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Tacoma, WA 98402

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NAS Whidbey Transport Model

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9722-9FAT5 - Groundwater Flow and Contaminant Transport at Area 6 Landfill, Naval Air Station Whidbey Island, Island County, Washington

Problem - The carcinogenic compound 1,4-dioxane was recently (2003) found in groundwater in the vicinity of the former landfill at Area 6, Operable Unit 1, Naval Air Station (NAS) Whidbey Island, Island County, Washington. The current extent and potential for further migration of 1,4-dioxane in the vicinity of Area 6 are not well known. Also at area 6, the Navy is exploring options to remove residual chlorinated volatile organic compounds (VOCs) from the vadose-zone source area that appear to be sustaining groundwater contamination, and they require information about how accelerated vadose-zone remediation may relate to accelerated groundwater remediation. The U.S. Geological Survey (USGS) previously constructed a three-dimensional MODFLOW groundwater flow model of Area 6 (Simonds, 2002). An updated and expanded groundwater flow and transport model for Area 6 and vicinity would facilitate exploring effective solutions to both the 1,4-dioxane and vadose-zone remediation problems.

Objectives - The primary objectives of this investigation are to construct and apply a groundwater flow and transport model for Area 6 to better understand the historical and potential future migration of 1,4-dioxane and how accelerated vadose-zone remediation may relate to accelerated remediation of VOCs in groundwater.

Relevance and Benefits - This investigation will be conducted as part of the U.S. Geological Survey Department of Defense Earth Science Program (DODESP) whose mission is to provide high quality, legally defensible, and timely earth science information to answer relevant questions about the Department of Defense environmental concerns, and to provide the data necessary for Federal, State, and local agencies to make informed decisions about environmental resources within their jurisdiction. This study addresses issues identified in the USGS Science Strategy document under the strategic science direction “The Role of Environment and Wildlife in Human Health: A System that Identifies Environmental Risk to Public Health in America,” and will provide information to advance the understanding of 1,4-dioxane transport processes in groundwater.

Approach - The primary tasks include compilation and quality assurance of historical Area 6 data from the Navy Technical Data Management System (TDMS) database; an inventory of domestic, Navy, and other wells located downgradient of Area 6; expansion of the existing three-dimensional hydrogeologic framework downgradient to Crescent Harbor (and Oak Harbor if necessary); collection of groundwater level and contaminant concentration data from Area 6 and downgradient wells; development (calibration, sensitivity analyses, and validation) of the expanded three-dimensional flow and transport model; application of the model to simulate past and potential future 1,4-dioxane plume migration and potential future migration and fate of VOCs in groundwater for different source-area remediation strategies; and documentation of the investigation and archiving the groundwater model to assure its availability for future use.

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