New methods have been developed and applied for dating porewater in hydraulically impermeable rock core samples (no groundwater mass flow) by means of stable isotopes, dissolved noble gases and of NaCl. Vertical profiles of dD and radiogenic 4He are presented for a claystone aquitard in the Gorleben area and for claystones and caprock in the Morsleben area (both in northern Germany). Isotope and noble gas data of conventional groundwater samples, pumped from the inflow of fracture water into the borehole fit very well in the obtained porewater profiles. In Morsleben these profiles show the occurrence of Holocene porewaters down to 100 m depth. In the depth range from 100 - 300 m, isotopically depleted palaeowaters of the last glacial period were found. In the claystones underneath and in the caprock formation, waters show noble gas ages increasing with depth up to 100 million years.