Distribution,cycling and mean residence time of 226Ra, 210Pb and 210Po in the Tagus estuary

Results for dissolved and particulate ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po in the Tagus river, estuary and coastal sea system show different distribution and chemical behaviour patterns for these radionuclides in the three aquatic environments. ²²⁶Ra from riverborne particles dissolves in the estuary and contributes to increased concentrations of dissolved ²²⁶Ra in estuarine water. In the estuary, dissolved ²¹⁰Pb and ²¹⁰Po from river discharge and atmospheric deposition are scavenged by suspended matter, which in turn becomes enriched in these nuclides in comparison with riverborne particles. As a result of these processes, the estuarine water flowing into the coastal sea contains enhanced concentrations of dissolved ²²⁶Ra, but is depleted in dissolved ²¹⁰Pb and ²¹⁰Po. Under average river flow conditions, mass balance calculations for dissolved ²¹⁰Po and ²¹⁰Pb in the estuary allowed their mean residence times to be estimated as 18 and 30 days, respectively. Due to the rapid sorption of these radionuclides on to settling particles, bottom sediments in the estuary represent a sink for ²¹⁰Pb and ²¹⁰Po from both natural sources and industrial waste releases. Results also suggest that partial re-dissolution of these radionuclides from bottom sediments and intertidal mudflats is likely to occur in the mid- and low-estuary zones. Nevertheless, box-model computations indicate that the discharge of ²¹⁰Pb and ²¹⁰Po into the coastal sea takes place mainly with the transport of sediment, whereas the discharge in the dissolved fraction can only account for one third of the activities entering the estuary in the soluble phase. Implications of these results to the cycling of radionuclides in phosphate waste releases into estuarine environments are discussed.     

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.     

Contamination and restoration of an estuary affected by phosphogypsum releases

The Huelva Estuary in Huelva, Spain, has been one of the most studied environmental compartments in the past years from the point of view of naturally occurring radioactive material (NORM) releases. It has been historically affected by waste releases, enriched in radionuclides from the U-decay series, from factories located in the area devoted to the production of phosphoric acid and phosphate fertilizers.Nevertheless, changes in national regulations forced a new waste management practice in 1998, prohibiting releases of phosphogypsum into the rivers. The input of natural radionuclides from phosphate factories to rivers was drastically reduced. Because of this there was a unique opportunity for the study of the response of a contaminated environmental compartment, specifically an estuary affected by tidal influences, after the cessation of the contaminant releases to, in this case, the Huelva Estuary (henceforth referred to as the Estuary).To investigate the environmental response to this new discharge regime, the specific activities of radionuclides ²²⁶Ra and ²¹⁰Pb in water and sediment samples collected in four campaigns (from 1999 to 2005) were determined and compared with pre-1998 values.From this study it is possible to infer the most effective mechanisms of decontamination for the Estuary. Decontamination rates of ²¹⁰Pb and ²²⁶Ra in the sediments and water have been calculated using exponential fittings and corresponding half-lives have been deduced from them. The cleaning half-life in the whole area of the Estuary is about 6 and 3.5 years for ²²⁶Ra and ²¹⁰Pb respectively.The observed trend clearly shows that contamination of the Estuary by natural radionuclides is now decreasing and radioactive levels in waters and sediments are approaching the natural background references. This work attempts to evaluate whether it can be expected that the decontamination of the enhanced levels of natural radioactivity in the Estuary can be performed via natural processes.     

Radioactivity of anthropogenic origin in the Tejo Estuary and need for improved waste management and environmental monitoring

The Tejo Estuary is a large water body surrounded by seven municipalities and industries with liquid effluent discharges containing contaminants that reach the estuary. This is the case for man-made radionuclides used in nuclear medicine, present in liquid effluents discharged by medical facilities. Radionuclide measurements in seaweeds, mussels, fish, water and sediments sampled along the North bank of the estuary revealed the presence of ¹³¹I and ^99mTc, originating from nuclear medicine facilities in Lisboa. Concentrations reached 90 Bq kg^?1 (wet weight) of ^99mTc in fish, and 18 Bq kg^?1 (wet weight) of ¹³¹I in mussels, and were even higher in the water. Another anthropogenic source of radionuclides is industrial waste, such as phosphogypsum the by-product of phosphoric acid production stockpiled at the South bank of the estuary. Main radionuclides present in phosphogypsum are ²²⁶Ra, ²¹⁰Pb, ²¹⁰Po and uranium. Concentrations of ²²⁶Ra in phosphogypsum are about 1000 Bq kg^?1, which is a high value in comparison with ²²⁶Ra in soils of the Tejo valley, <100 Bq kg^?1. ²²⁶Ra, in particular, is dissolved by rainwater from phosphogypsum stacks and seeps into the estuary. Other potential sources of radioactivity are discharges from naval nuclear powered vessels and merchant ships transporting radioactive materials that berth in the Lisboa harbour. A whole survey of the estuary indicated low concentrations of ¹³⁷Cs in sediments, mostly attributed to radioactive fallout. Since waste discharges are undergoing deep modifications due to enhanced urban waste treatment, but economic activities have changed and the use of radiopharmaceuticals increases, the periodic radioactivity monitoring of the Tejo Estuary is advised and should provide feedback to enhanced waste management.     

Experimental and modelling study on the uptake and desorption kinetics of 133Ba by suspended estuarine sediments from southern Spain

Dispersion of pollutants in aquatic environments depends on their uptake by suspended solids. This work deals with the uptake kinetics of 133Ba (gamma-emitter and a good analogue of 226Ra) by suspended estuarine sediments (which can be resuspended into the water column under certain conditions). This study presents a wide set of tracing experiments, including second tracing, decantation and desorption processes. The purpose is to characterize 133Ba uptake by sediments and to investigate the use and limitations of box models in order to describe the uptake kinetics. Water and sediment samples were collected in the Huelva estuary (Spain), where environmental 226Ra concentrations have been increased by two phosphate fertilizer industries. Samples were characterized by granulometric, organic carbon content, cation exchange capacity and XRF-EP analyses. Results revealed three-step kinetics, with characteristic times of minutes, hours and days. These results enabled the selection and calibration of a suitable box model and facilitated the testing of its use as a fully predictive tool.     

Trends of radionuclide sorption by estuarine sediments. Experimental studies using 133Ba as a tracer

Sediments play an important (but still poorly understood) role in the dispersion and final fate of radionuclides and other hazardous materials in aquatic systems. Adopting an experimental point of view, the present work deals with the transfer of a radioactive tracer from water to sediments. Sediments and waters were sampled in the Odiel and Tinto estuaries (South-West Spain) with anthropogenic-enhanced 226Ra concentrations. 133Ba was used as a tracer since it is a gamma emitter and a good analogue of the environmental behaviour of 226Ra. Laboratory experiments have been carried out to quantify the uptake kinetics of 133Ba by sediments in aqueous suspensions and by sediment cores under a water column at rest. Depth distributions of 133Ba in sediments have been studied with different contact times and using sediment samples with different grain sizes. The results reveal an important and fast uptake by suspended sediments (up to 40% in 10 min with a 20 g l(-1) suspension) and sediment cores (up to 70% in a few minutes). The kinetics of the uptake by suspended sediments could be reasonably described by a model of two parallel and reversible reactions followed by a weakly-reversible reaction. The total uptake and the rate of reaction decreased with the increment of grain size. Furthermore, uptake following diffusion through the interstitial water was hardly detectable beyond 1 cm depth. For the case with more experimental results, the depth distribution could be reasonably described by the analytical solution based on the use of an effective diffusion coefficient (4.2 x 10(-12) m2 s(-1)) and the measured intrinsic distribution coefficient (kd = 63 +/- 6 l kg(-1)).     

Performance assessment model development and parameter acquisition for analysis of the transport of natural radionuclides in a Mediterranean watershed

This paper describes the methodology developed to construct a model for predicting the behaviour of the natural radioisotopes of U, Th and Ra in a Mediterranean watershed. The methodology includes the development of the performance assessment model, obtaining water flow and radiological parameters based on experimental data and analysis of results. The model, which accounts for both water flows and mass balances of the radionuclides in a semi-natural environment, provides assessments of radionuclide behaviour in grassland and agricultural soils, rivers and reservoirs, including the processes of radionuclide migration through land and water and interactions between both. From field and laboratory data, it has been possible to obtain parameters for the driving processes considered in the model, water fluxes, source term definition, soil to plant transfer factors and distribution coefficient values. Ranges of parameter values obtained have shown good agreement with published literature data. This general methodological approach was developed to be extended to other radionuclides for the modelling of a biosphere watershed in the context of performance assessment of a High Level Waste (HLW) repository under Mediterranean climate conditions, as well as for forecasting radionuclide transport under similar Mediterranean conditions that will occur in the future in other areas. The application of sensitivity and uncertainty analysis was intended to identify key uncertainties with the aim of setting priorities for future research. The model results for the activity concentration in the reservoir indicate that for (238)U and (230)Th the most relevant parameter is the initial concentrations of the radionuclides in the reservoir sediments. However, for (226)Ra the most important parameter is the precipitation rate over the whole watershed.     

Modeling the dry-weather tidal cycling of fecal indicator bacteria in surface waters of an intertidal wetland

Recreational water quality at beaches in California and elsewhere is often poor near the outlets of rivers, estuaries, and lagoons. This condition has prompted interest in the role of wetlands in modulating surface water concentrations of fecal indicator bacteria (FIB), the basis of water quality standards internationally. A model was developed and applied to predict the dry-weather tidal cycling of FIB in Talbert Marsh, an estuarine, intertidal wetland in Huntington Beach, California, in response to loads from urban runoff, bird feces, and resuspended sediments. The model predicts the advection, dispersion and die-off of total coliform, Escherichia coli, and enterococci using a depth-integrated formulation. We find that urban runoff and resuspension of contaminated wetland sediments are responsible for surface water concentrations of FIB in the wetland. Model predictions show that urban runoff controls surface water concentrations at inland sites and sediment resuspension controls surface water concentrations near the mouth. Direct wash-off of bird feces into the surface water is not a significant contributor, although bird feces can contribute to the sediment bacteria load. The key parameters needed to accurately predict FIB concentrations, using a validated hydrodynamic model, are: the load due to urban runoff, sediment erodibility parameters, and sediment concentrations and surface water die-off rates of enteric bacteria. In the present study, literature values for sediment erodibility and water column die-off rates are used and average concentrations of FIB are predicted within 1/2 log unit of measurements. Total coliform are predicted more accurately than E. coli or enterococci, both in terms of magnitude and tidal variability. Since wetland-dependent animals are natural sources of FIB, and FIB survive for long periods of time and may multiply in wetland sediments, these results highlight limitations of FIB as indicators of human fecal pollution in and near wetlands.     

Concentrations of radionuclides in Lake Ontario water from measurements on water treatment plant sludges

The purpose of this investigation was to compare the results of indirect measurements of radionuclide concentrations in water with those measured directly. The levels of ¹³⁷Cs and ²²⁶Ra in western Lake Ontario nearshore waters were derived from high-resolution γ-spectrometric measurements on aluminum hydroxide sludge samples obtained from four water treatment plants. The concentrations of ¹³⁷Cs evaluated by this indirect technique averaged 0.036 compared with 0.023 pCi l⁻¹ measured directly, while those of ²²⁶Ra averaged 0.047 compared with the direct measurement of 0.03 pCi l⁻¹. The concentrations of a number of other radionuclides at these locations were also calculated from the sludge γ-ray measurements.     

Daily intakes of 238U, 234U, 232Th, 230Th, 228Th and 226Ra in the adult population of central Poland.

Activity concentration of the uranium and thorium series radionuclides was determined in foodstuffs and drinking water in central Poland. Annual and daily intake for the adult population was estimated from the concentrations determined and average annual consumption of food and water. The daily intakes (in mBq) were 22.1 (238U), 26.5 (234U), 2.38 (232Th), 4.06 (230Th), 11.2 (228Th) and 42.2 (226Ra). The intake of uranium isotopes occurred mainly with water; the main intake of thorium isotopes was with animal products, vegetables, cereals and potatoes, whereas 226Ra entered mainly with animal products, cereals and vegetables. From the intake and dose coefficients, the annual effective doses for the ingested radionuclides were calculated. The annual effective dose was 5.95 microSv, of which 72.4% originated from 226Ra.     

An examination of groundwater discharge and the associated nutrient fluxes into the estuaries of eastern Hainan Island, China using 226Ra

The nutrient concentrations and stoichiometry in a coastal bay/estuary are strongly influenced by the direct riverine discharge and the submarine groundwater discharge (SGD). To estimate the fluxes of submarine groundwater discharge into the Bamen Bay (BB) and the Wanquan River Estuary (WQ) of eastern Hainan Island, China, the naturally occurring radium isotope (²²⁶Ra) was measured in water samples collected in the bay/estuary in August 2007 and 2008. Based on the distribution of ²²⁶Ra in the surface water, a 3-end-member mixing model was used to estimate the relative contributions of the sources to these systems. Flushing times of 3.9 ± 2.7 and 12.9 ± 9.3 days were estimated for the BB and WQ, respectively, to calculate the radium fluxes for each system. Based on the radium fluxes from groundwater discharge and the Ra isotopic compositions in the groundwater samples, the estimated SGD fluxes were 3.4 ± 5.0 m³ s⁻¹ in the BB and 0.08 ± 0.08 m³ s⁻¹ in the WQ, or 16% and 0.06%, respectively, of the local river discharge. Using this information, the nutrient fluxes from the submarine groundwater discharge seeping into the BB and WQ regions were estimated. In comparison with the nutrient fluxes from the local rivers, the SGD-derived nutrient fluxes played a vital role in controlling the nutrient budgets and stoichiometry in the study area, especially in the BB.     

The role of suspended solids in the estuarine geochemistry of mercury

The behaviour of mercury in total water (soluble + particulate fractions) and suspended solids of the surface waters of the Wyre estuary, northwest England, has been studied during a tidal cycle. The estuary is contaminated by mercury derived from chlor-alkali effluents and other sources. The results from eight stations in the estuary show that quantitatively mercury in suspended solids dominates the mercury budget of the estuarine waters. At high chlorinity periods in the tidal cycle the mercury concentrations in suspended solids decreased significantly, and can be related to changes in the specific surface area and organic carbon contents of the suspended matter. A very good correlation between mercury and organic carbon concentrations in the suspended solids exists, and laboratory experiments support the conclusion that mercury is mainly associated with the organic phase of the suspended material and is not easily chemically desorbed during estuarine mixing processes.     

Transport and sources of metal contaminants over the course of tidal cycle in the turbidity maximum zone of the Hudson River estuary

Transport and source of metal contaminants (Ag, Cd, Cu, Pb and Zn) in the turbidity maximum zone of the Hudson River estuary were studied over the course of a tidal cycle in November 1994 and August 1995. This study showed that the metal/Fe ratios in suspended particles varied more widely than those in the local sediments, implying that sources of metal contaminants to the water column are not only from local sediment resuspension but also from lateral advection. Although the metal/Fe ratios oscillate with time at the anchor stations, a general increasing trend with salinity was found over the course of a tidal cycle, suggesting that the lower estuary could be a source of metal contaminants to the upper estuary regions. These results support that sediment resuspension and lateral advection are important factors responsible for transporting the lower estuary contaminants up river and redistributing metal contaminants in the estuary.     

Impact sédimentaire des travaux d'aménagement de l'estuaire de la Loire

The planning works realized in the Loire estuary (France) during the course of the twentieth century have brought about an important inflow of sea water to this site. The consequences can be observed through important changes in the location of the extreme limits of both the dynamic tide and the salinity front at 0.5‰; the dynamic tide has progressed from 63?km from the river mouth in 1903 to 100?km in 1992 and the salinity front from 38?km in 1948 to 70?km in 1991. Modifications of the hydraulic conditions have also affected the dynamics of the cohesive sediments present in the estuary. The purpose of this study is to determine the impact of the past and future planning works in the Loire estuary on the erosion and deposition of cohesive sediments in relation to hydraulic conditions. The principal sedimentary processes (transportation, deposition, settlement, erosion) are simulated by simplified models from measurements carried out both in situ and in the laboratory. The dynamics of the cohesive sediment in the Loire estuary are linked with the fortnightly cycle of the tide as follows: when the tidal coefficient falls below 70, almost all the suspended matter is quickly deposited and collects near the estuary bed to form a lens-shaped body of fluid mud which consolidates during 6?days of neap tides. Finally, as soon as the tide coefficient rises above 75, this body of fluid mud is taken again into suspension and only the fraction that was highly concentrated in the course of the settlement remains on the bottom. The sediments remaining on the estuary bed in the course of a fortnightly cycle of the tide represent the residual sedimentation rate over this period. Knowledge of the maximum tide current velocities in the estuary permits the determination of the maximum hydrodynamic stress on the bottom and the critical concentration of the mud susceptible to being eroded by this current, taking into account the yield stress of the mud. In other respects, the quantity of mud that is greater than this critical concentration per surface unit and time unit can be calculated according to a theoretical model developed by Sanchez and Grovel. This methodology allows the evaluation of the residual sedimentation rate representing a layer of 0.08?m per year in front of the harbour of Montoir for the hydraulic conditions of 1976, with a maximum current velocity on the surface of 2.40?m?s^–1. Following the dredging works realized between St-Nazaire and Donges after 1976, the level of the bottom has changed from –10.00 to –13.25?m (with respect to the hydrographic datum). These works have brought about a reduction in maximum velocities of 15% and an increase in the residual sedimentation rate of about 100%. This increase is in agreement with the theoretical estimation of this study. This study also describes the high sedimentation rate observed in a locally dredged berth and located in the zone of the estuary where a lens-shaped body of fluid mud is formed during slack tides. The sedimentation rate depends then on the frequency of the dredging works because the dredged zone can be filled up in a few days. Finally, a methodology is developed in order to estimate the erosion rate induced by an increase in current velocities. This methodology takes into account the vertical concentration gradient in the bed resulting from consolidation, the hydrodynamic stress linked to current velocity, the yield stress of sediment which varies with concentration and the critical hydrodynamic stress for erosion estimated from experimental measurements in laboratory. All the methodologies presented in this paper have been applied only to the estuary of the river Loire. Application to any other similar site is possible after laboratory experiments on local sediments in order to adjust the behavioural laws used to evaluate the permeability coefficient, the effective stress and the yield stress. This study shows that knowledge of hydraulic conditions and local sedimentary processes is very important for predicting the impact of planning works in an estuary.     

Modelling progressive hardening of shotcrete in convergence-confinement approach to tunnel design

The progressive hardening of shotcrete used for tunnel supports is of great importance because it influences the support response to ground movements and therefore the stresses induced in the shotcrete lining. This paper presents anew model, which can take into consideration the hardening of concrete and provides the convergence-confinement curve of the supported tunnel. The model is used to back-analyse the measurements taken in the Kielder Experimental tunnel and presented in technical literature. An axisymmetrial numerical model (FLAC code) has been also set up for comparison with the proposed model results. Both the in-situ measurements and the numerical results using FLAC are in good agreement with those computed using the proposed convergence-confinement method.     

Natural radionuclides in mineral waters

Concentration levels of natural radionuclides in mineral waters have been studied in several European countries. In the Federal Republic of Germany in recent years a nationwide investigation was carried out by several institutions.The concentrations of ²²⁶Ra, uranium isotopes, ²¹⁰Pb and ²¹⁰Po found are similar to those in other European countries. While for adults the health risk from drinking mineral water is comparable to the risk from terrestrial radiation, special consideration has to be given to the risk of infants when mineral water is used for preparing their food.     

The recent sedimentation history of the esk estuary,Cumbria, U.K.: The application of radiochronology

The sediments of the Esk estuary, Cumbria, contain a variety of radionuclides which are derived from the British Nuclear Fuels Ltd (BNFL) reprocessing plant at Sellafield, Cumbria. In principle the total ecosystem of the Esk estuary is labelled with a variety of radionuclides which we have studied in order to investigate fundamental process which take place in estuaries; also to improve our knowledge of relationships which exist between the rate of release of radionuclides in the BNFL source term and their distribution in the Esk. On the basis of defined sampling procedures we have dated the rate of sedimentation for the major sediment types of the system using six different radionuclides, and four independent methods, including a procedure of statistical modelling. The model is based upon procedures for matching the shape of the BNFL radionuclide release profile over a period of ～ 10 y, with that for the distribution of radionuclides in profiles of sediment cores. Good agreement is obtained for the match.In the Esk a large proportion of the radionuclides which are retained in sediments are associated with particulate matter and surfaces of sediment debris; in accreting sediments these deposits have not been disturbed during the past ～ 30 y.In modelling the releases of radionuclides from point sources, attention needs to be paid to the chemical and physical composition of the source term, as this can influence the subsequent distribution of radionuclides in nearshore and estuarine marine ecosystems.     

Heavy metal concentrations in water and bottom sediments of a Mexican reservoir.

Metal concentrations have been measured in water and sediments of the Jose Antonio Alzate Reservoir, Mexico, using EDXRF and ICP techniques. Spatial and temporal distributions of total metal levels have been identified; no organised pattern was detected for any particular metal concentration. Temporal variations of metal concentrations show evidence of the water self-cleaning capacity of the reservoir, despite the high-level metal contamination determined.     

Linearity assumption in soil-to-plant transfer factors of natural uranium and radium in Helianthus annuus L

The linearity assumption of the validation of soil-to-plant transfer factors of natural uranium and (226)Ra was tested using Helianthus annuus L. (sunflower) grown in a hydroponic medium. Transfer of natural uranium and (226)Ra was tested in both the aerial fraction of plants and in the overall seedlings (roots and shoots). The results show that the linearity assumption can be considered valid in the hydroponic growth of sunflowers for the radionuclides studied. The ability of sunflowers to translocate uranium and (226)Ra was also investigated, as well as the feasibility of using sunflower plants to remove uranium and radium from contaminated water, and by extension, their potential for phytoextraction. In this sense, the removal percentages obtained for natural uranium and (226)Ra were 24% and 42%, respectively. Practically all the uranium is accumulated in the roots. However, 86% of the (226)Ra activity concentration in roots was translocated to the aerial part.     

In situ gamma spectroscopy to characterize building materials as radon and thoron sources

In situ gamma spectroscopy is widely utilized to determine the outdoor gamma dose rate from the soil and to calculate the natural and artificial radionuclide concentration and their contribution to the dose rate. The application of in situ gamma spectroscopy in indoor environments can not supply quantitative information about activity concentration of radionuclides in building materials, but this technique can provide interesting information about building materials as radon source. In fact, a method based on analyses of gamma spectra data has been developed by the authors to provide, in field, quantitative estimation of disequilibrium in 226Ra and 228Ac sub-chains due to 222Rn and 220Rn exhalation. The method has been applied to data of gamma spectroscopy measurements carried out with HPGe detector (26%) in seven dwellings and one office in Rome. The first results of the data analysis show that, as regards especially the 226Ra sub-chain disequilibrium, different building materials (tuff, concrete, etc.) can show very different characteristics. If, in addition to the spectrometric data, other indoor environment parameters (indoor gamma dose rates, room dimensions, wall thickness, etc.) (Bochicchio et al., Radiat Prot Dosim 1994;56(1-4):137-140; Bochicchio et al., Environ Int 1996a;22:S633-S639) are utilized in a room model, an evaluation of 226Ra, 228Ac and 40K activity concentration and an indication of the exhalation features, by means of estimation of exhaled 222Rn activity concentration, can be achieved.