Noble gases as natural tracers of water circulation in the Paris Basin 1. Measurements and discussion of their origin and mechanisms of vertical transport in the basin

The concentrations and isotopic compositions of helium, neon, and argon were measured in 29 water samples collected from five superposed aquifers in the Paris Basin (Ypresian, Albian, Neocomian, Dogger, and Trias). In all these groundwater flow systems the data showed excesses of He-3, He-4, and Ar-40 above the solubility equilibrium with the atmosphere (air saturated water (ASW)), as well as vertical concentration gradients of these isotopes throughout the basin. The water of the Dogger and the Trias formations also had a Ne excess above ASW values. The mean rate of radiogenic and nucleogenic isotope production in the entire sedimentary sequence of the basin cannot produce the measured quantities of He-3, He-4, Ne-21, and Ar-40, if reasonable water residence times are considered. The total calculated production is estimated to represent less than about 13% of the measured values for all isotopes. The greater part (>87%) of the radiogenic and nucleogenic noble gas isotopes is thus believed to originate from the bedrock, and the isotopes are transported vertically through the entire basin. Three mechanisms of transport are considered: advection, dispersion, and molecular diffusion. For the Trias the radiogenic/nucleogenic production ratios of He-4/Ar-40 and Ne-21/Ar-40 are close to the mean production rate in the crust (4 +/- 3 and 0.96 x 10(-7), respectively). For the overlying Dogger, however, these ratios are much higher (He-4/Ar-40: 10 to 70; Ne-21/Ar-40: 8 x 10-7 to 23 x 10(-7)). We suggest that differences in the vertical diffusive flux of He-4, Ne-21, and Ar-40 are the reason for the high isotope ratios observed in the Dogger. In the vertical direction, He-4 is transported mostly by diffusion, whereas Ar-40 is transported mostly by advection. Neon 21 represents an intermediate situation. The distribution of He-3, He-4, Ne-21, and Ar-40 throughout the basin can be explained by the existence of only two sources: an atmospheric component contributed by recharge water and a radiogenic/nucleogenic component originating mainly in the bedrock. This concept is supported by two-dimensional cross-section modeling of the water flow in the Paris Basin and the advective, dispersive, and diffusive transport of these noble gases in its multiaquifer system presented in part 2 of this study.