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Newcomb, Dani

Rubenstein School of Environment and Natural Resources

2007

M.S.

Nonpoint source pollution by phosphorus and sediment is a wide-spread problem across the United States and specifically in Vermont and the Lake Champlain Basin. Best management of nonpoint source loading will likely involve a combination of land use and stream channel modifications, but few studies have comprehensively examined the relative importance of land use, streambank instability, and soil phosphorus. Thus, it is important to understand the associations between these characteristics, as well as their overall, relationship to watershed nutrient loading dynamics. The main objectives of this study were (1) to examine the impacts of land use at the watershed and near-stream scales on total suspended solids (TSS), total phosphorus (TP), and soluble reactive phosphorus (SRP), (2) to explore the links between geomorphic condition and phosphorus and sediment concentrations and loads throughout the watershed and at different spatial scales, and (3) to investigate the importance of soil phosphorus concentrations in stream banks in contributing to the overall phosphorus load. TP, SRP, and TSS samples were collected from eight sites located at tributary junctures and one site at the mouth of Hungerford Brook, a 50 km² watershed in the Lake Champlain Basin, under storm and baseflow conditions. Rapid geomorphic assessment (RGA) scores, land use, and soil phosphorus concentrations were collected for reaches upstream of sampling locations. Both nested and unnested design multivariate modeling was used to evaluate the importance of characteristics in the individual subwatersheds (unnested) or the entire upstream watershed (nested). SRP, TP, and TSS were predicted as both concentrations and instantaneous loads, using raw quantifications of subwatershed characteristics as well as these same characteristics standardized by the area of agriculture in the subwatershed. Correlation coefficients and principal components analysis were used to select variables that were used in Akaike information criterion (AIC) model selection and stepwise regression. Unnested variables used were agriculture, agriculture in a streamside buffer, proportion of corn, slope, channel degradation, and soil phosphorus. For the nested design, agriculture, agriculture in the buffer, channel aggradation, RGA score, and soil phosphorus concentrations were used. Best fit models were selected based on AICc scores and overall model R². n ANOVA was also performed on the percent difference between storm flow concentrations and average baseflow concentrations. Results indicate that phosphorus and sediment transport occurs mainly during storm events and concentrations greatly exceed state water quality standards. Concentrations of SRP and TP were significantly lower at the mouth of Hungerford Brook than in upstream subwatersheds, indicating that deposition and storage are occurring in this downstream part of the watershed. SRP concentrations appear to be best explained by agriculture in the riparian buffer, while TP and TSS are influenced by agricultural land use at multiple spatial scales. Agricultural land use was associated with increased stream instability. These findings suggest that additional phosphorus and sediment management, targeted at increasing stream stability and reducing impacts from agriculture, are needed in order to reduce the overall load traveling to Lake Champlain.