Nitrate attenuation in the Missoula Flood Deposits Aquitard (Willamette Silt) of the Willamette Valley, Oregon

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.