Non-radioactive liquid chemical waste was disposed at Material Disposal Area (MDA) L within Technical Area 54 (TA-54) at the Los Alamos National Laboratory (LANL) from the early 1960's until 1985. The surface of the site is currently used for Resource Conservation and Recovery Act (RCRA)-permitted chemical waste storage and for mixed waste storage under interim status authority. The major contaminant release at the site is a subsurface organic solvent vapor-phase plume consisting of several volatile organic compounds (VOCs) including 1,1,1-trichloroethane (TCA), trichloroethene (TCE), carbon tetrachloride, chloroform, tetrachloroethene (PCE), toluene, and benzene. Other contaminants that have been detected, but at much lower concentrations, and include chlorobenzene, xylenes, and 1,2,4-trimethylbenzene. TCA was found in the greatest concentration, and it also exhibits the greatest lateral and vertical extent in the organic vapor plume. The measured concentrations of TCA are almost an order of magnitude greater than values measured for TCE, the contaminant of second highest concentration. Under LANL's Environmental Stewardship-Environmental Remediation and Surveillance Program, extensive sampling and analysis have been conducted to determine the nature and extent of the plume, and a conceptual model to characterize the subsurface plume has been developed. Data analysis has shown that the site does not currently posemore » a potential unacceptable risk to human health or the environment. LANL proposes to conduct a study to determine whether a soil vapor extraction system can effectively remove VOC contamination from the subsurface vapor-phase plume at MDAL. Previous investigations conducted at MDAL on plume remediation include a Pilot Vapor Extraction Test (PVET), in which a soil vapor extraction (SVE) system was constructed and operated near the outer boundary of the plume. The results of this test demonstrated the potential effectiveness of SVE at MDAL. The proposed study entails installation of an active soil vapor extraction system to evaluate the rate of reduction of the contaminant concentrations immediately around the source terms. Active extraction will be conducted over an approximately four-month period, which should be complete by the end of March 2006. Continued subsurface monitoring of the vapor contaminant concentrations will capture soil vapor concentration rebound and will determine when or if additional extraction should take place. Rebound analysis will also provide valuable information on the nature of the source. Should future monitoring indicate an increase in the soil vapor concentrations, the emplaced extraction system may be used to remove and treat contaminants, ensuring that the subsurface plume does not increase in size or concentration. Continuous monitoring of the off gas will be conducted to ensure that all regulatory requirements are met. Data from this study will be used in the corrective measure evaluation for MDA L to assess the effectiveness of SVE as a remedy for remediation of the subsurface vapor-phase plume at MDAL. (authors)« less
Following the May 2000 Cerro Grande fire at Los Alamos, NM, surface water control structures were constructed near Los Alamos to mitigate the transport of contaminant‐bearing sediment toward the Rio Grande river due to increased runoff caused by the removal of vegetation by the fire. A low‐head weir was constructed in Los Alamos Canyon, 5 km to the east of Los Alamos, to capture contaminant‐bearing sediments and to allow runoff to pass downstream without significant ponding behind the weir. During construction of the weir, channel alluvium was removed and the underlying fractured basalt was exposed. To monitor any downward transport of contaminants into fractured basalt, and potentially downward to the regional groundwater, three boreholes (one vertical, and two angled) were installed for environmental monitoring. An innovative monitoring system was installed using FLUTe (Santa Fe, NM) liners for both vadose zone and perched groundwater zones. The vertical borehole intersects several perched water zones, and groundwater can be sampled from four ports. One angled borehole has an inflatable liner with sensors to measure relative water content. The second angled borehole was abandoned. Tracer tests were initiated in April 2002 and June 2003 with the application of solutions of potassium bromide and potassium iodide, respectively, onto the basin floor above the weir. The hydrogeologic characterization from drilling the boreholes, in conjunction with groundwater elevation and vadose zone moisture monitoring, and results from the tracer tests show that the subsurface hydrogeology is complex, and surface water and perched groundwater systems are in apparent close communication. Infiltration is rapid, and movement from the surface to the deepest perched zone (at a depth of 78 m) occurs within 8 to 14 d. Downward flow occurs predominantly via fracture flow.
Low-level radioactive waste (LLW) generated at the Los Alamos National Laboratories (LANL) is disposed of at LANL's Technical Area (T A) 54, Material Disposal Area (MDA) G. The ability of MDA G to safely contain radioactive waste during current and post-closure operations is evaluated as part of the facility's ongoing performance assessment (PA) and composite analysis (CA). Due to the potential for uptake and incorporation of radio nuclides into aboveground plant material, the PA and CA project that plant roots penetrating into buried waste may lead to releases of radionuclides into the accessible environment. The potential amount ofcontamination deposited on the ground surface due to plant intrusion into buried waste is a function of the quantity of litter generated by plants, as well as radionuclide concentrations within the litter. Radionuclide concentrations in plant litter is dependent on the distribution of root mass with depth and the efficiency with which radionuclides are extracted from contaminated soils by the plant's roots. In order to reduce uncertainties associated with the PA and CA for MDA G, surveys are being conducted to assess aboveground biomass, plant litter production rates, and root mass with depth for the four prominent vegetation types (grasses, forbs, shrubs andmore » trees). The collection of aboveground biomass for grasses and forbs began in 2007. Additional sampling was conducted in October 2008 to measure root mass with depth and to collect additional aboveground biomass data for the types of grasses, forbs, shrubs, and trees that may become established at MDA G after the facility undergoes final closure, Biomass data will be used to estimate the future potential mass of contaminated plant litter fall, which could act as a latent conduit for radionuclide transport from the closed disposal area. Data collected are expected to reduce uncertainties associated with the PA and CA for MDA G and ultimately aid in the assessment and subsequent prevention of radionuclide transport within the environment from the closed disposal area and potential exposure to site workers and the public.« less
Following the May 2000 Cerro Grande fire at the Los Alamos National Laboratory, concerns arose regarding increased sediment transport from denuded areas at the Laboratory to offsite locations. In order to mitigate potential increased offsite sediment transport, a low-head weir was constructed in the lower reaches of Los Alamos Canyon to collect the sediment load from runoff waters. During construction of the weir, surficial alluvial deposits were removed and the underlying fractured basalt bedrock was exposed. Although the weir was designed to allow for runoff through-flow, ponding was observed behind the weir on the exposed fractured basalt surface following heavy precipitation events. Therefore, the potential for enhanced subsurface infiltration from the weir ponding became a concern. Tracer tests were designed to evaluate the potential for increased hydrologic transport to subsurface perched water zones below the weir site. Potassium bromide and potassium iodide tracer tests were conducted at the weir site in April 2002 and June 2003, respectively, to evaluate the connectivity of surficial ponded waters with intermediate perched zones. Two angled and one vertical monitoring well were installed and instrumented to investigate the connection between surface water and perched waters. The tests revealed that the ponded surface water and the perched zones are well-connected. Bromide tracer was encountered down to a depth of 82 m, although the concentrations attenuated with depth. Infiltration can be rapid and travel times varied from 2 weeks to 2 months in the fractured basalt bedrock. Water level measurements obtained from four sampling ports in the vertical well indicated a response time between 1 day and 2 weeks for the intermediate perched water zones from the ponding events.
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