Thermal skin effect of pipes in streambeds and its implications on groundwater flux estimation using diurnal temperature signals

Heat tracing is widely used for quantifying groundwater flux across the river-streambed interface. This study investigates the potential for a thermal skin effect (TSE) whereby temperature measured inside a pipe buried in sediment is lagged and damped compared to temperature outside of the pipe, violating the assumption that monitored temperatures are representative of the saturated sediment. Numerical simulations show that 1-3 inch diameter pipes lead to an additional lag of tens of minutes to almost 1.5 h when diurnal temperature signals originating from the river and propagating into the sediment are monitored. The lag increases with pipe diameter and diminishes when steel is used instead of PVC. The temperatures taken from inside the pipe lead to substantial errors when they are used as targets for fitting with a forward heat transport model. The errors are large when the fit is made to the timing of the temperature signals but is small when the fit is made to the amplitude of the signal while ignoring the timing. This is because the TSE leads to little damping of the amplitude of the temperature fluctuations and mostly exhibits as extra lag. Therefore, methods that take advantage of the amplitude of the signal, such as time series analysis using amplitude ratios, are not sensitive to the TSE. Time series data can be corrected by subtracting the response time of the water-filled pipe or by adding the response time as a phase shift to analytical models.