Abstract:
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.
