be minuscule compared to natural backgroundlevels or even to norma! variations of background radiation. G. WATER AND AIR Background Information Contamination of water supplies does not constitute a major source of intake of radioactive fallout debris. In the case of surface water supplies there is a very large dilution factor. In the case of underground nuclear detonations the fission products are restricted largely to the immediate vicinity of the detonation due principally to two factors. Firstly, for underground shots to date approximately 90 percent of the fission products have been fixed in a glassy type of material formed by the detonation. Secondly, ion exchange between such key fission products as strontium 90 and cesium 137, and the soil resulted in almost all of the remaining activity being adsorbed within a matter of perhaps tens to hundreds of feet away from the source.” In addition to fission products, tritium may be formed in varying amounts. This radioisotope probably is not greatly influenced by the two factors mentioned and must depend upon the dilution factor for reduction of the concentration in the water—at least for under- ground detonations. For above ground or cratering shots, the tritium largely escapes into the atmosphere where very large dilutions occur. Theoretical caleulations suggest it may be possible for relatively high concentrations of tritium to be present in the amountof water immediately surrounding ground zero of some underground nuclear detonations.?! Essential to predicting potential contamination of ground water is the determination of the water movement. The most satisfactory method of obtaining the necessary data for this prediction is by drilling operations. Although these are ex- pensive operations they are carried on extensively at the , Air As long as the fallout material from atmospheric tests remains in the air some maybeinhaled andirradiate the lungs. This radiation dose to the lungs normally is less than external whole body exposure occurring after the fallout has been deposited on the ground. Also in general, inhalation is only a minor contributor to the intake of fallout debris into the body — ingestion is the much more important route. The whole body will also receive some exposure from the penetrating gamma rays while the fallout material is in the 18 that follows after the debris is deposited on the ground. However, this ratio of doses may not hold for events where most of the radioactivity that escapes beyond the test site is in the form of gases or finely suspended particles that are confined to a relatively shallow layer of air near the surface. Woter testing sites. air, but this dose will usually be small compared to the exposure Measurements of total fallout activity in air (called gross beta counts) provide only a crude alerting system. It is not a reliable procedure for predicting the amount of fallout to be deposited nor the amountof iodine 131 in milk.”? Because _ of the transitory nature of the fallout debris remaining in the air (and sometimes because of the particular choice of units used in expressing its concentration) what may sound like an alarmingly large amount may, in fact, result in only minor radiation doses. The Data Water The highest measuredfallout activity in water was at Upper Pahranagat Lake, Nev., in 1955 amounting to 0.14 millionth of a curie per liter.“ Since this was a total gross beta countit is difficult to give a precise estimate of the potential radiation dose. A crude analysis suggests that if this water had been stored and used as a sole supply for 70 years the total dose might be about one roentgen to the bones and one-quarter roentgen each to the thyroid and lower large intestine. No radioactive fission products nor inducedactivities including tritium from undergroundtests have been foundin underground water supplies at places of human consumption. Air The highest concentration of radioactive debris in the air in a populated areaoff-site (except for the Marshallese experience where measurements were made only after the passage of the cloud) was about 1.3 millionth of a curie per cubic meter averaged over the 24 hours the activity was present. This happened at St. George, Utah, on May 19, 1953. The estimated radiation dose to the lungs from inhaled fallout debris was less than 0.2 roentgen. The external whole body exposure from the fallout while it was still in the air was roughly estimated to be 0.025 roentgen—only about ‘4o0 of the whole body exposure that occurred after deposition of the fallout. Evaluation The concentrationsoffission products or tritium in the water supplies have not constituted major sources of radiation ex- posure to man. 745-413 O-64—4 Thereis a large dilution factor when surface 19