CERRO GRANDE ASH AS A SOURCE OF ELEVATED RADIONUCLIDES AND METALS

The Cerro Grande fire burned a large area in the eastern Jemez Mountains. The fire left a large amount of residual ash in burned areas. Sampling of ash and muck (post-fire sediments dominated by reworked ash) was conducted in locations representative of background conditions west (upstream) of the Laboratory. Ash and muck samples were also collected in the Viveash fire area (near Pecos, NM) for comparison. These data document the presence of elevated plutonium-239,240, cesium-137, and strontium-90 concentrations associated with global fallout from above-ground nuclear testing conducted primarily in the 1950s and 1960s. An increase in the concentrations of several naturally occurring metals (e.g., barium, manganese, and calcium) readily taken up into plant tissue is also observed. Radionuclide and metal concentrations increased by up to an order of magnitude in ash. The average concentration of cesium-137 in ash is 4.4 pCi/g, about 20 times the pre-fire background concentration. Flood deposits sampled kilometers from the mountain-front source of ash show persistent elevated concentrations of the radionuclide and inorganic constituents, including in watersheds unaffected by Laboratory operations. The data from the ash and muck samples are important for interpreting concentrations of radionuclides present in stormwater and sediment deposits, and are necessary for distinguishing fire-related constituents in stormwater and sediments from legacy contamination in canyons on the Laboratory. The ash and muck data provide a necessary post-fire baseline to support the assessment of potential impacts to the Laboratory and offsite (e.g., the Rio Grande and Cochiti Reservoir) from fire-related contaminants found in storm runoff. Much of the “contamination” transported to the Rio Grande may be unrelated to the Laboratory. Risk assessors should not, however, discriminate between sources of contamination in their assessments because the potential effects of exposure to radionuclides and metals are irrespective of their source. Knowledge of the source of contamination primarily guides the nature and location of potential mitigation measures.