Following the explosion of reactor 4 at the Chernobyl power plant in northern Ukraine in 1986, contaminated soil and vegetation were buried in shallow trenches dug directly on-site in an Aeolian sand deposit. These trenches are sources of radionuclide (RN) pollution. The objective of the present study is to provide constraints for the Chernobyl flow and RN transport models by characterising groundwater residence time. A radiochronometer H-3/He-3 method (t(1/2) = 12.3 a) and anthropogenic tracers including CFC and SF6 are investigated along with the water mass natural tracers Na, Cl, O-18 and H-2.The groundwater is stratified, as evidenced by Na and Cl concentrations and stable isotopes (O-18, H-2). In the upper aeolian layer, the Na-Cl relationship corresponds to evapotranspiration of precipitation, while in the underlying alluvial layer, an increase in Na and Cl with depth suggests both water-rock interactions and mixing processes. The H-3/He-3 and CFC apparent groundwater ages increase with depth, ranging from 'recent' (1-3 a) at a 2 m depth below the groundwater table to much higher apparent ages of 50-60 a at 27 m depth below the groundwater table. Discrepancies in H-3/He-3 and CFC apparent ages (20-25 a and 3-10 a, respectively) were observed during the 2008 campaign at an intermediate depth immediately below the aeolian/alluvial sand limit, which were attributed to the complex water transfer processes. Extremely high SF6 concentrations, well above equilibrium with the atmosphere and up to 1112 pptv, are attributed to significant contamination of the soils following the nuclear reactor explosion in 1986. The SF6 concentration vs. the apparent groundwater ages agrees with this interpretation, as the high SF6 concentrations are all more recent than 1985. The persistence of the SF6 concentration suggests that SF6 was introduced in the soil atmosphere and slowly integrated in the groundwater moving along the hydraulic gradient. The apparent age distribution in the lumped parameter models suggests an exponential or piston flow model in the upper geological section, followed by more pronounced mixing processes in the lower section. (C) 2012 Published by Elsevier Ltd.

Most published results concerning deuterium-hydrogen fractionation in plants are in the range 0.8-1, indicating no bioaccumulation of the heavy isotope. In spite this, an updated compilation of litterature data show that 77% of OBT/TFWT ratios measured in terrestrial plants and food items are greater than one, with a mean value of 1.92. On the other hand, OBT/TFWT ratios for aquatic samples do not show such a tritium anomaly, with 81% of the published ratios being less than I. This strongly suggests that the cause for excess tritium in terrestrial organic matter has to be found in the atmosphere. We have developed a simple model of tritium incorporation during plant growth, forced by the annual cycle of tritium in precipitation taken from the IAEA/ISOHIS database. The simulated distribution of the OBT/TFWT ratios for terrestrial samples shows many similarities with the observed one. Although other processes such as soil moisture with lower tritium content than atmospheric water vapour can be invoked, our results suggest that the annual tritium maximum which occurs in spring, during the growing season, is capable of creating substantial OBT/TFWT enrichments and has to be considered as well.

Studies on estimation of age of groundwater from the unconfined aquifer in sandy Quaternary deposits at the international radioecological experimental site at Chernobyl nuclear power plant site using "tritium — helium-3" (3H-3He) method are presented. Interpretation of the obtained data was carried out using groundwater flow modeling. A good agreement in isotope dating estimates and groundwater modeling data was obtained. Results of these studies suggest that the longCterm (during last 60 years) infiltration recharge rate at the first terrace of the Pripyat River constituted ≈ 200 mm/y. Successful application of 3H-3He method creates preconditions for its more wide application for studies of water exchange and radionuclide migration in groundwater at Chernobyl site.

The design, setup and performance of a mass spectrometric system for the analysis of low to very low-level tritium in environmental samples are described. The tritium concentration is measured indirectly by the (3)He ingrowth from radioactive decay after complete initial degassing of the sample. The analytical system is fully computer-controlled and consists in a commercial helium isotope mass spectrometer coupled with a high vacuum inlet system. A detection limit of 0.15 Bq/kg is routinely obtainable for sample sizes of 20g of water equivalent and an accumulation time of three months. Larger samples (and/or longer accumulation time) can be used to obtain lower detection limits. In addition to the benefit of a lower detection limit, another advantage of this non-destructive method lies in the simplicity of the analytical procedure which strongly limits the risk of contamination. An inter-comparison was successfully performed with the conventional beta counting technique on lyophilized grass samples, in a range of tritium concentrations of environmental interest. It shows that the (3)He mass spectrometry method yields results that are fully consistent with the conventional liquid scintillation technique over a wide range of tritium concentrations.

Owing to their inertness and contrasted composition in the various earth reservoirs, helium isotopes are powerful tracers of a number of processes pertaining to geophysics and geochemistry. Because sediments cover a large portion of the earth's surface, helium isotope geochemistry of sediment pore‐waters is of particular interest. In spite of this potential, its development has been hampered by the difficulty of collecting samples without gas loss and/or contamination problems. We developed a new method for the sampling and the quantitative extraction of dissolved helium from sediment pore‐waters, leading to the determination of 3He and 4He concentration profiles. Core sampling is non‐destructive (no squeezing). The principle of the method is to use standard copper tubes (1.2 cm OD/25 cm in length), subsequently sealed with clamps, to take mini‐cores along the sediment core immediately following its retrieval. In the lab, the sediment is transferred from the copper tube to a noble gas extraction line by applying pressurized helium‐free water at one end of the copper tube. This technique allows dissolved helium to be recovered and analyzed using standard procedures for water samples. Tests were carried out successfully on an artificial core equilibrated with air to check the extraction efficiency in the same conditions as for real cores. The validity of the method was further confirmed by acquiring a vertical helium profile from a real marine core from the Zaire deep‐sea fan, illustrating some possible applications.

The Creys-Malville nuclear plant, located on the left bank of the Rhône, was shut down in 1998. The facilities are currently in their initial stage of dismantling. In order to establish a baseline for tritium in the vicinity of the site prior to the main dismantling phase, we carried out a monitoring program between 2002 and 2005 in the main terrestrial and aquatic compartments of the local environment. Tritium levels in the groundwaters and in the Rhône waters correspond to the regional tritium concentration in precipitation. The data obtained for the terrestrial environment are also in good agreement with the regional background and do not show any specific signature linked to the nuclear plant. The various aquatic compartments of the Rhône (fish, plant, sediment) are significantly enriched in tritium both upstream and downstream of the power plant: although Tissue-Free Water Tritium concentrations are in equilibrium with the river water, the non-exchangeable fraction of organic bound tritium in plants and fishes shows values which outpace the river water background by one to two orders of magnitude, and up to four to five orders of magnitude in the sediments. This tritium anomaly is not related to the nuclear plant, as it is already present at the Swiss border 100km upstream of the site. Although fine particles of tritiated polystyrene entering the composition of the luminous paints used by the clock industry have been suspected on several occasions, the exact nature and the origin of this tritium source remain unknown and require further investigations.

Tritium concentration was measured in snow deposited at the GRIP site (central Greenland) and at the Vostok station (east Antarctica) from snow pits covering the period 1980-1990. The objective of the study was to investigate tritium concentrations in polar regions several decades after the bomb peak of the sixties and to put them in the context of available data for environmental tritium in the Arctic and the Antarctic over the last five decades. The tritium content of the samples was measured by mass spectrometry using the helium-3 regrowth method. In Antarctica, the tritium concentrations are in the range 70-110 TU. The comparison of the bomb tritium history at different locations show that tritium levels increase moving inland, where vapour pressure becomes extremely low and therefore more sensitive to the intrusion of stratospheric air masses highly enriched in tritium. Although most tritium fallout occurred in the Northern hemisphere, the tritium levels in central Greenland in the 80's, in the range 10-40 TU, are significantly lower than at Vostok. Unlike Antarctica, no such continental effect is observed in Greenland, due to the higher water vapour content of the air masses, as evidenced by the much higher snow accumulation rate. Whereas tritium fallout in Antarctica appears to occur as a result of direct injections of stratospheric tritium during winter, Arctic fallout are the result of the dominant spring injection of stratospheric air at mid-latitude, in line with the deposition of other stratospheric tracers. (c) 2006 Elsevier B.V. All rights reserved.

The results of an environmental survey of tritium in the vicinity of Creys-Malville nuclear plant are presented. The plant, which was shut down in 1998, is currently in its initial stage of dismantling. Measurements in the terrestrial environment do not provide any indication that tritium levels are significantly above the regional background. Tritium concentrations in groundwaters and in the river Rhône waters are at their background level too, with values in the range 0.94-1.64 Bq/l. In contrast, the various aquatic compartments of the river (fishes, plants, sediments) show enriched non-exchangeable bound tritium concentrations both upstream and downstream of the power plant, with values up to 14.7, 49 and 1495 Bq/kg (dry weight) respectively. Although fine particles of tritiated compounds entering in the composition of luminous paints have been suspected on several occasions, the nature and the origin of this tritium source, which is not related to the nuclear plant, remain unknown and will require further investigations.