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.

Principles and uncertainties of 14C age estimations for groundwater transport and resource evaluation.

Radiocarbon (14C) is useful for estimating groundwater ages for transport and water resource exploitation assessment. If the 14C content of dissolved inorganic carbon (14CDIC) is known, the age of groundwater can be estimated by applying a radiocarbon decay equation combined with an appropriate geochemical correction model. However, age determinations are subject to uncertainties caused by parameters which need to be estimated or assumed. Here, we discuss the principles of 14C-based groundwater age estimations and the corrections and errors that affect age determinations differently. Generally, the two factors that impact the results of 14C groundwater age are Type-1 and Type-2 errors. Type-1 errors are pulse-type changes on derived groundwater ages that are independent of the water age. Type-2 errors cause gradual changes on derived groundwater 14C ages that depend on the water age. The cumulative impact of these errors substantively reduces the accuracy and confidence of 14C age determinations and corrections. When using 14C for groundwater age, consideration of both error types along with the use of samples having a range of 14CDIC contents helps practitioners recognize and minimize 14C age uncertainty, especially for groundwater ages of <1000 and >30,000 years B.P.

A simple polymer electrolyte membrane system for enrichment of low-level tritium (3H) in environmental water samples.

Tritium (3H) is an essential tracer of the Earth's water cycle; yet widespread adoption of tritium in hydrologic studies remains a challenge because of analytical barriers to quantification and detection of 3H by electrolytic pre-concentration. Here, we propose a simple tritium electrolytic enrichment system based on the use of solid polymer electrolyte membranes (PEMs) that can be used to enrich 3H in 250-3000 mL environmental water samples to a 10-mL final volume. The IAEA PEM-3H system reported here can produce high enrichment factors (>70-fold) and, importantly, removes some of the deterrents to conventional 3H enrichments methods, including the use of toxic electrolysis and neutralization chemicals, spike standards, a complex electrolysis apparatus that requires extensive cooling and temperature controls, and improves precision by eliminating the need for tracking recovery gravimetrics. Preliminary results with varying operating conditions show 3H enrichments to 70-fold and higher are feasible, spanning a wide range of tritium activities from 5 to 150 TU with a precision of ∼4.5 %. Further work is needed to quantify inter-sample memory and to establish lower 3H detection limits. The IAEA PEM-3H system is open source, with 3-D CAD and design files made freely available for adoption and improvement by others.

Isotope fractionation of sandy-soil water during evaporation - an experimental study.

Soil samples containing water with known stable isotopic compositions were prepared. The soil water was recovered by using vacuum/heat distillation. The experiments were held under different conditions to control rates of water evaporation and water recovery. Recoveries, δ18O and δ2H values of the soil water were determined. Analyses of the data using a Rayleigh distillation model indicate that under the experimental conditions only loosely bound water is extractable in cases where the recovery is smaller than 100 %. Due to isotopic exchange between vapour and remaining water in the micro channels or capillaries of the soil matrix, isotopic fractionation may take place under near-equilibrium conditions. This causes the observed relationship between δ2H and δ18O of the extracted water samples to have a slope close to 8. The results of this study may indicate that, in arid zones when soil that initially contains water dries out, the slope of the relationship between δ2H and δ18O values should be close to 8. Thus, a smaller slope, as observed by some groundwater and soil water samples in arid zones, may be caused by evaporation of water before the water has entered the unsaturated zone.

A portable membrane contactor sampler for analysis of noble gases in groundwater.

To enable a wider use of dissolved noble gas concentrations and isotope ratios in groundwater studies, we have developed an efficient and portable sampling device using a commercially available membrane contactor. The device separates dissolved gases from a stream of water and collects them in a small copper tube (6 mm in diameter and 100 mm in length with two pinch-off clamps) for noble gas analysis by mass spectrometry. We have examined the performance of the sampler using a tank of homogeneous water prepared in the laboratory and by field testing. We find that our sampling device can extract heavier noble gases (Ar, Kr, and Xe) more efficiently than the lighter ones (He and Ne). An extraction time of about 60 min at a flow rate of 3 L/min is sufficient for all noble gases extracted in the sampler to attain equilibrium with the dissolved phase. The extracted gas sample did not indicate fractionation of helium ((3) He/(4) He) isotopes or other noble gas isotopes. Field performance of the sampling device was tested using a groundwater well in Vienna and results were in excellent agreement with those obtained from the conventional copper tube sampling method.

Reliable determination of oxygen and hydrogen isotope ratios in atmospheric water vapour adsorbed on 3A molecular sieve.

The isotope ratio of atmospheric water vapour is determined by wide-ranging feedback effects from the isotope ratio of water in biological water pools, soil surface horizons, open water bodies and precipitation. Accurate determination of atmospheric water vapour isotope ratios is important for a broad range of research areas from leaf-scale to global-scale isotope studies. In spite of the importance of stable isotopic measurements of atmospheric water vapour, there is a paucity of published data available, largely because of the requirement for liquid nitrogen or dry ice for quantitative trapping of water vapour. We report results from a non-cryogenic method for quantitatively trapping atmospheric water vapour using 3A molecular sieve, although water is removed from the column using standard cryogenic methods. The molecular sieve column was conditioned with water of a known isotope ratio to 'set' the background signature of the molecular sieve. Two separate prototypes were developed, one for large collection volumes (3 mL) and one for small collection volumes (90 microL). Atmospheric water vapour was adsorbed to the column by pulling air through the column for several days to reach the desired final volume. Water was recovered from the column by baking at 250 degrees C in a dry helium or nitrogen air stream and cryogenically trapped. For the large-volume apparatus, the recovered water differed from water that was simultaneously trapped by liquid nitrogen (the experimental control) by 2.6 per thousand with a standard deviation (SD) of 1.5 per thousand for delta(2)H and by 0.3 per thousand with a SD of 0.2 per thousand for delta(18)O. Water-vapour recovery was not satisfactory for the small volume apparatus.