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Isotopic characterization of low levels of uranium in environmental samples by mass spectrometry

Isotopic characterization of low levels of uranium in environmental samples by mass spectrometry

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Isotopic characterization of low levels of uranium in environmental samples by mass spectrometry

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  1. DOECAP ASP 2011 Workshop(September 21, Pleasanton, CA) Bradley K. Esser (Lead, Environmental Radiochemistry) Rachel Lindvall (Manager, Q-ICPMS Laboratory) Lawrence Livermore National Laboratory Isotopic characterization of low levels of uranium in environmental samples by mass spectrometry Lawrence Livermore National Laboratory, P. O. Box 808, Livermore, CA 94551 This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

  2. The real title is longer • Isotopic characterization of low levels of depleted uranium in groundwater, air particulate and soil samples by inductively-coupled plasma mass spectrometry • Isotopic characterization = Uranium-235 and -236 • Low levels = ambient background • Depleted uranium = U used for its p3a`roperties as a metal • Environmental samples • Dissolved U in groundwater & spring samples • Total U collected by air particulate samplers • Strong acid-leachable U in surface soil samples • Mass spectrometry = • single-collector quadrupole & multi-collector sector ICPMS

  3. Lawrence Livermore National Laboratory Site 300 Site 300 is a experimental test range.

  4. Testing at a Site 300 firing table

  5. Firing table gravels are sequestered in landfill pits

  6. Tritium released RCRA cap on Pit 7 and 30% of Pit 3 Firing table waste now shipped offsite El Nino rains & pit flooding Explosives experiments began at Building 850 Investigations began Pit 5 Pit 7 Pit 3 0 1955 1960 1965 1970 1975 1980 1985 1990 Pit operations

  7. Site 300 contaminant release sites Soil, bedrock, groundwater and surface water at Site 300 are contaminated with depleted uranium (DU) as a result of past releases to the environment. Groundwater contaminant characterization and cleanup goals are driven by the State of California non-degradation policy in addition to Federal regulation.

  8. Site 300 has a complex geology Natural U concentrations varies in different formation groundwaters =uranium concentration alone is not sufficient to attribute source. • Groundwaters with natural U • Concentrations range over 4 orders of magnitude (0.02 to 200 ppb) • >10% of wells produce water above the drinking water MCL (35 ppb) • Groundwaters with depleted U • Concentrations range over 4 orders of magnitude (0.02 to 400 ppb) • >75% of wells produce water below the drinking water MCL (35 ppb)

  9. Air particulate U is seasonally variable Are high air particulate U concentrations in the summer due to dry, dusty conditions or to site operations?

  10. We need a technique to quantify the presence of DU in order to • Unambiguously attribute anthropogenic U at a geologically complex site with variable groundwater U backgrounds • Define the edge of depleted U plumes • Identify depleted U in low-U samples • Identify natural U in high-U samples • Understand U concentration changes in time-series data • To distinguish different anthropogenic U sources

  11. Isotopic signatures in depleted uranium Uranium scarce, energy cheap = more cycles & more depleted tails Energy expensive, uranium available = fewer cycles & less depleted tails

  12. Commercially available uranium is depleted in U235

  13. Isotopic characterization of DU: Why not use alpha spectrometry?

  14. Isotopic characterization of DU: Why not use alpha spectrometry? • For characterization of depleted U in the environment, alpha spectrometry suffers from • Significant variability in natural 234U/238U • Poorer quantification of 235U and 236U U-236

  15. 234U is variable in natural U Natural U with 234U/234Uactivity ratio = 1 Depleted U

  16. Depleted U contains U-236 Depleted U does contain U236 Natural U contains nomeasurable U236

  17. We use ICPMS to determine uranium isotopic composition Multi-collector sector ICPMS Requires column chemistry for all samples & mass spectrometry expertise Used for soils, special projects and for nuclear forensics & attribution Provides the facility with calibrated spikes Single-collector quadrupole ICPMS Fast and efficient, can run groundwaters without cleanup chemistry Used for routine monitoring groundwater and air particulates for U isotopic composition & concentration

  18. We measure uranium isotope atom ratios to determine both isotopic composition and concentration (using isotope dilution) • Spike samples with known amount of a isotopically-enriched U233 spike solution that has been characterized for concentration and composition • Calculate detector deadtime • Calculate instrumental mass bias • Perform isotopic analysis with background correction, mass bias correction, uranium 233 spike correction and necessary quality control. We use internal LLNL SOPs to do this work

  19. As ICPMS instrumentation has improved, our methods have changed:Q-ICPMS for groundwater samples

  20. QC samples run with every batch • Uranium isotopic composition calibration standard • Purpose: correct for instrumental mass bias • Solution: NIST U005 or U010 (slightly depleted or enriched uranium) • Frequency: Two or three times per sample run • Data: Isotopic atom ratios (235U/238U, 234U/238U, 236U/238U) • Uranium isotopic composition validation standard • Purpose: demonstrate our ability to measure U isotope atom ratios • Solution: NIST 4321C (natural isotopic composition uranium) • Frequency: Two or three times per sample run • Data: Isotopic atom ratios (235U/238U, 234U/238U, 236U/238U) • Sample duplicate • Purpose: To demonstrate reproducibility • Solution: separate sample aliquot from same bottle; processed and run separately • Frequency: Every 12th sample

  21. QC samples run every quarter • Procedural blanks • Purpose: determine blank U introduced by precipitation, evaporation and dilution • Solution: Ultrapure water • Frequency: Once per quarter (3 total/quarter) • Data: Isotopic atom ratios (235U/238U, 234U/238U, 236U/238U) and concentration • Concentration and matrix standards • Purpose: demonstrate our ability to measure U concentration by isotope dilution • Solution: NIST 4321C (U concentration standard) + ultrapure water • Frequency: Once per quarter (3 total/quarter) • Data: Isotopic atom ratios (235U/238U, 234U/238U, 236U/238U) and concentration

  22. Other QC • Submitted field blanks and blind duplicates • Reagent blanks • Purpose: To determine spectral and non-spectral background levels • Solution: Dilute ultrapure nitric acid • Frequency: five to six times per sample run • Data: count rates on all collected isotope masses • Spike calibrations • Purpose: To validate spike solution concentrations • Solution: Isotopically-enriched spike sltn + concentration std sltn • Frequency: As needed • Data: 233U, 229Th concentrations in spike solutions • Performance tests & round robins • DOE MAPEP • We are an NWAL laboratory working with IAEA and the State Department

  23. Depleted Uranium Detection Limit (~0.1%) using MC-HR-ICPMS • U235/U238 (LLNL Isoprobe) • precision: 0.01 to 0.10 % (2-sigma) • accuracy: 0.03% (measured-true, 30 samples) • U236/U235 (LLNL Isoprobe) • detection limit: 20 ppm (3-sigma) • Detectability of depleted U (assuming 0.2% U235, 32 ppm U236) • Detectability with U235: ~0.015 to 0.15 % DU • Detectability with U236: ~0.15 % DU

  24. Figures of merit for Q-ICPMS of groundwater U

  25. Site 300 groundwater U isotopic composition by Q-ICPMS Over the last ten years, the Q-ICPMS laboratory has processed • ~2500 groundwater samples (from over 350 wells) These samples include • ~1200 samples with isotopic compositions within 2% of natural(representing ~360 wells) • ~1000 samples isotopic compositions more than 5% depleted relative to natural(representing ~120 wells)

  26. Natural and depleted U in Site 300 groundwater have similar concentration ranges High-U groundwater may contain only natural U Low-U groundwater may contain depleted U

  27. Depleted U wells are more likely to exceed the MCL More than 10% off wells producing natural U groundwater exceed the MCL More than 75% of wells producing depleted U groundwater do not exceed the MCL

  28. CASE STUDY: Site 300 B851 Firing Table AreaIdentifying DU in low-U groundwaters

  29. The B851 aquifer is deep and confined NUFT model(vadose-zonetransport): H3 = 70 yrs DU = 5,000 yrs

  30. Uranium in B851 groundwater is low and natural Uranium in one well (W851-08) spiked after the high rainfall associated with the 1997/98 El Nino event, and the isotopic composition changed

  31. Depleted & natural components in W851-08 Deconvolution of the isotopic signal showed that the increase in U concentration in W851-08 resulted from an increase in DU

  32. Fast vadose-zone transport of DU & tritium The increase in DU was accompanied by an increase in tritium far above rainfall activities.

  33. Pit 3 Pit 7 Pit 5 N depleted uranium E tritium 0 500 ft tritium & depleted uranium CASE STUDY: Pit 7 ComplexWhere is the DU coming from? NC7-48 Neroly Formation Cierbo sandstone Great Valley Sequence

  34. Can we use 235U to distinguish DU sources? Esser et al (2002) UCRL-JC-155182

  35. Depleted U may vary from pit to pit

  36. Natural U background concentrations are low

  37. Pit 3 contains more highly depleted U than Pit 5

  38. Extremely depleted U in Pit 3

  39. The 1997 El Nino flooded un-lined Pits 3 & 5 from below Pit 3 Pit 5 Pit 7 NC7-48 Alluvium As groundwater levels rose, groundwater U levels rose in the source monitor well Flow Neroly Bedrock

  40. Changes in isotopic composition indicate addition of DU Pit 3 Pit 5 Pit 7 NC7-48 Alluvium As groundwater levels rose, U levels rose… and U became more depleted Flow Neroly Bedrock

  41. Uranium isotopic data are consistent with hillslope hydrology Pit 3 Pit 5 Pit 7 NC7-48 Alluvium • Correlated changes in U concentration and composition in source monitor well NC7-48 are consistent with depleted U source from • Pit 5 under low water level conditions • Pit 3 under high water level conditions Flow Neroly Bedrock

  42. Is U236 a useful tracer? Reprocessed U Enrichment tails, spent U feedstock Tails, natural U feedstock

  43. In Site 300 soils & waters, U236 confirms the presence of DU, but does not provide additional constraints on origin

  44. Case Study: Air particulate DU as Site 300Is U236 a useful tracer? • Air particulate U236, U235 and U238 fall on the same trends as soil and water

  45. A fire at Site 300 in the summer of 2005 resulted in high particulate DU in filter samples from one station: WCP

  46. DU associated with this event had a distinct isotopic composition • Air particulate U236, U235 and U238 fall on the same trends as soil and water • In one set of samples, an anomalous U236 signal has been observed

  47. The source is unknown, but is consistent with commercial U • Air particulate U236, U235 and U238 fall on the same trends as soil and water • In one set of samples, an anomalous U236 signal has been observed • The isotopic signature of DU released in this event is within the range of commercially available DU signatures

  48. The source is unknown, but is consistent with commercial U • Air particulate U236, U235 and U238 fall on the same trends as soil and water • In one set of samples, an anomalous U236 signal has been observed • The isotopic signature of DU released in this event is within the range of commercially available DU signatures In Site 300 air particulates, U236 confirms DU, and may provide additional constraints on origin

  49. Conclusions • MC-ICPMS provides sensitive detection of depleted uranium • ~ 0.02% DU by 235U/238U • ~ 0.2% DU by 236U/238U • Q-ICPMS allows routine characterization of U isotopics in environmental samples • Allows quantification of DU in time-series data & plumes • Provides evidence for fast vadose-zone transport of DU • Uranium isotopics may allow source attribution • 235U in U.S. DU tails varies with time • 236U also varies and sometimes provides additional information

  50. Acknowledgements • Alan Volpe – ICPMS, geochemistry and inspiration • Eric Christofferson – Alpha counting • Scott Szchenyi – Quadrupole ICPMS • Mike Taffet, Vic Madrid, Zafer Demir, and Sevin Bilir (ERD) - Contaminant hydrogeology • Data management team • Ross Williams – Magnetic sector ICPMS • Richard Bibby, Everett Guthrie – Air particulate filter processing • Nathan Wimer - Depleted uranium isotopics • Anyone who I forgot