Denitrification removes nitrogen from watersheds under reducing conditions, but N₂O and CH₄, both greenhouse gases, can also be produced. The overarching hypothesis of my thesis was that hydric environments accumulate N₂O and CH₄ in groundwater and the vadose zone. To test the hypothesis, groundwater samples were taken monthly during 2007-2009 at 64 piezometers in 10 wetlands for analysis of excess N₂, N₂O, CH₄, and CO₂. Vadose zone gas and groundwater samples were taken during 2008-2010 at two riparian buffers and a hydrologically restored wetland.

The hydrology of the 10 locations was complex. A hydrologic connection across a transect was determined at one location where NO₃- significantly decreased, excess N₂ significantly increased, and moderate concentrations of N₂O and CH₄ accumulated. Within these 10 locations, three N₂O and four CH₄ hot spots were identified, and hot moments accounted for a large percentage of total accumulated N₂O and CH₄. I found evidence of CH₄ ebullition, the production of CH₄ bubbles in the vadose zone that strip other dissolved gases. The locations that accumulated the most dissolved CH₄ and N₂O were natural wetlands and riparian areas, respectively.

I measured both positive and negative excess N₂ concentrations in the vadose zone. Flux estimates ranged from -600 to 880 kg N ha-1 yr-1, which brackets missing N estimates at the watershed scale. These concentrations were calculated using N₂/Ar, and both gases are affected by physical processes. These calculated excess N₂ profiles could have been produced through either biological and/or physical mechanisms, and these processes currently cannot be distinguished. Less than 1% of the missing N on the transect scale, measured as the difference in N concentration between two piezometers, was accounted for by calculated diffusional fluxes from groundwater to the vadose zone.

The primary mechanism transporting gases from the vadose zone to the atmosphere was diffusion, but convection transported 20% of the calculated median CO₂ yearly flux. Increased production of N₂O and CO₂ was observed in the vadose zone after rainfall events. Overall, large concentrations of N₂O, CH₄, CO₂, and excess N₂ accumulated in the groundwater and vadose zone of these locations, supporting the overarching hypothesis.