Remediation of MGP-impacted Sediment in Ashuelot River
Weston & Sampson was retained by Eversource Energy (formerly Northeast Utilities Service Company) to design and oversee the remediation of manufactured gas plant (MGP)-impacted sediment in the Ashuelot River in Keene, New Hampshire. Coal tar was released to Mill Creek and, eventually, the river during a fire in the early 1900s. Tar-impacted sediment exists in the full extent of the creek and approximately 900 feet of the river. A thorough investigation of the chemical and toxicological state of the river sediments was conducted and remedial alternatives were evaluated under the New Hampshire Department of Environmental Services (NHDES) regulations to select the final remedial alternative of surface water diversion and visible tar-impacted sediment removal.
Weston & Sampson prepared all the necessary federal, state, and local permits for the project and designed the final remedy. A design report was submitted to NHDES outlining the proposed design and remedial approach. Numerous meetings with the Army Corps of Engineers, NHDES, and City of Keene representatives were held to develop the project and incorporate final details into the design. The selected remedy consisted of surface water diversion and excavation of tar-impacted sediment to a maximum depth of eight feet below the bottom of Mill Creek and the complete removal of tar-impacted sediment from the Ashuelot River. A total of approximately 40,000 tons of impacted sediment was removed for offsite treatment/disposal.
Surface water diversion techniques using sheet piling cofferdams were used to divert surface water flows. Sediment was removed to depths of eight feet in Mill Creek, and to depths of five feet in the Ashuelot River. A high-density polyethylene membrane barrier was used to cap the impacted sediment in areas ordinarily beyond the high water mark.
Due to high concentrations of cadmium in the sediment, Weston & Sampson designed and oversaw the stabilization of approximately 13,000 tons of sediment. These sediment management strategies saved the client approximately $1 million in hazardous waste disposal fees.
Weston & Sampson also designed groundwater dewatering/treatment systems, excavation support methods, and wetland restoration details.