soil. Large-scale leaching with sea water by the Lawrence Livermore group (Appendix B) has not yet proved effective (24). In most continental soils, cesium is very firmly fixed to clay minerals (25, 26). soils of the Marshall Islands, however, the fixation In the coralloid may be to organic matter, but the nature of the process is undefined (Appendix B). 4.2.2 Biological Extraction (Cropping). Since cesium may be concentrated in plants, the possibility exists of removing cesium from soil _ by cropping. The method does not seem practical. For example, assume that the plant specific-activity is three times that in soil, and that 1.5 kg/m* of plant material can be harvested annually. Then for Bikini's 2.4 km, some 3500 metric tons per year of plant material would have to be removed for 50 years to reduce rooting-zone cesium-137 activity from 29 pCi/g (the present level) down to 4.6 pCi/g (the liminal level). 4.2.3 Topping. A clean rooting zone may be created by topping contaminated soil with a fresh layer 50 cm or more thick, as might be needed. of roots If the topping layer is thick enough and fertile, large numbers of the edible plants contaminated layer below. will not penetrate from it into Nor would the tightly bound cesium-137 of the contaminated layer be expected to diffuse upwards into it. The plan would involvé removing and disposing of the vegetation currently in place $3 million), the - (cost, topping with 50 cm of dredged sediment from the lagoon, only practical the source (cost, $55 million), and conditioning and replanting the area thus treated (cost, $6-8 million), for a total cost of about $65 million. Two engineering, to after four years which, would with be required adequate to planning complete and the care, civil mature revegetation would develop over a period of 10 years. aad Topping, water. however, would not decontaminate the Furthermore, the roots of such plants as Messerschmidia, and mature breadfruit would penetrate result, the falling leaves of these Pisonia, into the contaminated depth. plants would contaminate the soil. 3000024 ground 31 As a surface