CLIENT King County Water & Land Resource Division
Location SeaTac, WA
The Des Moines Creek Regional Detention/ Retention Facility (RDF) is intended to provide stormwater storage, reduce peak flows and erosion, enhance habitat, and protect the Des Moines Creek Watershed.
During RDF’s initial design, the team identified elevated levels of arsenic in site wetland soils proposed for excavation. The Department of Ecology raised concerns about the potential impact of arsenic-bearing soils releasing arsenic to the surface water of Des Moines Creek.
The RDF design needed revision to address these potential impacts, which could have caused delays to the original construction schedule. King County called on Shannon & Wilson to incorporate a liner into the RDF design as a remedial measure. Working with design team members, we developed a creative solution that included reactive iron layer technology to inhibit the release of arsenic.
The liner was installed as a mitigation measure to protect against potential impacts to the environment. The project design required intensive agency coordination, treatability studies, dewatering, remedial design, and construction oversight. To maintain the original construction schedule, remedial design occurred concurrently with regulatory negotiations and treatability testing. Shannon & Wilson completed the design, studies, and regulatory discussions within six months, and the project met the original schedule. To date, there has been no reported issue of arsenic leaching to surface water.