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Acetylene Transport In Shallow Groundwater for Denitrification Rate Measurement

Dep. of Biology, George Mason Univ., Fairfax, VA, 22030;

J. L. Starr

USDA-ARS, Natural Resources Inst., Environmental Chemistry Lab., Beltsville, MD 20705;

T. B. Parkin

USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011 (parkin{at}nstl.gov).

^* Corresponding author (rjbragan{at}aol.com).

ABSTRACT

Nitrate (NO₃) concentrations frequently diminish in shallow groundwater (<3 m) when laterally transported through the soils of riparian zones and intermittent wetlands. Denitrification may be the major mechanism for this loss, but lack of suitable methods for direct measurement of denitrification rates in shallow groundwater limit the ability of field studies to verify this. The objective of this study was to determine if adequate acetylene (C₂H₂) concentrations can be maintained in the intermittently saturated, near surface soils found in riparian areas to temporarily block nitrous oxide reductase in denitrifying bacteria. Accomplishing this would allow use of the acetylene block method for denitrification rate assessment in the soils of riparian areas that are periodically saturated and with high potential for large denitrificafion rates. Method development and testing was conducted in the laboratory, using a 1.4 m₃ mesocosm filled with sandy subsoil with continuous forced-gradient radial flow. Movement of Br tracer and C₂H₂ injected into a central well was monitored by sampling from 25 and 50 cm distant radial wells. Both the tracer and aqueous C₂H₂ showed relatively uniform flow rates to the 25 cm wells, and then varied by a factor of two to the 50 cm wells. Acetylene concentrations exceeded those recommended to block NO_x reduction by denitrifiers at the N₂O stage. In situ denitrification rates were derived from monitored N₂O concentrations. An initial field test provided additional evidence that the procedure may be useful for direct measurement of denitrification losses in a variety of natural, shallow groundwater systems.