Groundwater Source Identification and Flow Model of the Dareh-Zar Copper Mine in Central Iran by Chemo-isotopic Techniques.

Seepage with AMD characteristics is observed in the Dareh-Zar copper mine in central Iran and more water inrush events are expected since the pit extends below the local groundwater levels. In order to properly design a dewatering system, it was necessary to determine the source of this groundwater and to establish a groundwater flow model. Thirty-nine water samples were collected from springs, qanats, observation wells, seepages, and permanent river water and analyzed for major ions, silica, Fe, Cu, and stable isotopes (18O and 2H). The electrical conductivity and pH of the water samples ranged from 403 to 4810 μS/cm and 3.3 to 8.6, respectively. The PCA and biplot diagrams confirmed the role of mineral weathering, redox reactions (Fe2+ release), and gypsum dissolution on groundwater chemistry outside of the pit and the effects of pyrite oxidation on weathering and dissolution reactions inside the pit. Based on hydraulic features inferred from the iso-potential map of the aquifer and cluster analysis of the chemical data, two distinct groundwater sources from the northwest and east of the mine, with fresh (Ca-HCO3) and brackish (Na-SO4) signatures, respectively, were identified as the possible sources of the Ca-SO4 groundwater in the mine pit. The dramatic difference in Na concentrations in most of the samples does not support groundwater evolution to Ca-SO4 types in the pit simply by mixing. Instead, the Ca-HCO3 groundwaters from the north and northwest areas likely evolve to the Ca-SO4 water-type in the pit due to pyrite oxidation. The stable isotopes indicated groundwater recharge zones at elevations ranging from 2479 to 2877 m above mean sea level, which is, on average, 207 m above the pit area and suggests that the north and northwest recharge zones are the primary source of the groundwater inrushes. These results are being used to help design a dewatering scheme for this mining area. [ABSTRACT FROM AUTHOR]