A report by an environmental consulting firm concluding that the Witherspoon Street site vacated by the University Medical Center of Princeton shows no evidence of soil or groundwater contamination was challenged Monday night at a special meeting of the Regional Planning Board. The study, carried out by Sovereign Consulting of Cherry Hill, also says that underground storage tanks at the former hospital site are not a major concern.

But an expert witness for the group Princeton Citizens for Sustainable Development, questioned by attorney Aaron Kleinbaum, said that the “due diligence” study carried out by Sovereign was not sufficient. The report examined records of underground storage tanks and the possibility of a septic system located under the parking garage, as well as asbestos in the empty building and hazardous materials on the site.

Allowing the developer AvalonBay to go ahead with its plan for a 280-unit apartment complex on the site without determining whether a septic system lies beneath the garage — which the Sovereign firm believes was either removed during construction of the garage or, if it exists, is now dormant — would not be the safest way to proceed, said James Peterson, who is president of Princeton Geoscience. “Septic issues still concern me,” he said. “Due diligence and a comprehensive site remediation report are two different things, with a very different approach.”

Mr. Peterson said that while the best time to have determined the existence of the septic tank was during the first phase of the investigation, it is still possible to delve further into the issue using hospital drawings and records that might show where septic tanks lie. “The lack of knowledge of the location of septic systems seems to me important,” he said. “If they’re unable to   find it, it’s not as if there’s no recourse. It’s very easy to conduct, and I would do that.”

The lengthy discussion, which included much comment and cross-examinations by AvalonBay senior vice president Ron Ladell, was the latest in a series that has the Planning Board trying to meet the December 15 deadline. Mr. Ladell has said the company is not willing to extend that deadline.

The next and final scheduled meeting on the proposal is tomorrow night. Should the Board decide it is not prepared to vote on the issue, it could be carried into 2013, which is when the current Board will be dissolved due to consolidation and a new one will be appointed. The Board’s attorney Gerald Muller has said that the Board can reject the proposal should AvalonBay refuse to grant an extension.

Board chair Wanda Gunning made time for members of the public who cannot attend tomorrow’s meeting to comment at Monday’s gathering. While much of the focus was on environmental issues, local residents also expressed their concerns about sustainability and design standards.

Architect Areta Pawlynsky drew enthusiastic applause for her brief power point presentation about the scope of the project. Showing the scale of the buildings as compared to existing houses in the neighborhood, she likened approval of the project as it stands to the famous and much maligned demolition of New York’s Penn Station in 1963. “This is not just an ordinary application,” she said. “This is our Penn Station moment.”

Harris Road resident Marco Gottardis, who has worked in hospital research laboratories, told the Board that standards today are much improved from those of the 1960’s and 1970’s. “There may be a contamination field that goes beyond the septic system,” he said, referring to waste from the hospital before stricter standards were in place.

Borough Council member Barbara Trelstad was the only citizen to speak in favor of the AvalonBay plan. “The hospital needs to sell the site now,” she said. “The chosen developer is before the Planning Board with a pliant application. It is smart growth. The questions raised tonight apply to any developer, and I think you need to bear that in mind,” adding that the project “provides affordable rental housing in our community.”

The Sovereign firm was hired last month to do an independent report on environmental documents related to the proposed complex. Kenneth Paul, a principal with the firm EcolScience, which AvalonBay hired to do its Phase 1 environmental report, testified that he is in full agreement with Sovereign’s conclusions. “Is there any evidence that the site is not suited [for the development]?,” Mr. Ladell asked him. Mr. Paul replied that there was not. “Are there any outstanding issues from an environmental point of view?,” Mr. Ladell continued. “There are not,” Mr. Paul said.

While the meeting was contentious at times, some who have issues with the AvalonBay plan came away feeling that some recognition of environmental concerns had been taken into account.

“Princeton Citizens for Sustainable Neighborhoods was pleased to see a thorough discussion at last night’s Planning Board meeting of what is the proper environmental remediation of the former hospital site before homes are built on it,” said Alexi Assmus, a member of the group, in an email. “We appreciate the public being given time to ask questions of the expert witnesses and applaud residents’ persistence in determining what testing has been performed to date, and their careful questioning of what the process will be to find possible contamination during construction. We thank AvalonBay for bringing their environmental experts to the evening meeting.”

Tomorrow night’s Planning Board meeting, at the Municipal Complex, begins at 7:30 p.m.