Sampling of sub-seafloor aquifers by a temporary well for CFC age dating and natural tracer investigations

Based on a sediment vibro corer, a tool for the sampling of sub-seafloor groundwater aquifers has been developed and successfully deployed in a coastal area of the western Baltic. The device was designed to obtain pure groundwater samples from coarse sediments to be used for tracer investigations and CFC age dating. Operated from a medium size research vessel, a well pipe tipped with a filter segment is vibrated into the sediment down to the aquifer. Groundwater entering the filter is pumped to the ship by a conventional submersible pump situated in the wellʹs filter tip. Groundwater is continuously analysed on board for O2, salinity, pH, Eh and temperature, prior to sampling for CFC and radioisotope analysis. All parameters indicate that pure groundwater had been obtained. CFC concentrations are very low suggesting that the groundwater of this shallow sub-seafloor aquifer recharged prior to 1950. This finding is in accordance with other hydrogeological evidence that this aquifer, located only 4–5 m below the seafloor, is connected to fairly deep confined sandy aquifers on land of Pleistocene or Miocene age. Applying the method described, it is possible to obtain sufficient sample volumes for analyses of natural groundwater tracers such as radon-222 and CFCs which can be used to trace submarine groundwater discharge as well as the origin of groundwater in such environments.