Mill Valley Middle School PEA moves to field sampling; DTSC and Marin County have approved the work plan

The Mill Valley School District’s planned modernization of Mill Valley Middle School moved into the field‑investigation phase after consultants said state and local regulators signed off on the PEA work plan.

Ryan Bass, an environmental consultant with Ninu and Moore, told the district’s community meeting that the site has been referenced in public discussion as a “Superfund” site but “in reality it is not” and explained that the Department of Toxic Substances Control (DTSC) and Marin County Environmental Health Services (the local enforcement agency, LEA) will provide oversight for the investigation and any follow‑up measures.

Dwayne Blamer, director of environmental services for Ninu and Moore, described the three‑step PEA process: a Phase I historical review (largely bypassed here because documents already exist), the current field sampling stage, and a third step that would identify remediation or mitigation if needed. He said the team has received the LEA’s approval and “as of yesterday we just received DTSC’s final approval on the work plan,” clearing the way for field work to begin and for a public review period after the PEA report is completed.

The field program described by consultants is extensive: 44 soil borings with more than 170 discrete soil sampling locations; installation of seven soil‑vapor wells sampled across seasons; grab groundwater sampling; and surface‑water sampling from the creek that crosses the northern site. The analytical suite will include volatile organic compounds, California Title‑22 metals (including arsenic, chromium and lead), polycyclic aromatic hydrocarbons, dioxins, pesticides, PCBs and asbestos. Consultants said they will compare results to conservative residential screening levels used by DTSC and the regional water board; if screening levels are exceeded, a site‑specific health‑risk assessment will follow.

“The residential screening levels are very overly conservative,” the consultant said, explaining the district will nonetheless use those conservative benchmarks to design any mitigation and to protect the most sensitive potential receptors (children 0–6 years old and the elderly).

Consultants also said the PEA will include a conceptual site model, boring logs, cross‑sections showing the lateral and vertical extent of waste and fill, an assessment of exposure pathways, and a sea‑level‑rise analysis to evaluate how rising water could interact with contaminants.

Several public questions focused on monitoring and the long‑term safety plan for an active campus. Ryan Bass and Dwayne Blamer described continuous air monitoring around the work area, wind monitoring, and audible alarms tied to regulatory thresholds; if alarms are tripped, work is suspended until mitigation is in place. Consultants also said methane and other potential vapor issues will be addressed by designing site‑specific controls (for example, ventilation under slab systems) if required by sampling results.

The PEA report and any supporting remedial or post‑closure land‑use documents will be submitted to DTSC and the LEA for review and then released for public comment. The consultants stressed that disturbance of a landfill cap does not automatically change federal Superfund designation; remediation and any cap‑breach work would proceed under the California regulatory framework with DTSC and LEA oversight.

Next steps: implement the approved sampling plan, complete laboratory analyses, prepare the PEA report for regulator review, and open a public comment period before any remediation decisions are finalized.