Dissolved gas analysis for assessing the fate of nitrate in wetlands

Dissolved gas analysis permits direct detection of ground water denitrification, technique we used in this study to assess the fate of nitrate in a riparian wetland. Dissolved argon (Ar) and dinitrogen (N-2) 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 ground water became more oxic in piezometers close to the aquifer layer, indicating upwelling of oxic ground water. Assessment of loss of nitrate and Ar in ground water within an upwelling zone indicated that shallow piezometers had significant N-2 loss through degassing. Most of the measured nitrate-nitrogen (NO3--N) loss of 205 muM in a piezometer nest could be accounted for by total N-2-N produced (169 muM N), calculated from changes in dissolved N-2 and estimated N-2 degassed. Degassing due to methane (CH4) production was also detected in some shallow piezometers within nests. This technique for analysis of dissolved gases in ground water can be applied to detect small changes in N gas concentration and aids in assessing the fate of nitrate along a ground water flow path.