M. García-León

The distribution of U, Th and 226Ra derived from the phosphate fertilizer industries on an estuarine system in Southwest Spain

This paper reports an extensive study of the presence of natural radioactivity around a phosphate fertilizer factory complex situated in an estuarine area of southwest Spain. The study has concluded that the wastes from such industries are the cause of the enhancement of natural radioactivity in the immediate environment. Thus, significantly high levels of U- and Th-isotopes and 226 Ra are detected in water and sediment samples collected in this area. These conclusions based on the enhanced isotopic concentrations are further supported by the measured U, Th and Ra isotopic activity ratios being quite different from any observed elsewhere in undisturbed estuaries. These isotope activity ratios appear to be very sensitive indicators of waste disposal practices in such environments

Determination of 99Tc in environmental samples

Abstract A method for the analysis of 99Tc in environmental samples has been developed. The pre-treatment of samples to avoid losses of technetium during ashing has been carefully studied. Hydrochloric acid is in this case superior to ammonia. Technetium-99m ( t 1 2 = 6.0 h ) is used as a radiochemical yield determinant. For large water samples technetium is reduced to +IV oxidation state and co-precipitated with iron hydroxide. Technetium is extracted from sulphuric acid solution with tributylphosphate and back-extracted with sodium hydroxide solution. Back-extraction with ammonia has also been shown to be successful. Technetium is electroplated from 2M sodium hydroxide solution (or from ammonium sulphate solution at pH 4–9) onto stainless steel discs. The radiochemical yield is determined either by counting conversion electrons from 99mTc using a surface barrier detector or by gamma-spectrometry using the 140 keV transition from 99mTc. After decay of 99mTc is measured by an anti-coincidence shielded GM-gas flow counter. The radiochemical yield of 10–100 g biological samples is 50–80%.

ON THE FRACTIONATION OF NATURAL RADIOACTIVITY IN THE PRODUCTION OF PHOSPHORIC ACID BY THE WET ACID METHOD

The fractionation of different natural radionuclides (U-isotopes,226Ra and210Po) in the process used for the production of phosphoric acid in some factories located in the southwest of Spain is analyzed. As a consequence, different ways of natural radionuclide liberation to the environment can be evaluated due to these industrial activities.

Enhancement of natural radioactivity in soils and salt-marshes surrounding a non-nuclear industrial complex

The existence of a very high extension (about 1000 ha) of phosphogypsum piles, sited in the estuary formed by the mouths of the Tinto and Odiel rivers (SW Spain), produce a quite local, but unambiguous radioactive impact in the surrounding salt-marshes. In these piles the main by-product formed in the manufacture of phosphoric acid is stored. The radioactive impact is generated by the deposition and accumulation of radionuclides from the uranium series that previously had been mainly leached or dissolved from the piles by waters that temporally can cover or cross them. Other means of impact, especially through the atmosphere, have been evaluated as negligible or not detectable.

Transfer of natural radionuclides from soils to plants in a marsh enhanced by the operation of non-nuclear industries

Two sampling campaigns were performed in 1993 at the marsh area (Odiel marsh) located in southwestern Spain, in the city of Huelva. Spartina densiflora and substrate soil (5 cm deep) samples were collected in several locations across the area in each campaign. Activity concentrations of 210Po, U-and Th-isotopes were determined in the plant and the substrate samples. The production of phosphoric acid from phosphate mineral in the vicinity clearly enhances the concentrations of these radionuclides in certain areas of the marsh. Moreover, concentrations in plants are affected by the concentration of the same element in its substrate. Indeed, high concentration levels in plants are coincident with high concentration in soils. However, concentration ratios (CR), defined as the ratio between the concentration of an element in the plant and of that in its substrate, are higher when substrate concentrations are low, whereas low CR values are found in areas where substrate concentrations are high. Moreover, both variables (CR and soil concentration) seem to be non-linearly related, at least, in the case of radionuclides from the 238U decay chain.

Modelling the dispersion of non-conservative radionuclides in tidal waters—Part 1: Conceptual and mathematical model

Abstract A 2D four-phase model to study the dispersion of non-conservative radionuclides in tidal waters, in conditions of disequilibrium for ionic exchanges, has been developed. At disequilibrium conditions, ionic exchanges cannot be formulated using distribution coefficients k d . Thus, kinetic transfer coefficients have been used. The model includes ionic exchanges among water and the solid phases (suspended matter and two grain size fractions of sediments), the deposition and resuspension of suspended matter and advective plus diffusive transport. In the second part of this work, which is presented in a separate paper, the model is applied to simulate 226 Ra dispersion, discharged from a fertilizer processing plant, in an estuarine system in the south-west of Spain.

U and Th speciation in river sediments

A study of the distribution of some natural radionuclides in different chemical fractions of river bottom sediments has been carried out. The study has shown that the majority of the total U in sediments is located in the non-residual fractions, while Th is more likely to be present in the residual ones. This difference between U and Th reflects largely a much higher mobility of U relative to Th in surface and near-surface environments. Coprecipitation with amorphous ferromanganese oxyhydroxides is the main process of U and Th incorporation from the water column into the soil particles. Moreover, the distribution of the radionuclides and the analysis of the Th/U mass ratios in different chemical fractions of sediments has made an unequivocal connection between the enhanced U content in river sediments and the wastes discharged into the rivers by the operation of fertilizer industries.

Radioactive impact of phosphate ore processing in a wet marshland in southwestern Spain

In this paper an extensive study of the presence of natural radionuclides in a marshland area located in the vicinity of a phosphoric acid production complex in southwestern Spain is presented. This marsh is a natural reservation where significant biological activity takes place. The marsh is highly affected by the inflow of water from the Odiel river which is enhanced with natural radionuclides from industries which use phosphate rocks as raw material for fertilizer production. Sediment samples, collected from the intertidal zone, showed that wastes from the phosphate industries have resulted in enhanced radioactivities in the marsh.

Modelling the suspended matter distribution in an estuarine system. Application to the Odiel river in southwest Spain

Abstract A numerical model which solves the advective-diffusive dispersion equation for suspended matter and includes the deposition and resuspension is presented. The model requires the simultaneous solution of the hydrodynamic equations under tidal dynamics and atmospheric forcing, using time steps of a few seconds. The model has been applied to the Odiel river (southwest Spain). The hydrodynamic module has been widely validated for neap and medium tides, whereas the dispersion was calibrated against the dissolved 226Ra dispersion pattern. This 226Ra is discharged to the Odiel river from a phosphate fertilizer factory. The model was able to reproduce the observed behaviour of the suspended matter in the estuary. The sedimentation rates have shown that a net, although slow, sedimentation is being produced. Sensitivity tests were inconclusive with respect to parameters describing settling and resuspension, as internal processes within the estuary are overridden by the high influx and efflux of particulate material from the sea.

Dating of marine sediments by an incomplete mixing model

Abstract A marine sediment core from the Kattegat area (Sweden) has been studied for dating purposes. The 137 Cs and 210 Pb profiles have been measured and described by a model of incomplete activity mixing. A time-dependent 137 Cs flux function for the sediment has been constructed from atmospheric data and the 137 Cs contents in water from the Sellafield nuclear reprocessing plant. It has been used as the 137 Cs input function for our model. The results show that the model clearly reproduces the measured profile.