The study of origin and residence times of Poitevin marsh waters using stable and radioactive isotopes such as 2H, 18O, 3H, 13C, 14C shows that the Poitevin marsh groundwaters salinity is due to seawater which was trapped in the sediments there is 7000 years BP ago. So, these groundwatersare distinguished by a great diversity of their age according to their geographical position. Thus, within Quaternary aquifer, waters are modern in the east and old in the west with residence times greater than 2500 years BP. On the other hand, in the upper Oxfordian formations under the Quaternary cover, they are old with residence times greater than 7000 years BP. So, waters of this territory can be subdivided into two groups. The modern waters arelocated upstream and in the eastern part of Poitevin marsh. The second group are old waters and are located in the western part of Poitevin marsh and under the Quaternary cover. The salinization process of Poitevin marsh groundwaters is therefore not the result of current marine contamination, but due to a marine invasion of the Poitevin marsh by old seawater during the Holocene, there is 7000 years B.P ago.