Assessing the efficacy of lime amendment to geochemically stabilize mine tailings

Historical mining practices in Butte, MT, resulted in deposition of tailings as overbank deposits in the Clark Fork River. This study used pore water chemistry, electron microprobe analysis, and geochemical modeling to compare the chemistry of these acid-generating deposits 5 years after liming with naturally revegetated and barren tailings. The sequence of weathering reactions is predictable, covering a continuum from acidic soils through the lime-amended soils to the naturally revegetated overbank tailings deposits. For example, sulfides and sulfates predominated in the untreated tailings mineral assemblage, while liming facilitated alteration of pyrite to ferrihydrite that sequestered weight percent concentrations of As, Cu, Pb, and Zn. Collocated pore waters collected using suction lysimeters installed in the surficial treated material (<30 cm deep) were alkaline (pH 7) and low in metals as compared to the pore water in unlimed barren tailings (pH <4.8). Geochemical modeling using MINTEQA4 was consistent with the electron microprobe results in that pore water was predicted to be oversaturated with respect to jarosite and gypsum under acidic conditions and with respect to calcite, gibbsite, ferrihydrite, and plumbogummite with increasing pH. This study demonstrates that lime amendment represents a viable long-term method to mitigate acid generation in tailings.