State of research: environmental pathways and food chain transfer.

Data on the chemistry of biologically active components of petroleum, synthetic fuel oils, certain metal elements and pesticides provide valuable generic information needed for predicting the long-term fate of buried waste constituents and their likelihood of entering food chains. Components of such complex mixtures partition between solid and solution phases, influencing their mobility, volatility and susceptibility to microbial transformation. Estimating health hazards from indirect exposures to organic chemicals involves an ecosystem's approach to understanding the unique behavior of complex mixtures. Metabolism by microbial organisms fundamentally alters these complex mixtures as they move through food chains. Pathway modeling of organic chemicals must consider the nature and magnitude of food chain transfers to predict biological risk where metabolites may become more toxic than the parent compound. To obtain predictions, major areas are identified where data acquisition is essential to extend our radiological modeling experience to the field of organic chemical contamination.

Critical considerations in the assessment of health and environmental risks - what we have learned from the nuclear experience.

The radiological experience has produced some sophisticated approaches to estimating the environmental risks of anthropogenic chemicals, based on well-validated models for both health effects and exposure assessment. As to exposure assessment, site- and situation-specific data must be acquired for quantitative predictions. However, the requirements for quantitative estimation are often overlooked, even with the radiological models. Metabolism or other chemical processes, also, may more profoundly alter a chemical contaminant than they would a radioelement. Suitably developed models provide useful guidance in deciding what kinds of data to get and how extensive particular data bases need to be.

Problems in evaluating radiation dose via terrestrial and aquatic pathways.

This review is concerned with exposure risk and the environmental pathways models used for predictive assessment of radiation dose. Exposure factors, the adequacy of available data, and the model subcomponents are critically reviewed from the standpoint of absolute error propagation. Although the models are inherently capable of better absolute accuracy, a calculated dose is usually overestimated by from two to six orders of magnitude, in practice. The principal reason for so large an error lies in using "generic" concentration ratios in situations where site specific data are needed. Major opinion of the model makers suggests a number midway between these extremes, with only a small likelihood of ever underestimating the radiation dose. Detailed evaluations are made of source considerations influencing dose (i.e., physical and chemical status of released material); dispersal mechanisms (atmospheric, hydrologic and biotic vector transport); mobilization and uptake mechanisms (i.e., chemical and other factors affecting the biological availability of radioelements); and critical pathways. Examples are shown of confounding in food-chain pathways, due to uncritical application of concentration ratios. Current thoughts of replacing the critical pathways approach to calculating dose with comprehensive model calculations are also shown to be ill-advised, given present limitations in the comprehensive data base. The pathways models may also require improved parametrization, as they are not at present structured adequately to lend themselves to validation. The extremely wide errors associated with predicting exposure stand in striking contrast to the error range associated with the extrapolation of animal effects data to the human being.

Relation between simultaneous Ca and Sr transport rates in isolated segments of vetch, barley, and pine roots.

Root segments of vetch, barley, and pine were exposed to a nutrient solution containing (85)Sr and (45)Ca tracers. Translocation was measured from solutions containing stable ions at concentrations of 2.5 mm Ca, and at either 0.5 mm or 2.5 mm Sr. Polar transport was established between 12 and 18 hr in barley, and between 16 and 22 hr in vetch. Acropetal transport remained below 5% of basipetal transport of tracer during these intervals. Transport in both vetch and barley usually declined before an elapsed time of 24 hr unlike corn, which maintained its steady state beyond 24 hr. Pine was radically different in that it showed no difference between acropetal and basipetal transport rates and had very low rates. Sr transport in all plants studied to date paralleled that of Ca and the ratio Sr:Ca transported was equal to the ratio Sr:Ca in the nutrient. In vetch, stable Ca transport was reduced to one-fifth when Sr concentration was increased from 0.5 mm to 2.5 mm. Yet stable Sr transport did not change, indicating that the effect on transport was not due to competitive inhibition. A similar effect was less pronounced in barley, but could not be detected in pine. The magnitude of the transport rates varied considerably among the various species, corn having the greatest followed by barley, vetch, and pine in decreasing order. Transport did not correlate with root weight or surface area; it amounted to from 0.03 to 0.60 nanomoles per hr in these experiments as compared to 7 nanomoles per hr previously established in corn (in all cases, 55 mm segments, sectioned 10 mm from apex).

Relation Between Calcium and Strontium Transport Rates as Determined Simultaneously in the Primary Root of Zea mays.

Root segments of Zea mays 55 mm long, were exposed to nutrient containing (85)Sr and (45)Ca tracers. Translocation rather than uptake was measured, using a newly-designed glass compartmentation system and validated tracer analytic model. Ca transport from solutions containing between 0.25 and 5.0 mm Ca was only slightly affected by concentration, but translocation from 0.25 to 0.05 mm solutions was markedly reduced. Maximum transport of strontium from nutrient containing 0.05 mm Ca was twice that from 2.5 mm Ca, and also twice the maximum calcium transported. Thus, under the condition simulating calcium depletion, i.e., 0.05 mm Ca, greater amounts of strontium were transported. In these cases the solutions also contained stable strontium at concentrations between 0.25 and 5.0 mm. In simultaneous determinations, the ratio of Sr to Ca moved was exactly equal to the ratio of their concentrations in nutrient solution, and there was no evidence of discrimination. Dinitrophenol reduced transport of Sr and Ca to an equivalent extent, amounting to between 2 and 9% of non-treated control levels.