COGEMA La Hague nuclear reprocessing plant is located in the North West of Cotentin peninsula near Cherbourg (France). This nuclear plant releases radioelements in atmosphere and in the English Channel. About 8.5 TBq.year-1 of radiocarbon are released as the liquid wastes through a pipe a few kilometres off sea shore, West of the reprocessing plant. Recent studies in the peninsula show anomalous higher radiocarbon contents in vegetation near the coast that have suggested a supplementary marine contribution through the degassing of the 14 C excess supplied by liquid releases of the nuclear plant. Carbon dioxide partial pressure, 14 C activities were measured in air and sea water in the Bay of Seine and around the COGEMA-La Hague nuclear reprocessing plant during three cruises in 2000 and 2002. Results show clearly that sea is a source of CO 2 and 14 C to the atmosphere. Higher 14 C concentrations in air and water related to the La Hague liquid wastes are clearly recorded. The aim of this paper is to show results of these oceanographic campaigns. Flux between seawater and atmosphere are calculated in the NorthWest Cotentin and in Bay of Seine.