ORIGIN OF RADIUM ORPHANED DURING HYDRAULIC FRACTURING OF BLACK SHALE

Waste water from the hydraulic fracturing (“fracking”) of Marcellus shale is extraordinarily saline and enriched in alkaline earth elements Ca, Sr, Ba and Ra. The radium isotopes ²²⁶Ra and ²²⁸Ra are produced from the decay of U and Th, respectively, and while the shale is enriched in both U and Th, wastewater is depleted in these elements. Nonetheless, the short half-life of ²²⁸Ra (5.8 years) and its high activity in wastewaters suggest that water-rock interaction during fracking has played a role in Ra release. Here, we determined the Ra isotope composition of Marcellus shale and sequentially-extracted shale leachates to investigate the mechanism of Ra release. We developed a high-sensitivity chemical separation procedure to purify Ra from a wide variety of sample matrices, and we used isotope dilution thermal ionization mass spectrometry (ID-TIMS) to measure ²²⁶Ra/²²⁸Ra ratios and Ra activities in these materials relevant to fracking waste production. Marcellus shale shows typical whole-rock ²²⁶Ra/²²⁸Ra atom ratios of 1000-1200. Wastewaters measured here and reported in literature shows ratios of 1000 to 2500 and as high as 5000, similar to or higher than bulk shale. Conversely, extremely low amounts of Ra are released from shale by leaching with pure water, and the Ra isotopic composition is very low. Measured here as 293±32, the ²²⁶Ra/²²⁸Ra composition of water-soluble Ra is almost 10x lower than that observed for produced water. Leaching black shale with weak acid similarly liberates little Ra with low isotopic ratio of 465±15. Among our extractions only saturating the shale with Ca²⁺ at neutral pH liberated substantial amounts of Ra with high isotopic composition, measured here at 2558±24. The majority of Ra in the rock (greater than 95%) is released during complete destruction of organics with hydrogen peroxide and dissolution of silicate clays with hydrofluoric acid; these extractions show Ra compositions of 1421±8 and 1065±12, respectively. We suggest that Ra in wastewaters co-produced with shale gas is (i) derived primarily from alpha-recoil of Ra from organic and clay fractions, (ii) hosted in strongly-bound exchangeable sites, and (iii) released during hydraulic fracturing by the interaction of shale with highly saline fracturing fluids.