U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Small-Scale Tracer Tests Applied to the Measurement of in situ Denitrification Rates in a Sewage-Contaminated Aquifer, Cape Cod, Massachusetts

Abstract

Small-scale natural-gradient tracer tests were used to measure in situ rates of denitrification in a nitrate-contaminated aquifer. Tracer solutions contained bromide, a conservative tracer, and acetylene, an inhibitor of nitrous oxide reductase. Breakthrough of the tracer solution and production of nitrous oxide were monitored at multilevel samplers approximately 10 meters downgradient from the injection multilevel sampler for a period of up to 40 days. Calculated rates of denitrification from two tracer tests were 620 and 410 nanomoles N₂O per liter of aquifer per day. These rates are on the low end of the range of reported rates in aquatic sediments, and are similar, but lower than, previous rate measurements made at the Cape Cod site by using flask and whole-core incubation techniques. Results of this study suggest that incubation techniques using aquifer sediments might slightly overestimate in situ denitrification rates in ground water. Peak concentrations of nitrous oxide lagged 2 days behind the peak concentrations of the conservative tracer in both tests. Work is currently underway to examine possible mechanisms for this result, and to model it by using a modified one dimensional advection-dispersion model containing a Michaelis-Menten term for nitrous oxide production.