[AHW32-01] Geologic controls on source water drive baseflow generation and geochemistry across watershed scales
★Invited Papers
Keywords:Groundwater, Baseflow, Dissolved-Carbon
Baseflow sources were quantified for six sub-watersheds in a structurally complex watershed in coastal California using endmember mixing model analysis and examined with respect to potential spatial controls (e.g., topography, geology, land use, and soil characteristics). Both the shallow zone (e.g., shallow subsurface contributions) and deeper groundwater reservoirs were important for stream discharge during low flow periods, and the distribution of baseflow sources across subwatersheds was directly related to geologic indices. A binary classification of underlying bedrock permeability (e.g., permeable versus not permeable) best explained the changes in the proportions of shallow subsurface water and groundwater inputs in individual subwatersheds through the seasonal baseflow recession. Dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and specific UV absorbance at 254 nm (SUVA254) were used to provide additional insight on endmember characteristics and their contributions to baseflow; stream DIC concentrations were broadly controlled by mixing of groundwater and shallow subsurface water endmembers with relatively static DIC concentrations, while stream DOC concentrations reflected both spatial and temporal changes in shallow subsurface water DOC endmember geochemistry. Results from this study show: (1) the importance of considering baseflow as a dynamic process with the potential to receive water from multiple sources, (2) the effect of geology on source geochemistry and flow contributions at the subwatershed scale during low flow conditions, and (3) the utility of using stream dissolved C concentrations for supplemental insight into geochemical variability in baseflow sources during low flow periods.