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 argo n were measured in 29 water samples collected from five superposed aqu ifers in the Paris Basin (Ypresian, Albian, Neocomian, Dogger, and Tri as). In all these groundwater flow systems the data showed excesses of He-3, He-4, and Ar-40 above the solubility equilibrium with the atmos phere (air saturated water (ASW)), as well as vertical concentration g radients of these isotopes throughout the basin. The water of the Dogg er and the Trias formations also had a Ne excess above ASW values. The mean rate of radiogenic and nucleogenic isotope production in the ent ire sedimentary sequence of the basin cannot produce the measured quan tities 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 i sotopes is thus believed to originate from the bedrock, and the isotop es are transported vertically through the entire basin. Three mechanis ms of transport are considered: advection, dispersion, and molecular d iffusion. For the Trias the radiogenic/nucleogenic production ratios o f 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 f or the high isotope ratios observed in the Dogger. In the vertical dir ection, He-4 is transported mostly by diffusion, whereas Ar-40 is tran sported mostly by advection. Neon 21 represents an intermediate situat ion. The distribution of He-3, He-4, Ne-21, and Ar-40 throughout the b asin can be explained by the existence of only two sources: an atmosph eric component contributed by recharge water and a radiogenic/nucleoge nic component originating mainly in the bedrock. This concept is suppo rted by two-dimensional cross-section modeling of the water flow in th e Paris Basin and the advective, dispersive, and diffusive transport o f these noble gases in its multiaquifer system presented in part 2 of this study.