Graduate Thesis Or Dissertation

 

Investigation of the hydraulic, physical, and chemical buffering capacity of Missoula Flood Deposits for water quality and supply in the Willamette Valley of Oregon Public Deposited

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  • The Willamette Silt is a surficial geologic unit composed of successive Missoula Flood Deposits that underlies 3100 km2 (1200 mil) of arable land in the Willamette Valley of Oregon. The Willamette Silt protects the underlying regionally important Willamette Aquifer from agricultural contamination while acting as a semi-confining unit and a diffuse recharge source. This primary study of the hydrogeologic and geochemical properties of the Willamette Silt incorporates extensive data collection, field work, 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. Initial calculations of flow regimes show that groundwater in the Willamette Silt (WS) at the field area flows at approximately 5.6x10 -7 m/s at a dip of 60 degrees downward toward deeply incised streams. At this rate, conservative agricultural species would be expected to reach the Willamette Aquifer approximately 23 years after fertilizer application to the surface. However, after more than 57 years of fertilizer application, the observed phosphorus and nitrate penetration fronts are located approximately half way through the Willamette Silt. Phosphorus is a non-conservative solute that is retarded through sorption to clay and silt particles, which allow the WS to act as a phosphorus sink. 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. Further understanding of the rate of propagation of the reduction-oxidation boundary will require more study. Numerical model analysis of a pump test conducted in the Willamette Aquifer shows that 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. Volumetric balance analysis shows that less than 1% of the water removed from the aquifer at a pumping well near the river was recharged to the Willamette Silt from the Pudding River.
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