Mill Valley school’s initial health testing shows low risk

Preliminary test results from soil, soil vapor and water samples taken at Mill Valley Middle School indicate a minute health or cancer risk from long-term exposure to a potential contaminant.

The test results, performed by Ninyo and Moore, an environmental engineering firm with offices in the Bay Area, have been sent to the California Department of Toxic Substance Control and the Marin County Environmental Health Services for a two-month review period.

“The health and safety of our students and staff is our top priority,” Superintendent Elizabeth Kaufman said. “We are fortunate to have DTSC, the agency that is setting national standards for evaluating school sites, providing oversight and direction.”

The long-term exposure identified in the test results is 25 years, according to the report. That means that staff or teachers, who might work at the site for many years, would be the only ones who might have that minimal risk. Students, who would only stay for three years from sixth to eighth grades, would not be at risk.

The results also indicate that contamination from the school site’s former use as a city burn dump and municipal landfill is generally blocked by on-site construction, landscaping, asphalt and a 2- to 3-foot cap of sandy clay placed underneath the current school when it was built in 1969.

The school district is planning to build a new middle school, possibly at the same site as its current school at 425 Sycamore Ave. Other potential sites are also being investigated.

The testing at the current middle school site is called a preliminary endangerment assessment. A PEA is done to make sure that school sites that were previously contaminated have been cleaned up or mitigated to a level that protects the students and staff who will occupy the new school.

Investigators looking at health hazard or cancer risks to students based on an assumed exposure scenario of six hours a day, 180 days a year for three years. The three years refers to middle school, which covers sixth through eighth grades.

For teachers and staff, however, investigators based risk on an assumed exposure of eight hours a day, 250 days a year, for 25 years.

“Current regulatory guidance requires risk assessments to be conservative in nature and to overestimate potential health risks,” said Adrienne LaPierre, a principal toxicologist with Haley and Aldrich, a national environmental consultant firm that did not prepare the risk assessment. “This means that the actual risks are likely to be significantly lower than those calculated in a human health risk assessment report.”

In preliminary recommendations, the draft PEA suggests that the Mill Valley School District should conduct additional sampling of soil vapor to account for seasonal variations and prepare a response plan to ensure long-term protection for future school campus users.

The draft response plan would include the installation of vapor mitigation systems underneath all new buildings to prevent vapors in soil from entering any buildings.

“Constructing a gas-barrier membrane system under the foundation of new buildings – similar to what was installed at the Mill Valley Community Center – and conducting continued monitoring is a common practice at closed landfill sites,” said Greg Pirie, deputy director of the Marin County Environmental Health Service. “This helps ensure the long-term health of the public and the environment.”

The preliminary endangerment assessment is a separate process from the draft environmental impact report, which is still pending and expected for initial release this summer. However, the PEA is related to the draft environmental report and can inform the DEIR.

Earlier this year, the school district announced that the draft environmental impact report was being delayed to accommodate review of additional potential sites for the new middle school.

Members of MVMS Modernization Mess, a Mill Valley school parents group, said Tuesday they would decline comment on the preliminary endangerment assessment until they finish reviewing the actual test results, which have been posted on the district website.

The results are available online at shorturl.at/vDSMi.

For many months, the parents group has lobbied the school district to look at other locations for the middle school other than the current location due to contamination concerns from the former dump site, and also flooding, sea level rise and odors from the Sewerage Agency of Southern Marin, which is adjacent to the middle school campus.

The Mill Valley preliminary endangerment assessment process collecting over 200 samples of soil, soil vapor and groundwater during November and December 2024.

These samples were analyzed for chemicals typically associated with municipal landfills including metals, volatile organic compounds, polycyclic aromatic hydrocarbons, dioxins, organochlorine pesticides, polychlorinated biphenyls and asbestos and petroleum products. The analytical results were in line with expectations for former landfill sites.

Data from these collection efforts was used to complete a human health risk assessment, or HHRA. The human health risk assessment is a mathematical model that uses very conservative and health protective assumptions to calculate the potential increased cancer risks from carcinogenic chemicals and health hazards from non-carcinogenic chemicals.

Mill Valley School District serves about 2,400 students at the middle school and five elementary schools.