HEAT TRACING OF TEMPORAL CHANGES IN VERTICAL FLUX THROUGH STREAMBED SEDIMENTS (Invited Presentation)

Rates of surface water and groundwater (SW-GW) interaction can be highly variable over time due to temporal changes in sediment hydraulic conductivity, storm events, and changes in stream stage due to natural or regulated river flow. There are few effective field methods to make continuous measurements of SW-GW exchange rates with the temporal resolution required in many applications. Controlled laboratory experiments were used to explore the accuracy of analytical solutions to the 1D heat transport model for capturing temporal variability of flux from propagation of a periodic temperature signal to depth. Column experiments were used to generate 1D flow of water and heat through saturated sand with a quasi-sinusoidal temperature oscillation at the upstream boundary. Measured flux rates were compared to modeled flux rates derived using the VFLUX computer model and the amplitude ratio between filtered temperature records from two depths in the column. Imposed temporal changes in water flux through the column were designed to replicate patterns of flux observed in the field. Field observations of temporal changes in flux were made over multiple days during a large-scale storm event and diurnally during seasonal baseflow recession. Temporal changes in flux that occur gradually over days, sub-daily, and instantaneously in time can be accurately measured using the 1D heat transport model, although those temporal changes may be smoothed over time. Filtering methods effectively eliminate artificial temporal flux patterns otherwise imposed by perturbations of the temperature signal, which result from typical weather patterns during field investigations. Although previous studies have indicated that sub-cycle information from 1D heat transport modeling is not reliable, this laboratory experiment shows that sub-cycle changes in flux can be observed using heat transport modeling. 1D heat transport modeling provides an easy-to-implement, cost effective, reliable field tool for making continuous observations of SW-GW exchange through time, which may be particularly useful for monitoring exchange rates during storms and other conditions that create temporal changes in hydraulic gradient across the streambed interface or changes in streambed hydraulic conductivity.