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Low Elevation Old Channel Features of the Willamette River Floodplain Support High Subsurface Denitrification Rates
Citation:
Forshay, Ken, Bart Faulkner, A. McElmurry, R. Brooks, P. Mayer, S. Cline, AND R. Adeuya. Low Elevation Old Channel Features of the Willamette River Floodplain Support High Subsurface Denitrification Rates. Presented at 97th Ecological Society of America Meeting, August 05 - 10, 2012.
Impact/Purpose:
Presentation for the 97th Annual Ecological Society of America Meeting on August 5-10, 2012 in Portland, OR
Description:
Background/Question/Methods: Large river floodplains are poor nitrate pollution buffers when polluted groundwater moves beneath biogeochemically retentive zones prior to entering the main channel. However, in floodplain regions with extensive backwaters and organic carbon accumulation, surface waters may interact with groundwater to support enhanced denitrification in the subsurface and provide a sink for nitrate pollution. Here, we evaluate the spatial heterogeneity and patterns of nitrate and denitrification in the shallow groundwater of a large river floodplain to determine the controls and find predictable indicators of enhanced denitrification to better identify habitats for restoration and preservation that enhance pollution removal. Green Island, part of the Willamette River floodplain, near Coburg, Oregon is under active restoration, led by the McKenzie River Trust, to hydrologically re-connect the Willamette to its historic floodplain and re-establish native vegetation. We measured rates of denitrification using an in-situ push-pull technique with 15N isotopic tracers injected into shallow groundwater ~0.5m below the water table across the floodplain from 0.5-4m below the sediment surface. These denitrification rates, combined with quarterly sampled nutrient concentration, ambient isotopes of N and O in nitrate, and evaluation of subsurface hydrology will help identify habitats that support enhanced denitrification and guide future restoration activities in the region. Results/Conclusions: Subsurface denitrification rates at Green Island span three orders of magnitude across the floodplain (Mean 57.1±17.8 µMol Kg-1D-1 n=31) with the greatest rates beneath wet habitats (Max 307.7) and lowest in those areas with the greatest depth to groundwater (Min 3.65 to below detection). Denitrification rates in the subsurface are correlated negatively with elevation (ρ=-0.48 p<0.01) and positively with both dissolved organic carbon (0.45 p<
URLs/Downloads:
ORD-000120 ABSTRACT FORSHAY_KEN.PDF (PDF, NA pp, 28.839 KB, about PDF)ORD-000120 PRESENTATION FORSHAY_KEN.PDF (PDF, NA pp, 6668.679 KB, about PDF)