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M I Sheppard

Publications and source records attributed to M I Sheppard.

11 recordsLinked to original sources
PubMed2006

Revision and meta-analysis of selected biosphere parameter values for chlorine, iodine, neptunium, radium, radon and uranium.

There is a continual supply of new experimental data that are relevant to the assessment of the potential impacts of nuclear fuel waste disposal. In the biosphere, the traditional assessment models are data intensive, and values are needed for several thousand parameters. This is augmented further when measures of central tendency, statistical dispersion, correlations and truncations are required for each parameter to allow probabilistic risk assessment. Recent reviews proposed values for 10-15 key element-specific parameters relevant to (36)Cl, (129)I, (222)Rn, (226)Ra, (237)Np and (238)U, and some highlights from this data update are summarized here. Several parameters for Np are revised downward by more than 10-fold, as is the fish/water concentration ratio for U. Soil solid/liquid partition coefficients, Kd, are revised downward by 10-770-fold for Ra. Specific parameters are discussed in detail, including degassing of I from soil; sorption of Cl in soil; categorization of plant/soil concentration ratios for U, Ra and Np; Rn transfer from soil to indoor air; Rn degassing from surface water; and the Ca dependence of Ra transfers.

Animals↗
PubMed2005

Parameterization of a dynamic specific activity model of 14C transfer from surface water-to-humans.

Carbon-14 is a particularly interesting radionuclide from the perspective of dose estimation because it mixes readily with stable CO2, and hence enters the food-chain as fundamental biomolecules. A model was developed for the situation of 14C releases to surface waters, where there are distinct changes in the water 14C activity concentrations throughout the year. The model computes the specific activity in water, phytoplankton, fish, crops, meat, milk and air, following a typical irrigation-based food-chain scenario. This paper describes the derivation of the required 14C-specific parameter values. Many of the key parameters are not commonly measured, at least not in the context of dose assessment. Thus, inference from other sources of data was required, and this is the scientific contribution described in this paper. The best estimates and appropriate measures of statistical dispersion are provided. This required consideration of both the temporal and spatial averaging domains to ensure they were correct for parameters as defined in the model. The model coupled with these parameter values represents several new developments for modelling 14C transfers.

Air Pollution, Radioactive↗
PubMed2005

Conceptual approaches for the development of dynamic specific activity models of 14C transfer from surface water to humans.

Carbon-14 is a particularly interesting radionuclide from the perspective of dose estimation. Many nuclear facilities, including power reactors, release 14C into the environment, and much of this is as 14CO2. This mixes readily with stable CO2, and hence enters the food chain as fundamental biomolecules. This isotopic mixing is often used as the basis for dose assessment models. The present model was developed for the situation of 14C releases to surface waters, where there are distinct changes in the water 14C activity concentrations throughout the year. Complete isotopic mixing (equilibrium) cannot be assumed. The model computes the specific activity (activity of 14C per mass of total C) in water, phytoplankton, fish, crops, meat, milk and air, following a typical irrigation-based food-chain scenario. For most of the biotic compartments, the specific activity is a function of the specific activity in the previous time step, the specific activity of the substrate media, and the C turnover rate in the tissue. The turnover rate is taken to include biochemical turnover, growth dilution and mortality, recognizing that it is turnover of C in the population, not a tissue or an individual, that is relevant. Attention is paid to the incorporation of 14C into the surface water biota and the loss of any remaining 14CO2 from the surface water-air interface under its own activity concentration gradient. For certain pathways, variants in the conceptual model are presented, in order to fully discuss the possibilities. As an example, a new model of the soil-to-plant specific activity relationship is proposed, where the degassing of both 14C and stable C from the soil is considered. Selection of parameter values to represent the turnover rates as modeled is important, and is dealt with in a companion paper.

Air Pollution, Radioactive↗
PubMed2001

Effect of pH on the sorption of uranium in soils.

This work was undertaken to study the influence of soil type and chemical composition on uranium sorption ratios (SR in 1 kg-1) in order to reduce the uncertainty associated with this parameter in risk assessment models. Thirteen soil samples were collected from three different locations in France under different geological conditions. Clay content varied from 7.0 to 50.0%, pH ranged from 5.5 to 8.8 and organic matter content from 1.0 to 4.6%. Soils were incubated at room temperature in polyethylene packets for 28 days in the presence of 1 mg U kg-1 soil. Sorption ratio values varied from 0.9 to 3198 for all soils with no significant effect of soil texture or of organic matter. However, soil pH was highly linearly correlated with (log SR) as a probable consequence of the existence of different uranium complexes as a function of soil pH. The sorption behaviour differences between UO2(2+) and UO2(2+)-carbonate complexes are so great that any other effect of soil properties on U sorption is hidden. Thus, soil pH should be the focus variable for reduction of the uncertainty associated with the soil Kd value used in environmental risk assessments, even for reducing the uncertainty in site-specific Kd values.

Hydrogen-Ion Concentration↗
PubMed1991

Mobility and plant uptake of inorganic 14C and 14C-labelled PCB in soils of high and low retention.

Quantifying and understanding the mobility of 14C and organic pollutants in soils is important, especially in the context of underground waste disposal. We studied migration of 14C applied as NaHCO3 (14C-CO3) and as 2,2',5,5' tetrachlorobiphenyl (14C-PCB) in carbonated, high-organic-matter-content and acidic, low-organic-matter-content undisturbed soil cores. The mobility of 14C-PCB depends on the profile distribution and amount of soil organic matter, whereas the mobility of 14C-CO3 depends primarily on the soil carbonate content. The solid/liquid partition coefficients (Kd) for 14C-CO3 were 6.7 and 1.2 mL g-1 for the two soils, respectively. For the 14C-PCB, the corresponding Kd values were 49 and 22 mL g-1. Plant/soil concentration ratios (CR) for inorganic 14C have previously been derived using overly conservative assumptions. Using plants grown in outdoor lysimeters, CR values for 14C-CP3 of 0.7 and 1.3, on a dry-weight basis, were measured for the two soils. These values are about 25-fold lower than the currently used values. The corresponding CR values for 14C-PCB were 0.014 and 0.088. For both 14C sources, there was evidence of atmospheric transfer from the soil to the plants. This was especially important for 14C-CO3, where it may have been dominant. Detailed modelling of 14C transport from underground waste disposal should include volatilization as a loss process from soil as well as a source for plants.

Carbon Radioisotopes↗
PubMed1988

Modeling estimates of the effect of acid rain on background radiation dose.

Acid rain causes accelerated mobilization of many materials in soils. Natural and anthropogenic radionuclides, especially 226Ra and 137Cs, are among these materials. Okamoto is apparently the only researcher to date who has attempted to quantify the effect of acid rain on the "background" radiation dose to man. He estimated an increase in dose by a factor of 1.3 following a decrease in soil pH of 1 unit. We reviewed literature that described the effects of changes in pH on mobility and plant uptake of Ra and Cs. Generally, a decrease in soil pH by 1 unit will increase mobility and plant uptake by factors of 2 to 7. Thus, Okamoto's dose estimate may be too low. We applied several simulation models to confirm Okamoto's ideas, with most emphasis on an atmospherically driven soil model that predicts water and nuclide flow through a soil profile. We modeled a typical, acid-rain sensitive soil using meteorological data from Geraldton, Ontario. The results, within the range of effects on the soil expected from acidification, showed essentially direct proportionality between the mobility of the nuclides and dose. This supports some of the assumptions invoked by Okamoto. We conclude that a decrease in pH of 1 unit may increase the mobility of Ra and Cs by a factor of 2 or more. Our models predict that this will lead to similar increases in plant uptake and radiological dose to man. Although health effects following such a small increase in dose have not been statistically demonstrated, any increase in dose is probably undesirable.

Background Radiation↗
PubMed1983

Technetium and uranium: sorption by and plant uptake from peat and sand.

The objectives of this study were to compare the effects of technetium and uranium on the yield and uptake, and to identify the organ of accumulation, of an edible leafy vegetable growing in sandy and peaty soils. In sand, where the soil's sorption capacity is negligible, technetium uptake is four orders of magnitude higher than from peat, suggesting no plant mediation of uptake and thus a constant concentration factor (greater than 50) in an oxidizing environment where technetium is continuously supplied. The technetium is predominantly translocated to the shoots. When soil fixation occurs, as in peat, this becomes the controlling factor in the plant uptake of technetium. In the case of uranium, plant mediation is more significant. Uranium uptake by Swiss chard is up to 80 times higher from sand than from peat. The uranium is restricted to the root system and may only be precipitated on the outer root membrane and may not accumulate in the roots.

Plants, Edible↗