Los Alamos National Laboratory (LANL) and Shaw Environmental, Inc. (Shaw) designed and constructed a multilayered permeable reactive barrier (PRB) to remove contaminants from shallow alluvial groundwater within Mortandad Canyon at LANL. This project was developed as a pilot project for LANL to conduct research and development and proof of concept and as such does not meet all identified target contaminant concentrations, but provides LANL data for future applications of the technology. Shaw worked jointly with LANL scientists in selecting the site, conducting a geotechnical and hydrogeologic investigation with contaminant characterization for waste disposal, preparing a design basis report, conducting geochemical and groundwater flow modeling, and preparing both conceptual and final detailed engineered designs. Geochemical modeling of the PRB multibarrier processes was conducted to predict influent and effluent contaminant concentrations and evaluate the potential for mineral precipitation and reduction of effective porosity in the barrier. A numerical model of groundwater flow was constructed to simulate hydrogeologic conditions in Mortandad Canyon and then used to simulate flow with the PRB in place. The Mortandad Canyon PRB is designed to remove radionuclides (americium-241, plutonium-238 and 239/240, and strontium-90), nitrate, and perchlorate from alluvial groundwater. The PRB consists of a funnel and gate constructed of sealable sheet piling driven through the alluvium and into the underlying volcanic tuff. The gate is designed as a braced cofferdam. The gate contains four sequential media cells consisting of lava rock gravel, mineral apatite (a calcium phosphate), biobarrier, and limestone gravel. The lava rock gravel will sorb colloids (sorbed with americium, plutonium, and strontium) from the alluvial groundwater. The apatite will remove soluble metals and radionuclides through sorption processes. The biobarrier serves as a host microorganisms that biodegrade nitrates and perchlorate. The limestone gravel functions to buffer the biobarrier effluent. In addition, there will also be sorption of soluble plutonium, americium, and metals within the biobarrier and limestone layer. A series of sampling ports and monitoring wells were installed within the reactive media cells. The purpose of the funnel is to direct shallow alluvial groundwater through the gate. This project was a joint effort between LANL and Shaw. The initial feasibility studies and bench scale treatability were conducted at LANL. The LANL laboratory data was used as the basis for design criteria. The hydrogeologic and geochemical modeling, engineering design, and construction were performed by Shaw with LANL guidance and input.
Assessment of optimum width and longevity of a permeable reactive barrier installed in an acid sulfate soil terrain