Paper No. 6
Presentation Time: 9:25 AM
TRITIUM PLUME DYNAMICS IN THE SHALLOW UNSATURATED ZONE ADJACENT TO A RADIOACTIVE WASTE DISPOSAL FACILITY IN AN ARID ENVIRONMENT—AMARGOSA DESERT RESEARCH SITE, NEVADA
Effective isolation of tritium (3H) and other contaminants at disposal sites in arid unsaturated zones requires improved understanding of transport processes and pathways. Previous studies documented an anomalously widespread (i.e., theoretically unexpected) distribution of 3H (> 400 m from burial trenches) in a dry, sub-root-zone gravelly layer (1–2-m depth) adjacent to a low-level radioactive waste site in the Amargosa Desert, Nevada that closed in 1992. The objectives of this study were to (i) characterize long-term, spatiotemporal variability of 3H plumes and (ii) quantify processes controlling 3H behavior in the sub-root-zone gravelly layer beneath native vegetation adjacent to the site. Geostatistical methods, spatial moment analyses, and mass flux calculations were applied to a spatially and temporally comprehensive, 10-yr data set (2001–11). In contrast with previous findings, our 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 3H dispersion. Plume mass diminished more rapidly than would be expected from radioactive decay alone, indicating net efflux from the plume. Estimates of upward 3H efflux via diffusive-vapor movement were more than ten times greater than 3H effluxes by either dispersive-vapor or total-liquid movement. Total vertical fluxes were more than twenty times greater than lateral diffusive-vapor fluxes, highlighting the importance of upward migration toward 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 (including radioactive decay and upward diffusive-vapor fluxes) substantially exceeded any continuing 3H contribution to the plume from burial trenches during the study period.