Nitrification in a Shallow, Nitrogen-Contaminated Aquifer, Cape Cod, Massachusetts

By Daniel N. Miller, Richard L. Smith, and John Karl Bohlke

ABSTRACT

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.