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:
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:
Mesh porewater sampler collector