Piezometric level

The NATA site consists of tailings overlying relatively low permeability clay soil, gravel-sand, and bedrock units. There are two aquifers one is confined between the bedrock and clay, and another unconfined aquifer which developed within the NATA tailings. Vertical exchange between aquifers is limited due to the clay separation. This is demonstrated by the higher piezometric levels of the unconfined aquifer compared to the confined one. 

Fig. 11.10 Piezometric levels (in masl) of the Albian aquifer. Recharge seems to occur in the peripheries, giving rise to down-gradient flow toward Paris in the confined Albian aquifer. This hydrological hypothesis has been tested by 14C dating (Fig. 11.11). (Following Evin and Vuillaume, 1970.)...

Source I Interstitial water or a confined low-conductivity aquifer in which solutions develop to high sulfate concentrations. The sulfate which may have originated in Paris (Montmartre) gypsum has a heavier isotopic composition, being probably the sulfate remaining partly through reduction. These are found to the west of Mt. Mesly, down the piezometric level. 

We may consider that at present the flow system is stable in the whole system, i.e. the aquifer receives and supplies the same flow. The water volume corresponding to the recent lowering of piezometric levels remains quite small as compared to the flow rates of intake and output. 

Numerous hydraulic tests were carried out, during the various project phases, either in an isolated borehole or between boreholes. Hydraulic experiments consisted of pulse tests, constant head injection and/or extraction and constant rate injection/extraction using borehole intervals delimited in FBX 95001, FBX 95002, BOUS 85.001 and BOUS 85.002 (Figure 1). Both water flow and piezometric level measurements for borehole intervals are available. Continuous pressure monitoring has allowed detecting crosshole responses during pulse testing these have then been used to calibrate the hydraulic conductivities of fractures. 

Local values are obtained by processing the mean values of piezometric levels. Distances between piezometric tappings were 10 cm in the zone of maximum drag reduction and 20 cm in the rest of the duct. 

The groundwater divide and piezometric level are at a higher elevation than that of the proposed top water level. In this situation, no significant water loss takes place. 

Sagging-collapse sinkhole formation (Figure 3a) is possible with any hypsometric positions of overkarst groimdwater level and karst water piezometric level. 

Phreatic piping-collapse sinkhole formation (Figure 3b) is possible only if overkarst ground-water level is higher than karst water piezometric level. 

Figure 3. Sinkholes mainly caused by dissolution, ascend of overkarst groundwater level (GVL) or/and descend of karst water piezometric level (KWPL). (a) Sagging-collapse sinkhole, (b) Phreatic piping-collapse sinkhole.

The communication established between the shallow aquifer and one of the gypsums through the sinkholes systems (Figure 22), resulted to a rapid circulation towards the hole of the parking, both of the water housed in the cavities of the gypsum as of the Quatemaiy aquifer, resulting in a decrease of the piezometric levels up to 6 meters in some areas. 

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