Envirogeochemical exploration for "NORM" wastes: quadrupole inductively coupled plasma mass spectrometric measurements of thorium and uranium isotopes

Abstract

Naturally occurring radioactive material consists of 238U, 235U and 232Th and series daughters; it is released into the environment from many anthropogenic sources. The use of quadrupole ICP-MS is well-suited to site investigations of releases of naturally occurring radioactive material through measurements of the ratios 232Th∶230Th, 230Th∶238U and 234U∶238U. Each ratio measurement may potentially indicate the contrast between the geochemical background and the affected environment. 232Th∶230Th was measured in samples prepared by fusion or selective extraction; Th was concentrated using TRU-Spec extraction resin and isotopic measurements were made with 5–10 µg Th. The precision is ≈1% (2σmean) and 10% for 232Th∶230Th of 1 000 and 100 000–200 000, respectively. Thorium isotopic measurements were applied to studies of accumulation of 230Th in phosphate-fertilized lawns. The ratio 230Th∶238U was determined via combining separate measurements of 230Th∶232Th and 232Th∶235U or 232Th∶238U; a 230Th∶238U precision of 1–2% (2σmean) is reported for a phosphate fertilizer sample. This 230Th∶238U measurement scheme requires no synthetic yield tracers, and is applied to studies of deposition of U mill tailings in an affected river basin. The ratio 234U∶238U was determined in natural waters following a two-stage extraction with Chelex 20 and UTEVA resins; a precision of ≈0.3–0.5% (2σmean) is attained for sample concentrates containing 15–25 µg U. 234U∶238U measurements are useful in distinguishing between dissolved U from natural weathering versus mining/milling sources.