Tritium Plume Dynamics in the Shallow Unsaturated Zone in an Arid Environment

Abstract

Effective isolation of tritium ( 3 H) 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 3 H (>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 longterm, spatiotemporal variability of 3 H plumes; and (ii) quantify the processes controlling 3 H 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)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(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 3 H efflux via diffusive-vapor movement were >10´ greater than by dispersive-vapor or total-liquid movement. Total vertical fluxes were >20´ 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 3 H contribution to the plume from the LLRW facility during 2001 to 2011 and suggest that the widespread 3 H distribution resulted from transport before 2001.

Multimodel analysis of anisotropic diffusive tracer‐gas transport in a deep arid unsaturated zone

“…Maples et al . [] found that depletion of the gas‐phase tritium plume at the ADRS was greater than expected from standard estimates of diffusivity and radioactive decay. The high lateral diffusivity detected in the current tracer experiment may have contributed to mass‐loss from the plume.…”

“…These equations have yielded feasible values of isotropic gas diffusivity in numerous experiments spanning small distances (e.g., 1 m) and time scales (hours to days) [ Werner et al ., ] as well as at larger vertical scales [ Weeks et al ., ]. Yet, some observations of lateral transport of gases in deep unsaturated sediments have been difficult to explain using mathematical and numerical models based primarily on diffusion with MQ‐type equations [ Striegl et al ., ; Mayers et al ., ; Walvoord et al ., ; Maples et al ., ]. Additional studies of vertical and lateral gas transport in deep unsaturated sediments are needed to better understand how broadly the common assumption of diffusive transport with MQ‐type relationships applies [ Weeks et al ., ; Carrigan et al ., ; Thorstenson et al ., ; Walvoord et al ., ; Walvoord et al ., ].…”

Spatial Fingerprinting of Biogenic and Anthropogenic Volatile Organic Compounds in an Arid Unsaturated Zone

“…Site operators excavated sediments, deposited waste, and backfilled the trenches. The unlined trenches received containers of solid waste, solidified liquid waste (in Portland cement), and some direct discharge of liquids (Striegl et al, 1996; Maples et al, 2013). The waste trenches ranged from 2 to 15 m deep.…”

“…A 1‐yr controlled tracer test at the ADRS using SF 6 showed that gas diffusion was a primary transport mechanism at scales of tens of meters, and pronounced density effects were not observed (Walvoord et al, 2014). Studies of longer term transport of plumes at ADRS have indicated greater extents of contamination than expected from pure diffusion (Striegl et al, 1996; Mayers et al, 2005; Walvoord et al, 2008; Maples et al, 2013), possibly stemming from additional transport processes such as interactions of barometric pumping (advection) with geological layering (Green et al, 2015). The molecular mass of the gases influences the steady‐state vertical distribution of stable isotopes at the site (Seltzer et al, 2017).…”

“…1B) was a low-level radioactive waste (LLRW) facility, active from 1962 to 1992. This area received mixed waste including LLRW, VOCs, acids, bases, and other hazardous chemicals; regulations before 1985 did not require separation of LLRW from other waste (Maples et al, 2013). Site operators excavated sediments, deposited waste, and backfilled the trenches.…”