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L'exploitation du charbon dans le Nord-Pas-de-Calais (France) a engendré le dépôtd'importantes quantités de résidus miniers sous forme de terrils, essentiellement constituésde schistes houillers. Le lessivage de ces stériles par les eaux météoriques et l'oxydationdes sulfures de fer contenus sont susceptibles de contribuer à l'enrichissement en sulfateset métaux associés de l'aquifère de la craie, principale ressource en eau de la région. Desanalyses chimiques et isotopiques (S & C) ont été effectuées sur des prélèvements d'eauen amont et en aval hydraulique des sites d'étude ainsi que sur la fraction minérale desterrils. Afin de déterminer les modalités d'infiltration des eaux météoriques au sein desterrils, des prospections radio-magnétotelluriques, couplées à des mesures de perméabilitéet de granularité ont été effectuées. Les analyses ont permis de mettre en évidence que lelessivage des terrils entraîne un flux d'ions sulfate et de carbone vers l'aquifère de lacraie. L'approche géophysique a permis de mettre en évidence un phénomène d'infiltration deseaux météoriques. Elle a montré en outre l'existence de barrières de perméabilité enprofondeur. Il apparaît donc que les eaux météoriques, lorsqu'elles s'infiltrent dans leterril, ne peuvent pénétrer à plus de quelques mètres de profondeur. L'existence de zonesimperméables, en limitant l'infiltration des eaux en profondeur, limite également laquantité de sulfure potentiellement oxydable et donc la quantité de sulfates entraînée versl'aquifère de la craie.In the Nord-Pas-de-Calais region (France), coal mining activity has induced a build-upof many mine tips. The tip materials are dominantly composed of siltstones, locally rich iniron sulfide. Weathering of pyrite might be expected to release sulfate ions and associatedmetals within the run-off waters down to the underlying aquifer, which is composed of athick Cretaceous chalk formation. The objectives of this study were twofold: (1) todetermine the possible role of the mine tips in the sulfate mineralization of the chalkaquifer and; (2) to assess the amount of waste material that can be leached and may supplysulfate ions to the water table.Two sites were selected for this study. Site 1 rests directly on the Senonian-Turonianchalk, whereas site 2 lies on sandy-clayey Cenozoic formations overlying the chalkformations (Figs. 1-2). Water samples were collected within the chalk aquifer (Fig. 3),which represents a free water table except for where the almost impermeable Cenozoicformations confine this table (site 2). Rock samples were collected at the surface and at adepth of <12 m at both sites. Various analyses were performed on these samples includingmineralogical analyses carried out on both the bulk fraction and the clayey fraction, aswell as elementary analyses of total carbon, total sulfur and CaCO3contents. Elemental analyses were carried out by Inductively Coupled Plasma-Atomic EmissionSpectroscopy (ICP-AES: major and minor elements) or Inductively Coupled Plasma-MassSpectrometry (ICP-MS: trace elements). Chemical and isotopic (C, S) analyses were performedon water sampled from upstream and downstream of the mine tips. In situ measurements werealso carried out during sampling. Finally, to assess the degree of rainwater seeping throughthe coal mine tips, two radio-magnetotelluric surveys were carried out in September andNovember, 1999 following rainy periods. Permeability measurements and grain-size analyses ofsubsurface samples were also performed at site 2.The carbon and sulfur contents showed superficial leaching on the mine tips (Fig. 4).The use of sulfur isotopes as tracers of the sulfate origin allowed identification of twosources for the two sites: a "mine tip" source with a slightly negatived34 S (-2.8‰ to -3.9‰), which corresponds to the oxidation of sulfidescontained by the Carboniferous shales, and another source (d34 S=-20‰)corresponding to the gypsum of the Cenozoic formations, which was only present at site 2(Figs. 5 & 6).This study outlined different behavior for the tips of the two sites. At site 1, wherethere is a free water-table zone, the mine tip leachates carry sulfate ions directly to thewater table, whereas in the case of a confined aquifer zone such as the one present at site2, a proportion of the sulfate was reduced once exported to the water table (the redoxpotential showed negative values; Table 1). This suggestion of bacterially-mediatedreduction is supported by the d34 S of the sulfate content in the watertable. The bacterial activity was fueled by the organic carbon release that accompanies thesulfur leaching on the mine tips. This carbon contribution was confirmed by the 14C activitythat characterized the chalk aquifer waters at the upstream region of the mine tip andnoticeably decreased downstream. The decrease is a result of the supply of "dead carbon"from the mine tips (Fig. 7).The oxidation of pyrite also results in H+ production. However,the pH decrease observed downstream from the sites was very slight. Waters derived fromleaching of the mine tip seeped through the buffered environment of the chalk aquifer. Thedistribution of metal content showed no surface to depth gradient for samples taken fromboth sites. The only evidence of "neutral acid mining drainage" (NAMD) was the sulfateamounts exported, and the increase in Mg, Ca, HCO3- and Sr contentsobserved downstream from the sites (Table 2a-b).The decrease in the apparent resistivity of radio-magnetotelluric profilesdemonstrated that rain waters could deeply penetrate in some parts of the tip at site 2(Fig. 9). Considering the constant nature of the waste material (grain size and porosity), adecrease in resistivity accompanied by an increase in conductivity between the two surveysindicated water seepage. Permeability measurements showed the occurrence of deeppermeability barriers (Fig. 10), limiting not only rainwater seepage, but also the amount ofmobilizable sulfide and consequently the amount of sulfates exported to the chalk aquifer.Grain size is not the only reason for the permeable or impermeable nature of waste material- the grain ordering and the compaction of levels at depth also have a role

Impact des terrils houillers sur la qualité des eaux souterraines (bassin minier Nord-Pas-de-Calais, France) : approche géochimique et isotopique