Estimating fluid residence time in deep sedimentary aquifers is one of the most challenging problems in hydrological sciences. The Dogger limestone aquifer of the Paris Basin (1500-2000 m depth) has been intensively studied for its low-enthalpy geothermal and oil resources (Aubertin et al., 1967); Rojas et al., 1989; Wei et al., 1990; Marty et al., 1988; Matray et al., 1994), yet no method proved to be fully adapted to date the Dogger groundwaters, and indirect approaches like radiogenic noble gas accumulation (Marty et al., 1993), 14C (Wei et al., 1990) and pressure head measurements (Aubertin et al., 1967) led to somewhat contradictory view. Previous hydrological modelling suggested a wide range of fluid residence times from 103 to 105 yrs (Aubertin et al., 1967; Wei et al., 1990). These relatively fast hydrological ages were inconsistent with the chemistry of the waters (Rojas et al., 1989), their stable isotopic composition (Matray et al., 1994), and the high radiogenic helium contents (Marty et al., 1988; Marty et al., 1993), which suggest older ages on the order of 106 a. Hence independent techniques are needed for constraining the fluid movement in this aquifer.
We have sampled four geothermal wells tapping waters in the Dogger aquifer close to the Paris metropolitan area and in the South, at Fontainebleau. The abundance of the atmosphere-derived noble gas Ne, Ar, Kr and Xe has been measured. It is known that these gases are dissolved in the groundwater at the recharge and their solubility is temperature-dependant (Andrews and Lee, 1979). Based on the noble gas recharge paleotemperatures, we can estimate the recharge period and consequently the residence time of the waters in the aquifer. The present study represents also one of the first attempts to use this method in a deep aquifer subjected to macrodispersion (mixing) and megadispersion (leakage through aquifers) of the original noble gas signal, being the majority of the works focused in very young confined aquifers.
The calculated noble gas paleotemperatures range from 15šC to 25.9šC (±1.5 šC) and show a good correlation with the d18O, which is conservative in this aquifer (Matray et al., 1994). The points falls close to the linear relationship d18O-paleotemperatures calculated for the maritime western Europe (Evans et al., 1984) at present. The calculated paleotemperatures are 5-15šC higher than those expected for a Holocene (interglacial) or Late Pleistocene (Wurm glacial) recharge, and are consistent with the d18O values, depleted in light isotopes, which indicate warmer conditions than those prevailing during most of the Quaternary (Matray et al., 1994). The highest temperature (25.9šC) suggests clearly sub-tropical climate conditions which are characteristic of the Early Cenozoic (Eocene-Oligocene) (Savin, 1977). The noble gas recharge paleotemperatures increase with the distance from the southern recharge and the salinity of waters. The lowest temperature has been recorded in freshwaters close to the actual recharge (Fontainebleau), while the highest temperature has been measured in saline waters, located at the center of the basin. The temperature distribution is consistent with recent hydrological modelling of the Dogger aquifer (Menjoz et al., 1993), which show higher fluid velocities in the Fontainebleau area, while the center of the basin is hydrodynamically confined and where the highest salinities have been recorded. Here, it is thought that invasions of Triassic brines have taken place (Matray et al., 1994). This distribution suggests that the original Dogger recharge noble gas signal has been affected by leakage of older and warmer saline paleofluids from Trias. However, mass balance calculation indicate that the Trias paloecomponent in the Dogger is the 2-10% by mass. This contribution have not affected markedly the original Dogger noble gas signal and we propose that the Dogger meteoric waters, responsible for the dilution of the invading Triassic brines, have also a Tertiary origin. The present-day bowl-shaped geometry of the Paris Basin has been reached in the Eocene time with only minor changes in the Oligocene (Matray et al., 1994). Such conditions could allowed preservation of warm Tertiary meteoric waters in its central deepest part. Successive mixings with more recent freshwaters from the southern recharge are probably responsible for the actual temperature distribution in the basin. Such a mixing could explain the contradictoy views (hydrological versus geochemical approach) concerning the timescale of Dogger fluid movement.
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