HR: 0800h
AN: H51A-0341    [Abstracts]
TI: Nitrate attenuation in the Missoula Flood Deposits Aquitard (Willamette Silt) of the Willamette Valley, Oregon
AU: Arighi, L
EM: louisarighi@ucscalumni.com
AF: Oregon State University, Dept. of Geosciences 104 Wilkinson Hall, Corvallis, OR 97331-5506
AU: * Haggerty, R
EM: haggertr@geo.oregonstate.edu
AF: Oregon State University, Dept. of Geosciences 104 Wilkinson Hall, Corvallis, OR 97331-5506
AU: Myrold, D D
EM: david.myrold@oregonstate.edu
AF: Oregon State University, Dept. of Crop and Soil Science 3017 ALS, Corvallis, OR 97331
AU: Iverson, J
EM: jiverson@golder.com
AF: Golder Associates, 9 Monroe Parkway, Suite 270, Lake Oswego, OR 97035
AU: Baham, J E
EM: john.baham@oregonstate.edu
AF: Oregon State University, Dept. of Crop and Soil Science 3017 ALS, Corvallis, OR 97331
AU: Madin, I P
EM: ian.p.madin@dogami.state.or.us
AF: Oregon Department of Geology and Mineral Industries, 800 NE Oregon St., Suite 965, Portland, OR 97232
AU: Arendt, J
EM: ARENDT.Jack@deq.state.or.us
AF: Oregon Department of Environmental Quality, 750 Front St. NE Suite 120, Salem, OR 97301-1039
AB: Low-permeability geologic units may offer significant chemical and hydraulic protection of adjacent aquifers, and are important for managing groundwater quality, especially in areas with significant non-point source contamination. Nitrate in the Willamette Valley is attenuated across the Willamette Silt, a semi-confining unit overlying a regionally important aquifer. To quantify the main mechanism responsible for nitrate attenuation, soil cores were taken at 19 locations, and profiles of nitrate concentrations were constructed for each site. In 7 locations a sharp, major geochemical transition - a "redoxcline" - is present near the base of the Willamette Silt; this redoxcline is characterized by a color change from red-brown to blue-gray, an increase in iron(II) concentration, a rise in pH, and the appearance of carbonate minerals. At all sites where a significant surface input of nitrate was detected, the nitrate signal was attenuated before reaching the base of the silt. Denitrifier Enzyme Activity assays from one site show no denitrification potential in the profile, suggesting that a non-biological mechanism is responsible. We suggest that iron(II) is reducing the nitrate abiotically to nitrite, and that the blue-gray reducing zone of Willamette Silt is indicative of the presence of sufficient iron(II) for the reaction to go forward. To increase the usefulness of this study to regional water management agencies, a thickness isopach map of the reduced zone was created both for the northern and southern Willamette Valley to help determine areas where nitrate is most likely to be attenuated.
DE: 0469 Nitrogen cycling
DE: 0471 Oxidation/reduction reactions (4851)
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
SC: Hydrology [H]
MN: Fall Meeting 2005