A review of current knowledge of the transfer of radiostrontium to milk and possible countermeasures. Final report.

In this review we describe current information on the transfer of radiostrontium to milk, and critically evaluate available countermeasures to reduce radiostrontium contamination of milk. Levels of radiostrontium in milk respond rapidly to those in the diet. The transfer of radiostrontium to milk is determined by calcium intake and status. Under normal ranges of dietary calcium intakes the transfer of radiostrontium to milk is likely to be inversely proportional to that of the dietary calcium intake. Therefore, the usefulness of conventional transfer coefficients for radiostrontium is limited, and predictions could be misleading. A relationship was noted between calcium intake and radiostrontium transfer to milk, which might allow improved estimation of radiostrontium transfer to milk. The simplest and most effective countermeasure to reduce radiostrontium activity concentrations in milk is to provide dairy animals with uncontaminated feed, this has the added advantage of being effective for other radionuclides. If there is a limited supply of such feed preference should be given to dairy animals over animals reared for meat. If the affected area is very large or deposition occurs in late spring it will be difficult to supply uncontaminated fodder to large numbers of dairy cows and the use of alternative countermeasures needs to be considered. The most practical, simple method of producing fodder or pasture grass with sufficiently low radiostrontium levels would be to apply lime to contaminated soils. The effectiveness of liming will depend on the prevailing calcium status of the soil; since most UK agricultural soils are not deficient in calcium the effect of liming may not be high. On organic soils liming may increase plant uptake of radiostrontium, and therefore the effect for major organic soil categories would need to be checked before application. The application of organic matter may also be effective, but the concomitant effect on radiocaesium needs consideration. Removing or diluting the top layer of contaminated soil, either by surface ploughing, or preferably skim and burial ploughing should also be considered, although the latter would not currently be possible since such ploughs are not available in the UK. The use of additives given to ruminants to reduce radiostrontium in milk is an alternative countermeasure which should be considered, particularly if difficulties are encountered with supplying uncontaminated feedstuffs for dairy animals. Such countermeasures also have the advantage that they are rapidly effective, in contrast to many of the soil-based countermeasures. Furthermore, countermeasures based on additives are generally easy to administer to dairy animals, which are routinely handled twice daily, and potentially more cost-effective than soil-based treatments. Increasing the calcium intake by a factor of two should decrease radiostrontium levels in milk by a concomitant amount. However, relatively small increases in stable calcium intake are unlikely to achieve a substantial reduction. Other suggested additives to reduce the transfer of radiostrontium to milk are either of limited effectiveness (e.g. clay minerals) or need further investigation (e.g. calcium alginate) before they could be considered as a practically feasible alternative to calcium. The effectiveness of available countermeasures varies. Radiostrontium activity concentrations decline rapidly in milk when feeding uncontaminated feed, and the rates of loss are determined by the biological half-life of radiostrontium in ruminant milk. Soil treatment can give maximum reduction of ten fold, but a factor of two or three is more common. Feeding dairy cows enhanced levels of Ca will give maximum reduction factors of two to three. Potentially higher reduction factors could be achieved if selective, and appropriate Sr binders were available. A summary of potentially effective countermeasure options which could be adopted is given below