Tracage chimique et isotopique des eaux souterraines en relation avec les eaux de lixiviation de terrils, bassin minier du Nord-Pas-de-Calais (France)
Authors: Denimal Sophie , Barbecot Florent , Dever Laurent , Tribovillard Nicolas , Meilliez Francis 
Published: June 2005

Introduction. - In the Nord-Pas-de-Calais region (northern France), the mining activity linked to the coal extraction resulted in the existence of many mine spoils. Most of the time, the choice of the mine spoil location has been made without knowing the potential contamination of the aquifers contained in the underlying formations by substances which can be solubilized and released in relation with the weathering of these mine spoils. The aim of this study is to determine the possible role of the mine spoils in the sulfate mineralization of the chalk aquifer. Previous works [Bernard, 1979; Droz, 1985] allow to consider several possible sources of sulfate in the coal basin: the dissolution of evaporitic minerals present in the Carboniferous limestones Formation which can locally contaminate per ascensum the chalk aquifer; the dissolution of gypsum contained in the Tertiary formations (Argiles de Louvil, Sables d'Ostricourt); sulfates of anthropic origin linked to the waste water discharge and/or to the agricultural practices; finally, the weathering of mine spoils which leads to the oxydation of the pyrite contained in the Carboniferous shales, and can release sulfate ions that may be transferred to the aquifer. Two sites have been selected 30 km to the south of Lille: site 1 lies directly on the Senonian-Turonian chalk while site 2 lies on the sandy-clayey Tertiary formations overlying the chalk formations (fig. 1-3). Geology and hydrogeology. - The waters have been sampled within the chalk aquifer (fig. 2). This water table forms the main resource in drinking water, currently exploited. The chalk aquifer is a free water table except where the Tertiary formations make this water table confined, as is the case for site 2. The recharge of the chalk aquifer is made by the percolation of the impluvium through the microporosity of the chalk with an infiltration velocity of 0.5 to 0.7 m per year [Vachier et al., 1979]. The chalk aquifer flow is schematically SW-NE to the Orchies basin (fig. 1). This flow is caused by the piezometric depression in the Orchies basin linked to industrial pumpage. Since the decrease of these pumpages, we can observe a rise of the chalk aquifer piezometric level evaluated to 10 m in the studied area. Material and methods. - Rock samples have been collected on the surface of the two sites and below it at site 1. Mineralogical analyses have been carried out on the bulk fraction as on the clayey fraction, the elementary analysis of total carbon and total sulphur has also been performed. Two water sampling surveys have been carried out on 19 sites. Sampling has been made from piezometers which reach the chalk aquifer and that are placed close to the mine spoils but also in a radius of 5 to 10 km around. Chemical and isotopic analyses have been made on the waters sampled at hydraulic upstream and dowstream of the mine spoils. In situ measurements have been carried out during the sampling. Piezometric levels have been taken at all the sampling sites. Results and discussion. - The carbon and sulphur contents have shown a superficial leaching of these elements on the mine spoils (fig. 9). The use of isotopic geochemistry and in particular the sulphur isotopes as tracers of the sulfate origin has permitted to identify two contribution sources at the two sites: a "mine spoil" source with a delta 34 S weakly negative (delta 34 S = -2,8 per mil, -3,9 per mil) which corresponds to the oxidation of sulfides contained by the Carboniferous shales and another source (delta 34 S = -20 per mil) corresponding to the gypsum of the "Sables d'Ostricourt" which is present only at site 2 (fig. 7, 10 & 11). This study has revealed the different behaviour of the two sites: for site 1, with a free water-table zone, the mine spoil leaching carries sulfate ions directly to the table whereas in the second site, with a confined aquifer zone, a part of the sulfate ions are reduced once exported to the table (the redox potential presents negative values; fig. 6). This bacterial reduction is made possible by the organic carbon leaching concomitant to the sulphur leaching on the mine spoils. This carbon contribution has been confirmed by the 14 C activity analysis: 14 C activity is characteristic of the chalk aquifer waters at the upstream of the mine spoil and noticeably lower downstream, this decrease can be linked to the "dead carbon" supply from the mine spoils (fig. 8). This organic carbon is involved in bacterially-mediated sulfate reduction (2CH 2 O + SO 4 (super 2-) --&gt; 2HSO 3 (super -) + H (super +) + HS (super -) ). Conclusion. - Both studied mine spoils release sulfate ions to the Chalk aquifer, in response to the weathering and leaching of their surficial part. Using an isotope study-grounded approach, the influence of the mine spoils upon the sulfate enrichment of the water table can be distinguished from that of Cenozoic formations sometimes present between the chalk aquifer and the mine spoil basis. No other sulfate source seems to be involved in the studied area. Together with sulfate ions, the mine spoils export dissolved organic carbon. Both chemical species sustain bacterial activity (sulfate reducers) that develops where the water table is made confined by overlying, almost impervious, Cenozoic deposits. In that case, reducing conditions in the aquifer decrease the amount of sulfate ions present. Thus the simultaneous supply of sulfate and carbon by the mine spoils leads to a self-purification process where the aquifer is confined.