Acid Rock Drainage Treatment Sludge Disposal in Mine Voids and Pit Lakes: Analysis of Environmental Risks

            This paper describes adverse environmental impacts that may arise from inadequate disposal of acid rock drainage (ARD) sludge. Treatment of ARD at large-scale mine sites produces large quantities of ARD treatment sludge. Sludge disposal is an important component of long-term environmental management, and disposal within mine voids and pit lakes is often considered. The sludge consists of amorphous to microcrystalline iron and aluminum hydroxides and hydroxysulfates, which precipitate in response to addition of a strong base such as sodium hydroxide or lime to ARD. Because sludge also contains toxic metals and metalloids removed from ARD, the chemical stability of the sludge in relation to hydrogeochemical conditions present in mine voids and pit lakes must be considered as part of any decision to dispose of sludge in these locations. Sludge stability is relevant to preventing release of CERCLA hazardous substances into the environment and to complying with regulatory requirements associated with RCRA and the Underground Injection Control program.

           Evaluations of potential sludge disposal in pit lakes or mine voids have been completed at Gilt Edge Mine Superfund Site. These evaluations present a case study that demonstrates the process and data-needs associated with sludge disposal evaluations, and the importance of considering sludge stability when evaluating long-term disposal options. The results of sludge analyses, extraction tests, and geochemical equilibrium modeling have indicated that sludge stability is dependent on the hydrogeochemical conditions of the environment in which it is stored. Sludge stored within strongly acidic conditions has the potential to become a secondary source of contaminants into the environment, after existing neutralization potential in the sludge has been exhausted. Under these conditions, dissolution of sludge particulates and/or desorption of metals and metalloids from sludge particulates may occur, which might lead to mobilization of contaminants into the environment via surface water or groundwater pathways.