Formatted Title:

Assessing the Effectiveness of the Completed Lower Fox River Remedy: Updated Data through 2022

Background/Objectives:

The Lower Fox River (LFR) extends 39 miles from the outlet of Lake Winnebago over a series of locks and dams northward to the mouth of the river, where it discharges into Green Bay/Lake Michigan. Polychlorinated biphenyls (PCB) were discovered in the LFR in the 1970s. The regulatory process divided the site into five operable units (OUs) beginning at the outlet of Lake Winnebago, including Little Lake Butte des Morts (OU1), proceeding through the Lower Fox River (OUs 2 to 4), and ending at Green Bay (OU5). Separate Records of Decision (RODs) were issued to address legacy PCB contamination in sediments for OU1 and OUs 2 to 5. The initial RODs required extensive dredging throughout OUs 1, 3 and 4, but were subsequently amended following a comprehensive pre-design investigation and initial design and construction activities to incorporate more cost-effective and environmentally protective engineered caps and sand covers. The ROD Amendments also incorporated post-dredge sand covering to manage residuals, which otherwise would have limited the effectiveness of the remedy. The primary RA over most of OU2 and OU5 is monitored natural recovery (MNR). Remedial construction activities in OU1, OU2/3, and OU4/5 were completed in 2009, 2011, and 2020, respectively.

Approach/Activities:

A site-wide baseline monitoring program, including sediment, water column, and fish tissue sampling, was conducted in 2006-2007, and post-construction monitoring has been performed to evaluate remedy effectiveness. Progress towards achieving the risk reduction objectives in the LFR is assessed through monitoring of the individual pathways of fish tissue (human health, ecological and young-of-year), surface water, and MNR sediment quality. Multiple rounds of long-term monitoring (LTM) have been conducted for OUs 1 to 3, along with upstream background LTM in Lake Winnebago (LW). The first post-baseline round of LTM for OU4/5 was conducted in 2021. The first full round of LTM, encompassing all OUs (LW, OU1, OU2, OU3, OU4, and OU5) in a single event, was completed in 2022.

Results/Lessons Learned:

Achievement of LTM objectives is determined through statistically established exit criteria and supported by a decision framework including a weight-of-evidence evaluation. Surficial sediment samples in OU2 and OU5 support a conclusion that MNR of surface sediments is occurring. Substantial progress towards achieving the surface water and fish tissue exit criteria in OUs 1 to 4 has been demonstrated during these LTM events. OU4 surface water concentration met the 90% SWAC reduction criteria in 2021 and 2022. Based on the positive results collected during all LTM events, as well as the baseline event, young-of-year fish tissue monitoring was discontinued in OU1 and OU4, and ecological fish tissue monitoring was discontinued in OUs 1 to 5 after the 2022 LTM event. While, as anticipated, surface water recovery has been more rapid than fish tissue recovery, generally decreasing trends in OUs 1 to 3 are projected to continue and indicate that the remedy, as amended, was effective, and that in certain OUs either baseline, background or risk-based targets are likely to be met within the 30-year period envisioned in the ROD. Although post-remedial monitoring in OU4 and OU5 has just begun (2021 and 2022), fish and water quality data indicate that they will also achieve these goals within the 30-year timeframe. Results and trends for OUs 1 to 5 for baseline through 2022 will be provided.

Paul Montney

Affiliation

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Assessing the Effectiveness of the Completed Lower Fox River Remedy: Updated Data through 2022