Laboratory study of trace element release and environmental risks from shale-fracturing fluid interactions

Hydraulic fracturing (fracking) of black shales such as the Marcellus has become a major method of natural gas extraction in the United States and globally. The process involves high-volume injection of hydraulic fracturing fluid (HFF) into deep rock formations, initiating a sequence of geochemical reactions that alter shale mineralogy and mobilize potentially hazardous trace elements over several timescales. The laboratory experiments reported herein explore how interactions between HFF and shale during stages of the fracking process-including the shut-in period, initial flowback, long-term gas production, and post-closure-affect the release and redistribution of a large suite of trace elements. Experimental results show that during shut-in, acid additions rapidly dissolve carbonates and sulfides, subsequently allowing sulfide oxidation; evidence of clay transformation, barite precipitation, and secondary iron mineral precipitation is seen. During the initial production phase, most elements are released via direct acid dissolution to flowback waters; oxoanion forming elements (P, As, Mo, Th) are mobilized somewhat later. These processes lead to the partial or complete loss of some trace elements (such as lanthanides, Zn and Sr) from the solid phase. Others such as As, Mo, and U are redistributed into more labile mineral forms, raising concerns about their long-term environmental mobility and potential for drinking water contamination, particularly as well networks expand and age.