The U.S. Environmental Protection Agency plans to strengthen cleanup regulations for airborne toxins along a contaminated area covering much of Mountain View's North Bayshore.

In a public meeting last month, EPA officials presented plans to update their cleanup strategy for the Teledyne/Spectra Physics Superfund site, which extends north of the city's Rex Manor neighborhood up into the western side of North Bayshore.

Like Mountain View's other contaminated areas, underground aquifers along the Teledyne site are polluted with trichloroethylene (TCE), an industrial degreaser used in the area's bygone semiconductor industry. Since the early 1990s, the companies responsible for the pollution have been tasked with treating the groundwater to remove traces of TCE, which is known to cause cancer.

In recent years, EPA officials have acknowledged TCE can also present public health risks if it evaporates and becomes airborne. These airborne toxins are considered particularly harmful if they accumulate inside buildings, especially homes or offices where people could be spending prolonged periods of time. Pregnant women, particularly during the first trimester, are considered especially vulnerable to even short-term exposure.

This so-called vapor intrusion would be a new focus for EPA officials, but they pointed out they have already been monitoring it. Since around 2015, about 45 homes in the area have been sampled, some of which needed fixes to their ventilation systems to ensure harmful compounds weren't building up, said Angela Sandoval, EPA project manager. In some cases, homes were found to have cracks in their foundation slabs, allowing the contaminated vapors to seep inside.

Previously, EPA officials were consulted by the city of Mountain View whenever new construction or remodel projects were proposed within the Superfund zone. EPA officials would lend advice on how to reduce exposure risk, although this step was never explicitly included in the Superfund guidelines. Under the new cleanup standards, this vapor intrusion review would be formalized as part of the cleanup plan.

The updated cleanup plan would also emphasize bioremediation, which involves injecting fortified microbes into the groundwater to break down hazardous compounds into harmless byproducts. EPA officials said past trials of bioremediation in the North Bayshore area showed a dramatic reduction in TCE, going from 300 parts-per-billion to four parts-per-billion within a few years.

For about 20 years, the cleanup effort had instead focused on pumping and treating the groundwater, but this was later found to be largely ineffective. Simply leaving the pollution in the ground and letting it naturally break down was found to be just as effective, according to EPA officials.

Mountain View's municipal drinking water comes mainly from Hetch Hetchy and does not draw upon local groundwater.

Sandoval emphasized that the EPA cleanup plan was showing real promise to someday restore the groundwater, possibly someday bringing it to federal drinking water standards.

"Our cleanup plan has the potential to reduce the cleanup time frame from hundreds of years to decades," she said. "These remedies have been proven to be very effective."

More information on the new proposed guidelines can be found at the EPA website.