been observed in nature when some kinds ofrock, heated by the sun, are hit by a sudden shower.! This mechanism for the airborne plutonium observed over the PPO pieces seems to be confirmed by the striking results (Table I) of an experiment in which air particulate samples were taken during a rain se- quence. The average concentration duringthefirst 5 min ofrain was 104 times the prerain concentration. Ht then dropped continuously until it reached the prerain level about 1 h after the end of the rain. The principal cause for the rapid decrease is gravitational settling of the particles. The rain washes some of the plutonium out ofthe air but it cannot be the only removal mechanism because the decrease continues after the rain has stopped. A repetition of the experiment, but with dehumidifier turned off, produced similar rates of decrease, in- dicating that pickup by the dehumidifier is not the principal removal mechanism. The results in Table III were obtained by counting the fiber glassfilters with a 2-7 gas proportional counter. Some of the filters were subsequently dissolved, and a more accurate analysis by liquid scintillation counting showed that most of the initial activity came from relatively large particles caught on the filter surface and therefore countedefficiently by the proportional! counter. Later, the large particles settled out, and the remaining smaller particles penetrated deeper into the filter and were countedless efficiently.This experiment was repeated twice, with Millipore membranefilters of two different pore sizes, and at two air collection points within the chamber, in order to obtain samples for particle size and agglomeration studies. The airborne plutonium concentrations were similar to those measured in the first experiment (Table III). The first rainwater that percolated through the soil supporting the large fuel pieces contained 1.8 uCi of plutonium. This very rapid plutonium breakthrough indicated that some of the material that spalled from the fuel is in the form ofvery small particles or a colloid. Plutonium ions would not be stable in solution at the slightly basic pH (about 8). and if present would be held in the soil by some ion- exchange type of mechanism. The amountof plutonium found in the rainwater that. percolated through the soil supporting the large pieces is strongly influenced by the climate, as is shown in Table IV. Each weekly rain is equivalent to a 32-mm rainfall and deposits 27 liters of water. Under summer conditions, about 6 liters of rain- water percolated through the soil. This water contained an average of 0.7 uwCi of plutonium. Winter rain produced more percolated water, because of lower evaporation losses, and a higher plutonium content, the early winter rains yielding as much as 18 wCi of plutonium. It seems that the higher summer temperatures dried the soil more between TABLE ITI AIRBORNE PLUTONIUM CONCENTRATIONS rains so it became saturated later during a rain. Because of this lower water flow in summer. fewer DURING A RAIN IN AN ENVIROAMENTAL plutonium dioxide particles were washed through CHAMBER CONTATNING LARGE PTECES OF the soil. At the beginning of the winter, some ofthe PLUTONIUM DIOXIDE Collection Time Pu Concentration 1 24 3 2 5 3x 104 3 5 6x 10 Filter No. 4 5 6 7 (min) 10 10 15 15 (pCi/m*) 3 4x 10° 3x 10° 9x 10° 8 x 107 particles left in the soil during the summer were washed through, giving higher results for the first Remarks Before rain First 5 9 180 10 860 2x 10° HUMID CLIMATE EFFECTS ON AMOUNT OF PLUTONIUM CARRIED BY WATER PER- min of rain COLATING THROUGH SOIL (Large Pieces from MHFT-12) Last 15 min of 50 Table IV Climate Winter Conditions 0-17 % Av Percolated Water per Rain (liters) 11 Av Pu fuci) 9 70 - 100% Ri rain Summer 20 - 40 °c 6 87 - 96% RH OP AKG VEGs 0.7