Transport of iodine and cesium via the grass-cow-milk pathway after the Chernobyl accident.

More than 150 data sets giving time-dependent concentrations of 131I and 137Cs in feed and milk of cows after the Chernobyl accident are evaluated using a minimal compartmental modeling approach. Transfer of cesium via the grass-cow-milk pathway is adequately described by a three-compartmental model. No unique model results for 131I, as a compartment with slow secretion of 131I into milk, are identified for some datasets only. Frequency distributions of weathering half-lives on grass and of equilibrium feed-to-milk transfer coefficients are approximately lognormal. Mean values of weathering half-lives on plants are 9.1 +/- 0.6 d for iodine and 11.1 +/- 0.8 d for cesium, in good agreement with means established from experiments performed before 1986. Mean values of equilibrium feed-to-milk transfer coefficients are 3.4 +/- 0.4 10(-3) d L-1 for 131I and 5.4 +/- 0.5 10(-3) d L-1 for 137 Cs. Both are lower than means calculated from the pre-Chernobyl data base. Plausible explanations of the differences include (1) reduced availability of fallout compared to soluble tracer; (2) underestimation of post-Chernobyl transfer coefficients by some experiments concluded too early to record slow transport processes; and (3) reduced transfer of 131I compared to long-lived iodine isotopes due to decay during fixation in the thyroid. Feed-to-milk transfer of 131I is related to milk yield, but no influence of milk yield and type of feed on transfer is apparent for cesium.