Port of Tacoma Remedial Investigation and Feasibility Study

Slag from a former smelter was used as road ballast during the development of this former log yard, now owned by the Port of Tacoma.

In 1974, slag from a former smelter was used as road ballast during the development of this former log yard, now owned by the Port of Tacoma. The site was capped in the late 1980s to abate metals leaching from the slag and to protect water quality in adjacent Wapato Creek. Because of today’s more stringent water quality standards, the Port and former lessee Portac, Inc., entered into a new Agreed Order (AO) with the Washington State Department of Ecology to conduct an RI/FS that meets Model Toxics Control Act (MTCA) requirements.

As the common consultant to the Port and Portac, GSI designed and implemented four multimedia sampling events to evaluate the nature and extent of contaminants in soil, groundwater, porewater, sediment, and surface water in this tidally influenced environment. The results of the RI indicated that while the natural geologic and geochemical conditions of the site minimize the migration of arsenic in groundwater, seepage of rain water through the cap is resulting in groundwater that is in contact with slag. Some contaminated perched water reaches the creek via the existing stormwater conveyance system, which acts as a preferential pathway for contaminant migration.

The FS evaluates several remedial alternatives to enhance the cap and minimize added infiltration, prevent further migration of perched groundwater through the stormwater conveyance system, and enhance the long-term attenuation of arsenic in groundwater. The accelerated RI/FS was completed in 2 years; Public Review Drafts of the RI/FS reports were submitted to Ecology in February 2018.

Now that a Settlement Agreement has been reached between the Port and Portac, GSI continues to serve as a consultant to the Port and is working on several interim stormwater and groundwater management solutions to minimize migration of arsenic to surface water. GSI also prepared the Cleanup Action Plan (CAP) in 2020 to meet the Port’s site redevelopment objectives.

GSI’s work involved:

  • Working in accordance with MTCA requirements.
  • Designing and implementing a multimedia remedial investigation of soil, groundwater, sediment, porewater, and surface water.
  • Using custom-made nylon mesh diffusion samplers to collect porewater in a tidally influenced environment.
  • Developing specialized anoxic sample collection and arsenic testing techniques, including batch adsorptions tests, sequential extraction tests, and a study to characterize iron-arsenic precipitates.
  • Conducting a feasibility study on an accelerated schedule.
  • Designing stormwater and groundwater management and treatment options.
  • Preparing a Cleanup Action Plan consistent with the Port’s long term redevelopment plans and approved by Ecology.


Tacoma, Washington


  • Remedial investigation and feasibility study
  • Cleanup Action Plan
  • Passive porewater sampling
  • Specialized geochemical testing
  • MTCA compliance

GSI implemented a comprehensive RI, which illustrated that arsenic in groundwater is strongly attenuated under the geologic and geochemical conditions at the site.

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