Engineered Cap Chemical Isolation Layer Monitoring: The Lower Fox River

摘要

Background/Objectives. The Lower Fox River extends 39 miles from the outlet of Lake Winnebago over a series of locks and dams to the mouth of the river where it discharges into Green Bay. The Lower Fox River is the most industrialized river in Wisconsin; since the early 1900s, water quality has been degraded by expanding industries and communities discharging sewage and industrial wastes into the river. PCBs were discovered in the Lower Fox River in the 1970s. Due to their persistence in the environment, PCBs remain the focus of current remediation efforts in the river. The river and bay are divided into five operable units (OUs 1 through 5). Approach/Activities. OU3 extends 7 miles from Little Rapids Dam to the De Pere Dam. The Remedial Action (RA) in OU3 began in 2009 and was completed in 2011 and consisted of dredging and capping. Two Cap Types, A and B, were placed in OU3. The Type B Caps require long-term monitoring (LTM) of the chemical isolation layer (CIL) as part of the approved LTM. Cap design for OU3 follows USACE guidance and requires a “clean” CIL to retard movement of dissolved fraction PCBs from the underlying sediment up into the water column. The CIL selected for Type B Caps is a medium sand. Type B Cap design consists of a minimum 6 inch layer of medium sand covered by a minimum 4 inch layer of armor stone (1.5 inch minus material). As water provides a medium for continuing environmental transport to Lake Michigan and beyond, and fish represent a valuable ecological resource, the effectiveness of sediment remediation in the Fox River will ultimately be determined by a long-term reduction in the concentration of PCBs in water and fish. Properly functioning CILs in the capping system is therefore an integral part of the overall performance of the OU3 RA. The cap monitoring scope of work for the OU3 LTM includes sampling the CIL of Type B caps for PCB congeners at several locations to assess if a “clean” CIL is present. In addition, grain size analyses will be performed to assess correlation of PCB concentration to particle size distribution in the CIL samples. Divers will be deployed to remove the armor layer stone prior to CIL sample collection using coring technology. Results/Lessons Learned. This presentation will discuss the details of the LTM CIL sampling design and implementation, provide a brief overview of the Type B Cap design, and compare the OU3 2012 CIL sampling results to results from CIL samples collected at the time of Type B Cap placement in 2011 (prior to armor stone placement).