Minewater remediation at a French zinc mine: sources of acid mine drainage and contaminant flushing characteristics

Abstract

Recent closure of a zinc mine in southwestern France has resulted in flooding of workings and discharge of contaminated minewater from the workings, polluting a small stream. The minewater discharge is of near neutral pH and is contaminated with high levels of zinc, cadmium, manganese, iron and sulphate. With the exception of iron, concentrations of these contaminants have fallen dramatically since the discharge arose. The main source of recharge to the mine is likely to be overlying aquifers associated with near-surface weathering. A classification is proposed to explain the hydrogeochemical sources of contaminants and the changes in discharge water quality with time. Primary sources related to active sulphide oxidation dominate the minewater chemistry in the unflooded workings, whilst secondary sources related to remobilization of secondary minerals dominate in the flooded workings. Prediction of contaminant flushing rates has been carried out on the basis of this classification. Zinc and cadmium levels are expected to reach stable medium-term levels within 2–5 years of the discharge first occurring. Potential remediation strategies include plugging of the discharging mine portal and flooding of upper workings, coupled with conventional chemical and wetland treatment.

Novel passive treatment for mine water discharges

“…This effect holds either for complexation by OH ÿ in solution (equations (4) and (5)) or, analogously, by hydroxy-or oxy-ions associated with the hydrated mineral surface (equations (6) and (7)). Surface-catalysed oxidation of Fe(II) is generally initiated in the presence of Fe(III) oxide surfaces above pH 5, where significant adsorption of Fe 2+ occurs owing to formation of the surface complex XFeOFe 2+ .…”

“…1 Cessation of dewatering and other impacts of mine abandonment owing to the decline of mining has led to major polluting mine water discharges in Europe and elsewhere. Recent descriptions of such discharges include those in China, 2 South Korea, 3 France, 4 the UK 5 and the United States. 6 Acidic mine water discharges can have a profound influence on the surrounding environment and pose a serious threat of longterm environmental degradation.…”

Chemical containment of mine waste

“…To define the acceptable load or treatment objective for the discharge, a simple mixing model can be employed to calculate the mean downstream contaminant concentration for a particular discharge contaminant loading (Sadler 1997). Two approaches can be taken depending on the availability of data.…”

“…This measure is effective where the primary mechanism of acid generation or ferruginous water generation is active oxidation of sulphides under unsaturated conditions. For example at the Roquis mine, St Salvy, France, flooding of the underground workings has limited sulphide oxidation (Sadler 1997). However, flushing of underground workings has led to initially low water quality due to the dissolution of stored secondary products.…”