GEOCHEMICAL AND BIOGEOCHEMICAL MECHANISMS OF CONTAMINANT ATTENUATION IN A MULTI-LAYERED PERMEABLE REACTIVE BARRIER

A multi-layered permeable reactive barrier (PRB) has been installed in Mortandad Canyon, on the Pajarito Plateau in the north-central part of LANL, to demonstrate in-situ treatment of contaminants while mitigating possible vulnerabilities from downstream contaminant movement within alluvial and deeper perched groundwater. Mortandad Canyon was selected as the location for the PRB because the flow of alluvial groundwater is constrained by the geology of the canyon, a large network of monitoring wells already exists along the canyon reach, and the hydrochemistry and contaminant history of the canyon is well-documented. The PRB incorporates a sequence of four reactive media layers to immobilize or destroy a suite of contaminants present in alluvial groundwater, including Sr-90, Pu-238, 239, 240, Am-241, perchlorate, and nitrate. The four sequential media cells consist of gravel-sized scoria, apatite, pecan shells and cottonseed with an admixture of gravel (biobarrier), and limestone.

Design elements of the PRB are based on laboratory-scale treatability studies and on a field investigation of hydrologic, geochemical, and geotechnical parameters. The PRB was designed with the following criteria: 1-day residence time within the biobarrier, 10-year lifetime for the PRB, minimization of surface water infiltration and erosion, optimization of hydraulic capture, and minimization of excavated material requiring disposal. Each layer has been ported to allow sampling of water and reactive media, and monitor wells are located immediately adjacent to the up- and down-gradient edges of the barrier. Preliminary results indicate that both nitrate and perchlorate are being reduced by microbial activity in the biobarrier.