The discovery of a cancer-causing chemical at a construction site last year prompted Stanford University to redesign its new faculty-housing development on California Avenue.

It also triggered concerns from residents in the adjacent College Terrace neighborhood about whether the underground toxins may have migrated toward their own homes.

The proposal by Stanford to move seven homes in the planned University Terrace development from "hot spot" areas where trichloroethylene (TCE) was discovered won the City Council's approval Tuesday night, despite requests from College Terrace residents that the project be put on hold until the issue is better understood.

The residents' concerns were heightened last month, after tests of 19 College Terrace homes showed a detectable level of TCE in six. In some cases, the contaminant was found in samples taken from crawl spaces or basements, suggesting that it was not caused by a chemical solution (such as a degreaser) used by the homeowner.

No one debate Tuesday the fact that TCE, which causes cancer and birth defects and generally travels in soil vapors, should be taken seriously. The main question that the council wrestled with was whether approving Stanford's revised map for the 180-home project would in any way hinder potential future efforts by College Terrace residents to protect themselves from TCE.

Cast in an unfamiliar role as soil scientists, council members concluded that it would not. To that end, the council voted to support Stanford's proposal to redesign its site map and added conditions calling for the university to equip the roughly 40 new homes closest to hot spots with further protections against toxic chemicals. This could be sub-slabs under the homes' foundation, a ventilation system or a depressurization system.

The decision was informed by extensive testimony by a team of scientists from the state Department of Toxic Substances Control (DTSC). Facing persistent questioning by the council, state officials assured council members that they are reviewing the report from the College Terrace residents about the neighborhood's TCE levels and will issue a response soon.

Whatever mediation is proposed for those sites, they said, will not change the situation at Stanford's construction site at 1601 California Ave. The state agency had concluded that the toxicity issues found at the construction site were sufficiently addressed by Stanford through the university's decision to reconfigure the development.

But some residents questioned whether this solution goes far enough. Lenny Siegel, executive director of the Center for Public Environmental Oversight, reviewed Stanford's risk assessment for the California Avenue site and concluded in a memo last year that the document "understates the risk from TCE vapor intrusion" and that the university's plan "does not sufficiently mitigate the risk of vapor intrusion."

The recent finding of TCE at the College Terrace homes added a fresh wrinkle. Though Stanford's and the state's analyses suggested that the chemical at the construction site would not travel to the adjacent neighborhood because the soil is predominantly clay rather than sand, Siegel argued in a June 26 memo that the assertion that TCE has not moved "is not borne out by the evidence."

The analysis commissioned by College Terrace residents "calls into question the finding that relocating the buildings on the project site is sufficient to meet regulatory requirements," Siegel wrote. The standard distance for relocating buildings away from elevated soil-gas levels is 100 feet, rather than the 50 feet proposed by Stanford, Siegel wrote.

Siegel recommended including in the new homes "sub-slab depressurization" systems that prevent vapors from entering homes by forcing vapors to migrate elsewhere. Such a system typically involves a horizontal pipe with perforation installed under a building, connected to vertical pipes above the roof line, and fans that divert the soil gas away from the residence.

Another option is a passive system (which, unlike an "active" system, does not need to be maintained by the homeowner) known as "sub-slab venting," which achieves the same goal as depressurization but relies on outdoor air pressure rather than fans.

Thus far, Stanford has been hesitant about adding the new systems, arguing that simply relocating the homes away from the toxic sites is sufficient and noting that the state agency did not require the barriers. By moving the planned homes, Stanford has taken a "conservative" approach toward making sure the faculty members living in the new houses would be unharmed by TCE, said Chris Wuthmann, Stanford's project manager for the University Terrace project.

"We'd prefer not to make it necessary for homeowners to have to check or maintain the system beneath the foundation slabs," Wuthmann told the council.

Siegel didn't fully buy this explanation.

"They said they don't want to do it because they don't want homeowners to check the manometer to see that it's working," Siegel said. "It's like saying people can't deal with methane detectors or smoke alarms. It's rather simple to deal with. It's a standard way of mitigating."

Councilman Cory Wolbach, siding with Siegel, pushed for Stanford to include these mitigations to the 40 homes that would be closest to the toxic site. After some debate, the council voted 7-to-1, with Councilman Eric Filseth dissenting and Councilman Tom DuBois recusing himself (his wife works at Stanford), to include Wolbach's proposal in its approval. The council then voted 8-to-0 to approve Stanford's map amendment for the housing project.

The decision came despite requests from College Terrace residents that the decision be delayed until the state toxics department responds to the neighborhood's findings. College Terrace resident Fred Balin said approving Stanford's amendment without waiting for the DTSC's analysis of the College Terrace data a "complete abdication of responsibility" on the city's part.

"Listen to what you have to tonight; get a sense of the project (but) under no circumstances approve it," Balin said.

But the council opted to move the project along after being reassured by state regulators that two toxic sites -- at College Terrace and University Terrace -- can and should be evaluated separately and that there is no reason to have the issue at the former hold up the construction at the latter.

Jovanne Villamater, hazardous substances engineer at the DTSC, said she does not expect the agency's response to the residents to have any effect on its approval of Stanford's plan for dealing with TCE at the construction site.

"If we were to find out that there is an impact on the neighborhood, we'd try to find a way to mitigate what's going on," Villamater said. "But I don't see it as impacting the decision that was already made."

The reassurances from state experts satisfied the council members, who openly acknowledged their lack of expertise on the topic of toxic soil vapors. Wolbach called the testimony from experts "an education for all of us."

Vice Mayor Greg Scharff, who made the motion to approve Stanford's request, concurred.

"This is difficult," he said. "None of us are experts in this area."

Ultimately, Scharff proposed granting Stanford the amendment to the project map, which the council had initially approved two years ago. The approval, he argued, "doesn't put the neighbors in any worse situation across the street."

Councilwoman Liz Kniss agreed and, citing the reassurances from the state agency, said the council should follow the recommendation of its planning staff and approve Stanford's redesign.

"This is troubling and I feel bad for the neighbors, but we have listened to an hour and half of testimony and it's pretty much stayed the same," she said.