Analysis and simulation of the geochemical transfers within an aquifer: the Beauce ground-water and the alteration of the Fontainebleau Sands

Aims. - Ground-water mineralisation results mainly from alteration of the c onfining 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 aqu ifer have been studied to highlight such ground-water/aquifer interactions. The Beauce ground-water is contained in thick limestone series (Oligo-Mioce ne) interlayered with the Fontainebleau Sands (Stampian). The sands show va rious facies related to the geomorphology : dark and reduced facies are res tricted to the bottom of the Formation in the center of the plateaux, oxidi zed yellowish facies occur in the vadose zone beneath the plateaux, and ble ached white facies at the edge of the plateaux. These Facies result from mo dern alterations due to the ground-water and the recharge waters. The water and rock chemistry allows to investigate the diverse stages of the geochem ical and weathering processes that occur within the aquifer, and to calcula te accurate mass balance of the dissolved and exported elements. Hydrochemistry of the Beauce ground-water. - Each compartment of the aquife r can be characterised by one type of water. (1) The aquifer is recharged b y two types of water. The waters coming from the limestone plateau are calc o-carbonaceous, oxygenated and lightly mineralised, especially in silica : those coming from the sandy soils near the outcropping sand area are more a cidic, 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 alte ration 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(-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 contai ns hydrogen sulfur and is oxygen free (Eh around +140 mV), with relatively high Fe and SO4 contents related to the pyrite oxidation. It is also charac terised by a relatively high SiO2 and cations contents resulting from the a lteration of the alumino-silicates. (4) The ground-water of the bleached sa nds aquifer is strongly diluted by the recharge waters from the sand-v soil s. It remains oversaturated with respect to quartz and has the ability to h ydrolyse the alumino-silicates. Geochemical modelling. - The ground-water/aquifer interactions have been mo delled. The modelisation takes into account the kinetics of water/rock inte ractions 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. Th e pyrite oxidation and the << buffering >> calcite dissolution directly dep end on oxygen availability in the ground-water. Muscovite remains stable. f eldspars and glauconite are partially dissolved and Al-smectite forms. The composition of the resulting simulated oxydized sands agrees with the analy ses. (2) The leaching of these oxydized sands by the calco-carbonaceous rec harge 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 r and by the acidic recharge water brings about to simulate bleached sand. Th e 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 bala nce calculations. The discrepancy mainly comes from the simulation of exces sive amounts of clay minerals and iron oxides. This precipitation excess ma y be related to the fact that the precipitation kinetics and/or of the orga nic complexations have not been taken into account in the model. Even if th eie is discrepancy in the mass balance, the thermokinetic simulations valid ate 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 alter ation processes and indicates that the alte rations are presently active. T he white facies results from the progressive alteration of dark primary san ds. This bleaching leads to a complete change of the geochemical characteri stics 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.