Plant-Based Plume-Scale Monitoring Reveals the Extents and Pathways of Tritium Transport

Andraski, B.J., Michel, R.L., Halford, K.J., Stonestrom, D.A., and Abraham, J.D.

Cost-effective methods are needed to detect contamination near radioactive-waste and other contaminated sites. Such methods should be capable of providing an early warning of contaminant releases and be accurate and robust enough for monitoring the long-term performance of waste-isolation facilities and remediation measures. Plant-based methods were developed adjacent to a closed low-level radioactive waste (LLRW) facility in the Amargosa Desert, Nevada. Objectives were to (i) characterize and map the spatial variability of plant-water tritium, (ii) develop empirical relations to predict subsurface tritium contamination from plant-water concentrations, and (iii) gain insight into transport pathways and processes. Tritium was selected because it is a common radionuclide disposed at radioactive waste sites and it is a good tracer of water movement. Solar-distillation and solid-phase-extraction were used to collect and prepare plant (creosote bush, Larrea tridentata) foliage water for direct-scintillation counting. The maximum plant-water tritium concentration was 4,890 Bq/L; background values averaged 2.5 Bq/L. Geostatistical analysis showed that plant concentrations were spatially correlated to a distance of 380 m. Simple-contour and kriged maps of plant concentrations identified "hot spots" that were verified by soil-water-vapor measurements. Empirical linear relations between plant water and soil-water-vapor concentrations measured at the 0.5- and 1.5-m sampling depths were used to map the spatial distributions of root-zone and sub-root-zone tritium, respectively. Results showed that tritium migration away from the waste source primarily occurs in the gas phase with preferential transport through a dry, gravelly layer beneath the root zone, from which it moves upward and is subsequently released to the surface environment. Shallow and deep geologic units controlling preferential transport through the unsaturated zone were mapped by direct-current electrical resistivity imaging. Our study is apparently the first to document such extensive (> 300 m) subsurface gas-phase transport of tritium away from a disposal area. Sampling plant water for tritium requires one-fifth the time of soil-gas samples, reduces equipment costs, and provides a volume-integrated (versus point) sample that reflects the soil volume exploited by the plant's roots. The plant-sampling approach is likely to be transferable to additional species and environments; site-specific testing can be used to develop and evaluate the accuracy of predictive relations between plant and subsurface tritium concentrations. Plant-based plume mapping revealed the extents and pathways of tritium transport at the Amargosa Desert site. Process-based numerical models have failed to reproduce this observed transport by as much as one order of magnitude. Better process understanding is needed to support long-term monitoring of contaminated sites.