TOXIC ELEMENT MOBILIZATION FROM BLACK SHALES

The U.S. has rapidly expanded production from unconventional black shale deposits using hydraulic fracturing (“fracking”) techniques. During hydrocarbon extraction through operations and ultimately at closure there is the potential to impact the environment if appropriate mitigation measures are not implemented. Minimizing/avoiding surface and ground water-related impacts is critical. Unconventional hydrocarbon deposits contain high concentrations of potentially toxic trace elements and their interaction with water during hydraulic fracturing or during weathering may be a cause for environmental concern. It is critical to understand the mineralogy and the element distribution and enrichment in black shales in order to predict trace element mobility and potential risk to human health and the environment.

We have compiled quantitative information on elemental distribution and mineral abundances of black shales and prepared enrichment diagrams, distribution diagrams, and correlation plots which have helped to understand and interpret how these abundances have the potential to cause environmental impacts. This data was then used to identify elements under appropriate geochemical conditions that have the potential to be toxic and to mobilize from the rock to the water.

Initial geochemicial and mineralogic observations of the entire data set of black shales as well as individual deposits include; 1) Mineral abundance changes very little during early diagenesis, although during burial diagenesis significant mineralogical changes are observed, 2) Certain black shales like the Eagle Ford are carbonate-rich while others like the Bakken are carbonate-poor and this alkalinity is very important in the neutralization of acidity from pyrite-rich shales 3) Black shales are generally enriched in Mo, Bi, Se, Cd, As, and Sb and depleted in Na relative to average crustal abundance. Trace and major element distributions from the Bakken, Eagle Ford, Marcellus, New Albany, and Woodford were evaluated individually and showed similar trace element enrichment but wide ranges in enrichment in Ni, U, and Zn.

Black shale mineralogic and elemental data will ultimately aid in the understanding of the environmental fate and transport of trace elements into surface and ground water systems.