Water characteristics of the Upper Blackfoot River with inactive mine workings : Mike Horse, Anaconda and Carbonate report / for ASARCO (Northwestern exploration division)

Abstract

Several old mine workings exist in the Upper Blackfoot river where operations started in the late eighteenth century and ceased in 1964. There has been concern about the possibility of effluents from these abandoned workings and mining wastes adversely affecting the surface water quality of the Blackfoot river. Accordingly, an investigation of the characteristics of the eff.luents · from the Carbonate tailings pond and the portals of the Mike Horse and Anaconda mines was carried out. The composition of the waters leaving these potential sources of contamination was compared to the water characteristics in the Blackfoot river below the discharge point of the abandoned workings. It was discovered that although the ore of these mines and part of the wastes are pyritic, the effluent contains very little acidity (40 mg/1 equivalent CaC03 in the Carbonate tailings pond: 60 mg/1 equivalent CaC03 in the Anaconda portal). As a result of contact with country rock and highly alkaline ·waters in the drainage basin, all other waters sampled have alkalinity concentrations ranging from 32 to 98 mg/1 CaC03 • The sulphate concentrations in the portal effluents correspond closely to those expected for calcium and magnesium sulphate solubilities. The very low concentrations of iron found suggest that active acid mine drainage is no longer affecting the effluents. Furthermore, there are indications that no relationship exists between the characteristics of the mine effluents and the water in the Upper Blackfoot river below the abandoned mine workings. Elevated concentrations of zinc and manganese are evident in the water in the immediate vicinity of the portals of the Mike Horse, the Anaconda and the associated waste dump. Environmental degradation of the water is limited to about 0.8 miles of Mike Horse creek and Beartrap creek between the Mike Horse and the Anaconda. Natural cleansing processes take place in this affected portion of the creeks through the growth of attached algae which absorb and fix precipitate emerging from the portals. The Carbonate tailings area is partially colonized by indigenous vegetation cover which provides protection from oxygen, thereby curtailing extensive acid generation. If the aim is to improve the water quality of the Upper Blackfoot river, there is no justification for extensive reclamation measures, as any such activity could disturb the waste material. This may result in a new onset of acid generation which could adversely affect surface water quality. As well, earth moving activities may increase the suspended solids loading to the Blackfoot river. Any consideration of reclamation measures for aesthetic purposes must carefully incorporate the promotion and maintenance of the natural biological cleansing mechanisms which are presently in place.