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A Hydrogeologic Framework for Understanding Watershed-Scale Surface Water and Groundwater Interactions in Western Oregon, USA Public Deposited

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  • Stream water quality, including stream temperature, has become an increasingly important issue in recent years. Warming streams can harm aquatic ecosystems by pushing fish from traditional breading grounds, forcing them to migrate, adapt, or perish. Recent research from Scripps Institute of Oceanography at UC San Diego has shown that climate change is influencing surface water temperatures more than expected. Oceans, lakes, and streams are warming at faster rates than previously predicted, which could disrupt ocean cycles and have additional worldwide impacts on climate. These are realities that politicians and land managers must prepare for, especially in times of increasing water demand. This project, conducted in the Oak Creek Watershed, near Corvallis, Oregon, USA, presents a unique opportunity to study how geologic and anthropologic variables might influence hydrologic attributes including groundwater flow and stream temperature. Our goals were to characterize the hydrogeologic framework of the Oak Creek Watershed geographical area, to examine the connection between surface water and groundwater along the extent of the stream profile, and to examine the combined effects that geology, land use, and elevation may have on surface water and groundwater temperature. We used previous geologic and hydrologic research done in the Oak Creek region, logs from deep explorations wells located near the project site, along with field measurement data to create a geologic conditions map and hydrogeologic cross-sections of the Oak Creek region. A hydraulic potentiomanometer was developed and deployed along the stream profile to investigate surface water-groundwater relationships and identify areas of the stream that might be gaining or losing streamflow due to groundwater interactions. Static water levels from groundwater monitoring wells (18) installed throughout the watershed and publicly available well logs (620) were used to create a potentiometric surface map of the Oak Creek Watershed and surrounding region. These products were then applied to create a conceptual model of shallow, deep, and lateral groundwater flow throughout the watershed and surrounding region. Evaluation of the effect of the Oak Creek Watershed hydrogeologic framework on stream temperatures was done through analysis of data collected from multiple air (4), stream (21), groundwater (4) and stage (1) sensors installed at select locations along the length of the stream. Data was collected every hour, converted to 7-day average of daily maximum (7-DADMax) values, and then grouped based on geology, land use, or elevation variables. Data from individual sensors in each group were averaged to create a daily 7-DADMax value that was then compared using graphing and box plots, Pearson correlation, and analysis of variance (ANOVA) statistical models. Results indicated that geology is the primary factor controlling surface water-groundwater connections within the Oak Creek Watershed and surrounding area. Contrasting permeability architecture between highly fractured volcanic basalt juxtaposed against sedimentary geology influences the flow dynamics of groundwater as it moves down-gradient through the watershed. This was shown to be exaggerated near the Corvallis Fault interface, which acts as a hydraulic barrier and deflects groundwater. Potentiomanometer results suggest the decrease in permeability at the fault zone slows down-gradient groundwater flow and produces an expanse of elevated groundwater levels upslope of the fault. Statistical analysis of temperature data collected throughout the watershed revealed a linear relationship between stream and ambient temperatures, and between stream reaches grouped based on geology, land use, or elevation variables. The most significant influence on stream temperature was shown to be ambient temperature and stream gradient. Test groups in higher elevations showed greater statistical difference in means than test groups from lower elevations. There was no apparent statistical impact of geology, land use, or groundwater on stream temperature. It is most probable these components along with other external variables influence the connection between surface water and groundwater and consequently stream temperature. This project demonstrated that geology is the key component regulating surface water-groundwater interactions, while stream temperature has been shown to be chiefly related to ambient temperatures and baseflow conditions. It is probable then that a combination of these components control watershed hydraulic interactions and water quality. The results of this research may be useful to watershed and land managers considering improving surface water quality and the connections between surface water and groundwater, or to agencies interested in aquifer storage and recovery projects in nearby fractured basalt aquifers.
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  • Hall, Jesse L. (2020). A Hydrogeologic Framework for Understanding Watershed-Scale Surface Water and Groundwater Interactions in Western Oregon, USA. Oregon State University
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