A rapid method to determine ²²⁶Ra concentrations in Marcellus Shale produced waters using liquid scintillation counting

Concentrations of naturally occurring radioactive material (NORM) in Marcellus Shale produced water presents a challenge for effective management and treatment, because of the vast fluid volumes generated. With an increased emphasis on beneficial reuse and resource recovery from the produced waters, a rapid, yet reliable, method for quantifying radium in these produced waters is needed. The high total dissolved solids (TDS) concentration introduces difficulties when measuring ²²⁶Ra by recommended EPA methods that were specifically developed several decades ago for drinking water. While other techniques for measuring radium in these high-TDS fluids have since been developed, these newer techniques often require extensive and complicated pre-concentration steps; and they thus require extensive analytical chemistry skills, utilize hazardous chemicals like hydrofluoric acid, demand long holding times or measurement times, and require high sample volumes. We present a rapid method for ²²⁶Ra measurements in high-TDS produced waters by liquid scintillation counting, which has been corroborated herein by concurrent gamma spectrometry analyses. Samples were prepared for analysis by evaporating the fluid and re-suspending the evaporate with acidified distilled deionized water prior to liquid scintillation counting for 1 h. This protocol yielded radium recoveries ≥93%. Per this protocol, the alpha and beta spectra of ²²⁶Ra and its daughters were computationally separated by alpha-beta discrimination and spectrum deconvolution. The minimum detectable activities of ²²⁶Ra was 0.33 Bq/L (9.0 pCi/L) when the counting time was 60 min and the sample volume was 4 mL. Nine produced waters of varying TDS and radium concentrations from the Marcellus Shale Formation were analyzed by this method and compared with gamma spectroscopy; and these yielded comparable results with an R² of 0.92. The reduced sample preparation steps, low cost, and rapid analysis position this as a well-suited protocol for field-appraisal and screening, when compared to comprehensive radiochemical analysis. We offer that for a given produced water region, routine and local liquid scintillation analyses can be compared and calibrated with infrequent gamma spec analyses, so as to yield a near-real time protocol for monitoring ²²⁶Ra levels during hydrofracturing operations. We present this as a pragmatic and efficient protocol for monitoring ²²⁶Ra when produced water samples host low levels of ²²⁸Ra—since the progeny of ²²⁸Ra can significantly confound the LSC analyses.