Natural attenuation study of the impact of acid mine drainage (AMD) in the site of Carnoules

This study was carried out on the AMD being emitted from the tailings dam at Carnoules (Gard, France), an old polymetallic mine (Zn, Pb). The tailings dam is located at the end of a valley where the tributary Reigous originates. Consequently, the water source is continuously polluted with AMD producing acidification and metal solubilisation in the tributary Reigous. The fact that natural attenuation is a major abatement process for soluble metals (Fe, Zn, Pb, Mn) and As was demonstrated by determining their concentrations in water along the tributary. After 750m, percentage abatements as high as 85%, 80%, 98%, 72% and 94% respectively were observed. These values were shown to fluctuate during the year. The majority of the metal and arsenic removal is achieved within the first part of the tributary emerging from the dam (first 60 meters). In order to explain the rapid abatement which can not be satisfactorily explained by the local physical and chemical properties of the water (pH, O-2, temperature or metal levels) biological data were collected. Biological studies showed that this short distance was also the most populated in terms of bacteria (i.e. Thiobacilli) and protozoa, as revealed by adenosine triphosphate (ATP) determinations, microscopic observations and counts.