One goal of the U.S. Department of Energy Office of Legacy Management (LM) Applied Studies and Technology (AS&T) program is to share new knowledge. LM and the International Atomic Energy Agency (IAEA) often share advances in applied science. In May 2016, AS&T scientist Dr. Jody Waugh traveled to Madrid, Spain, to give a keynote presentation at the IAEA International Conference on Advancing the Global Implementation of Decommissioning and Environmental Remediation Programmes. The presentation was a synopsis of ecological engineering remedies that LM is evaluating at former uranium processing sites in the southwestern United States.

Ecological engineering remedies are based on the concept that natural ecological processes at legacy sites can be beneficially enhanced or manipulated. Dr. Waugh presented test cases for four ecological engineering remedies that may be candidates for international applications.

Hydraulic Control of Groundwater

LM is responsible for characterizing and remediating groundwater at several former uranium mill sites. Groundwater contamination at these sites is primarily due to large volumes of processing liquids seeping from tailings impoundments during milling operations. AS&T evaluated evapotranspiration (ET) by native plants to hydraulically control groundwater flow as an alternative to pump-and-treat remedies at three legacy sites in Arizona and New Mexico. Scientists characterized the sites’ plant ecology, strategically transplanted native desert phreatophytes (deep-rooted desert plants that withdraw groundwater), and developed a mathematical model that combines satellite imagery and ground measurements of plants to estimate landscape-scale ET. Results suggest that by managing livestock grazing and planting native phreatophytes, ET from healthier vegetation can control upland recharge, enhance groundwater discharge, and thereby sustainably help control groundwater flow and contaminant transport.

Uranium mill tailings ecological engineering remedies.


 

In Situ Plant and Microbial Remedies for Soil and Groundwater

Uranium processing fluids leaching from tailings left residual contamination in soil and groundwater at many legacy sites. AS&T combined phytoremediation and microbial cycles to reduce nitrate and ammonium levels at one former uranium mill site in Arizona. Contaminants were leaching into groundwater from a bare soil area where a tailings pile had been removed. Scientists planted and lightly irrigated this source area with two species of native shrubs, and then discontinued irrigation. ET decreased leaching, and total soil nitrogen levels dropped more than 80 percent over 15 years. Nitrogen isotope analyses indicated that the drop resulted from coupled microbial nitrification and denitrification processes. Scientists also greatly enhanced rates of microbial denitrification in groundwater by injecting ethanol, which also reduced sulfate and uranium levels and led to a current AS&T investigation of ethanol injection to enhance microbial isolation of uranium in groundwater.

Land Farm Phytoremediation of Groundwater

AS&T evaluated land-farm phytoremediation as a pump-and-treat approach for nitrate, ammonia, and sulfate contamination in groundwater at a former mill site in Arizona. Scientists irrigated a planting of native shrubs with nitrogen-contaminated groundwater pumped from an alluvial aquifer. Plant uptake and microbial denitrification and nitrification cycles kept nitrogen levels from building up in the land-farm soil, plant growth and transpiration limited nitrate and ammonia recharge and leaching back into the aquifer, sulfate pumped from the plume remained in the soil profile as calcium sulfate, and the land farm produced a native seed crop that residents could use for rangeland revegetation and mine-land reclamation.

Evapotranspiration Covers for Tailings Disposal Cells

Finally, AS&T is evaluating ET covers as an alternative to conventional covers for tailings disposal cells. ET covers consist of thick, fine-textured soil layers that retain precipitation, which is seasonally removed by plants. Capillary barriers composed of coarse-textured sand and gravel placed below this soil “sponge” can enhance soil water storage capacity and limit unsaturated flow. The sustainability of ET covers depends, in part, on the establishment and resilience of a diverse plant community. Scientists used a series of increasingly larger test cells to design an ET cover for a uranium mill tailings disposal cell in Utah. The design used sandy clay loam soil from the site and native shrub-steppe vegetation. The test cells were designed to continuously measure percolation at a field scale and to provide comprehensive soil water balance monitoring. Results were used to design the final cover for the disposal cell. Subsequent monitoring over 16 years demonstrated exceptional hydraulic performance of the ET cover.

LM and IAEA plan to continue sharing knowledge about remediation science applications. IAEA invited Dr. Waugh to return and share more information on ecological engineering remedies with member states. LM has hosted IAEA tours of AS&T field studies at legacy sites in Colorado, Utah, and Arizona in the past, and will continue to do so in the future.