Uptake of elements of concern by plants growing on remediated uranium mill tailings

Background/Question/Methods

The U.S. Department of Energy (DOE) is investigating ecological changes in engineered earthen covers that were designed to limit radon exhalation and rainwater seepage from uranium mill tailings for hundreds of years.  Rock armoring fine soil in engineered covers slows evaporation and traps dust and organic matter, thereby providing water and nutrients for plant germination and survival.  Risk managers need to understand the tradeoffs of potential benefits and detriments of plants encroaching on engineered covers.  Plant transpiration can offset increases in hydraulic conductivity and percolation flux caused by soil-forming processes.  However, effects of (1) soil-forming processes and transpiration drying on radon flux, and (2) contaminant uptake and accumulation by plants rooted in engineered covers, are largely unknown.  We measured levels of uranium, selenium, and other elements of concern in above ground tissues harvested from plants growing on engineered covers and in nearby references areas at seven uranium mill tailings sites in the western U.S.  Reference area soils matched the fine soils used to construct the engineered covers.  Element levels in plant tissues were compared with Maximum Tolerance Levels (MTLs) set for livestock by the National Research Council, and with other risk thresholds. 

Results/Conclusions

For some elements at some sites, levels were higher in plants growing on engineered covers than in the reference area plants, indicating possible mobilization of these elements from the tailings into plant tissues.  For example, at five of the seven sites, uranium was higher in native shrubs growing on than off engineered covers.  However, except for selenium, element levels in plants both on and off covers were well below MTLs.  Selenium levels above MTLs in plants at two sites, known to have seleniferous soils, can be attributed to cover soils and not tailings.  Selenium biomagnification over the long term and toxicity at higher trophic levels, unrelated to uranium processing, is possible in these areas.  However, as with seleniferonus soils elsewhere in the West, the land could be managed safely for forage production.  The results suggest that uptake of elements of concern by plants growing on these engineered covers does not pose an exposure risk.  DOE will use these results, coupled with results of ongoing soil water balance and radon attenuation studies, to inform long-term vegetation management decisions at uranium mill tailings sites.