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Study Data
Arighi Thesis |
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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. |
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