Environmental Health - Toxic Substances
U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 3 of 3--Subsurface Contamination From Point Sources, Water-Resources Investigations Report 99-4018C
Nitrification in a Shallow, Nitrogen-Contaminated Aquifer, Cape Cod, Massachusetts
By Daniel N. Miller, Richard L. Smith, and John Karl Bohlke
Little is known about nitrification in ground-water environments when compared to marine systems and surface soils. Ground-water geochemistry near the upper boundary of a shallow, sewage-contaminated ground-water plume on Cape Cod, Mass., indicated a transition zone where O2 [24 micromolar (µM)] and NH4+ (37 µM) coexist. The occurrence and rate of nitrification in this zone were investigated by a combination of isotopic, biogeochemical, microbial, and molecular techniques. 15N values of the NH4+ increased from +13 per mil (‰) within the NH4+ plume to +31‰ in the transition zone consistent with partial nitrification of the NH4+ . Core incubations under nitrifying conditions demonstrated that nitrifying organisms were present and indicated a low, but measurable potential activity. Molecular analysis of core DNA also specifically detected Nitrosomonas eutropha DNA in sediment extracts. A small-scale, natural-gradient tracer test was conducted with 15N-enriched NH4+ and Br- as tracers. Transport of NH4+ was at least four times slower than transport of the conservative Br- tracer. A low nitrification rate (13 to 96 nanomole per liter aquifer per day) was calculated from the natural-gradient tracer test data. From this study, we conclude that nitrification can occur in ground-water environments and can play a significant role in the speciation and transport of nitrogen.