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

Abstract

Air monitoring at Area G, the low-level radioactive waste disposal area at Los Alamos National Laboratory, revealed increased air concentrations of 239Pu and 241Am at one location along the north boundary. This air monitoring location is a couple of meters north of a dirt road used to access the easternmost part of Area G. Air concentrations of 238Pu were essentially unaffected, which was puzzling because both 238Pu and 239Pu are present in the local, slightly contaminated soils. Air concentrations of these radionuclides increased about a factor of ten in early 1995 and remained at those levels until the first quarter of 1996. During the spring of 1996 air concentrations again increased by a factor of about ten. No other radionuclides were elevated, and no other Area G stations showed elevations of these radionuclides. After several formal meetings did not provide an adequate 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 within a meter or so of the air stations. Then, during early 1996, the dirt road was rerouted such that its new path was directly over the unknown contamination. It appears that the trenching brought contaminated material to the surface and caused the first rise in air concentrations and then the rerouting of the road over the contamination caused the second rise, during 1996. We also found that during 1976 and 1977 contaminated soils from the clean-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 source of the air contamination, were very low in 238Pu which explains why we saw very little 238Pu in the increased air concentrations. A layer of gravel and sand was spread over the contaminated area. Although air concentrations of 239Pu and 241Am dropped considerably, they have not returned to pre-1995 levels.