A COMPARISON OF INFORMED AND UNINFORMED SAMPLING USING HEAT AS A PRELIMINARY INDICATOR OF FOCUSED GROUNDWATER DISCHARGE

Using heat as a tracer to determine the spatial distribution of groundwater inflow into streams has improved sampling approaches and advanced understanding of fluid fluxes in the streambed. One method that has been applied in streams over the past decade is distributed temperature sensing using a fiber-optic cable (FO-DTS). FO-DTS has been used successfully to identify focused groundwater discharge for fluid sampling and vertical flux analysis (“informed measurements”). However, results from informed measurements have not been formally compared to discharge measurements made at pre-determined locations in the same reach (“uninformed sampling”). We deployed an FO-DTS cable along the left, center, and right sides of a small meandering sand-bed stream in the Nebraska Sand Hills. At locations of obvious thermal anomalies, we measured head gradient and estimated hydraulic conductivity to make informed estimates of vertical groundwater discharge (n = 20). We also made uninformed vertical groundwater discharge measurements at approximately 50-m intervals along the cable (n = 10 for left, center, and right sides, respectively; 30 total). A non-parametric U-test showed that the median groundwater discharge from all informed locations (0.17 m d^-1) was significantly higher than for all uninformed locations (0.05 m d^-1). For the left side of the stream, statistical analyses showed that median groundwater discharge was significantly higher for informed sampling (0.31 m×d^-1, n = 10) compared to uninformed sampling (0.02 m×d^-1, n = 10). Median discharge from informed and uninformed sampling on right and center of the stream did not yield significantly different results. However, temperature anomalies were more modest and overall magnitude and variability in groundwater discharge was lower in the center and right of the stream indicating discharge processes there were generally below the detectable threshold for streambed interface FO-DTS. Overall, our results show quantitatively the benefit of using FO-DTS to better capture variability in groundwater fluxes through streambeds. We will also present results regarding FO-DTS informed sampling to characterize vertical groundwater fluxes through a sand bar in a larger river channel, where heat tracing is not typically applied.