The development of unconventional shale gas in Pennsylvania generated large quantities of highly mineralized water. While the geochemical evaluation and wastewater treatment technologies for the Marcellus produced water have been intensively studied, the origin of elevated salinity as well as other key components (e.g., Ba<sup>2+</sup> and Ra<sup>2+</sup>) is still the subject of scientific controversy. This study summarizes current debate on the origin of salinity in produced water and focuses on elucidating geochemical processes that contribute to naturally occurring radioactive materials (NORMs) in Marcellus Shale produced water. Isotopic dating (or radiometric dating) is a powerful tool to estimate the age (residence time) of brine constituents, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products. The Ra isotopes fingerprint of Marcellus flowback water was collected from the measurement in this study and other publicly available databases. Isotopic dating based on Ra-228: Ra-226 and Th-228: Ra-228 (or Ra-224: Ra-228) ratios was developed in this study to estimate the residence time of radium isotopes in liquid phase. Results showed that the mean residence time of radium in liquid phase is between 0.4-6 years, suggesting the radium leaching from the shale is a relatively rapid process and its concentration in produced water may increase with time. In addition, this study reviewed the flowback water management strategies and identified the possible environmental risks associated with the radioactivity of wastewater and solid waste. Results showed the Radium concentration in wastewater can be maintained at an acceptable level with proper pre-treatment (e.g., sulfate or carbonate precipitation). However, solid waste generated by centralized treatment facilities and in storage impoundments contains Ra-226 at concentrations ranging from several hundreds to more than ten thousand pCi/g and must be properly disposed. Finally, current disposal strategies and potential disposal options for radioactive solid waste are discussed.

Origin of radioactivity in Marcellus Shale flowback water and potential concerns with radioactivity in wastes generated by unconventional gas industry

249th National Meeting (2015)

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