Abstract: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 1.84 Wm À1°CÀ1 , instead of field data that ranged from 0.62 to 2.19 W m À1°CÀ1, produced a mean overestimation of 2.33 cm dÀ1 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 À1 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

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Groundwater discharge to a brackish lagoon (Ringkøbing Fjord) was quantified with seepage meters along four transects perpendicular to the shore during four seasons in 1 year. The objectives were to develop a conceptual model of the spatiotemporal variability of seepage and how landscape characteristics and saltwater intrusion affect seepage fluxes. The electrical conductivity of the groundwater was measured in vertical sediment profiles up to 3 m below the lagoon bed to assess the effects of the freshwater-saltwater interface location on flux distribution. The main differences compared with existing theoretical models are a lower discharge near the shore line (0-5 m) and two discharge peaks more offshore (15-20 and >25 m), which allowed the development of a conceptual model that is different from the classical concepts introduced for marine or lake environments and which can be representative of other similar areas. These differences are explained by the vegetation and organic material deposition in the near-shore environment reducing discharge, the presence of the saline wedge leading to upward flow, and the increase of recirculation of brackish water due to wave pumping in areas offshore most exposed to the wind. The seasonal variability in discharge is smaller than the spatial variability between and within transects along the shoreline. Based on the conceptual model, fresh water discharge in a 20-m-wide fringe was calculated to be between 66 and 388 L·day −1 ·m −1 shore line, corresponding to a difference of a factor of six due to the variations in coast morphology and local characteristics of the area. The seasonal changes were responsible for changes of only 8% to 75% depending on the location. KEYWORDS groundwater discharge to lagoon, seepage meter, spatial-temporal variability, variable salinity