Acid Rock Drainage in Pyrite-rich Rock Formation at the Subsurface

Abstract Acid Rock Drainage (ARD) is an environmental hazard associated with pyrite oxidation in rock excavations and excavation spoils. The oxidation process is initiated when pyrite, in the form of dust and fine crystals, is exposed to aqueous oxygen to release low pH acidic metal-laden leachate. The reaction is exothermic, and the heat generated is known to cause spontaneous combustion in open pit coal mines. Until recently, pyrite oxidation was considered as purely an aerobic process with the presence of aqueous oxygen as a necessary pre-condition. However, recent studies have demonstrated that the reaction can proceed in anoxic environments in the presence of an effective oxidant besides oxygen. Rock blasting in quarry and open cut mines has the tendency to induce fracturing of rock mass to significant depths below the ground surface. These fractures open the subsurface to atmospheric oxygen intrusions and runoff infiltrations. In a pyrite-rich rock mass, this creates conducive environment for pyrite oxidation and acidic leachate into the groundwater. The lower pH conditions that results are conducive for the solubilization of ferric oxides and hydroxides and the release of ferric ions in the groundwater. Ferric ions are highly soluble at low pH conditions, typically below pH 3, but precipitates out of solution when the pH rises above 3.0. In a fractured rock mass, runoff infiltrations would cause the water table and the groundwater pH to rise and dissolved ferric ions to precipitate out of solution. The rise of the water table into the fractured zone would be accompanied by groundwater seepage and the deposition of ferric ion precipitates as orange/yellow coloration along seepage paths. In this study, the extent of blasting-related rock mass fractures has been quantified by rock engineering principles. The results are linked to the origin and causes of pyrite oxidation related ARD environmental hazard in an abandoned quarry pit. The study outcome shows that the subsurface is equally susceptible to pyrite oxidation related environmental hazards with potential adverse consequences on rivers and streams.