Abstract:Based on a sediment vibro corer, a tool for the sampling of sub-seafloor groundwater aquifershas been developed and successfully deployed in a coastal area of the western Baltic. Thedevice was designed to obtain pure groundwater samples from coarse sediments to be used fortracer investigations and CFC age dating.Operated from a medium size research vessel, a well pipe tipped with a filter segment isvibrated into the sediment down to the aquifer. Groundwater entering the filter is pumped tothe ship by a conventional submersible pump situated in the well's filter tip. Groundwater iscontinuously analysed on board for O2, salinity, pH, Eh and temperature, prior to sampling forCFC and radioisotope analysis. All parametres indicate that pure groundwater had beenobtained.CFC concentrations are very low suggesting that the groundwater of this shallow sub-seaflooraquifer recharged prior to 1950. This finding is in accordance with other hydrogeologicalevidence that this aquifer, located only 4-5 metres below the seafloor, is connected to fairlydeep confined sandy aquifers on land of Pleistocene or Miocene age.Applying the method described, it is possible to obtain sufficient sample volumes foranalyses of natural groundwater tracers such as radon-222 and CFCs which can beused to trace submarine groundwater discharge as well as the origin ofgroundwater in such environments.Keywords: submarine springs, groundwater, pockmark, submarine well, CFC, radon