Significant Project Features
- Excavation of 2,750 CY of upland soil contaminated with MGP tar, petroleum, NAPL, and PAHs
- Mechanical dredging of 3,300 CY of sediment in the Seneca River contaminated with MGP tar, petroleum, NAPL, and PAHs
- Management and coordination of T&D activities for the off-site disposal of MGP-impacted soils, former MGP structures, and debris
- Installation of 21,820 square foot low land cover system of stone mix, canal fill, and 6-inches of cobble
- In-Situ stabilization of 4,500 CY of MGP-impacted soils
- Design, installation, operation and maintenance of a temporary 50-GPM wastewater treatment system to treat water during remediation activities
- Achieved over 8,600 safe workhours without a lost-time incident
History & Location Details
The Seneca Falls Former MGP Site is located in a mixed residential and commercial area of Seneca Falls Village. The site is bordered to the east by residential properties, to the south by the Seneca River and Canal, to the west by a gasoline filling station, and to the north by Fall Street. Sevenson’s scope of work under this contract required excavation and mechanical dredging of sediments contaminated with MGP tar, petroleum NAPL and PAHs from the Seneca River and impacted soil removal and insitu stabilization of impacted soils in the Upland area of the site.
Sevenson performed the required utility locates and mark outs, installed soil erosion and sediment controls, performed clearing and grubbing, and installed a temporary access road as required to access the low land areas due to the steep elevation. Site preparation also included construction of sediment dewatering pad, dredge pads, a load out platform and a temporary wastewater treatment plant. A turbidity curtain was installed to minimize resuspension and migration of suspended sediment during marine operations. Sevenson performed both bathymetric and topographic surveys. A bathymetric survey was conducted prior to dredging in order to determine the target depth within each area of the dredge cell.
The flow of the river is controlled by a lock system which is based on the level of Seneca Lake in Geneva, so during dredging operations, the river flowed anywhere from 100 CFS (cubic feet per second) to 1,600 CFS. The river, in this area, has a depth of about 25’-30’. The area Sevenson dredged was approximately 10’ to bedrock. Sevenson utilized land-based equipment for dredging 3,300 CY of sediment. A long-front excavator with an environmental bucket equipped with GPS technology was used to ensure the target depths were achieved.
In preparation for off-site disposal, Sevenson was required to amend the 3300 cubic yards of sediment placed into the dewatering pad by stabilizing with lime kiln dust. The sediment mixture was then loaded onto dump trucks provided by NYSEG and removed for offsite thermal treatment. Any water generated from excavation or remediation operations was transported to the WWTP which consisted of a trailer-mounted system capable of treating up to 50 gallons per minute. The treated water was discharged into the Seneca River.
Surface soil excavation was performed prior to the initiation of ISS activities in the upland area. Both surface and subsurface debris was encountered as this area contained many former manufactured gas plant remnants including, steel tanks, former foundations, asbestos covered pipe, etc. All of these obstructions were removed prior to ISS. A total of 2750 CY of MGP-impacted soils were excavated to depths ranging from 2 ft to 4 ft in the Upland area.
Sevenson was required to stabilize 4500 CY of subsurface soils in the upland area. To perform this work Sevenson set up a conventional batch plant consisting of (2) horizontal silos and a grout mix plant that was used to prepare and deliver the required reagent mix each day. Upon completion of the area, Sevenson used both an excavator with a conventional bucket for primary ISS mixing, and a second excavator with a transverse mixing head for finish mixing in each of the designated ISS cells. The Upland ISS area was divided into appropriately sized cells that enabled between 250-350 yards per day to be mixed on average. Upon completion of ISS operations, the excess swell material was excavated to design grade to allow for the installation of the final cover system and shipped off site to ESMI for thermal treatment.
Restoration began with the installation of a sediment cap in the river using clean fill to original grades. GPS technology was used for elevation control. Low land restoration consisted of the installation of fill, topsoil and riparian plantings. Upland backfill consisted of a granular fill with an 8-inch top of dense grade aggregate.