DEUTERIUM AND OXYGEN-18 SAMPLING IN TANDEM WITH CONTINUOUS TEMPERATURE LOGGING INCREASES UNDERSTANDING OF SPATIAL AND TEMPORAL PATTERNS OF STREAM EXCHANGES WITH GROUNDWATER

Seasonally and spatially varying sources of groundwater are examined in terms of impacts on cross-sectional patterns of fluxes through the streambed. Stable isotopic species of water containing deuterium (D) and oxygen-18 (O-18) were analyzed in combination with results from advanced thermal modeling to explore systematic changes in local and regional groundwater conditions on streambed fluxes at different locations along Fish Creek, a 25-km tributary of the Snake River, WY. Streambed piezometer cross-sections were established in three hydrologically distinct reaches of the creek in order to (1) periodically sample surface water and groundwater for D and O-18 and (2) continuously monitor streambed temperatures and hydraulic heads. Isotopic analyses were used to identify spatially varying seasonal groundwater sources, which were then compared to the magnitudes and directions of streambed fluxes derived from calibrated two-dimensional simulations of streambed temperature and pore-water flow. At the watershed scale, this analysis revealed systematic variations in the D / O-18 of groundwater sources caused by varying degrees of evaporative fractionation; while at the local scale, the analysis indicated that shifts in cross-sectional streambed flux patterns are caused by seasonal changes in groundwater levels, spatially varying hydraulic conductivities, and subtle variations in surface topography for the downstream case of low vertical hydraulic gradients. Combined analysis of isotopic and heat tracers illustrates how complexity in streambed fluxes is produced by temporal and spatial variability in regional and local groundwater flow. An important implication of this complexity is that traditional differential stream discharge measurements (“seepage runs”) lack the resolution to identify variability in streambed fluxes at potentially many sections of a stream, potentially impacting understanding of water-resource issues such as stream water quality and ecosystem dynamics.