SOIL DISTURBANCE EXPERIMENT DATED: 8 December 1977 DOE/ERSP TECH NOTE NO. 4.0 AUTHOR: F. Tomnovec, EG&G During the Enewetak cleanup program various people have been concerned with the measurements taken with the EG&G IMP. Their concern was with the effect of the road (which is bulldozed and cleared of heavy brush) on the IMP's measurements. The road is necessary for the surveyors to stake out and establish a grid system. The IMP travels this road, pausing at each stake to make a measurement. The resultant radiation grid is used by DRI to establish certain radiation patterns, which will be used in determining the land areas that need soil removal to lower the level of radioactivity to a reeommended level. During IMP measurements at Pearl it was evident that high radiation fields of 69Co could be from neutron induced activation in steel, which was used extensively for building, and also in the tower housing the nuclear event. Any steel debris that could be neutron activated could have been originally close enough to be contaminated by the fireball, and then ejected outward by the blast or later human efforts. It was decided to send in the lst RADCON Team and the 84th Engineers to remove all visible metal debris. In some eases large steel I beams were bulldozed out of the ground. When the operation was complete the radiation levels had been reduced. The 60Co had been removed by the removal of the steel, but the decrease in the 2414m was questionable. Table B-4-1 shows the results of the debris removal at three stake positions. In an effort to explain that the decrease was solely from the removal of the metal debris, Table B-4-2 was constructed. This table compares the measurement station with several stations that are adjacent. Station 1-N-1 looks quite similar to its adjacent neighbors, Pictures taken at the site show extensive brush removal, but only track marks seem to be the major evidence of soil disturbance. One can postulate that the removal of the metal debris was also the principal reason for the removal of the 241Am, Station 2-S-1 indicates only an 11% reduction of the 241A4m , yet the soil appears to be disturbed as much as 1-N-1. The removal of the metal debris sharply reduced the contribution from the 60Co, Station 5-S-3 was the least disturbed of the three stations, yet somehow the 2414m was dramatically reduced. Some debris was also removed as evidenced by the reduction in 60Co, The lack of a simple way to remove the metal debris by the use of a dozer, without removing the thick heavy brush which conceals the debris, brings up the inevitable question: Did the disturbance of the soil by the dozer reduce the 2414m? To help answer this question an experiment was performed to progressively disturb the soil, and measure the effect by taking an IMP measurement after each disturbance. The area chosen was island Pearl, station 5-N-1. This station is one of the areas that had been used in the previous brush attenuation experiment. A 70 ft. diameter cirele had been carefully cut by hand out of the dense underbrush. A soil sampling program had also been conducted at this station, both on the surface and at 10 and 20 cm in depth. The results of the measurements are presented in Table B-4-3. The most startling fact is the small effect of removing the top four inches of soil in the road. The reason can be found if one examines Table B-1-4 of Tech Note 1.0. The effect of the road on the radiation field seen by the detector is 17.4%. Table B-4-4 is the soil sampling data on Pearl as a function of depth. Table B-4-5 is the same data, but the data has been averaged for the two samples A & B. Table B-4-6 presents the data as a ratio of the subsurfaces to the surface activity. From this table we can expect on the average that after removal of the top 4 inches there will still be 66% of the activity of the top soil exposed. The original activity measured by the IMP over this undisturbed soil was 20.6 pCi/g. The road is responsible for 17.4% of the radiation field from this cleared area. The contribution of the road to the radiation field was 20.6 pCi/gX 0.174 equals 3.58 pCi/g. The remainder of the cleared area accounts for 17.02 pCi/g of the radiation field. The effect of the removal of the top 4 inches of the B-4-1