Evolution of the aqueous geochemistry of mine pit lakes – Blanzy–Montceau-les-Mines coal basin (Massif Central, France): origin of sulfate contents; effects of stratification on water quality

The flooding of two mine pit lakes (Lake St. Louis and Lake Fouthiaux) situated in the Blanzy–Montceau-les-Mines coal basin (Massif Central, France) has been under close scrutiny for 5 a. The aim was to understand: (i) what sources can influence the lake water geochemistry; (ii) what is the role of lake stratification on water chemistry; (iii) how the lake water chemistry evolves during flooding. To answer these questions, T°, conductivity, pH and Eh depth profiles were measured with a specific probe (Seabird®) in the lakes (seven surveys between July 1998 and February 2002). At the same time, water samples were collected every 5 m along the profiles and were analysed for Na+, Ca2+, K+, Mg2+, image, F−, Cl−, image, Fe, Cu, Ni, Zn, Co, Cd and Mn. The geological and hydrogeological contexts play a major role on the water chemistry of the lakes and three sources of influence have been distinguished : (i) the contribution of underlying mine groundwater, enriched in image, due to the leaching of sulfide oxidation products from old flooded underground works; (ii) the seepage through coal mine spoils also containing sulfides; (iii) to a lesser extent Triassic formations, containing gypsum, present in the Fouthiaux area, which lead to a image enrichment of lake waters. All these influences result in an enrichment in image, showing essentially a neutralised acid mine drainage process, that is characterised by neutral to slightly alkaline waters, and an absence of high metal contents (except for Fe and Mn) which are non soluble at the pH and Eh conditions present in the lakes. The two studied lakes were initially meromictic and ions such as image or Mn can show high concentrations at depth, in the lake layers which are not subject to winter mixing. During the studied period, Lake St. Louis remained meromictic and non removed elements continue to have high concentrations in deep zones. In contrast, Lake Fouthiaux, initially meromictic, became holomictic with time and the bottom layer did not maintain the enrichment in image, Mn, and other elements shown at the beginning.