LEACHING OF COAL MINE TIPS AND SULPHATE TRANSFER TO THE CHALK AQUIFER OF A COAL BASIN IN "-PAS DE CALAIS" () - CONTRIBUTION OF GEOPHYSICS

by Sophie Denimal*, Laurent Tribovillard*, Roger Guerin**, Konstantinos Chalikakis**, Pierre Andrieux**, Yves Albouy?", Jacques Vassal'?

*Lab.de sedimentologie et geodynamique, Univ. des Sciences et Technologie, (France) ** UMR Sisyphe, DESS de geophysique appliquee, U.P.M.C., Paris (France) OOInstitut de Recherche pour le Developpement, Bondy (France)

PROBLEM TO BE SOLVED In the Nord-Pas-de-Calais region (Northern France), mining activity related to coal extraction resulted in the presence ofnumerous mine tips. Weathering ofthose wastes leads to oxidation of the pyrite contained in the Carboniferous shales and thus releases sulfate ions that may contribute to the sulfate enrichment ofthe chalk aquifer, the main drinking water resource of the region. Two sites have been studied, the first one is located above a free water-table zone (site 1) whereas the second one lies above a confined aquifer (site 2). The aim of this study is to determine the contribution ofthe mine tips to the sulphate mineralisation ofthe chalk aquifer. Previous works [Bernard, 1979; Droz, 1985] tend to prove that there may be several sources of sulphate in the coal basin, related either to the local geology (carboniferous limestones, gypsum), or to human activity or to the mine tips.

HYDROGEOLOGICAL AND HYDROCHEMICAL RESULTS Total carbon and sulphur analyses have shown a superficial leaching ofthese elements on the tips. Analysis of groundwater sulphate content downstream and upstream of the two sites confirms the impact of the mine tips on the chalk waters (Fig. 1). Use of sulphur isotopes as tracers for sulphate origin has permitted to identify two sources (Fig. 1): a « mine tip» source 34S 34S with weakly negative values for 8 (8 = -2,8%0, -3,9%0) which corresponds to the 34S oxidation of sulphides contained within the Carboniferous shales and a second source (8 = -20%0) related to the gypsum ofthe « Sables d'Ostricourt» (Droz, 1985).

>,{ ::~.~~>...... '-- ..;. 1235/0.71 .,",;' 3300~0 d200/3.71 30 r-:-:-::"~O""'" c 1175=5/r='C :""::.2"=""0.3::'11 1 1115/·1.11 o Alluvial depos its Site 2 _ Loam on "Sables d'Ostricourt" _ Sables d'Ostricourt , a Loam on "Argiles de Lou"jl" [2J Argiles de Loam on Senonian Chalk g Henin Beaumont Site 1 o Senonian Chalk Ix 1vlX =sulphate content (mgn) Scale 1:50000 V =534S value L500 m Fig. I: Geological map ofthe studied area .. Sulphate contents and S34S values ofwaters sampled in the Chalk aquifer The geological map of the studied area (Fig. 1) shows that the extension of the tertiary formations is not precisely known, especially around mine tip 2. Actually, alluvial deposits and limons mask the limits between clays ("Argiles de Louvil") and sands ("Sables 34 d'Ostricourt"). In order to explain the "anomalous" Ô S value observed at site 2, i.e. to verify the presence of sands close to the mine tip and to better estimate their extension, geophysics has been proposed. GEOPHYSICAL RESULTS The results of ten electrical soundings (l0 ES) and thirty time domain electromagnetic soundings (30 TDEM) which have been performed around six locations, from mine tip 2 to the north-east, south ofOignies, can be summarized as follows: 1-Tertiary formations above the chalk : They are present at aIl six sites, i.e. most probably everywhere to the north-east of site 2, as it is shown by three typical ES sounding 34 curves (Fig 2). The hypothesis conceming the Ô S anomaly east of tip 2 is thus confinned. 34 This result however implies similar values for Ô S measurements, yet to be carried out in other wells to the north-east. 2-Interpretation of electrical soundings: A realistic geological cross section can be drawn solely from electrical soundings without any additional information: it shows a regularly deepening top ofthe chalk tQwards the north-east, from 13 to 30 meters. The tertiary fonnations consists in a remarkably uniform layer of clay with an average thickness of 10 meters immediately above the chalk, topped by a regularly increasing thickness of sands, from 1 or 2 meters close to the tip to 10 meters close to the village ofOstricourt. However, because of the non uniqueness of the result of ID inversion, a different section could be drawn; for instance for ES F (Fig 3a), two rather extreme solutions are possible, where the depth to the chalk may be equal either to 18 or to 35 meters. 3- Joint inversion ofES and TDEM: It appears that the so-called realistic cross-section drawn above is not compatible with the IDEM sounding curves. As a matter of fact compatibility between the two sets ofdata is generally impossible with a 5 or 6 layer model; 8 to 10 layers are sometimes required. Ts this geologically sound ? Yes if the superficial and tertiary formations consist in inter-bedded thin layers ofsand and clay. The depth to the chalk in the example shown below (Fig 3b) is constrained to 30 meters.

Rho(ohm-m) Rho(ohm~) ,.. 0.1 1 10 100 1000 0.1 1 10 100 1000 a a a ~saDd clay i • l:Ia I}\ ~ 1•• -10 ·10 ~. -la.... •• 'T' ,-- ~"":. 10 -:t--~~__I:I 1 ~~ III • l- 1 ln ·20 - ï ln .20 Electr1cal soundlngs 1 I 1:1 A ~~ 1 l~ r- ~ D Fig.3a: Fig.3b: •F -40 -40 ES E S+TDEM

,. ABn (m) '.0 '00. -sa -sa Fig. 2: Examples ofelectrical soundings Fig. 3: Inversion ofES F and IDEM F TEMPORARY CONCLUSIONS - FUTURE WORK The results brought by geophysics do confirrn the geological model inferred from isotopic analyses; i.e. the presence of tertiary formations above the chalk, close to tip 2. Implications on newly drilled wells to the north-east must be verified however. The intrinsic limitations of electrical soundings are emphasised: a realistic cross section can always be drawn from sound geological hypotheses without the help of additional information where the general trends will usually be acceptable whereas the details and the depths may prove to be far away from reality. Joint inversion of ES and TDEM soundings may often prove difficult to perforrn; muIti-thin­ layers models are generally required. Full joint interpretation ofthe two data sets will be soon carried out and its results will be compared a posteriori to direct information brought by newly drilled wells. Denimal S., Tribovillard L., Guérin R., Chalikakis K., Andrieux P., Albouy Yves, Vassal Jacques. (2001). Leaching of coal mine tips and sulphate transfer to the chalk aquifer of a coal basin in "Nord-Pas de Calais" (France) : contribution of geophysics. In : Proceedings of the 7th meeting environmental and engineering geophysics. Bondy (FRA) ; Lausanne : ORSTOM ; EEGS, 2 p. multigr. Meeting Environmental and Engineering Geophysics, 7., Birmingham (GBR),

2001/09/02-06.