Tracers Of Environmental Processes Research Articles

Biased estimates of the isotope ratios of steady‐state evaporation from the assumption of equilibrium between vapour and precipitation

AbstractStable water isotope ratios are measured as a tracer of environmental processes in materials such as leaves, soils, and lakes. Water in these archives may experience evaporation, which increases the abundance of heavy isotopologues proportionally to the gradients in humidity and isotope ratio between the evaporating water and the surrounding atmosphere. The isotope ratio of the atmosphere has been difficult to measure until recently, and measurements remain scarce. As a result, several assumptions have been adopted to estimate isotope ratios of atmospheric water vapour. Perhaps the most commonly employed assumption in terrestrial environments is that water vapour is in isotopic equilibrium with precipitation. We evaluate this assumption using an eight‐member ensemble of general circulation model (GCM) simulations that include explicit calculation of isotope ratios in precipitation and vapour. We find that across the model ensemble, water vapour is typically less depleted in heavy isotopologues than expected if it were in equilibrium with annual precipitation. Atmospheric vapour likely possesses higher‐than‐expected isotope ratios because precipitation isotope ratios are determined by atmospheric conditions that favour condensation, which do not reflect atmospheric mixing and advection processes outside of precipitation events. The effect of this deviation on theoretical estimates of isotope ratios of evaporating waters scales with relative humidity. As a result, the equilibrium assumption gives relatively accurate estimates of the isotope ratios of evaporating waters in low latitudes but performs increasingly poorly at increasing latitudes. Future studies of evaporative water pools should include measurements of atmospheric isotope ratios or constrain potential bias with isotope‐enabled GCM simulations.

Environmental radioactivity measurements and applications – Difficulties, current status and future trends

For several decades natural and artificial radioactivity in the environment have been extensively studied all around the world. Nuclear accidents – mainly that of Chernobyl – have led to the development of the field of radioecology, while detector systems and techniques – with predominant that of γ-spectrometry – have been continuously developed through the years to meet researchers' needs. The study of natural radionuclides that was originally limited to 226Ra, 232Th and 40K was then extended to include radionuclides such as 234Th, 210Pb, 235U and 7Be, which allowed the study of radioactive equilibrium. Besides their importance from the radiation protection point of view, many radionuclides are also used as tracers of environmental processes, such as aerosol and transportation of air masses studies (7Be, 10Be, 22Na), soil erosion, sedimentation and geochronology (210Pb, 137Cs), marine ecosystems studies and studies related to climate change. All these studies require specialized samplings strategies and sampling preparation techniques as well as high quality measurements, while the improvement of detection limits is often of vital importance. This work is a review of environmental radioactivity measurements and applications, mainly focused in the field of γ-spectrometry, for which difficulties and limitations will be presented, together with future trends, new challenges and applications.

New Gamma-Spectrometry Technologies for Environmental Sciences

HPGe-spectrometers operating in underground laboratories represent the most important recent achievements in the radiometric sector mainly because of better sensitivity, which enabled to decrease a sample size by about a factor of ten, and to carry out new applications of radionuclides as tracers of environmental processes.

New Analytical Technologies for New Science

Recent developments in ultra low-level radionuclide analyses using radiometrics and mass spectrometry methods, which have had important impacts on new applications of radionuclides as tracers of environmental processes are discussed.

Short-term seasonal variability in 7Be wet deposition in a semiarid ecosystem of central Argentina

The 7Be wet deposition has been intensively investigated in a semiarid region at San Luis Province, Argentina. From November 2006 to May 2008, the 7Be content in rainwater was determined in 58 individual rain events, randomly comprising more than 50% of all individual precipitations at the sampling period. 7Be activity concentration in rainwater ranged from 0.7 ± 0.3 Bq l −1 to 3.2 ± 0.7 Bq l −1, with a mean value of 1.7 Bq l −1 (sd = 0.53 Bq l −1). No relationship was found between 7Be content in rainwater and (a) rainfall amount, (b) precipitation intensity and (c) elapsed time between events. 7Be ground deposition was found to be well correlated with rainfall amount ( R = 0.92). For the precipitation events considered, the 7Be depositional fluxes ranged from 1.1 to 120 Bq m −2, with a mean value of 32.7 Bq m −2 (sd = 29.9 Bq m −2). The annual depositional flux was estimated at 1140 ± 120 Bq m −2 y −1. Assuming the same monthly deposition pattern and that the 7Be content in soil decreases only through radioactive decay, the seasonal variation of 7Be areal activity density in soil was estimated. Results of this investigation may contribute to a valuable characterization of 7Be input in the explored semiarid ecosystem and its potential use as tracer of environmental processes.

Radioactive nuclides as tracers of environmental processes

Relatively short-lived radioactive nuclides of cosmogenic origin, such as7Be (T1/2=53.3 d),32P (14.3 d),33P (25.3 d),35S (87.4 d) as well as22Na (2.6 y) have come to be recognized as potential tools to trace environmental processes, such as precipitation, washout (precipitation scavenging), resuspension, atmospheric particle deposition and deposition patterns of airborne contaminants, aerosol deposition and aerosol trapping by above ground vegetation (air-to-vegetation transfer). Of the above radionuclides,7Be as well as210Pb of terrestrial origin and137Cs, a fission product radionuclide, are commonly used for determining the atmospheric particle deposition parameters. It was found that the deposition velocity of7Be-associated aerosol particles varied from 0.3 to 0.8 cm. s-1(average 0.5 cm. s-1), the washout ratio for7Be varied from 103 to 375 (average 144), the resuspension factor for the7Be-associated particles varied from 1.6. 10-4to 4.2. 10-4m-1(average 2.3. 10-4m-1) and the air-to-vegetation transfer rate for7Be varied from 5258 to 16180 m3.kg-1(average 9856 m3.kg-1) for grass, gramineae or poaceae the species. For the137Cs-associated aerosol particles the deposition velocity varied from 1.3 to 6.3 cm. s-1(average 3.4 cm. s-1), the washout ratio for137Cs varied from 284 to 3810 (average 1295), the resuspension factor for the137Cs-associated aerosol particles varied from 0.1. 10-4to 1.2. 10-4m-1(average 0.6. 10-4m-1) and the air-to-vegetation transfer rate for137Cs varied from 39638 to 345279 m3.kg-1(average 181197 m3.kg-1) for grass, gramineae or poaceaethe species.

Temporal changes of 7Be and 210Pb concentrations in surface air at temperate latitudes (40°N)

Atmospheric concentrations of 7Be and 210Pb were measured for 15 years (1987–2001) in ground-level air at Thessaloniki, Northern Greece (40°38′N, 22°58′E). Mean activity concentrations of 7Be and 210Pb were 5.02 mBq m −3 and 664 μBq m −3, respectively, characteristic of the latitude of 40°N. Monthly atmospheric concentrations of 7Be showed a strong seasonal trend with the highest values being observed in the summer and the lowest in the winter period. Multiple regression analysis of the data of 7Be concentrations and a number of meteorological parameters revealed that the sunspot number and temperature are the most significant parameters affecting the concentrations of 7Be in surface air. The observed strong positive correlation between the mean monthly concentrations of 7Be and the temperature confirms that the increased rate of vertical transport within the troposphere, especially during the warm months, has as a result to carry down to the surface layer air masses enriched in 7Be. Highest values of the mean monthly atmospheric concentrations of 210Pb were observed in the autumn and lowest in the spring period. The positive correlation that was observed between 210Pb and 7Be concentrations during the summer months suggests that these two radionuclides could be used together as tracers of environmental processes.

Comparison of cosmogenic radionuclide production and geomagnetic field intensity over the last 200 000 years

The production rate of cosmogenic radionuclides such as 10Be or 14C is known to vary as a function of the geomagnetic field intensity. It should, therefore, be possible to extract a record of palaeofield intensity from the deposition record of these radionuclides in marine or terrestrial sediments and ice cores. Field intensity variations, however, are not the only factor that has influenced the cosmogenic radionuclide records. In the case of 14C, variations of the global carbon cycle, caused by reorganization of the ocean circulation patterns from the last glacial to the present interglacial, are superimposed. 10Be is not affected by these variations because it is not part of the carbon cycle, but its deposition rates in marine sediments vary as a function of lateral sediment redistribution and boundary scavenging intensity. A global stacked record of 10Be deposition rates, corrected for sediment redistribution by normalizing to 230Thex, was shown to remove most of the disturbances, and provides a record of 10Be production rate variations over the last 200 000 years, which translates into geomagnetic field intensity variations. This dataset is compared with palaeofield intensities reconstructed from marine sediments by palaeomagnetic methods, from variations in atmospheric 14C/12C derived from independent calibrations of 14C ages, such as U/Th dating and tree ring chronology, and from 36Cl and 10Be fluxes in polar ice cores. Potential influences of the Earth’s orbital parameters and insufficient correction for orbitally triggered climate variations on the palaeointensity reconstructions are assessed. It is argued that the palaeointensity records derived from marine sediments are not significantly affected by these factors.

Silver-108m in Biota and Sediments at Bikini and Eniwetok Atolls

THE occurrence1 of the long lived silver radionuclide 108mAg (T½>100 yr) in biota from the Pacific Ocean has been held to suggest that the silver radionuclide ratio 110mAg/108mAg may be useful as a tracer of environmental processes. It also appears1 that large amounts of 110mAg and 108mAg were not produced during the 1958 test series or earlier, and that the production activity ratio 110mAg/108mAg was 162, a ratio derived solely from the thermal neutron activation of stable silver. We consider that their observations require reconsideration.