Validation of a gamma-spectrometric method for the measurement of ^226,228Ra in environmental media relevant to the offshore oil and gas industry

Summary

The offshore production of oil and gas leads to naturally occurring ^226,228Ra being brought to the surface along with produced water. Assessment of potential environmental exposures, if any, requires accurate, precise and practical methods for the measurement of low concentrations of ^226,228Ra. A method for the simultaneous measurement of ^226,228Ra in marine sediments, biota and produced water at a sensitivity of 0.01 pCi/g using high-sensitivity, high-resolution gamma-spectrometry was subjected to an interlaboratory exercise using U.S. commercial laboratories to validate its accuracy, precision, robustness, and sample throughput. The prescribed method involves direct counting for solid samples and a simple Pb(¹³³Ba,Ra)SO₄ co-precipitation procedure for water samples followed by gamma-counting. Analytical results received from the participating laboratories were subjected to data analysis and statistical evaluation to validate the overall performance of the prescribed method. Relatively good precision and high accuracy of data were achieved when the participating laboratories followed the prescribed procedure closely. The consistency of results among laboratories was not correlated to the ^226,228Ra concentrations in the samples. Most of the results that failed the acceptance criteria were either due to the absence of geometric and secular equilibrium between ²²⁶Ra and its signature decay daughters, or due to insufficient counting statistics when laboratories are working close to their detection limits. No significant bias or systematic errors were observed, except for produced water samples where results were biased high compared to the known values.