Estimating groundwater discharge to surface waters using heat as a tracer in low flux environments: the role of thermal conductivity

Analytical modelling of heat transport was used to address effects of uncertainty in thermal conductivity on groundwater-surface water exchange. In situ thermal conductivities and temperature profiles were measured in a coastal lagoon bed where groundwater is known to discharge. The field site could be divided into three sediment zones where significant spatial changes in thermal conductivity on metre to centimetre scale show that spatial variability connected to the sediment properties must be considered. The application of a literature-based bulk thermal conductivity of 1.84Wm(-1) degrees C-1, instead of field data that ranged from 0.62 to 2.19Wm(-1)degrees C-1, produced a mean overestimation of 2.33 cm d(-1) that, considering the low fluxes of the study area, represents an 89% increase and up to a factor of 3 in the most extreme cases. Incorporating the uncertainty due to sediment heterogeneities leads to an irregular trend of the flux distribution from the shore towards the lagoon. The natural variability of the thermal conductivity associated with changes in the sediment composition resulted in a mean variation of +/- 0.66 cm d(-1) in fluxes corresponding to a change of +/- 25.4%. The presence of organic matter in the sediments, a common situation in the near-shore areas of surface water bodies, is responsible for the decrease of thermal conductivity. The results show that the natural variability of sediment thermal conductivity is a parameter to be considered for low flux environments, and it contributes to a better understanding of groundwater-surface water interactions in natural environments. Copyright (C) 2015 John Wiley & Sons, Ltd.