Measuring the Thermal Conductivity of Sediments for the Estimation of Groundwater Discharge to Surface Waters with Temperature Probes

Abstract:

Using temperature probes is a common exploratory method for studying groundwater-surface water interaction due to the ease for collecting measurements and the simplicity of the different analytical solutions. This approach requires to define the surface water temperature, the groundwater temperature and a set of parameters (density and specific capacity of water, and thermal conductivity of sediments) that can be easily extracted from tabulated values under the assumption that they are homogeneous in the study area. In the case of the thermal conductivity, it is common to apply a standard value of 1.84 Wm^-1 C^-1 corresponding to sand. Nevertheless the environments where this method is applied, like streambeds or lake/lagoons shores, are sedimentary depositional systems with high energy and biological activity that often lead to sediments dominated by organic matter or sharp changes in grain size modifying greatly the thermal conductivity values. In this study, the thermal conductivity was measured in situ along transects where vertical temperature profiles were collected in a coastal lagoon bed receiving groundwater discharge (Ringkøbing Fjord, Denmark). A set of 4 transects with 10-20 temperature profiles during 3 different seasons was analyzed together with more than 150 thermal conductivity measurements along the working transects and in experimental parcels of 1 m² where the cm scale spatial variability of the thermal conductivity was assessed. The application of a literature-based bulk thermal conductivity of 1.84 Wm^-1 C^-1 instead of field data that ranged from 0.62 to 2.19 Wm^-1 C^-1, produced a mean flux overestimation of 2.33 cm d^-1 that, considering the low fluxes of the study area, represents an increase of 89 % and up to a factor of 3 in the most extreme cases. The changes in thermal conductivity can alter the estimated fluxes hindering the detection of patterns in groundwater discharge and modifying the interpretation of the results.