University
Nitrate pollution dominantly from septic tank/cess pool sewage and lawn fertilizer has become a concern for Suffolk County Groundwater Authority (SCWA) water supplies. Munster (2004) and Bleifuss (1998) have used anions, cations and d15N and d18O isotopes to distinguish between nitrate contamination sources from lawn fertilization and sewage. Munster, oral communication 2008, suggests that in more densely populated areas using septic tanks sewage is the dominant source of nitrate. Porter et al (1980) suggested that Suffolk County groundwater shows a deficit of nitrate given the high input of nitrogen originating from sewage and lawn fertilizer. Nitrogen from a septic tank/cess pool system enters the unsaturated zone as ammonium. Some of the ammonium in the anaerobic zone around a cess pool can be retained by absorption on the mineral grains and organic material. Nitrogen can also be removed by ammonia volatilization in this zone or later by denitrification after oxidation to nitrate. Given the low pH and oxidizing conditions in the unsaturated zone, ammonia volatilization is unlikely to account for the entire nitrogen deficit. Porter et al (1980) suggested that this imbalance can be explained at least in part by denitrification.Xu (2005) found, based on dissolved oxygen measurements, that Suffolk County groundwater is oxic, not allowing for denitrification. However, Eh/pH measurements suggest that there are reducing environments which would allow denitrification. We plan to measure denitrification by quantifying excess N2 by calculating changes in N2:Ar ratios. Infiltrating groundwater has the same N2:Ar ratio as precipitation. Denitrification produces excess N2 in the groundwater. This excess can be measured using Membrane Inlet Mass Spectrometry (MIMS) which is suitable for detecting even small extents of denitrification.
2008 NGWA Conference on Eastern Regional Ground Water Issues
