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United States Department of Agriculture

Agricultural Research Service

Title: Dissolved Gas Analysis for Assessing the Fate of Nitrate in Wetlands

Authors
item Mookherji, Swati
item McCarty, Gregory
item Angier, Jonathan

Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 15, 2002
Publication Date: N/A

Interpretive Summary: Riparian forest buffer strips along streams are important for removing agricultural nitrogen from water to improve the quality of rivers, lakes and coastal waters. Use of buffer strips is an important part of best management practices for improving water quality in the Chesapeake Bay watershed. The Natural Resources Conservation Service (NRCS) has initiated an important task to develop two million miles of buffers along our nation rivers and streams. The lack of direct methods for measurement of removal of agricultural nitrogen in groundwater makes it difficult to routinely monitor effectiveness of buffers for cleanup of agricultural nitrogen from groundwater. We developed a relatively simple and precise method based on the measurement of dissolved gas concentrations in groundwater where increases in nitrogen gas is indicative of nitrogen removal from water. This method is capable of accurately measuring nitrogen removal from groundwater by detecting small changes in nitrogen gas concentrations. It can be utilized for assessing seasonal and spatial variation of nitrogen concentrations in groundwater and for evaluating effectiveness of riparian buffers.

Technical Abstract: Dissolved gas analysis permits direct detection of groundwater denitrification, a technique we used in this study to assess the fate of nitrate in a riparian wetland. Dissolved argon (Ar) and dinitrogen (N2) were measured in transects of nested piezometers installed at different depths within upwelling regions of a riparian wetland. The method uses the Ar content in the water as a natural inert tracer for assessing background content of N2 from the previous air-water equilibrium. Within the wetland under study, anoxic to suboxic groundwater became more oxic in piezometers close to the aquifer layer, indicating upwelling of oxic groundwater. Assessment of nitrate and AR loss in groundwater within an upwelling zone indicated that shallow piezometers had significant N2 loss through degassing. Most of the measured nitrate-nitrogen (NO3-N) loss of 205 uM in a piezometer nest could be accounted by the total N2-N produced (169 uM N) which was calculated from changes in dissolved N2 and estimated N2 degassed. Degassing due to methane (CH4) production was also detected in some shallow piezometers within the nests. This technique for analysis of dissolved gases in groundwater can be applied to detect small changes in N gas concentration and will aid in assessing the fate of nitrate along a groundwater flow path.

Last Modified: 7/30/2014
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