1009-1011 Gravenstein Highway North Removal Action Workplan

1009-1011 Gravenstein Highway North Removal Action Workplan

The Department of Toxic Substances Control (DTSC) has approved the 1009-1011 Gravenstein Highway North Removal Action Workplan (RAW). The subject RAW is associated with a residential development project located on approximately 6.1 acres situated between Gravenstein Highway and Hurlbut Avenue in the City of Sebastopol (Site). The Site, which is currently vacant, is located in an urbanized area that includes mixed commercial and residential development, a public charter school campus, a regional trail, and several mature trees. The RAW addresses the presence of arsenic, lead, and organochlorinated pesticides (OCPs) in soils at concentrations above residential screening levels or California Hazardous waste levels. It is anticipated that approximately 10,517 cubic yards, or approximately 15,775 tons (based on the assumption that a cubic yard of soil weighs approximately 1.5 tons), of material contains chemicals of potential concern (COPCs) above residential use screening at the Site. Remediation activities to be implemented as part of the RAW will include excavation of the impacted soil outside of the protected tree line (followed by confirmation soil sampling), on-Site burial and capping of impacted soils, groundwater depth monitoring, and incorporation of a deed restriction and land use covenant (LUC). The excavations to remove impacted soils are generally proposed to an approximate depth of two feet below ground surface (bgs), with additional excavation to four feet bgs to remove arsenic-impacted soil in certain areas of the Site. Confirmation soil sampling and analysis will be conducted to verify that cleanup criteria were met at the excavation bottom and perimeter. Excavation would continue until confirmation sampling shows that the cleanup criteria have been met. Once confirmation soil sampling is completed and the final volume of the impacted soil is determined, an on-site burial cell(s) will be excavated to match the volume of the impacted soil. Soil excavated from the on-site burial cells will be stockpiled and used as part of the clean soil cap. Once the burial cells are completed and the site improvements near the building are finalized, one (1) piezometer will be installed downgradient of each burial cell group location, for a total of two (2) piezometers. The piezometers will characterize the post-remediation depth to water below the Site. The depth to water will be monitored semi-annually for the first two years, and thereafter will be decreased to an annual gauging event. Results from the gauging events will be submitted by the Home Owners Association (HOA) as part of the annual Operation & Maintenance Plan reporting. A deed notification and LUC will be recorded in order to specify an on-site burial cell location(s) along with the areas within the driplines of protected trees where impacted soil is to remain above the site cleanup criteria. The LUC will ensure that impacted soil above the cleanup criteria that remains on-site will not be disturbed by installation of planned utilities and proposed building foundations in the future during redevelopment activities. The City of Sebastopol’s tree ordinance also prohibits tree removal or soil disturbance activities over these established, non-irrigated trees.

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