Radiocarbon integrity of dissolved inorganic carbon (DIC) samples stored in plastic and glass bottles: implications for reliable groundwater age dating.

Various approaches based on the natural variations of carbon isotopes (14C and 13C) in dissolved inorganic carbon (DIC) are routinely used to study groundwater dynamics and to estimate recharge rates by deriving groundwater ages. However, differences in 14C activities in groundwater samples collected repeatedly from the same wells and discordantly young 14C groundwater ages compared to noble gases led some authors to question the validity of radiocarbon dating. Poor sampling protocols and storage effects (14C contamination) for radiocarbon analysis are a critical factor in explaining age determination discrepancies. We evaluated the impact of storage protocols on carbon isotope exchange with atmospheric carbon dioxide by comparing glass versus standard plastic field sampling bottles for various storage times before radiocarbon and 13C analyses. The 14C bias after 12 months in pre-evacuated glass vials was minimal and within analytical precision. However, storage of DIC samples in plastic sampling bottles led to marked changes in 14C and 13C contents (up to ∼15 pmC and ∼ 5 ‰, respectively, after 12 months), meaning contamination led to younger groundwater age estimations than it should have been. Protocols for sampling and storing DIC samples for radiocarbon using pre-evacuated glass bottles help avoid atmospheric 14CO2 contamination and microbial activity.

Global and local meteoric water lines for δ17O/δ18O and the spatiotemporal distribution of Δ'17O in Earth's precipitation.

Recently, δ17O and its excess (Δ'17O) have become increasingly significant "triple-oxygen-isotope" indicators of distinctive hydrological processes in hydrology and climatology. This situation mirrors the research regarding δ18O and δ2H in the 1960s towards a solid theoretical base and a surge in application examples and field studies worldwide. Currently, systematic global measurements for δ17O in precipitation are still lacking. As a result, attempts have been made to define a Global δ17O/δ18O Meteoric Water Line (GMWL), often by using regional or local datasets of varying systematicity. Different definitions of the global reference slope (λref) for determining Δ'17O values have been proposed, by ongoing debate around a proposed consensus value of 0.528. This study used worldwide samples archived in the IAEA Global Network of Isotopes in Precipitation (GNIP) to (a) derive a δ17O/δ18O GMWL based on four-year monthly records from 66 GNIP stations, (b) formulate local δ17O/δ18O meteoric water lines (LMWL) for these stations' areas, and (c) evaluate regional and seasonal variations of Δ'17O in precipitation. The GMWL for δ17O/δ18O was determined to be δ'17O = 0.5280 ± 0.0002 δ'18O + 0.0153 ± 0.0013, in keeping with the consensus value. Furthermore, our results suggested that using a line-conditioned 17O-excess is a viable alternative over the global λref in the context of regional hydrology and paleoclimatology interpretations; however, without challenging the global λref as such.

Balancing precision and throughput of δ 17O and Δ'17O analysis of natural waters by Cavity Ringdown Spectroscopy.

δ 17O and Δ'17O are emerging tracers increasingly used in isotope hydrology, climatology, and biochemistry. Differentiating small relative abundance changes in the rare 17O isotope from the strong covariance with 18O imposes ultra-high precision requirements for this isotope analysis. Measurements of δ 17O by Cavity Ringdown Spectroscopy (CRDS) are attractive due to the ease of sample preparation, automated throughput, and avoidance of chemical conversions needed for isotope-ratio mass spectrometry. However, the CRDS approach requires trade-offs in measurement precision and uncertainty. In this protocol document, we present the following:•New analytical procedures and a software tool for conducting δ 17O and Δ'17O measurements by CRDS.•Outline a robust uncertainty framework for Δ'17O determinations.•Description of a CRDS performance framework for optimizing throughput, instrumental stability, and Δ'17O measurement precision and accuracy.

High spatial resolution prediction of tritium (3H) in contemporary global precipitation.

Tritium (3H) in Earth's precipitation is vigilantly monitored since historical nuclear bomb tests because of radiological protection considerations and its invaluable role as a tracer of the global water cycle in quantifying surface, groundwater, and oceanic fluxes. For hydrological applications, accurate knowledge of 3H in contemporary local precipitation is prerequisite for dating of critical zone water and calibrating hydrogeologic transport and groundwater protection models. However, local tritium input in precipitation is hard to constrain due to few 3H observation sites. We present new high-spatial resolution global prediction maps of multi-year mean 3H in contemporary "post-bomb" (2008-2018) precipitation by using a robust regression model based on environmental and geospatial covariates. The model accurately predicted the mean annual 3H in precipitation, which allowed us to produce global 3H input maps for applications in hydrological and climate modelling. The spatial patterns revealed natural 3H in contemporary precipitation sufficient for practical hydrological applications (1-25 TU) but variable across continental regions and higher latitudes due to cumulative influences of cyclical neutron fluxes, stratospheric inputs, and distance from tropospheric moisture sources. The new 3H maps provide a foundational resource for improved calibration of groundwater flow models and critical zone vulnerability assessment and provides an operational baseline for quantifying the potential impact of future anthropogenic nuclear activities and hydroclimatic changes.

Comparative evaluation of 2H- versus 3H-based enrichment factor determination on the uncertainty and accuracy of low-level tritium analyses of environmental waters.

Analysis of low-level tritium (3H) in environmental waters requires pre-concentration using electrolytic enrichment prior to decay counting. Accurate and precise electrolytic enrichment factors (EF) are required to determine the sample's environmental 3H concentration. Two methods are used to determine EFs: i) the Spike Proxy Method (SPM) and ii) the Deuterium Method (DM) with each having several modalities. We conducted a comparative assessment of four EF strategies using 250 mL and 500 mL electrolytic enrichment of three low-level 3H proficiency water standards (0.5-7 TU) to see which strategy gave the most accurate 3H results based on z- and Zeta-scores. Our comparative evaluation revealed the DM offers consistently superior 3H results, with more precise EF determinations compared to the three SPM strategies. The DM gave the best z-scores with an EF relative combined uncertainty of about 0.5‰ and a negligible contribution to the overall uncertainty budget due to the EF determination. Moreover, the DM can improve productivity by eliminating the spike and gravimetric procedures from routine analyses and can give rapid cell enrichment performance feedback prior to decay counting. We recommend low-level tritium laboratories consider adopting the DM into their 3H sample enrichment and analysis operations.

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 that has been 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: The proficiency test helped identify recurrent weaknesses and potential solutions. Strategies for performance improvements 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 tritium 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. >50×).