LOW FLUX LIMITATIONS ON THE USE OF HEAT AS A TRACER IN GROUNDWATER-SURFACE WATER INTERACTIONS

The movement of water across the groundwater-surface water (GW-SW) interface is essential for the health and function of water bodies. GW-SW interactions control a variety of processes including nutrient cycling, pollutant attenuation, and water supply. To quantify these fluxes, heat as an environmental tracer is often used. A well-established and commonly used method, heat as a tracer relies on the advective transport of heat across the GW-SW interface. Multiple methodologies have been developed that rely on the collection of temperature time series data that are collected from at least two unique depths a set vertical distance apart. Signal processing techniques are then used to analyze either the amplitude ratio or phase shift between the upper and lower temperature sensor to determine flow direction and fluxes. As the flux across the GW-SW interface approaches zero, conduction becomes the dominant component of heat transport, resulting in the quantification of erroneous flux values. The impacts of these near-zero flux rates on the amplitude ratio method have not been fully explored, and the lower limit at which this method is able to accurately quantify fluxes has not been identified. To examine the limitations of this method, synthetic temperature time series data were generated at varying precisions for various combinations of head gradients, sensor spacing, and thermal properties of the sediment using a numerical model. Fluxes were subsequently quantified using the amplitude ratio method and the dimensionless Péclet number, the ratio of advective to conductive heat transport, for each scenario. Using the Péclet number, criteria were established to identify the extent to which conduction controls the transport of the thermal regime. Results provide bounds on when and under what conditions these generated fluxes can be considered true and representative of the system.