In-Situ Method to Determine Soil Thermal Conductivity at Sites Using Thermal Monitoring to Quantify Natural Source Zone Depletion
At petroleum release sites, Natural Source Zone Depletion (NSZD) rates can be calculated based on measured vertical temperature gradients above and below the methane oxidation zone if the thermal conductivity (KT) of the soils is known. Current practice is typically to estimate the thermal conductivity using literature values based on soil type, or less frequently, using an instrument applied ex-situ to soil cores brought to the surface. The first method is not site-specific, while the second method results in a one-time measurement that can be variable and inaccurate. In this study, an in-situ method was applied to calculate soil thermal conductivity based on seasonal change in soil temperatures . This method requires at least four complete seasons (one year) of high frequency (e.g., daily) temperature data collected to measure NSZD rates, data which are commonly collected at many NSZD thermal monitoring sites. When applied at four locations across two sites, the thermal conductivity values obtained using this in-situ method were comparable to literature values and less variable than ex-situ thermal conductivity measurements. Overall, the in-situ method yielded thermal conductivity values for the four locations and various depth intervals between 0.30 and 1.37 W/m-K in the vadose zone, and 1.25 to 1.94 W/m-K for locations in the capillary fringe or saturated zone, similar to literature estimated values. The in-situ method did not appear to be reliable when using temperature data from very shallow depths (<0.6 m) due to highly variable temperature signals, or from below depths of 7.3 and 5.8 m bgs for Site A and Site B, respectively, due to the attenuated temperature variations (amplitude of 0.5 °C) below these depths. This in-situ method appears to be a useful alternative to ex-situ measurements or literature estimates. In addition, the insitu method can be used to track changes in thermal conductivity that may occur from year to year due to changes in soil moisture content.