Radium and radium-daughter nuclides in carbonates: a brief overview of strategies for determining chronologies

Natural radionuclides behaviour in drinking groundwaters from Castilla y León (Spain); radiological implications

Since the coming into force of the European Council Directive 51/2013 EURATOM and its transposition into the Spanish legislation, the presence of radioactive substances in drinking waters must be kept under surveillance to ensure that the health protection requirements are met. Driven by this regulatory framework, in an attempt to know the starting point from which to design surveillance plans, the groundwaters intended for human consumption of Castilla y León (Spain) have been radiologically characterised by using both low-level γ-ray and α-particle spectrometry to determine the activity concentration of the natural radionuclides needed to account for the indicative dose estimation. This extensive research has comprised the radiological characterisation of more than 400 drinking water samples from one of the European Union's largest regions. Furthermore, the gross α and gross β activities have been analysed. Results showed a high geographical variability that can be related to the hydrogeological formations where the groundwaters come from. The uranium isotopes, 234U and 238U, are the main radionuclides present in the analysed drinking waters reaching values up to 2000 mBq/L, in the southwestern and western of Castilla y León, where U-rich minerals are part of the host rock. High 210Pb and 226,228Ra occurrences are found in the low permeability igneous and metasedimentary hydrogeological formations of Salamanca province. From a public health protection point of view, 4.4% of the total drinking water samples from intakes exceeded the Indicative Dose parametric value of 0.1 mSv, which is a not negligible number of samples, being very likely related to granitic and metamorphosed host rock under specific local conditions. This fact highlights the need for research and consideration of special surveillance of the groundwaters from these areas.

Age-Dating of Foliage and Soil Organic Matter: Aligning 228Th:228Ra and 7Be:210Pb Radionuclide Chronometers over Annual to Decadal Time Scales

The 228Th:228Ra ratios of foliage and organic soil horizons evolve with time following a predictable radioactive decay law and thus provide a new chronometer for absolute age-dating of plant and soil organic matter. Preferential uptake of 228Th (t0.5 = 1.9 years) and 228Ra (t0.5 = 5.9 years) by canopy tree species, ferns, and mosses, drives disequilibrium in the 232Th-228Ra-228Th radioactive decay series within forest vegetation and organic soils. With examples from northeastern USA, we verify a new 228Th:228Ra age model by demonstrating its concordance with the fallout radionuclide chronometer 7Be:210Pb in the 0 to 5-year time frame [R2 = 0.87, RMSE = 0.5 years]. At our locality, canopy tree species assimilate 228Th with a typical initial ratio (228Th:228Ra)0 ∼ 0.3, but in several instances, both deciduous and coniferous tree species show a preference for Th over Ra with (228Th:228Ra)0 exceeding 5. While the 228Th:228Ra system is restricted to organic soil horizons, concordance of 228Th:228Ra with established 7Be:210Pb and 241Am bomb-pulse chronometers establishes a coherent age-dating system of soil organic matter based on three independent chronometers and five particle reactive metals, and spanning 0-200 years in time scale that encompasses both organic and mineral soils to depths of up to 30 cm. Concordance indicates that these metals all follow common processes of organometallic colloid formation and migration and, in conjunction with 14C, may open new opportunities to understand soil pedogenic processes that regulate the storage of carbon and atmospheric metals such as Pb and Hg.

Measuring effective radium concentration with large numbers of samples. Part II--general properties and representativity

Effective radium concentration EC(Ra), product of radium concentration and radon emanation, is the source term for radon release into the pore space of rocks and the environment. Over a period of three years, we performed more than 6000 radon-222 accumulation experiments in the laboratory with scintillation flasks and SSNTDs and we obtained experimental EC(Ra) values from more than 1570 rock and soil samples. With this method, which allowed the measurement of EC(Ra) from large numbers of samples with sufficient accuracy and uncertainty, as detailed in the companion paper, the dependence of the emanation factor on temperature and moisture content is revisited. In addition, with such a large EC(Ra) dataset, dispersion of EC(Ra) can be studied at sample-scale (cm to dm) and at scarp-scale (m to tens of m). Furthermore, we are able to discuss the representativity of obtained EC(Ra) values at field-scale, and to investigate the spatial variations of EC(Ra) over kilometric scales, within geological formations and across formations and faults. This experimental study opens new perspectives in the understanding of radium geochemistry and illustrates the importance of studying the radon source term with large numbers of samples for the modelling of geological and environmental processes, and also for the assessment of the radon health hazard.

Removal of radium from groundwater using a modified bauxite refinery residue

Radium (Ra) removal by an unconventional sorbent, a modified bauxite refinery residue (MBRR), is investigated for a groundwater extracted in Missouri, USA. The MBRR treatment causes substantial reductions of both gross α and combined Ra activities from 0.955 ± 0.005 and 0.66 ± 0.005 Bq L to below detection limits (0.037 Bq L or 1 pCi L). Column breakthrough occurs at 0.555 Bq L for gross α and 0.185 Bq L for combined Ra (15 and 5 pCi L; USEPA's maximum contaminant levels) after 54 and 40 d run time, respectively. At 84 d the MBRR media continues to remove 24.3% of raw water gross α and 39.7% of the combined Ra. The treatment effluent has an initial pH of 10.9, outside the USEPA guides (6.5-8.5); this may be readily mitigated by posttreatment acid injection, or by raw water blending. The MBRR simultaneously removes other potentially hazardous trace elements (e.g., Cu, Zn, and Fe) to extremely low concentrations. In addition, toxicity characteristic leaching procedure testing of spent MBRR suggests that metals are bound tightly, such that it is nonhazardous, permitting cost-effective disposal to landfill without special confinement or storage. Consequently, MBRR may be utilized as an alternative adsorbent for treating Ra-contaminated groundwater.