Increased contaminant mobility through wind erosion following forest disturbance

Abstract

In May of 2000, the Cerro Grande fire burned roughly 30% of Los Alamos National Laboratory (LANL), including areas containing a variety of soil-bound radioactive and chemical contaminants. Post-fire tree thinning caused additional reductions in vegetation and disturbed surface soils. The dramatic loss of vegetation and litter cover, coupled with significant disturbances to surface soils, prompted concerns for the safety of the public and LANL workers. The unknown effects of these forest disturbances on wind-driven resuspension of these contaminated soils were of particular concern because of potential for increased inhalation exposures. This study used a variety of aerosol measurements in burned and tree-thinned areas to assess dust flux and associated inhalation doses to LANL workers. Results show that wind-driven dust flux was significantly greater in severely and moderately burned areas relative to unburned areas, by more than one order-of-magnitude initially and by 2-3 times a year after the fire. Unexpectedly, the elevated dust fluxes did not decrease during the second and third years in burned areas, apparently because ongoing drought delayed post-fire recovery. In the thinned areas, wind erosion was significantly elevated, but there were indications of reduced dust flux during the second year after thinning. Regarding the special concern for burned sites containing depleted uranium (DU), we measured 7-day partition coefficients (Kd) values that ranged from 276-508 mL g-1, which suggests that the DU would generally remain in surface soils being available for wind erosion, if exposed. Finally, we estimate a relative increase of about 1-4% in PM-10 dust emission across the entire LANL resulting from the fire. Calculations of relative inhalation dose from DU suggest a high-end estimate of a 26% increase for LANL workers on severely burned areas. Despite the potential increased doses, the estimated annual dose was < 1 μSv, which is far below the dose limits for occupational or public exposures. Beyond implications of exposure to contaminated soils, our results highlight the importance of considering wind- as well as water-driven transport and erosion, particularly following disturbance, for ecosystem biogeochemistry in general and human and ecological risk assessment in particular.