Fractured Bedrock DNAPL/ Dissolved Phase Plume Conceptual Model Development at the Eastland Woolen Mill Superfund Site, Corinna, Maine

The Eastland Woolen Mill (EWM), which straddled the East Branch of the Sebasticook River in Corinna, Maine, was listed on the United States Environmental Protection Agency (USEPA) National Priority List in July of 1999. The site is currently undergoing clean-up activities. The United States Army Corps of Engineers New England District is managing site activities under a cooperative interagency agreement with USEPA. Chlorinated benzenes released during EWM manufacturing processes resulted in extensive contamination to fractured meta-sedimentary bedrock underlying the Site. Chlorinated benzene dense non-aqueous phase liquids (DNAPL) have migrated vertically at the Site into underlying bedrock. A chlorinated benzene groundwater plume exists in bedrock. Under natural conditions, bedrock groundwater discharges to overburden and then to the river. This paper presents the evolution of the EWM Site fractured bedrock conceptual model and its’ value in decision-making from the early stages of the remedial investigation up to the planning and implementation of Site remedial activities. Initial Site Investigation (SI) activities, including potentiometric surface mapping and groundwater sampling from existing residential wells, indicated localized bedrock groundwater contamination down gradient and cross gradient of the EWM. Several potential secondary source areas were also identified, in addition to the primary source areas beneath the EWM. The initial conceptual model developed after the SI suggested that a deep and discretely fractured bedrock system was controlling plume migration, which was expected to discharge along the river. A remedial investigation that included photo-lineament analysis, bedrock mapping, monitoring well installations, borehole geophysical logging (optical, acoustic tele-viewer, and heatpulse flow meter), low-flow straddle packer and multi-level groundwater sampling, a groundwater pumping test, and potentiometric surface mapping was undertaken to better define contaminant distribution, fate and transport. Key observations made from the borehole geophysical data set show the dominant set of hydraulically active fractures were along sedimentary bedding planes, which strike from E-W to NE-SW and dip 60-65 degrees to the south and southeast. The bedding plane fractures are only weakly cross connected by a smaller set of axial plane fractures and north to northwesterly trending joints, accounting for the high degree of anisotropy noted in a groundwater pumping test and cross-gradient plume alignment. The resulting average strike of combined joints and bedding plane fractures is roughly North 50 East. A pumping test conducted near the source area indicated a drawdown ellipse axis of North 43 East. The majority of the hydraulically active fractures (i.e., fractures with measurable flow under stressed conditions) in the aquifer are shallow relative to the depth of documented contamination. Hydraulically active fractures are generally within the top 130 feet of the bedrock aquifer, yet documented contamination extends nearly 300 feet deep under the former mill. A few hydraulically active fractures have been identified within the aquifer at depths down to 250 feet below the bedrock surface. Investigations were concluded to 600 feet at one location. Groundwater sampling showed the extent of groundwater contamination was approximately four-times greater cross gradient to the southwest and northeast than in the downgradient direction that had been established by potentiometric measurements. Several residential wells were significantly impacted cross gradient (southwest) from the primary source area. Investigation of other potential sources of contamination near these well did not identify chlorinated benzene contamination. Deep DNAPL penetration is suspected down dip along bedding plane fractures under the EWM source area. The similarity between the orientation of the aquifer anisotropy and the elongated cross gradient plume suggested that contaminated groundwater, while under a hydraulic stress, had moved preferentially, from deep within the source area, to the southwest along a narrow band of interconnected bedding plane fractures. Groundwater modeling confirmed that the high degree of anisotropy in the aquifer allowed distant domestic well pumping stresses, to pull contamination over 550 feet to the southwest along these deep, fracture pathways. Contamination in deep fractures was confirmed in the farthest wells.