MINE-DUMP SIMULATORS TO STUDY ACID MINE DRAINAGE PROCESSES

Mine-dump simulators (MDS) were constructed to study the evolution of solids and solutions associated with acid-mine drainage (AMD). MDS are analogs of mine dumps that can be monitored and controlled under laboratory conditions and sampled more frequently than actual field sites. The simulators, meter-scale boxes filled with 100’s of kg of mine waste, are designed and instrumented to collect geochemical, mineralogical, geophysical, and microbiological data. Simulators with different types of mine waste (e.g., igneous- vs carbonate-hosted) permit observation of multiple aspects of acid drainage generation, such as the buffering capacity of the deposit host-rock.

Reaction progress can be readily followed using the simulators. Upon addition of DI water to the solids, Fe³⁺ is the dominant aqueous iron species (>90% Fe_[tot]) in the earliest stages of dissolution. After about 5 days, Fe²⁺ becomes dominant (>50% Fe_[tot]), indicating dissolution of Fe²⁺-bearing minerals such as pyrite. Initially high dissolved oxygen (DO) values are lowered to near zero in the bottom of the simulator as oxygen is consumed by weathering reactions [e.g., oxidation of sulfides] while the aqueous SO₄ concentration rises. Many trace elements are released into solution, and all but Pb remain at detectable levels during the course of a saturation cycle. Thus, a solid sink for Pb exists, probably PbSO₄ (anglesite) and/or a Pb-bearing jarosite.

The MDS have provided some new insights into acid-mine drainage processes. For example, efflorescent sulfate minerals were formed in the MDS that were not observed in the field. These highly-soluble, metal-bearing solids are products of reaction pathways typically not considered in AMD. About 10 days after saturation, geophysical sensors detected a large (100’s mV) self potential (SP) anomaly that likely represents passing of a redox boundary across the electrode arrays as the waste dried. To evaluate the overall effects of microbial activity, paired MDS’s (one sterile, one nonsterile) with otherwise identical waste material were wetted. The nonsterile pH was a full unit lower than the sterile control, suggesting microbial activity has a significant role in acid production from mine dumps.