Much of the NTS area is underlain by caliche hardpans, which appear at the depth to which precipitation may reach, the result of carrying small amounts of calcium salts in solution downward, salt precipitation by water evaporation into the soil atmosphere, and then loss to the open atmosphere above. Wallace and Romney have pointed out the possibility that Pu may also be taken into this cycle, which is, of course, an efficient route for removing an element from the mineral cycles of vegetation occurring there. The possibility that Pu may reach the subsurface aquifers on a time scale short of geological time appears very remote, or if it did, that that water would be moved to a locality where drinking or irrigation occurs is also remote. In the very long terms that approach short geological time periods, there appear to be other forces at work which may reduce Pu availability to bioenvironmental cycles. This is the general geological process known as orogenic isostacy. mountains rise, are eroded into the valleys, By this process, filling the valleys at approximately the same rates as the mountains rise. As a result, present valley surfaces will eventually be covered by weathered materials from the surrounding mountains and, with the covering, the Pu-contaminated surface will also be covered. The possibility is only of interest because of the very long half- life of Pu, which is itself approaching geological time. Finally, there is the consideration of cost. or may not be needed, Fencing, which may is certainly a very small expenditure relative to most of the methods considered at present. In addi- tion, large amounts of soils will need to be moved and places will have to be found to deposit it at further expense. There will be losses in transport, equipment will be contaminated, and there will still be a potential problem in preventing redistribution even after the soils are at a new location. to the fiscal costs, there may be other costs. In addition Wallace and Romney (1975) have pointed out some of the possibilities of longterm damage to ecosystems and those arguments will not be repeated here. 177