New Soil From Mine Tailings Through Biosolid Amendment

Pepper, I.L., The University of Arizona


Abstract Copper mine tailings are essentially crushed rock and a perfect inorganic matrix resembling soil. However, such tailings are dissimilar to soil since they contain almost zero organic matter content and extremely low microbial populations. However, addition of large amounts of Class A biosolids to copper mine tailings resulted in a functional soil with respect to microbial characteristics which were sustainable over a ten year period, enabling extensive revegetation of the tailings. Specifically such amendment resulted in 4 log10 increase in bacterial content relative to unamended tailings. Microbial activities such as nitrification, sulphur oxidation and dehydrogenase activity were enhanced and sustained throughout the study period. 16Sr RNA clone libraries obtained from community DNA suggest that tailings amended with biosolids achieve diversity and bacterial populations similar to native soil bacterial phyla ten years post application. Hence we have shown that new soil can be derived from mine tailings through biosolid amendment.

Keywords: biosolid amendment, mine tailings, soil microbial characteristics

Introduction Mining of ores for copper is a significant industry throughout many years of Arizona. During this process, large amounts of material are initially removed to provide access to the ore itself. These deposits are termed ‘overburden’ and result in human made mountains of material often to a depth of 35m. Such degradation of land is a major concern in many countries (Harris 2003).

Mine tailings are essentially crushed rock, and as such are a perfect inorganic matrix resembling soil. However, unweathered mine tailings are unique, unlike any natural soil. Tailings have very low cation exchange capacities and almost zero organic matter content. Left alone, mine tailings are unsightly and only allow for scant vegetative growth. This in turn leads to erosion via wind, which can create dust storms that create human health hazards via air pollution. Tailings have very low macronutrients for plant growth, especially nitrogen, and conversely can contain high concentrations of heavy metals, which can be particularly problematic in tailings where the pH is low. In such instances, soil fertility, and microbial communities are drastically reduced (Seaker and Sopper, 1988).

Therefore, there is a need for reclamation of tailings to allow for subsequent revegetation and ecosystem stability. One potential solution allowing for the reclamation of mine tailings is the use of biosolids which are produced at every municipality throughout the State and Nation (Marx et al., 1995). Interestingly, biosolids represent another human-made waste that needs to be disposed of or utilized. The term biosolids implies treatment to produce Class A or Class B biosolids that meet the land-application standards in the Part 503 Environmental Protection Agency regulations (U.S. EPA 1994). Class A biosolids are treated so that they contain nondetectable levels of pathogens (NRC 2002).

The objective of this study was to evaluate whether the addition of a large input of organic material in the form of Class A biosolids, to mine tailings, would result in a new soil with functional microbial characteristics.

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