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

Naturally occurring radioactive material consists of U-238, U-235, and Th-2 32 and series daughters; it is released into the environment from many anth ropogenic sources. The use of quadrupole ICP-MS is well-suited to site inve stigations of releases of naturally occurring radioactive material through measurements of the ratios Th-232:Th-230, Th-230:U-238 and U-234:U-238. Eac h ratio measurement may potentially indicate the contrast between the geoch emical background and the affected environment. Th-232:Th-230 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 mug Th. The precision is approximate to1% (2 sigma (mean)) and 10% for T h-232:Th-230 Of 1 000 and 100 000-200 000, respectively. Thorium isotopic m easurements were applied to studies of accumulation of Th-230 in phosphate- fertilized lawns. The ratio Th-230: U-238 was determined via combining sepa rate measurements of Th-230 :Th-232 and Th-232:U-235 Or Th-232:U-238; a Th- 230:U-238 precision of 1-2% (2 sigma (mean)) is reported for a phosphate fe rtilizer sample. This Th-230:U-238 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 U-234: U-238 was determined in natura l waters following a two-stage extraction with Chelex 20 and UTEVA resins; a precision of approximate to0.3-0.5% (2 sigma (mean),) is attained for sam ple concentrates containing 15-25 mug U. U-234:U-238 measurements are usefu l in distinguishing between dissolved U from natural weathering versus mini ng/milling sources.