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Title: ACETYLENE TRANSPORT IN SATURATED SOIL MESOCOSM FOR IN SITU DENITRIFICATION RATE MEASUREMENT

Author
item BRAGAN, ROBERT
item Starr, James
item Parkin, Timothy

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/8/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: Riparian area and wetland denitrifying activity offer opportunities to diminish excessive groundwater nitrate concentrations but methods to measure denitrification rates in such areas have been inadequate to date. Researchers and managers are unable to routinely differentiate the effects of plant uptake, dilution by deeper groundwater, and denitrification. Uncertainty continues about the remedial effects of riparian and wetland areas. We developed a new, shallow groundwater denitrification rate measurement method by applying the widely accepted acetylene block incubation procedures directly to saturated soils using a cluster of wells. One well is used to inject acetylene saturated water that flows through intervening soils to be sampled at other wells in the cluster. The pulse is repeatedly sampled to measure accumulated nitrous oxide. This provides a direct measurement of denitrification rates occurring within the waterlogged soil. Testing of this approach was done at Beltsville in a field scale chamber filled with soil. The results demonstrate that the method should be effective in measuring shallow groundwater denitrification rates. A subsequent paper will demonstrate that the method was successfully applied to a riparian buffer site on Maryland's Eastern Shore.

Technical Abstract: Nitrate concentrations frequently diminish in shallow groundwater as it is laterally transported through the rhizophere of riparian zones and intermittent wetlands. Denitrification may be a major mechanism for this loss, but methods for direct measurement of denitrification have been limiting. The objective of this study was to determine if C2H2 can be introduced and maintained in a groundwater system in concentrations high enough to efficaciously block nitrous oxide reductase. Field-scale method development and testing was conducted in the laboratory, using a 1.4 m3 mesocosm filled with sandy subsoil. Movement of Br and C2H2 injected into a center well was monitored by sampling from 25- and 50-cm radial wells. The Br tracer showed relatively uniform flow rates to the 25-cm wells, and then varied by a factor of two to the 50-cm wells. Aqueous C2H2 moved to all wells in a breakthrough curve (BTC) pattern similar to that of the Br tracer. Most of the C2H2 BTC concentrations exceeded that required to sto denitrification at the N2O stage. In situ denitrification rates were derived from BTCs of N2O, following the pulsed injection of C2H2. These results suggest that this procedure would be successful for direct measurement of denitrification losses in natural shallow groundwater systems.