Air monitoring data reveal previously unknown contamination at radioactivewaste disposal area, Los Alamos National Laboratory

Air monitoring at Area G, the low-level radioactive waste disposal area at Los Alamos National Laboratory, revealed increased air concentrations of (P U)-P-239 and Am-241 at one location along the north boundary. This air moni toring location is a couple of meters north of a dirt road used to access t he easternmost part of Area G. Air concentrations of (PU)-P-238 were essent ially unaffected, which was puzzling because both Pu-238 and Pu-239 are pre sent in the local. slightly contaminated soils. Air concentrations of these radionuclides increased about a factor of ten in early 1995 and remained a t those levels until the first quarter of 1996. During the spring of 1996 a ir concentrations again increased by a factor of about ten. No other radion uclides were elevated, and no other Area G stations showed elevations of th ese radionuclides. After several formal meetings did not provide an adequat e explanation for the elevations, a gamma-survey was performed and showed a small area of significant contamination just south of the monitor location . We found that in February 1995, a trench for a water line had been dug wi thin a meter or so of the air stations. Then, during early 1996, the dirt r oad was rerouted such that its new path was directly over the unknown conta mination. It appears that the trenching brought contaminated material to th e surface and caused the first rise in air concentrations and then the rero uting of the road over the contamination caused the second rise. during 199 6. We also found that during 1976 and 1977 contaminated soils from the clea n-up of an old processing facility had been spread over the filled pits in the vicinity of the air monitors. These soils, which were probably the sour ce of the air contamination, were very low in Pu-238 which explains why we saw very little Pu-238 in the increased air concentrations. A layer of grav el and sand as spread over the contaminated area. Although air concentratio ns of Pu-239 and Am-241 dropped considerably, they have not returned to pre -1995 levels.