CHEMICAL CYCLES AND HEALTH RISKS OF SOME CRUSTAL NUCLIDES.

摘要

Within the field of geochemistry there have been rapid advances in the ability to observe and describe natural systems interactions at all scales. Analysis of energy systems impacts on the environment requires quantification of short and long term local, regional and global modifications. This dissertation describes and utilizes an approach for assessing long term health risks due to dispersion of naturally occurring radionuclide series and chemical toxins by normal and altered landscape chemical cycles. In particular, the health risks resulting from geochemical mobilizations of arsenic, lead, uranium and radium are considered. Based on a review of toxic waste hazard measures and risk assessment studies, a general expression is developed for quantifying health risks imposed by the introduction of toxic materials to components of the total environment. This general measure deals with long term interactions within and between the internal human environment and the external biogeochemical environment.; Health hazards are expressed as dose factors which convert environmental concentrations into a corresponding dose field (organ doses in rad for radionuclides; daily intake for toxic elements). The dose field is translated into population health risk expressed as lifetime cancer risk for carcinogens and average blood levels for other toxins.; The external environment is modelled by considering the nature of the earth system and the manner in which elements are distributed and mobilized within the system. The landscape cell (or prism) is presented as a tool for visualizing and mapping toxic material cycles near the crustal surface. The overall process is incorporated in the GEOTOX code which is a geochemical systems model for describing the dynamics of crustal toxins within a landscape and the resulting health risks. GEOTOX is used to investigate the response of regional landscapes to increased soil and rock inventories of ('238)U, ('226)Ra, arsenic and lead. It is found that each decay series of element imposes a hazard by its behavior in the total environment that can not be quantified by a simle measure of toxicity. GEOTOX is used to follow continuous releases of the four toxins to steady state. The results provide the source fluxes in g/km('2)/y(and Ci/km('2)/y) to soil or groundwater that result in an incremental increase of population cancer risk of 10('-5) per individual. For lead the corresponding blood level increase is set at 5 g/100ml. GEOTOX is also used to model the risk field induced by the migration of the uranium series from a uranium ore body through a hypothetical landscape. A review of the genesis of anomalous uranium zones in sedimentary rock is presented and forms the basis for proposing the reference ore body.