ANALYSIS OF ISOTOPIC AND THERMAL TRACERS TO EXAMINE REGIONAL VERSUS LOCAL GROUNDWATER CONTRIBUTIONS TO SEASONAL STREAMBED FLUX PATTERNS

Seasonally and spatially varying sources of groundwater are examined in terms of impacts on cross-sectional patterns in fluxes through the streambed. Stable isotopes of water are analyzed in combination with thermal tracing techniques to examine the influence of changes in local and regional groundwater conditions on streambed fluxes across three cross-sections of Fish Creek, a tributary of the Snake River, WY. Streambed piezometer cross-sections were established across the width of the channel to sample surface water and groundwater for deuterium (D) and oxygen-18 (¹⁸O), along with continuously monitored temperatures and periodically monitored water levels for the three hydrologically different stream reaches. Isotopic analyses are used to identify spatially varying seasonal groundwater sources, and compared to magnitudes and directions of streambed fluxes derived from calibrated two-dimensional simulations of streambed temperature. At the watershed scale, this analysis reveals systematic variations in the D and ¹⁸O of groundwater sources due to evaporative fractionation; while at the local scale, analysis indicates that cross-sectional streambed flux variability is due to several factors, including recharge-related, seasonally varying source fluctuations in groundwater levels, cross-sectional variation in streambed hydraulic conductivities, and for the special case of low vertical hydraulic gradients, variations in streambed surface topography. Combined analyses of isotopic and heat tracers illustrate that complexity of streambed fluxes is influenced by both temporal and spatial variability in regional versus local groundwater flow patterns. Results confirm that traditional reach-scale streamflow measurements, such as differential stream-discharge measurements (aka seepage runs), often lack the resolution to identify variability in streambed fluxes that impact localized biogeochemical processes, cumulatively leading to overall stream and near-stream water quality as well as resulting stream and benthic ecology on the reach scale.