Proficiency testing of 78 international laboratories measuring tritium in environmental waters by decay counting and mass spectrometry for age dating and water resources assessment.

RATIONALE: Tritium (3 H) is an important hydrological tracer commonly used for over 60 years to evaluate water residence times and water dynamics in shallow/recent groundwaters, streams, lakes and the ocean. We tested the analytical performance of 78 international laboratories engaged in low-level 3 H assays for water age dating and monitoring of environmental waters. METHODS: Seven test waters were distributed by the IAEA to 78 international tritium laboratories. Set 1 included a tritium-free groundwater plus three ultra-low 3 H samples (0.5-7 TU) for meeting groundwater dating specifications. Set 2 contained three higher 3 H-content samples (40-500 TU) suitable for testing of environmental monitoring laboratories. RESULTS: Seventy of the laboratories used liquid scintillation counting with or without electrolytic enrichment, seven utilized 3 He accumulation and mass-spectrometry, and one used gas-proportional counting. Only ~50 % of laboratories demonstrated the ability to generate accurate 3 H data that was precise enough for water age dating purposes. CONCLUSIONS: TRIC2018 helped identify recurrent weaknesses and potential solutions. Strategies for performance improvement of 3 H laboratories include: a) improved quantification of 3 H detection limits and analytical uncertainty, b) stricter quality control practices in routine operations along with care and recalibration of 3 H standards traceable to primary NIST standards, c) annual assessment of enrichment factors and instrumental performance, and d) for water age dating purposes the use of electrolytic enrichment systems having the highest possible 3 H enrichment factors (e.g. > 50x).