Analyse et simulation des transferts geochimiques au sein d'un aquifere; la nappe de Beauce et l'alteration des Sables de Fontainebleau

Abstract

Abstract Ground-water mineralisation results mainly from alteration of the confining aquifer along its flow path. Such alteration may deeply alter the aquifer formations and have to be recognized to avoid misinterpretation of the altered facies. The Beauce ground-water and the Fontainebleau Sands aquifer have been studied to highlight such ground-water/aquifer interactions. The Beauce ground-water is contained in thick limestone series (Oligo-Miocene) interlayered with the Fontainebleau Sands (Stampian). The sands show various facies related to the geomorphology: dark and reduced facies are restricted to the bottom of the formation in the center of the plateaux, oxidized yellowish facies occur in the vadose zone beneath the plateaux, and bleached white facies at the edge of the plateaux. These facies result from modern alternations due to the ground-water and the recharge waters. The water and rock chemistry allows to investigate the diverse stages of the geochemical and weathering processes that occur within the aquifer, and to calculate accurate mass balance of the dissolved and exported elements. Hydrochemistry of the Beauce ground-water: Each compartment of the aquifer can be characterised by one type of water. (1) The aquifer is recharged by two types of water. The waters coming from the limestone plateau are calco-carbonaceous, oxygenated and lightly mineralised, especially in silica; those coming from the sandy soils near the outcropping sand area are more acidic, oxygenated and very lightly mineralised. (2) The ground-water of the limestone aquifer are oxygenated, oversaturated with respect to quartz and near equilibrium with calcite and cristobalite. Silica comes from the alteration of the clay minerals and/or the cherts in the limestones. The ground-waters are of calco-carbonaceous type, have pH values approx 6.5. They are characterised by a relative low mineralisation (average 460 mg.kg (super -1) ), an oversaturation with respect to quartz and the presence of dissolved oxygen (Eh around +450 mV). (3) The ground-water of the dark sands aquifer contains hydrogen sulfur and is oxygen free (Eh around +140 mV), with relatively high Fe and SO 4 contents related to the pyrite oxidation. It is also characterised by a relatively high SiO 2 and cations contents resulting from the alteration of the alumino-silicates. (4) The ground-water of the bleached sands aquifer is strongly diluted by the recharge waters from the sandy soils. It remains oversaturated with respect to quartz and has the ability to hydrolyse the alumino-silicates. Geochemical modelling: The ground-water/aquifer interactions have been modelled. The modelisation takes into account the kinetics of water/rock interactions and was done by successive steps. (1) Alteration of the dark sands by the ground-water leads to dissolution of the pyrite and the calcite. The pyrite oxidation and the "buffering" calcite dissolution directly depend on oxygen availability in the ground-water. Muscovite remains stable, feldspars and glauconite are partially dissolved and Al-smectite forms. The composition of the resulting simulated oxydized sands agrees with the analyses. (2) The leaching of these oxydized sands by the calco-carbonaceous recharge water leads to simulate leached sands of similar composition than the unsaturated sands beneath the plateau. Muscovite remains stable; smectite, kaolinite and hematite form. (3) The final leaching of the former leached sand by the acidic recharge water brings about to simulate bleached sand. The muscovite stability, smectite dissolution and kaolinite precipitation are in conformity with the composition of the white sands. Nevertheless, the modelisation is not fully in agreement with the mass balance calculations. The discrepancy mainly comes from the simulation of excessive amounts of clay minerals and iron oxides. This precipitation excess may be related to the fact that the precipitation kinetics and/or of the organic complexations have not been taken into account in the model. Even if there is discrepancy in the mass balance, the thermokinetic simulations validate the reaction path which leads to the Beauce ground-water mineralisation and the Fontainebleau Sands bleaching. Discussion: Ground-water chemistry thus provides information on the alteration processes and indicates that the alterations are presently active. The white facies results from the progressive alteration of dark primary sands. This bleaching leads to a complete change of the geochemical characteristics of the Fontainebleau Sands. These alterations come with the export of a large amount of material corresponding to a geochemical lowering of the Beauce Plateau of at least 10 m during the Plio-Quaternary period. This example illustrates the importance of the sub-surface alterations due to ground-water flow, that are difficult to gauge from outcrop observation only.