Water flowing through Mortandad Canyon downstream of Laboratory operations now will flow through a Permeable Reactive Barrier — a huge column of pollution-capturing materials — before proceeding farther downstream. Waters that encounter the PRB will be scrubbed of radionuclides such as strontium-90; americium-241; plutonium 238, 239 and 240; and uranium isotopes as well as chemicals such as perchlorate, nitrate and heavy metals.
Mortandad Canyon is the location of the effluent stream from the Laboratory's Radioactive Liquid Waste Treatment Facility. The RLWTF discharges about 60,000 gallons of treated effluent per week on average. Recent improvements to the RLWTF have greatly reduced concentrations of chemicals and radionuclides in the effluent. But legacy discharges have left behind residual contamination in canyon soils and in the shallow groundwater in some areas. Because some of these residues are water soluble, they can be remobilized as water flows down the canyon. The Permeable Reactive Barrier provides a simple, inexpensive technology to help control legacy contamination.
The decision to improve the quality of effluent from the RLWTF and to install the PRB were done by the Laboratory's own initiative as part of the Laboratory's ongoing environmental stewardship commitment.
The PRB requires no personnel or energy resources to operate. The PRB is essentially a trap dug into the ground. Within the four walls of the 27-foot-deep PRB chamber are layers of low-cost materials specifically formulated to capture contamination. Water flows through layers of fine lava rock, calcium phosphate, pecan shells and cottonseed, and limestone before continuing downstream. Each layer treats the water for a specific set of contaminants.
The chemistry behind the treatment methods is well established and technically proven.
Treatment materials within the PRB will last for about 10 years. After their cleaning power has been exhausted, the layers can be excavated easily and sent to an appropriate disposal facility. New materials can then be added and the PRB is good for another 10 years.
To assess the effectiveness of the PRB, Laboratory environmental scientists can take samples of water and materials inside the treatment box. In addition, water samples will be taken upstream and downstream of the PRB for analyses. Moreover, the PRB is strategically located between wells that monitor the alluvial groundwater within Mortandad Canyon.
Because nothing like the PRB has ever been used on Laboratory property before, Los Alamos environmental researchers will carefully monitor all aspects of the technology to determine if it would be useful in other areas and at other sites across the nation.
The barrier cost about $900,000 to install, and should cost very little to maintain each year.
Note to news media/editors: Photo available on-line at: http://www.lanl.gov/worldview/news/images/prb.jpg
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