Hydrologic response and radionuclide transport following fire at semiarid sites

Infrequent, high-impact events such as wildfires, droughts, biological shifts, floods, and mechanical disturbances can greatly change land surfaces, including vegetative cover and soil characteristics, which in turn can trigger high rates of hydrologic erosion and associated transport of sediments and sediment-sorbed contaminants. Where persistent soil contamination exists, infrequent mobilization of contaminants may dominate in determining long-term risks to human and ecological receptors. Among these infrequent events, fire stands out as having the capacity to cause large increases in sediment transport. This study measured runoff, sediment yield, and mobility of sediment-sorbed contamination (137Cs) on burned and unburned plots at the Waste Isolation Pilot Plant, New Mexico (WIPP), and the Rocky Flats Environmental Technology Site, Colorado (RFETS). Results showed that 137Cs transport from burned plots was up to 22 times greater than that from unburned plots at WIPP and 4 times greater at RFETS. Associated runoff was up to 12 times greater on burned plots at WIPP and sediment yields up to 6 times greater. Further, 137Cs concentrations in transported sediments were enriched compared with parent soils (expressed as enrichment ratio) by a factor of 2.3 at WIPP, and 1.3 at RFETS. However, enrichment ratios were not significantly different in sediments from burned and unburned plots. Our results provide new data on the effects of fire on the transport of sediment-sorbed contaminants, and demonstrate that rare events such as fire can greatly increase contaminant mobility.