A discovery of toxic vapors in the middle of a construction site has prompted Stanford University to redesign portions of its new faculty-housing development on California Avenue.

The project, known us University Terrace, includes 180 homes and is one of two developments that Stanford is entitled to build under a 2005 agreement with the City of Palo Alto. In April 2014, the city gave its official approval to the project, which includes single-family homes, duplexes and condominiums.

Recently, however, the project at 1451-1601 California Ave. suffered an unexpected setback: the discovery of trichloroethylene (TCE) in the soil. The compound, most often used as a degreaser for industrial operations, has been associated with kidney, liver and cervix cancers, according to the Environmental Protection Agency.

Stanford alerted its faculty and the residents in the nearby College Terrace neighborhood in the fall. It also alerted the state Department of Toxic Substances Control, which is now reviewing the university's planned measures to address the discovery.

Jean McCown, Stanford's assistant vice president in the Office of Government and Community Relations, said the university has also submitted a "slightly revised subdivision map to Palo Alto to reflect the adjustment of the location of a small number of the homes, as recommended by our consultant."

The discovery of toxic chemicals is, in itself not surprising, given the site's location in the Stanford Research Park, the home of many industrial operations. In 2004, when Stanford performed its initial environmental assessment for the site, it identified low levels of toxic chemicals, including polychlorinated biphenyl (PCB) in the soil. More PCB was discovered at the 1601 California Ave. site in 2015, when Theranos' commercial lease for the property expired and the building slab was removed, allowing new soil tests. The contaminated soil was summarily removed, new materials were put in and the site was retested to ensure the chemicals are below the allowable level, according to the university.

The TCE discovery, however, presents a thornier challenge. The chemical can move as a vapor through soil and can pass through cracks and other openings in a building's structure, possibly affecting indoor air quality, according to Stanford. To address these impacts, the university has agreed to place 15 feet of clean soil over areas where elevated TCE levels were found, seal utility corridors to prevent vapor migration, put in vapor barriers under all homes and move homes away from areas where elevated TCE levels exist, according to the university, which created a "frequently asked questions" (FAQ) page to address the topic.

The plan was submitted to the Department of Toxic Substances Control, which would have to approve the mitigation. Stanford expects to hear back from the state agency this month or early 2016.

Meanwhile, College Terrace residents have been pursuing their own investigation on the topic. After Stanford alerted the residents last month about the TCE discovery, two members of the College Terrace Residents Association Board of Directors met with Stanford officials to discuss the findings of site analyses.

It was determined that the concentration of TCE was generally around 45 parts per billion (ppb) at 45 feet deep, well below the 130 ppb threshold used by the Water Quality Control Board to determine whether the chemical can get into indoor air. However, sampling at some areas had indicated a level significantly above 130 ppb, according to the neighborhood newsletter. Stanford hopes to mitigate these areas by covering them with up to 15 feet of clean soil to create what the university calls a "robust vertical buffer," according to the FAQ page.

"Over time, TCE naturally attenuates and the best and most effective way to deal with the unlikely event of vapor intrusion is to configure the project to create a buffer zone, place up to 15 feet of clean fill in areas with elevated TCE, and install protective vapor barrier systems underneath homes," according to the FAQ page.

Fred Balin, a College Terrace resident who wrote the update in the newsletter, said Stanford's actions and the recent investigations by College Terrace residents have suggested that the discovered TCE does not pose any danger to other parts of the neighborhood. Stanford, he said, appears to be doing the right thing in informing the public about its discoveries and in pursuing mitigations. At the same time, he and other residents want to make sure they remain informed, he said.

"We're not panicking, but we want to get all the information we can in terms of our neighborhood," Balin told the Weekly.

Palo Alto officials are also now in the process of reviewing Stanford's response. The city's Current Planning Manager Jodie Gerhardt told the Weekly that Stanford submitted on Wednesday a proposed amendment to the subdivision map. Staff will be reviewing the amendment to make sure the application remains consistent with the city's zoning code.

Under local law, amendments to parcel maps can be approved by the planning director and a city engineer without requiring new public hearings, provided that the revised project complies with the zoning code, remains consistent with the city's Comprehensive Plan, does not add any new lots, units or buildings and does not increase the subdivision's environmental impacts.

Stanford, for its part, does not expect the reviews to significantly delay the opening of the new homes. According to a webpage devoted to the project, the first homes are expected to open in the spring of 2017. The remaining homes would be completed by the second half of 2018.