Graduate Thesis Or Dissertation

A GIS study of Benton County, Oregon, groundwater : Spatial distributions of selected hydrogeologic parameters

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  • Benton County has experienced substantial growth in the past 30 years, and is expected to continue growing (BCWP 2008). Continued development has occurred in the Willamette Valley, housing development in the nearby hills of the Coast Range is growing. New houses in the foothills of the Coast Range may not use municipal water supply and are generally supplied by domestic or community wells, which pump groundwater sourced from the local uplifted formation of the Siletz River Volcanics (SRV). Continued population growth is expected to rely heavily on the groundwater resources of the SRV. The SRV are a series of accreted Tertiary submarine and subaerial basalt formations stretching from Northern California to Vancouver Island, composed of porous pillow basalt flows with interbedded semi-impermeable silts and shales. Flow occurs via basalt fractures and interflow zones in the SRV. The aquifer structure and flow mechanisms result in discontinuous perched and confined aquifers. Due to the structure and higher gradients of the flow zones in the SRV, aquifers are presumed to be heterogeneous, anisotropic, and leaky. Wells usually penetrate multiple saturated zones before sufficient yield is provided to supply a domicile. The complex, unpredictable nature of the SRV has discouraged hydrogeologic studies, and the majority of studies are performed by consultants to evaluate a new water supply. The most recent comprehensive study of groundwater in Benton County that included the SRV as a water-bearing unit was a USGS Water-Supply Paper by F.J. Frank in 1974. The substantial recent development of the SRV has provided a large amount of new data, including well logs digitized by OWRD. This thesis characterizes the SRV by developing hydrogeologic spatial datasets for Benton County. The study applies GIS methods to spatially distribute well log entries by Public Land Survey System (PLSS) units and by Address, resulting in a representative subset of the original data, with good spatial coverage, moderate resolution, and decent accuracy. Wells located within the SRV are spatially subset to provide insight about the formation, and spatial interpolations of common hydrogeologic parameters are performed leveraging the well distributions. This study found that (relative to Benton County): the SRV have a lower well density, a higher percent of wells in the SRV have positive yields (84%), SRV wells have lower average well yields (19-22 gpm), appear to have a higher frequency of confined groundwater, and have much lower mean specific capacity (0.03 - 0.3 gpm per ft). More importantly, this study has taken a first step towards accomplishing some of the data needs established by Benton County. Additionally, fundamental LIDAR and spring location datasets were prepared for upper Oak Creek Watershed in association with this study, opening the door for subsequent topography-groundwater studies.
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