[en] Thermal neutrons produced by cosmic rays or nuclear weapon tests interact with atmospheric nitrogen resulting in the formation of radiocarbon which, after oxidation into carbon dioxide, follows the natural carbon cycle. The partial pressure of carbon dioxide in the soil is several times that in the atmosphere due to plant root respiration and decay of organic matter. Water absorbs biogenic carbon dioxide while percolating through the unsaturated zone. The carbon content of groundwater is mainly in the form of bicarbonate ions. The extraction of carbon from water sample as barium carbonate is carried out in the field. Benzene is synthesised from the carbonate sample. The activity of radiocarbon in the synthesised benzene is determined by using a liquid scintillation analyzer. Details of sampling procedure, benzene synthesis, counter calibration and treatment of sample data have been given. 7 figs. (author)