AGU Abstract: Spatial heterogeneity in isotopic signatures of baseflow in small watersheds: implications for understanding watershed hydrology

In a few weeks, I’ll be giving the following talk at the American Geophysical Union Fall Meeting in a session on Groundwater/Surface Water Interactions: Dynamics and Patterns Across Spatial and Temporal Scales. My talk will be in Moscone West 3014 at 11:05 am on Wednesday, December 15th, 2010.

Spatial heterogeneity in isotopic signatures of baseflow in small watersheds: implications for understanding watershed hydrology
A. J. Jefferson

Time series of stable isotopes of oxygen and hydrogen in stream water are widely used to characterize watershed transit times and flowpaths, but synoptic sampling of multiple locations within a watershed can also provide useful information about heterogeneity of stream water sources and groundwater-surface water interactions that may affect interpretations of watershed hydrology. Here I present results of same-day baseflow sampling campaigns in low-relief, 0.1 to 100 km² watersheds. More than half of less than 5 km² forested and urban watersheds sampled in this study had variability in ?²H exceeding 2‰ and ?¹⁸O variability exceeding 1‰, substantially larger than the analytical uncertainty. In some cases, the heterogeneity was extreme, with ?²H varying by >10‰ over 150 m in one stream. Some isotopic perturbations occur in conjunction with stream conductivity and temperature changes, and such zones likely reflect localized contributions from fractured crystalline bedrock. In the urban 100 km² watershed, mainstem baseflow isotopes were relatively homogeneous, but ?²H varied by more than 10‰ across tributaries, suggesting that subwatersheds are fed by water with different sources or transit times. Some urban streams were isotopically similar to the municipal water supply, suggesting that water main leakage and wastewater discharge may be locally significant contributors to baseflow. The isotopic heterogeneity of small streams and watersheds suggests that an understanding of groundwater-stream interactions is needed to correctly interpret isotope-based inferences about watershed transit times and flowpaths.