The Willamette Silt is a surficial hydrogeologic unit, composed of successive layers of fine grained Missoula Flood Deposits, that underlies 3100 km2 (1200 mi2) of arable land in the Willamette Valley. The Willamette Silt (WS) protects the underlying regionally important Willamette Aquifer (WA) from agricultural contamination while acting as a diffuse recharge source to the WA and a buffer to depletion of streams bottoming in the WS. This primary study of the hydrogeologic and geochemical properties of the Willamette Silt incorporates extensive data collection, field techniques, laboratory analyses, and numerical modeling to provide a characterization of the hydraulic parameters, groundwater flow regime, agricultural leachate penetration, and buffering capacity of the unit.

Numerical model analysis of a pump test conducted in the Willamette Aquifer reveals that the storage capacity of the Willamette Silt provides a source of diffuse recharge to the WA under stressing conditions. Further, the low hydraulic conductivity of the unit provides a hydraulic buffer to depletion of streams bottoming in the WS under pumping stress generated in the underlying WA. Numerical model volumetric balance analysis is used to demonstrate the hydraulic buffering capacity of the WS under differing stress scenarios.

Calculations of flow regimes in the Willamette Silt indicate that conservative agricultural species would be expected to reach the Willamette Aquifer approximately one year after fertilizer application to the surface. However, after more than 18 years of inorganic fertilizer application, the observed nitrate penetration front is located approximately half way through the Willamette Silt. The nitrate penetration front is coincident with a geochemical reduction-oxidation boundary, giving reason to believe that the WS is preventing nitrate (a highly soluble, non-sorbing tracer) transport through facilitation of autotrophic denitrification at this boundary. If this hypothesis proves true, the rate at which the reduction-oxidation boundary is propagating downward through the Willamette Silt is essential information for managing the water quality of the WA and streams bottoming in the WS.