An alternative method for Ra determinations in water

Concentrations of 226Ra and 224Ra in 13 wells distributed throughout McCulloch and Mason counties in the Hickory Aquifer of the Llano Uplift Region of West-Central Texas are reported. Measurable alpha-particle activity is present in all wells, with seven wells having 226Ra radioactivity concentrations greater than 185 Bq m-3 (5 pCi L-1). An alternative methodology for measuring 226Ra, 224Ra and 228Ra is described. The EPA-approved methodology for estimating total Ra is shown to be invalid for aquifers containing significant levels of 224Ra. Alpha-particle activity measurements made in the interval of 12 to 300 h after Ra isolation lead to self-consistent solutions for radioactivity concentrations of 226Ra and 224Ra, with negligible contributions from 228Ra. Radioactivity concentrations of 228Ra can be calculated from grow-in terms for this isotope used with alpha-particle activity measurements at post-isolation times significantly longer than 800 h. Comparison of the 226Ra radioactivity concentration with that reported previously by the Texas Department of Health for a single well indicates acceptable agreement. However, the radioactivity concentration attributable to 228Ra for the same well was found to be in significant disagreement with the Texas Department of Health value.