Some of the most significant and enduring problems of the historic mining in the San Juan Mountains are the soil and water quality degradation associated with abandoned mine tailings, waste rock piles and draining adits. These sites contribute highly mineralized soil and acidic water to local watercourses through the erosion of contaminated soils; primarily due to lack of vegetative cover. The redistribution of mineralized soils through fluvial and aeolian processes has had significant, deleterious effects on water quality and the biota of Animas River headwaters.
A major impediment to reducing the amount of pollution coming from mining works is due to the difficulty of re-establishing vegetation and active soil processes on sites that are devoid of vegetation, and have conditions that are generally hostile to plant colonization. The challenges of re-growing vegetation at these sites are multiple, and include: acidic soils, harsh climatic conditions, and adverse physical conditions associated with soil compaction and low water retention.
To address the challenge of re-growing vegetation at mine sites, we sought to determine the suitability of biochar as a soil remediation tool for abandoned mine lands in the San Juan Mountains, near Silverton, Colorado. Our study assessed the effects of biochar and straw compost amendments on the mobility and toxicity of metalliferous contaminants in soils at abandoned mine sites. We conducted field and container trials to examine the effect of biochar treatments on vegetation establishment and growth, soil water holding capacity, and soil leachate chemistry. Field sites were located on Forest Service and Bureau of Land Management abandoned mine lands and other disturbed soil sites. Sites were located near Silverton, CO at elevations of 2,800 to 3,700 meters, with soil conditions that ranged from pure waste rock to partially reclaimed forest soil.
Our results indicate that when compared to seeding alone, the addition of a 30% by volume biochar soil amendment increased vegetation cover on acid affected soils by 240%. Conversely, on non-acid soils no increase or substantially less cover was observed when compared to straw mulch alone. Biochar treatments increased water holding capacity in all soils by 90% - 180%. Seed emergence was faster with more emergence and longer sprouts (350% - 670% increase in length) at 120 hours for treatments that contained biochar when compared to soil only. However, the greatest numbers of seeds emerging, with the longest sprouts were in treatments that contained both biochar and straw mulch. Above ground biomass was positively affected by biochar additions, with acid mine sites showing a 192% increase in biomass. The effect of biochar on above ground biomass on non-acid sites was small with a magnitude increase of 6%-11%.
The results for the effect of biochar on soil leachate chemistry are mixed, with some analytes showing a decrease in leachate concentration at 40 days (Al, Fe), some showing an increase in concentration (Cu) and most showing very small or no differences (Cd, As, Mn, Ni, Pb, Zn) in leachate concentration when compared to a soil only control. Confirmation of these results may indicate that biochar, when combined with organic mulch, may be a useful tool for soil restoration at abandoned mine sites, or at other locations with acidic soils.
Still unresolved questions include:
- What are the optimal application rates?
- What is the effect of biochar additions on the accumulation of carbon over the long term?
- What is the effect on metal mobility at the field scale?
- How should land managers overcome some of the practical obstacles to mechanized use of biochar at abandoned mine sites?
Click the links below for further reading:
Colleen Rostad and David Ruther Ford - 2011 Biochar for Soil Fertility and Natural Carbon Sequestration USGS Fact Sheet
Kas Dumroese and Jeremy Pinto - 2011 Potential Use of Biochar as an Amendment to Substrates USFS Research Project
Beesley et al. 2011 Environmental Pollution A review of biochars’ potential role in the remediation, revegetation and restoration of contaminated soils
Emma Marris 2006 Nature Putting the Carbon Back: Black is the New Green
Johannes Lehmann 2007 Frontiers in Ecology Bio-energy in the Black