On the limits of heat as a tracer to estimate reach-scale river-aquifer exchange flux

For the past few decades, heat has been used to estimate river-aquifer exchange flux at discrete locations by comparison of river and groundwater temperature. In recent years, heat has also been employed to estimate reach-scale river-aquifer exchange flux based only on river temperature. However, there are many more parameters that govern heat exchange and transport in surface water than in groundwater. In this study, we analyzed the sensitivities of surface water temperature to various parameters and assessed the accuracy of temperature-based estimates of exchange flux in two synthetic rivers and in a field setting. For the large synthetic river with a flow rate of 63 m³ s^-1 (i.e., 5.44 × 10⁶ m³ d^-1), the upper and lower bounds of the groundwater inflow rate can be determined when the actual groundwater inflow is around 100 m² d^-1. For higher and lower fluxes, only minimum and maximum bounds, respectively, can be determined. For the small synthetic river with the flow rate of 0.63 m³ s^-1 (i.e., 5.44 × 10⁴ m³ d^-1), the bounds of the groundwater inflow rate can only be estimated when the actual groundwater inflow rate is near 10 m² d^-1. In the field setting, results show that the inflow rate must be less than 100 m² d^-1, but a lower bound for groundwater inflow cannot be determined. The large ranges of estimated groundwater inflow rates in both theoretical and field settings indicate the need to reduce parameter errors and combine heat measurements with other isotopic and/or chemical methods.