Lakeside Landfill Conceptual Site Model Refinement

Lakeside is a closed landfill that is managed by DEQ under a settlement agreement with the former owner/operator. In the 2011 Record of Decision (ROD), DEQ determined that contaminated groundwater seepage to the Tualatin River posed potentially significant adverse effects. This was based on the assumption that groundwater discharged to the river with little or no reduction in contaminant strength. Proposed remedial elements included a costly hydraulic barrier consisting of 12 shoreline groundwater extraction wells to minimize leachate migration to the river.

DEQ brought in GSI to determine whether the assumption about groundwater contamination was valid. To evaluate concentrations of contaminants and evaluate direct toxicity at the point of exposure to benthic invertebrates, we designed and implemented three rounds of porewater and surface water sampling along the banks of the river. The third round included the use of nylon mesh diffusion samplers to collect shallow porewater and conduct ex situ porewater bioassays. GSI used concurrent groundwater, porewater, and surface water data to update an existing conceptual site model. Our analysis found that:

1. Contaminant concentrations in landfill leachate were significantly reduced before discharge to the river.
2. Remedial action levels generally were achieved within shallow porewater.
3. Exposure of bioassay test organisms to porewater had little to no impact on the organisms’ survival or reproduction.

Following these findings, DEQ issued a ROD amendment in May 2017, replacing the requirement for hydraulic containment through groundwater pumping with monitored natural attenuation, by far a more cost-effective solution.

GSI’s work involved:

• Investigating groundwater, porewater, and surface water contaminant concentrations and geochemistry.
• Using custom-made nylon mesh diffusion samplers to collect porewater in a tidally influenced environment.
• Using site porewater to conduct ex situ bioassay tests.
• Refining the existing conceptual site model and evaluation of the groundwater-to-porewater-to-surface water pathway.
• Analyzing data and finding that contaminants in groundwater were not impairing surface water quality or habitat for macroinvertebrates, leading to the conclusion that a costly hydraulic barrier would not be needed.