The riverbed in Dunnett’s Cove, a 10-acre area in Bangor, Maine, was heavily polluted with non-aqueous phase liquid (NAPL) droplets and sheens from tar that migrated from river sediment to the surface. The tar was deposited by a manufactured gas plant (MGP) that operated upstream from the river for well over 100 years. Prior to TRC’s involvement, there was no comprehensive view of the extent of the problem, nor of the potential issues surrounding remediation of the tar deposit. Developing an effective remedy was further complicated by the rocky nature of the riverbed, a 12- to 20-foot tidal range, the complex physical process influencing tar migration, and the presence of sensitive habitats within the river.
TRC’s solution restored the waterfront, with turnkey services ranging from a comprehensive investigation of the tar deposit’s source, the nature and extent of the contamination, and human health and ecological risks, through design and construction of the innovative sediment cap that now controls the coal tar migration. The innovative NAPL-Trapping Cap (subsequently awarded a patent, No. 8,419,314) redirects migrating tar to a controlled accumulation area, where it is trapped while the gas is vented to the atmosphere. The cap is constructed of an impermeable clay layer, a transmission layer, a venting system for gases, and a stone armor layer that resembles the Maine shoreline.
TRC managed the construction in a phased remedial approach that included site preparation, dredging, and capping. Equipment and materials were mobilized to the site, pilings installed in the river, and treatment buildings erected. Approximately 7,000 cubic yards of sediment were removed, treated, dewatered, and hauled to a landfill for disposal. Remaining sediment was entombed beneath a thick cap to permanently prevent migration of coal tar from sediment to surface water.
TRC was able to cost-effectively define the problem and significantly reduce the scope of remediation compared to other more expensive alternatives. Construction was completed in 5 months, and the City was able to hold their annual American Folk Festival.