干旱区生态环境知识资源中心

Effective isolation of tritium (H-3) and other contaminants at waste-burial facilities requires improved understanding of transport processes and pathways. Previous studies documented an anomalously widespread (i.e., theoretically unexpected) distribution of H-3 (>400 m from burial trenches) in a dry, sub-root-zone gravelly layer (1-2-m depth) adjacent to a low-level radioactive waste (LLRW) burial facility in the Amargosa Desert, Nevada, that closed in 1992. The objectives of this study were to: (i) characterize long-term, spatiotemporal variability of H-3 plumes; and (ii) quantify the processes controlling H-3 behavior in the sub-root-zone gravelly layer beneath native vegetation adjacent to the facility. Geostatistical methods, spatial moment analyses, and mass flux calculations were applied to a spatiotemporally comprehensive, 10-yr data set (2001-2011). Results showed minimal bulk-plume advancement during the study period and limited Fickian spreading of mass. Observed spreading rates were generally consistent with theoretical vapor-phase dispersion. The plume mass diminished more rapidly than would be expected from radioactive decay alone, indicating net efflux from the plume. Estimates of upward H-3 efflux via diffusive-vapor movement were >10x greater than by dispersive-vapor or total-liquid movement. Total vertical fluxes were >20x greater than lateral diffusive-vapor fluxes, highlighting the importance of upward migration toward the land surface. Mass-balance calculations showed that radioactive decay and upward diffusive-vapor fluxes contributed the majority of plume loss. Results indicate that plume losses substantially exceeded any continuing H-3 contribution to the plume from the LLRW facility during 2001 to 2011 and suggest that the widespread H-3 distribution resulted from transport before 2001.