Submitted to: Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/10/1997
Publication Date: N/A
Interpretive Summary: Land and water quality managers need to know where denitrifying bacteria can be relied upon to remove excessive amounts of nitrate from groundwater. Better test procedures are needed to do this, particularly for high probability sites with shallow groundwater and forest cover as commonly occur in riparian areas. This paper reports on field tests of new procedures that we developed using the acetylene block technique for direct measurement of the denitrification product nitrous oxide in shallow groundwater. This research reports the first application of the acetylene block technique to freely flowing shallow groundwater, without disrupting soil structure, microbial ecology, and groundwater hydrology. This research, conducted at a riparian site on Maryland's Eastern Shore, not only demonstrates the method, but showed nearly 2 mg/L/day of nitrate removal from shallow groundwater near and in the riparian forest. Rates of denitrification in a nearby grass-covered field were about 20 times smaller. The ability to measure on-site variations in denitrification rates will help in the management of nitrate contaminated groundwater. Use of this cluster well procedure needs to be coordinated with hydrologic information regarding nitrate enriched surface runoff and groundwater flow paths.
Technical Abstract: Denitrification effects on shallow groundwater nitrate loads vary greatly depending upon site specific rates of microbial metabolism and locations of nitrate enriched groundwater flow paths. The objective of this research was to field test in situ procedures for applying the acetylene block method to measure shallow groundwater denitrification rate. Injection and sample well procedures were developed in a soil filled, laboratory chamber during the first part of this study. In this paper we report results of field testing in three sets of cluster wells installed in field, grass buffer strip, and riparian woods. A pulse of bromide, as a water tracer, and acetylene, to block denitrification at the nitrous oxide stage, were injected into the center well. Tracers and nitrous oxide produced were monitored at sample wells 50 to 60 cm from the injection well. Adequate acetylene concentrations (0.4 mM) were maintained at 6 of 7 well clusters tested. Acetylene moved an average of 27% more slowly than Br, and nitrous oxide 25% more slowly than acetylene. Highest denitrification rates of 1.6 and 1.8 mg-N/L/day were observed at the top of the water table at the grass edge and within the riparian forest. Rates in an adjoining field and 2 m deep into the water table were less than 5% of the highest rates. These studies demonstrated the use of cluster well procedures to conduct an in situ acetylene block test for denitrification rates in shallow groundwater areas. The results provide a means to assess site denitrification capacity with little disruption of soil structure and site hydrology.