Theme: 10. Characterization, Fate and Transport

Topic Area: 10d. Conceptual Site Models: Improvements in Development and Application

Formatted Title:

Reevaluating the Conceptual Site Model of a Shoreline Chlorinated Solvent Plume in Groundwater

Background/Objectives:

Naval Base Kitsap Keyport is located on a small peninsula in the central portion of western Puget Sound in Kitsap County, Washington. The former landfill comprises approximately 9 acres on the west side of the installation and was built in an estuary wetland that continues to border the landfill to the west and south.

The landfill was the primary disposal area for domestic and industrial wastes generated on the base from the 1930s until it was closed in 1973. Liquid plating waste, waste paint, thinners, and strippers from the plating, paint and stripper shops were disposed of in the southwest area of the landfill. The primary contaminants of concern (COCs) are chlorinated hydrocarbons (cVOCs, primarily trichloroethene, cis-1,2-dichloroethene, and vinyl chloride) and 1,4-dioxane. The selected remedy of phytoremediation and natural attenuation was predicated on a conceptual site model (CSM) that emphasized cVOCs within the waste body, to 15 feet below ground surface, at two “source areas.” The remedy included two phytoremediation plantations at identified hotspots and natural attenuation. Long-term monitoring indicates that the restoration timeframe is unreasonable and a revised CSM was needed to optimize the remedy.

Approach/Activities:

Updating the CSM required developing a dense data set spanning a wide range of media, including 267 tree core samples; 61 membrane interface probe (MIP) direct-push borings; field screening at the parts per billion level of continuous soil cores along with grab soil and groundwater samples for laboratory analysis collected at 173 locations using direct-push and sonic continuous coring. Permanent groundwater monitoring wells were also installed at hotspots and to define lateral and vertical extents using both conventional construction and continuous multi-channel tubing (CMT) wells. Additional investigation efforts included sediment, porewater, and surface water sample collection and analysis; thermal imaging of groundwater seeps; geophysical investigation of lateral and temporal changes in the groundwater/surface water interface in the intertidal zone and anomalies within the landfill; aquifer performance testing; environmental sequence stratigraphy and three-dimentional (3D) spatial analysis. Laboratory analyses included target cVOCs, conventional chemistry parameters and microbial populations to allow assessment of biodegradation potential.

Results/Lessons Learned:

The CSM developed based on the now extensive data set differs substantially from that used to select the original remedy, revealing cVOC contamination to 100 feet below ground surface in a complex channelized geologic sequence of paleotidal and paleofluvial deposits. The geologic and chemical distribution models rendered in 3D, show preferential flow pathways along paleochannels transporting cVOCs at depth beneath the adjacent marine embayment. Temporal electrical resistivity studies demonstrate how freshwater in paleochannels, likely transporting cVOCs, reaches 1000 feet offshore beneath the marine embayment at low tides, with saline groundwater flooding those channels back toward the landfill boundaries at high tide. In the shallow subsurface, the revised CSM shows orders of magnitude higher cVOC concentrations discharging from groundwater to wetland surface water at the landfill boundary compared to what was known at the time of remedy selection. This revised CSM transforms the understanding of the site, as defined in the ROD, and indicates additional remedial action will be needed to control contaminant migration.

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Amanda Rohrbaugh