Mine water geochemistry and biogeochemical modeling
Résumé
In mine tailings sites, where sulfide minerals are in contact with atmospheric humidity and meteoritic water, oxidation of sulfides generate acidity leading to mobilization of metals and metalloids through the soil to surface and underground waters. The resulting acid mine drainage can impair dramatic environmental problems (i.e., toxicity of arsenic III, etc.). In the old gold mine of Chéni (France), large physical, chemical and microbiological datasets are available: mineralogical compositions of the tailings [1], long-term survey of drainage waters chemistry [2] and microbiological characterizations [3]. The pH/Eh of sampled waters through the tailing column (unsaturated and saturated zones) is in the range of 2.9-7.6 and 0-700mV respectively. Analysis of these data permitted to identify the key processes of mine water behavior. Numerical simulations, using the geochemical software PHREEQC [4], were performed aiming to identify the key mechanisms and understand those coupling processes. First analysis of geochemistry show different redox potentials for each element (Fe, As, O2, N, S). Such redox disequilibrium, known in natural waters [5] is rarely take into account in modeling. In our work, redox decoupling of the thermodynamical database is used to model redox behavior of mine water. Biological oxidation of arsenic was identified as an active and sustained process in such systems. Experimental results of biological As oxidation was used to develop a thermokinetic model coupling geochemistry and biologic activity [6] and reproducing the pH-dependant activity of bacterial population. Finally, we elaborate a mixed biogeochemical model to reproduce the behavior of an acid mine drainage, which takes into account major processes including bacterial activities, precipitation of Fe-containing minerals (Schwertmannite, ferrihydrite, jarosite) and surface complexation. [1] Roussel (1998) PhD thesis, Univ. Limoges [2] Bodénan et al. (2004) Appl. Geochem. 19 pp. 1785-1800 [3] Battgalia (2002) J. Appl. Micr. 93 pp. 656-667 [4] Parkhurst et Appelo (1999) USGS Rep. 99-4529 [5] Lindberg and Runnels (1984) Science 225 pp. 925-927 [6] Jin and Bethke (2005) Geoch. Cosm. Acta 69 pp. 1133-1143