Graduate Thesis Or Dissertation

Stream Chemistry in the Oregon Coast Range: Spatial and Temporal Variation

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  • Hydrological and biogeochemical processes occurring throughout a watershed are typically reflected in stream water chemistry, including in coastal watersheds. While Oregon Coast Range streams have been studied in the past, there is a dearth of knowledge in terms of seasonal temporal trends and spatial variability that this study seeks to address. Assessment of potential future impacts of climate change and anthropogenic influence on streams in the region could be complicated by lack of baseline chemical data in areas of low disturbance. Samples of water were collected and analyzed monthly from eight streams in the central Oregon coast throughout the year of 2021. Data were collected on sample pH, specific conductance, temperature, major anions (Br-, Cl-, F-, NO3-, PO43-, SO42-), and total concentrations of a range of elements, some associated with major cation species (Ca, Mg, Na, S, Si, Al, B, Ba, Fe, K, Li, Mn, Sr). The streams sampled included Cummins Creek, Little Cummins Creek, Gwynn Creek, Cape Creek at Cape Perpetua, Ten Mile Creek, Big Creek, China Creek, and Cape Creek at Heceta Head. The findings reported here support initial spatial and temporal hypotheses for the year 2021, in that seasonal variation in stream chemistry is correlated with discharge as ions follow patterns of dilution or enrichment depending on source, while spatial variation is correlated with relative stream size in terms of cross-sectional area. As expected on the long-term temporal scale, there was a statistically significant decrease in NO3-concentration from 1991-2017-2021, in line with suggested reduction in nitrogen-fixing red alder stands. However, in contrast to initial predictions, evidence was found to suggest that temporal trends between 1991-2017-2021 indicate further calcium and magnesium depletion rather than expected accumulation, though this may be explained by sharply rising discharge patterns on these consecutive dates. The primary focus and purpose of this study is to provide a baseline dataset of seasonal patterns in stream water chemistry in a range of Oregon Coastal streams, building upon and complementing data in the existing literature. This dataset could assist in the future development of nutrient budgets for these coastal watersheds, and help initiate further research and monitoring of stream chemistry in the region.
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