Expedited Design and Construction Completed in a Quarter of the Time

Issaquah is a Seattle-area suburb that provides drinking water to its customers using a combination of four groundwater wells and purchased regional surface water supplies. As part of U.S. Environmental Protection Agency’s Third Unregulated Contaminant Monitoring Rule, the City detected perfluorinated alkyl substances in its 946-litres-per-minute Well No. 4 supply, the primary compound being perfluorooctane sulfonate. 

In response to these detections, the City shut down Well No. 4 along with the adjacent, uncontaminated 4,353 litres-per-minute Well No. 5 to avoid PFAS migration. 

With summer quickly approaching, the City sought an expedited solution that would allow it to meet peak water demands. The City engaged HDR professionals to quickly evaluate a number of temporary and long-term treatment alternatives to eliminate the PFAS contamination from Well No. 4 so that both wells could be operating before water demands increased. 

To meet this schedule, we worked closely with city and state regulators to quickly design, permit and assist construction and startup of a treatment system. Features were installed so the system could be expanded to include Well No. 5 if that aquifer ever became contaminated. The installed system consists of two pressure filters operated in series, each filled with 9,072 kilograms of granular activated carbon.  

The time between City Council authorisation and sending granular activated carbon-treated water to distribution was only 77 days. The treatment system has successfully reduced perfluorooctane sulfonate and perfluorooctanoic acid concentrations to non-detectable levels in all treated water samples since startup.