Watershed restoration limitations at the abandoned reclaimed Alta Mine, Jefferson County, MT

Abstract

Abandoned hardrock metal mines can have an antagonistic effect on soil productivity, vegetation, and water quality. Specifically, abandoned mines that actively generate acidity in soil are phytotoxic due to low pH and increased bioavailability of heavy metals. Arsenic concentrations in mine soils are often elevated, but may not be as mobile as heavy metals at low pH. Acid mine drainage migration from abandoned mines is problematic because it leads to water quality impairments that limit water use for certain activities (i.e. stock watering and irrigation). In this work, a previously reclaimed abandoned lead and silver mine (Alta Mine Jefferson County, MT) was characterized for its persistent impacts on soil, vegetation, and water quality. A progressive monitoring effort linked offsite water quality impacts to deep underground mine workings, shallow ground water, and metalliferous soils found at the Alta mine. Vegetative cover was measured in 16 transects in conjunction with 30 soil pits excavated on the reclaimed site. By regression and analysis of variance, sparse vegetative cover was significantly (p<0.1) linked to pH and acid generation potential. To overcome acidic soil conditions, lime and compost amendments were tested on site. The amendments significantly (p<0.1) neutralized soil acidity; however, a corresponding increase in vegetative cover was not observed. Erosion of the bare unstable slopes caused greater than anticipated seed bank loss that precluded vegetation establishment. The successful establishment of a dense vegetative cover on the abandoned mine could prevent erosion and water quality impacts due to sedimentation. Vegetation may also have minor impacts on landscape sources of arsenic and heavy metals that were identified in the study; but the most significant source of water quality impairment, deep underground mine workings, will persist under any land treatment.