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.