The influence of geomorphic setting on ground water denitrification in forested riparian wetlands
Riparian wetlands are potentially significant landscape features for controlling groundwater nitrate export to surface waters. Reach- and watershed-scale variability hinder efforts to predict riparian denitrification potential and manage riparian zones based on functional assessments. Groundwater denitrification is influenced by the presence of subsurface carbon and by flow paths. Of the three major geomorphic settings of glaciated regions (glacial till, glacial outwash, and alluvium), outwash and alluvium were considered. Riparian wetlands situated in outwash and alluvium are frequently characterized by low-gradient, deep, stratified sediments transmitting high groundwater fluxes, with considerable retention times possible. Research objectives were to (1) compare the vertical pattern and extent of groundwater denitrification in riparian wetlands in outwash vs. alluvium, and (2) assess the potential for groundwater interaction with biologically active zones in these settings. To quantify groundwater denitrification we modified an in situ method based on 15N-enriched nitrate. We measured denitrification rates at 65, 150, and 300 cm depths in wetland soils at four sites (two per setting). No significant difference was found in denitrification magnitude or pattern between settings. At three sites there was no significant difference in denitrification among depths. At deeper depths, rates were significantly higher closer to the stream. At each site we measured piezometric heads along a transect perpendicular to the stream during a period of sustained high water table. We used a groundwater model to estimate associated flow paths, and fluxes through the riparian subsurface. Modeling results showed no relationship between setting and flow patterns. At all sites evapotranspiration dominated the hydrologic budget. Outflux to the stream was <10% of the total outflux at all sites. A nitrogen budget using a hypothetical upland development scenario showed three of the four sites could potentially remove or store ∼25% of the upland groundwater N load, with ∼75% carried further downgradient to emerge as either evapotranspiration or baseflow. Minimal N would have reached the stream. Low flux to the stream during periods of elevated evaporative demand suggests that non-hydric and till riparian areas provide much of the baseflow at this time, arguing for protection of infiltration and natural flow patterns throughout the watershed. ^
Hydrology|Agriculture, Forestry and Wildlife|Environmental Sciences
Dorothy Q. N Kellogg,
"The influence of geomorphic setting on ground water denitrification in forested riparian wetlands"
Dissertations and Master's Theses (Campus Access).