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% (2σ_mean) 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% (2σ_mean) 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% (2σ_mean) 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.