Shallow Groundwater Denitrification Rate Measurement by Acetylene Block

Abstract

AbstractWe field tested a short term, in situ method for measuring highly variable shallow groundwater denitrification rates using a methodology developed in an earlier, laboratory phase of this study. A pulse of tracer solution was injected and subsequently sampled within well clusters 50 cm in radius. The injectate contained dissolved acetylene (C2H2) to block denitrification at the nitrous oxide (N2O) stage of reduction. Six well clusters were established at two depths along a riparian area transect through field, grass, and woodland ecotypes. Bromide tracer, C2H2, and accumulated N2O were monitored at sample wells downgradient from each injection well. Adequate C2H2 concentrations (0.43 mM) to block N2O reduction between injection and sample wells were attained in five of the six well clusters tested. Acetylene peak concentrations moved laterally an average of 19% more slowly than the Br peak, N2O peak concentrations an average of 8% after C2H2 peaks. Highest denitrification rates of 2.8 and 3.3 mg N L−1 d−1 were observed near the surface of the water table within the grass buffer and riparian forest. Shallow groundwater denitrification rates in the adjoining field and 2 m deep into the saturated zone at all three ecotypes were <25% of the highest rates. Acetylene block procedures were thus used to determine in situ denitrification rates within a few days time with little disruption of soil structure or associated microbial communities. Portable equipment allows application at remote sites independent of line power and road access.