Experimental and modelling study of artificial radionuclides (239Pu, 241Am and 99Tc) uptake by suspended matter in environmental waters located in the south of Spain

The interest on transfer coefficients studies have increased recently, since they are important parameters required understanding and reliably modelling the dispersion of conservative and non-conservative radionuclides in aquatic environments. The approaches, based in the implementation of the uptake kinetics of dissolved radionuclides by solid particles, are more appropriate than those based in the use of the distribution coefficients, k^. In this work, we present a series of tracing experiments to study the uptake of Pu, Am and Tc in natural aqueous suspensions from three aquatic systems (Gergal reservoir, Guadalquivir river, and the estuary of Tinto river) located in the South of Spain. The kinetic transfer coefficient for direct sorption depends on the total available surface of particles and on the concentration of active sites in the surface layer (what depends on the mineral composition, free edges, pores, coatings, etc.). In order to compare results from different environments and to fix the conditions of applicability of the derived coefficients, it is necessary to handle the particle size spectra and the mineral composition of natural occurring suspended loads. The time dependent uptake curves, covering up to a large period, are fitted to the numerical solutions calculated with different models of the uptake kinetics.ENRES

Hydrochemistry and 222Rn Concentrations in Spring Waters in the Arid Zone El Granero, Chihuahua, Mexico

Water in arid and semi-arid environments is characterized by the presentation of complex interactions, where dissolved chemical species in high concentrations have negative effects on the water quality. Radon is present in areas with a high uranium and radium content, and it is the main contributor of the annual effective dose received by humans. The objective of this study was to evaluate concentrations of 222Rn and the water quality of spring waters. Water was classified as calcium sulfated and sodium sulfated. Most of the water samples with high radon concentrations presented higher concentrations of sulfates, fluorides, and total dissolved solids. 222Rn concentrations may be attributed to possible enhancement of 226Ra due to temperature and salinity of water, as well as evaporation rate. In 100% of the sampled spring waters the 222Rn levels exceeded the maximum acceptable limit which is proposed by international institutions. Aridity increases radiological risk related to 222Rn dose because spring waters are the main supply source for local populations. The implementation of environmental education, strategies, and technologies to remove the contaminants from the water are essential in order to reduce the health risk for local inhabitants.CIMAV (Centro de Investigación en Materiales Avanzados

Application of a liquid scintillation counter to the measurement of tracing radioactivity in experiments on suspended matter uptake in aquatic environments

The experiments on the study of the kinetic transfer coefficients of conservative and non-conservative radionuclides, in aquatic environments, need to determine radionuclide activity in large number of samples. Different radioactivity measurement techniques can be selected for analyses, but the liquid scintillation technique is preferred due to its easy sample treatment and its low time consumption. Samples are initially traced with a known activity of a selected radionuclide (2 3 9Pu, M IAm or "Tc). The transference of radionuclides, from the water to the suspended matter, is studied through the determination of the residual activity in aliquots of water sample, which are successively collected with time. In this work, we present the experimental procedures used for the 2 3 9Pu, 2 4 1Am and "Tc activity measurement by liquid scintillation counting, using a Wallac Quantulus 1 2 2 0 spectrometer. Results of the application of this technique to traced water samples are also shown in this study.ENRES

Self-cleaning in an estuarine area formerly affected by 226Ra anthropogenic enhancements: numerical simulations

A numerical model of the Odiel–Tinto estuary (Spain) has been developed to study the self-cleaning process that was evidenced from 226Ra measurements in water and sediments collected in the period 1999–2002, after direct releases from a fertilizer complex ceased (in 1998). The hydrodynamic model is first calibrated, and standard tidal analysis is carried out to calculate tidal constants required by the dispersion code to determine instantaneous water currents and elevations over the estuary. In this way, long-term simulations may be carried out. The dispersion code includes advective/diffusive transport of radionuclides plus exchanges with bottom sediments described through a kinetic approach. The dispersion model is first tested by comparing computed and measured 226Ra concentrations over the estuary resulting after releases in the Odiel and Tinto rivers. Next, it is applied to simulate the self-cleaning process of the estuary. The time evolution of radium concentrations in bed sediments is in generally good agreement with observations. The computed sediment halving time of the estuary is 510 days, which also is in good agreement with that estimated from measurement

Self-cleaning in an estuarine area formerly affected by 226Ra anthropogenic enhancements

The estuary of the Odiel River has been affected by both direct discharges of phosphogypsum (radium enriched industrial waste) and dissolution and weathering of the exposed piles where this radium enriched waste was stored. In 1998 the waste management policy for industries changed. The direct discharges stopped and the new phosphogypsum piles were well protected against dissolution processes, avoiding any transference of radium into the environment. This work presents a study of the evolution with time (1999–2002) of the levels of 226Ra in river water and sediment samples with the new waste management policy. A liquid scintillation technique was used to measure the 226Ra activity concentration in sediment samples. A gas-proportional counter was also used to measure the 226Ra activity concentration in river water samples. The main conclusion is that a systematic and continuous decrease of the activity concentration of 226Ra with time in the Odiel River estuary is occurring. Thus, a possible self-cleaning in the estuary, once the direct waste discharges were avoided, can be inferred

Hydrochemistry and Uranium Concentration in Brackish Groundwater from an Arid Zone, Chihuahua, Mexico

In arid zones, the principal water supply is from groundwater, which can present high concentration of salts, heavy metals, and radioactive elements. The aim of the study was to determine isotopic uranium concentration in groundwater samples with high concentration of salts and its association with other chemical species. Samples were taken from wells with high salt content. The 238,234U radioisotope concentrations were determined by liquid scintillation and alpha-particle spectrometry. In addition, the physical-chemical parameters were recorded in situ; whereas the dissolved ions and elemental composition were measured by UV-Vis and X-ray fluorescence spectrophotometry, respectively. To obtain isotopic uranium concentrations, three radiochemistry procedures were carried out. An ANOVA test was performed to compare the results from procedures, as well as an analysis of Pearson correlation was used between parameters to obtain their associations. Statistically, the U isotopic concentrations did not show differences (p-value 0.82) between procedures. 238U and 234U showed mean concentrations of 6.7 mBq mL-1 and 16.6 mBq mL-1, respectively, with an Activity Ratioby up 7.2. The groundwater under study showed high concentration of TDS, calcium, sulphate, chloride, nitrate, and nitrite. Isotopic U concentrations tend to increase with NO3>Zn>Cl>Br>SO4>Cu>T>SDT>P; meanwhile their contents decrease with T>Cl->NO2>Fe. These findings help us to understand the uranium behavior in groundwater with high salt contents as well as the influence of agricultural supplies on chemical species presents in groundwater

239Pu sorption by suspended matter from Hueznar reservoir (southern Spain): Experimental and modelling study

Kinetic transfer coefficients are important parameters to understand and reliably model the behaviour of non-conservative radionuclides in aquatic environments. This report pertains to a series of radiotracer experiments on 239Pu uptake in natural unfiltered waters from the Hueznar reservoir (southern Spain). The experimental procedure ensures the preservation of environmental conditions and the dissolved Pu activity was measured by liquid scintillation technique. The data suggest that the main pathways for Pu uptake consist of two parallel and reversible reactions. We studied the effects of suspended matter concentrations and the specific surface area on the kinetic transfer coefficients.ENRES

Evaluation of the radioactive impact of the phosphogypsum wastes used as amendment in agriculture soils

Some 3x109 kg of phosphogypsum (PG) wastes are annually generated by two fertiliser-production factories in Huelva (south-western Spain). PG has relatively high concentrations of 226Ra and other radionuclides, with an special concern due to the 222Rn emissions. These wastes could be used to improve the fertility of agriculture soils in a large former marsh area of the Guadalquivir river. Thus, it is interesting to study the levels and behaviour of natural radionuclides within this system to evaluate the radioactive impact of this amendment. An agronomical test is being conducted by one of the authors in an experimental farm in Lebrija (Seville). The soils are treated with 13 and 26 t ha-1 of PG, 30 t ha-1 of manure. Each treatment was repeated twice and continued for two years with beetroot and cotton plant production. We are measuring 226Ra (by alpha counting and gamma spectrometry) and U isotopes (by alpha spectrometry and ICP-MS analysis) in drainage waters, soils and vegetal-tissues samples. The PG used in the treatment has 620 ± 70 Bq kg-1 of 226Ra. The drainage waters have 226Ra contents similar to those from non-contaminated natural waters, but the uranium concentrations are one order of magnitude higher. Our results are suggesting that the major uranium input comes from the application of phosphatefertiliser. No significant levels of radionuclides were found in the vegetal tissues.ENRES

Phosphogypsum Amendment Effect on Radionuclide Content in Drainage Water and Marsh Soils from Southwestern Spain

Phosphogypsum (PG) is a residue of the phosphate fertilizer industry that has relatively high concentrations of 226Ra and other radionuclides. Thus, it is interesting to study the effect of PG applied as a Ca amendment on the levels and behavior of radionuclides in agricultural soils. A study involving treatments with 13 and 26 Mg ha−1 of PG and 30 Mg ha−1 of manure was performed, measuring 226Ra and U isotopes in drainage water, soil, and plant samples. The PG used in the treatment had 510 ± 40 Bq kg−1 of 226Ra. The 226Ra concentrations in drainage waters from PG-amended plots were similar (between 2.6 and 7.2 mBq L−1) to that reported for noncontaminated waters. Although no significant effect due to PG was observed, the U concentrations in drainage waters (200 mBq L−1 for 238U) were one order of magnitude higher than those described in noncontaminated waters. This high content in U can be ascribed to desorption processes mainly related to the natural adsorbed pool in soil (25 Bq kg−1 of 238U). This is supported by the 234U to 238U isotopic ratio of 1.16 in drainage waters versus secular equilibrium in PG and P fertilizers. The progressive enrichment in 226Ra concentration in soils due to PG treatment cannot be concluded from our present data. This PG treatment does not determine any significant difference in 226Ra concentration in drainage waters or in plant material [cotton (Gossipium hirsutum L.) leaves]. No significant levels of radionuclides except 40K were found in the vegetal tissues.ENRESA AGF97-1102-CO2-01Comisión Interministerial de Ciencia y Tecnología (CICYT) AMB97-1720-C