Starting with the mammals, we begin to get ecological effects such as limitation of fertility at accumulated doses as low as 50 roentgens, and we can go up to as high as over a million roentgens in our attempts to sterilize an area by killing off the bacteria. Now, bacteria in the soil, for instance, are needed for the healthy agricultural quality of the soil. So that we actually are concerned with this whole range of interest when we think of ecological problems. Now, some of these, of course, are going to be thrown out when we look at the various levels of attack and realize that we don't come to these. Nevertheless, these problems at least have to be looked at. The chart, indicating an overall range of 50 to 1 million roentgens, implies that minor effects (such as reduction in fertility) in mammals may be observed at the lower dosage whereas in excess of 1 million roentgens would be needed to kill bacteria. RADIOSENSITIVITY OF A SPECIES It is also important to know the extent of radiosensitivity of one species during different phases of its life cycle. (See fig. M-4.) (Fig. M-4 illustrates this and follows:) FIGURE M-4.-RADIOSENSITIVITY OF DROSOPHILA You will notice that the adult drosophila has a particular sensitivity as measured here of 85,000. But if you will look at the other aspects of the life cycle of this particular organism, you find that there is a sensitivity in the embryo as low as 170 to 200. So it becomes obvious that the radiosensitivity of the adult insect is not enough. Whenever we come to a particular life form which is of interest to us it is important that we have the radiosensitivity profile of the whole life cycle of the organism if we are to be really qualified to say what the radiosensitivity in this organism is. This is just an illustration of that. The chart shows the response of drosophila to X-rays. It is obvious that only knowledge of the resistence of the adult would be very misleading. The reproductive behavior of a species must also be considered in assessing effects. Bacteria, for example, will repopulate an area very quickly even though only a small number survive. It is also worth mentioning that small organisms might be killed by external beta radiation which would cause only local surface lesions in large animals. This should be watched for in the evaluation of fallout patterns from an ecological point of view. Figure M-5 shows the radiosensitivity of several species of seed plants. TABLE M-1.-Tolerance of various plants to chronic gamma radiation 1 Dose rate is in roentgens/24-hour day; however, the actual dosage/day averaged about 90 percent of the dose rate shown. ? This dose rate is not necessarily the lowest rate which will produce a severe effect. This is a highly selective list from the literature. The literature, itself, is very, very sparse in terms of the food crops that we are concerned with. There is a widespread range of sensitivity. Actually, this is a cumulative dose with exposure given daily for weeks and going from a dose rate per 24-hour day of 20 roentgens up to 4,100 roentgens to encompass the differences in sensitivity of particular plants to radiation. Now, the same thing exists with regard to seeds themselves. Here are dormant seeds and if you will notice the critical dose, which is the beginning of reducing the ability of seeds to grow, we have a range from 5,000 roentgens for rye, going to 100,000 roentgens for cabbage. (See fig. M-5.) FIGURE M-5.-RESPONSE OF DORMANT SEEDS TO VARYING DOSES OF COBALT 60 GAMMA RAYS AS MEASURED BY SEEDLING GROWTH IN A GREENHOUSE 1 Critical dose is the lowest dose-reducing growth below that of unirradiated controls. One more thing that I would like to put in, just to give you the range of doses which are important, is that when we get into the sensitivity of mammals to radiation we are now talking about the killing dose for 50 percent of the mammals within 30 days, and the numbers run 281 up to 805, starting with dogs and going through the rabbit. (See table M-2, p. 340.) 73266-61-23 Man's 50-percent lethal dose is anybody's guess, but we usually use about 450 roentgens or so. The point is that the mammals are actually very, very low on the sensitivity scale compared to plants. EFFECT OF SHELTERS When you think about the survival of people versus the survival of other aspects in the economy, particularly the agricultural economy, whether or not we have shelters makes a big difference. For instance, if you do not shelter the population it is certainly simple and obvious that plant life will survive far beyond any kind of human life or animal life but once you shelter then you will allow plant population to be exposed to tremendous amounts of radiation and then you have to take into account different levels of radiation. There is a wide spread in the range (up to 200 times). The response of seedlings of various plants also gives indication of considerable variation (20 times). This brief survey on comparative radiosensitivity should enable one to appreciate the difficulty in assessing total radiation effects. However, for any particular level of nuclear attack the fallout contour patterns should indicate whether the levels of serious consideration for ecological effects are being approached. As part of the necessary knowledge for handling post-attack problems, the catalog of comparative radiosensitivity will have to be considerably expanded. This knowledge, combined with data on the passage and concentration of isotopes through various food chains may eventually lead to reasonable predictability of radio-ecological effects. In complete detail, this is a long way off. Selectively, for items of great concern (e.g., feed crops, certain insects, etc.) concentrated research should be done in the immediate future. Very little of this has actually been done. I do not think the essential crops, like wheat, actually have more in the way of a complete radiation profile through all the different stages. There is a tremendous amount of basic research going on in this field, but not quite from the civil defense point of view, not quite from the possible attack recuperation point of view. Certain gaps are left and this is the point I am trying to bring out today. These programs have to be given a slightly different orienta RECONSTITUTION OF LAND AFFECTED BY FALLOUT Over most of the land (forest, chaparral, grassland), time is the only recourse for reducing radiation to a semblance of its original level. Natural decay of isotopes, leaching and fixation in the soil, and washing away will bring about an eventual return to low levels of radioactivity. There appears to be but one item for man's consideration concerning the radiation level. If reseeding or restocking is to be attempted, the introduced material should be relatively unaffected by the existing and projected radiation profile. For example for the kind of airplane reseeding that could be done, one would want to know that the residual radiation as it is projected will not essentially do away with the effort of the original reseeding. The possibility of actively reconstituting land for biologically productive use following radioactive fallout would seem, for economic reasons, to be restricted to croplands. The work of the U.S. Department of Agriculture on this problem is important. STRONTIUM 90 DECONTAMINATION 90 In areas where cropland is rendered unfit for growing food for human consumption, Sr90 will probably be the principal contaminant. I might add parenthetically, that the Special Subcommittee on Radiation of the Joint Committee on Atomic Energy under Chairman Holifield has held many hearings on this subject and has been responsible in large part for motivating the large scale effort now in progress toward solution of the Sr90 problem. Where Sr-90 reaches significant levels, the initial external dose rate may be 3003,000 roentgen-hour. Work here will have to be curtailed for a considerable period of time until the dose rate gets low enough to be safe for men to enter the area. This ties in with the need for agricultural surpluses to get us over the early stages before we can go in and do something about agricultural land. The possible methods for handling the Sro contamination include physical removal of topsoil, removal of Sr90 by cropping, and leaching or fixing the Sr90. It is also possible to grow crops that can be contaminated without causing future danger (e.g., cotton, tobacco, etc.). The nonphysical methods of Sr.90 removal are not very promising as yet. Chemical fixation, leaching, and cropping all have serious limitations and further research is called for. For example, leaching of Sr90 from soil requires several tons of gypsum or lime as well as large amounts of water and fertilizer per acre. Cropping would require 10 to 20 successive crops under most favorable conditions to obtain a significantly high percentage of removal. Some of the Department of Agriculture experiments on physical removal are shown in figure M-6, p. 342 and table M-3, p. 342. FIGURE M-6.-PERCENTAGE OF DECONTAMINATION BY REMOVAL OF CROPS AND MULCHES Treatment Raking mulch, 10 tons per acre.. Raking mulch, 5 tons per acre_. Raking mulch, 2 tons per acre--- Cutting and removing sod__. Flail chopping soybeans and some soil, after mowing__ Mowing soybeans_. Mowing Sudan grass-- Least significant difference (0.05). 1 Only 1 replicate was available for these treatments. The stated least significant difference therefore does not apply to these values. TABLE M-3.-Percentage of decontamination by scraping surface soil following various treatments They indicate that the methods are effective. However, these methods are apparently costly and one problem still remaining is what to do with the radioactive material removed. Burial in ditches near the original site is probably the best solution now available. I would like to show you some of the results. This was done with barium 140 as the radioactive test object and indicates a percentage of decontamination by removal of crops and mulches, and with a raking, 10 tons to the acre, they were able to accomplish 100 percent removal and various other things that they have done down to 29 percent removal. Apparently this is not too good a method in that it interferes with the general manner in which farmers actually work their farms. In other words, apparently it is not to the farmer's liking or it is uneconomical to spread these mulches around but, rather, they like to work them into the soil to increase the fertility of the soil. Now, this represents the kind of physical things that were done in that they cut off and scraped the surface of the soil, and they did various things like putting asphalt on it, and they checked the various kinds of things like plowland, discland, seedbeds, and in a general way, the chart just shows that you can get 75 to 85 or 90 percent removal of the strontium 90 or the radioactive material by physically going over this land and scraping it. So we do have methods for handling this problem, at least, in the essential cropland areas. |