The effect of topography driven groundwater flow on deep subsurface temperatures in the Roer Valley Graben (southern Netherlands)

Numerous heat flow and thermochronological studies have documented suspected thermal effects of deep groundwater flow. However, the high uncertainty and heterogeneity of subsurface permeability hampers model studies of the thermal effect of groundwater flow. New subsurface temperature data from the Roer Valley Graben, a Cenozoic rift basin in the southern Netherlands, show up to 20 °C variation in temperature in the upper 1000 m of the basin's sediments, which could be related to groundwater flow, as they do not coincide with any known trends in thermal conductivity of the sediments or crustal thinning. Groundwater salinity data could offer additional constraints on the extent of topography driven flow systems. Data from the Roer Valley Graben show that the fresh-salt water boundary is located up to 250 m below the base of Pliocene-Quaternary continental deposits, which suggests freshening of the underlying marine sediments by meteoric water. The lowest values of groundwater salinity are located in the southern part of the basin, which coincides with the lowest subsurface temperatures. We explore the extent and thermal effect of topography driven groundwater flow in the basin using the numerical model code Rift2D, which couples groundwater flow, heat flow and solute transport. Model permeability was based on 2500 core-plug and pumping test data. The distribution of permeability was adjusted to match the simulated to the observed groundwater salinity data. Model results show that topography driven flow decreases temperatures in the upper ~1000 m of the basin by values of up to 20 °C, in spite of the relatively low relief of 130 m. The strong effect of the topography driven flow system results from relatively high permeability of the unconsolidated clastic infill and a low continuity of shallow marine and lacustrine clayey sediments. These result imply that topography driven flow can significantly alter subsurface temperatures in basins even in cases where only a limited relief is present. The fresh-salt water interface (left) is located at a greater depth than the base of Pliocene-Quaternary continental deposits (right), suggesting flushing of underlying marine formations by a topography-driven flow system.