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.