Formatted Title:

C-Street Slip Feasibility Study and Remedial Actions Options Report-Sediment, St. Louis River Area of Concern, Superior, Wisconsin

Background/Objectives:

In the most recent St. Louis River Area of Concern 2022 Remedial Action Plan, specific management actions were identified to remove beneficial use impairments (BUIs) and eventually delist the St. Louis River Area of Concern (SLRAOC). From the nine beneficial uses that were listed as impaired in the SLRAOC, three were identified as relating to the C-Street Slip as key risk drivers:

BUI No. 1 – Fish Consumption Advisories – mercury
BUI No. 4 – Degradation of Benthos – total PAHs and mercury
BUI No. 5 – Restrictions on Future Dredging – mercury, benzo(a)pyrene, naphthalene, arsenic

A Remedial Actions Options Report-Sediment (RAOR) was developed to evaluate the feasibility, effectiveness, and cost of remedial options for the C-Street Slip based on the findings from historic investigations and a 2022 AECOM supplemental site investigation. Remedial alternatives were evaluated and an alternative recommended to mitigate identified BUIs.

Approach/Activities:

Data from historic investigations, a pre-design investigation and the 2022 AECOM supplemental site investigation were evaluated for quality assurance and consolidated in an EQuIS Solutions database. The sediment is contaminated by PAHs, mercury, arsenic and to a lesser extent, volatile organic carbons (VOCs). Remedial target areas and data gaps were modeled using Earth Volumetric Studio version 2022.10.2 (EVS), and ArcGIS software (Esri, version 10.8.1) and ArcPro (Esri, version 2.9). The contaminated sediment results represent a three-dimensional (3-D) model of the analytes limited to below the bathymetric surfaces surveyed in July 2022. The 3-D plumes were generated with an interpolation process called kriging. Results were used in EVS volumetric tools to provide the volume of contaminated sediment in cubic yards (CY). The target sediment areas in the C-Street Slip were delineated in two dredge management units (DMUs); the MGP DMU which encompasses most of the PAH- and VOC-impacted sediments and the Mercury DMU which contains mercury-impacted sediments not related to MGP releases.

Results/Lessons Learned:

Considering the RAOs, remedy selection criteria, and data available at the time of RAOR development, the selected option was a hybrid dredging alternative. This option includes removing approximately 30,000 CY of sediment in the C-Street Slip with total PAH concentrations greater than the midpoint effect concentration (MEC) of 12 milligrams per kilogram (mg/kg) and/or mercury concentrations greater than 0.59 mg/kg. In addition, sediment with benzo(a)pyrene greater than industrial direct contact not-to-exceed concentrations and arsenic concentrations greater than Wisconsin soil background threshold values (BTVs) was targeted for removal. The cost estimate for the hybrid alternative is approximately $7,545,070 for dredging of both DMUs with an additional cost for installation of sheet-pile wall(s) at the head of C-Street Slip to stabilize infrastructure associated with a greater than 20-ft dredge prism. The project schedule plans for completion of sediment remedial design (RD) in 2024 with sediment remedial action (RA) following in 2025. In addition, the upland RA is in progress at the time of RAOR submittal.

Jen Stark

Affiliation

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(Group 1, Poster Board #49) C-Street Slip Feasibility Study and Remedial Actions Options Report-Sediment, St. Louis River Area of Concern, Superior, Wisconsin