EARTHWORMS AND RADIONUCLIDES, WITH EXPERIMENTAL INVESTIGATIONS ON THEUPTAKE AND EXCHANGEABILITY OF RADIOCESIUM

The potential influence of earthworm activity on the mobility, of radi onuclides in soils and their subsequent availability for uptake by pla nts and transfer to higher trophic levels is briefly reviewed. The acc umulation of caesium by the earthworm Aporrectodea longa from soil and from plant litter was investigated in laboratory experiments, as was the effect of reworking (through burrowing and ingestion) soil and soi l with added organic material, on the extractability of caesium (ammon ium acetate extraction). Soil was spiked with Cs-134, organic matter w ith Cs-137. In soil-fed worms, most of the radioactivity measured was eliminated with the gut contents; 5-25% of the ingested radioactivity was retained or assimilated. Loss of caesium from sail-fed worms follo wed a two component curve, with an initial rapid loss clue to gut clea rance (half-life of loss (Tb1/2) of about 0.2-0.6 days) and a slower l oss of assimilated caesium (Tb1/2 of 15-26 days). Loss rates of assimi lated caesium from worms fed on fragmented apple leaves were found to have half-lives of 18-54 days. Assimilation of caesium from apple leav es was higher than from soil, ranging from 55-100% of the activity mea sured before gut clearance. Dry weight transfer factors (concentration in worm tissue/concentration in substrate) for worms cleared of their gut contents were similar for the two substrates 0.04 and 0.04 for tw o loss experiments with worms fed on radioactive soil, and 0.03 and 0. 05 for worms fed on apple leaves. After three months of reworking soil and soil/organic mixtures, A. longa was found to have no measurable e ffect on the extractable fraction of caesium. If earthworms have any s ubtle effects that were masked by changes in availability that occurre d when the spiked soil and organic substrates were mixed together. Onl y about half of the extractable fraction in soil was recovered when so il was mixed with organic material suggesting that some of the labile fraction in soil had become complexed with organic material. This exch ange occurred in substrate mixtures with and without worms. The limita tion of chemical extraction procedures is discussed and suggestions fo r further work are presented.