Method for exploiting and/or monitoring an underground formation comprising at least one reservoir containing water, at least one gas species and at least one atmospheric isotope of at least one rare gas being present in dissolved form in the water of said reservoir, characterized in that at least the following steps are carried out: i- at least one gas sample is taken at a water collecting zone of said reservoir, which zone is situated at the surface of said formation, said sample being taken in such a way as to avoid any contamination with the air, using a sampling cylinder made up of a tubular body made of metal or of glass, isolated from the outside by a valve at each end; ii- at least the concentration of at least said gas species and the concentration of at least said atmospheric isotope of said rare gas in said gas sample are measured; iii-the concentration of said gas species present in said reservoir in dissolved form is determined by means of a model of said concentration of said gas species in dissolved form, said model being a function of said concentration of at least said gas species in said gas sample and of said concentration of said atmospheric isotope in said gas sample, wherein said model is based on an analytical method relying on a formula of the type: C j T = 1 / K Hj + C i air / C i v. K Hi amb − 1 / K Hi z. C j v where: - C j T is said concentration of said gas species of interest j; - K Hj is the Henry constant relating to said gas species of interest j; - C i air is the concentration of said atmospheric isotope i in the air; - C i v is said concentration of said atmospheric isotope i measured in said sample; - K Hi amb is the Henry constant relating to said atmospheric isotope i for surface ambient conditions; - K Hi Z is the Henry coefficient of said atmospheric isotope i at the depth z; - C j v is said concentration of the gas species of interest j measured in said sample, or else said model is based on an iterative numerical method involving resolving a direct problem on each of the iterations of said iterative numerical method and optimizing an objective function measuring a discrepancy between said measured concentrations and concentrations calculated by said resolution of said direct problem. i- at least one gas sample is taken at a water collecting zone of said reservoir, which zone is situated at the surface of said formation, said sample being taken in such a way as to avoid any contamination with the air, using a sampling cylinder made up of a tubular body made of metal or of glass, isolated from the outside by a valve at each end; ii- at least the concentration of at least said gas species and the concentration of at least said atmospheric isotope of said rare gas in said gas sample are measured; iii-the concentration of said gas species present in said reservoir in dissolved form is determined by means of a model of said concentration of said gas species in dissolved form, said model being a function of said concentration of at least said gas species in said gas sample and of said concentration of said atmospheric isotope in said gas sample, wherein said model is based on an analytical method relying on a formula of the type: C j T = 1 / K Hj + C i air / C i v. K Hi amb − 1 / K Hi z. C j v where: - C j T is said concentration of said gas species of interest j; - K Hj is the Henry constant relating to said gas species of interest j; - C i air is the concentration of said atmospheric isotope i in the air; - C i v is said concentration of said atmospheric isotope i measured in said sample; - K Hi amb is the Henry constant relating to said atmospheric isotope i for surface ambient conditions; - K Hi Z is the Henry coefficient of said atmospheric isotope i at the depth z; - C j v is said concentration of the gas species of interest j measured in said sample, or else said model is based on an iterative numerical method involving resolving a direct problem on each of the iterations of said iterative numerical method and optimizing an objective function measuring a discrepancy between said measured concentrations and concentrations calculated by said resolution of said direct problem. where: - C j T is said concentration of said gas species of interest j; - K Hj is the Henry constant relating to said gas species of interest j; - C i air is the concentration of said atmospheric isotope i in the air; - C i v is said concentration of said atmospheric isotope i measured in said sample; - K Hi amb is the Henry constant relating to said atmospheric isotope i for surface ambient conditions; - K Hi Z is the Henry coefficient of said atmospheric isotope i at the depth z; - C j v is said concentration of the gas species of interest j measured in said sample, - C j T is said concentration of said gas species of interest j; - K Hj is the Henry constant relating to said gas species of interest j; - C i air is the concentration of said atmospheric isotope i in the air; - C i v is said concentration of said atmospheric isotope i measured in said sample; - K Hi amb is the Henry constant relating to said atmospheric isotope i for surface ambient conditions; - K Hi Z is the Henry coefficient of said atmospheric isotope i at the depth z; - C j v is said concentration of the gas species of interest j measured in said sample, or else said model is based on an iterative numerical method involving resolving a direct problem on each of the iterations of said iterative numerical method and optimizing an objective function measuring a discrepancy between said measured concentrations and concentrations calculated by said resolution of said direct problem.