Soil Vapor Monitoring at NEC’s Ninth Street Property Points to Next Steps in Cleanup

by Jennifer Kalt 

Results from sampling in November are the latest in the NEC’s multi-year effort to clean up contamination at its former headquarters near the Arcata Plaza.

SHN Staff Geologist Roger Klakken drills the first boring to place soil-vapor monitors along I Street in Arcata. Photo by J. Kalt, Nov. 18, 2020.

On Nov. 18 and 19, NEC’s consultants (SHN Geologists & Engineers, Inc. of Eureka) conducted the first sampling effort of a grant awarded by the State Water Resources Control Board in 2018. The $607,714 grant from the Site Cleanup Subaccount Program will enable a thorough remediation of NEC’s Ninth Street parcel. 

The contamination stems from a dry cleaning business located on the property until sometime in the 1980s. Perchloroethylene (also known as “perc” or PCE) is a persistent contaminant that is often difficult and expensive to remediate, since its density causes it to sink into groundwater. During the tenure of the business, PCE leaked into the soil and groundwater below. The owners of the dry cleaners are deceased, leaving the NEC responsible for the cleanup. 

The site of NEC’s former headquarters on Ninth Street has remained vacant since 2001, when a fire destroyed the building. Soil and groundwater contamination from a former dry cleaning business was discovered while NEC made plans to rebuild. Photo by J. Kalt, Nov. 18, 2020.

SHN placed 34 soil-gas vapor samplers into borings 3’ below the surface for five days. Half were placed on NEC’s parcel, which has been vacant since the building burned down in 2001. Another 17 samplers were placed around the perimeter to identify and measure off-site movement. The samples were analyzed for various standard Volatile Organic Compounds (VOCs) and Semi-VOCs, including PCE.

The results show that PCE concentrations are highest in the central and western portion of the NEC parcel. Results also show high PCE concentrations over a larger area, with potential offsite migration primarily to the west toward “I” Street and Jolly Giant Creek, a Humboldt Bay tributary. The plume appears to be parallel to the PCE plume from another former dry cleaner one block away at 10th and H Streets. The owners of that site are currently developing cleanup plans. 

SHN assessed the results of the soil-gas investigation along with historical data to evaluate proposed locations for soil-gas monitoring wells, soil sampling, and indoor/outdoor ambient air samples. 

Boring locations in Bret Harte Alley, which is crisscrossed with underground pipelines and power lines. Photo by J. Kalt, Nov. 18, 2020.

These results will help determine the best locations for installing groundwater monitoring wells. Laboratory studies using soil samples from the site will identify the best remediation agent. In addition, two rounds of indoor and outdoor air samples will be analyzed. Each step will be done in close consultation with the Regional Water Quality Control Board, which will make recommendations on cleanup options, such as soil excavation, soil vapor extraction, or injection with a remediation agent. It may take several years to fully remediate the site before any plans can be made to rebuild or sell the property.

Thanks to the State Water Resources Control Board for awarding funding from the Site Cleanup Subaccount Program, a grant program established by the State legislature in 2014 to clean up contaminated sites where those responsible for the contamination cannot be held accountable. 

More information, including the sampling results, maps, and historical data, are posted on the Regional Board’s GeoTracker website at

For details on previous cleanup efforts at the site, see State Grant Awarded to Clean Up NEC’s 9th Street Parcel in the June/July 2018 issue.


Jennifer Kalt is the Director of Humboldt Baykeeper and has been a member of NEC’s Ninth Street Committee for many years.