Hillslope Dissolved Organic Matter Transport and Transformation in a Semi-Arid Headwater Catchment

Date_______________ The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline ABSTRACT Dissolved organic matter (DOM) is a ubiquitous mixture of compounds formed from the degradation of both terrestrial and microbial materials. It is a key link across the terrestrial-aquatic interface in headwater catchments, and therefore is intimately linked with the hydrologic connectivity of the catchment to the stream. This study uses fluorescence spectroscopy and parallel factor analysis (PARAFAC) to evaluate the mobility of specific chemical constituents of DOM during snowmelt. Monitoring occurred on a daily basis within the shallow soil (10 – 25 cm depth) and the stream during snowmelt and was compared to approximately bimonthly groundwater samples (~18 m depth). Results suggest that a transition occurred in the stream during snowmelt from DOM dominated by protein-like material to more humic-like material. This transition is indicative of an engagement of DOM originating from shallow soils during snowmelt. Dissolved organic carbon (DOC) normalization of these loadings suggest that the peak in DOC concentration seen in the stream is mainly controlled by the non-fluorescent fraction of DOM. This study identifies a humic-like component fluorophore in the soil and the stream that is traditionally seen only in soil DOM. We propose that the presence of this component in our stream samples during high flow in a small headwater catchment but not in larger systems during variable flow, suggests that this humic-like component is preferentially processed within the stream. These results indicate that shifts in hydrologic connectivity of iv different watershed units to the stream are a major control on DOM export from the watershed and that DOM mobility is unique to its chemical composition. v ACKNOWLEDGEMENTS I first would like to thank my primary advisor, Dr. Holly Barnard, for her consistent guidance and support throughout the thesis process. She not only contributed to the written product, but engaged in critical and intellectual discussion that furthered the process-based thinking that allowed this thesis to come to fruition. I would also like to thank my co-advisor, Dr. Diane McKnight, for providing intellectual conversations regarding fluorescence spectroscopy, PARAFAC, and dissolved organic matter processes as a whole. This guidance was an invaluable asset to the completion of this thesis. I thank Dr. Mark Williams for his intellectual contribution to the understanding of …