Hanover — A probable carcinogen that leaked from Dartmouth College’s one-time burial ground for lab animals has spread to a nearby stream, marking the first time the substance has been detected in surface water.

The level of contamination college contractors measured last month was well below the state limit for water quality.

The chemical compound — 1,4-dioxane — is presumed to have migrated underground from Rennie Farm, the site off Hanover Center Road where Dartmouth’s medical school in the 1960s and ’70s buried thousands of pounds of carcasses involved in radioactive experiments.

Last fall, the same compound was found in a family’s private well at levels that were twice the state standard. The stream where it was detected last month runs behind that same property.

“If you have concerns about your well water, we would be happy to arrange sampling and testing for 1,4-dioxane, even if your well has been tested previously,” Maureen O’Leary, Dartmouth’s director of environmental health and safety, said in an email sent to neighbors on Wednesday afternoon.

The 1,4-dioxane found at Rennie Farm likely was a component in chemical solvents; the Environmental Protection Agency classifies it as a probable human carcinogen.

O’Leary said in her email that contractors on Aug. 18 tested six places along a stream that feeds into Hewes Brook, a tributary to the Connecticut River.

Results at one location, which the college called Stream-3, showed levels of 1,4-dioxane at roughly one-sixth the state standard for water quality, which is 3 parts per billion. The other five tests were negative — that is to say, below the detection limit of 0.25 ppb.

The college received these results on Aug. 25, O’Leary’s email said. The following day, Dartmouth collected six more samples along the stream — all of which came back negative.

Going forward, O’Leary said, college officials are expediting the design and installation of a pump-and-treat system that, as part of their remediation plan, would siphon away contaminated water from the Rennie Farm and the surrounding area.

O’Leary said the school also planned to monitor an expanded number of wells on a more frequent basis: every month, instead of every other month.

Asked on what schedule Dartmouth would expedite the treatment system, college spokeswoman Diana Lawrence said O’Leary was referring to “the currently expedited effort to install the groundwater extraction system; the system will be installed as rapidly as possible.”

“Our goal is to have the system operating at the end of the year,” Lawrence said in an email. “The system is very complex and requires numerous permits, highly specialized equipment, and the construction of infrastructure at the site.”

Lawrence added that the system would require approval from the New Hampshire Department of Environmental Services, the state agency overseeing the cleanup.

Water regulators at NH-DES could not immediately be reached for comment Wednesday.

The apparent spread of the 1,4-dioxane comes a week after Dartmouth finished re-excavating the dump site, after an 2011 effort to remove contaminated carcasses was found this summer to have been incomplete.

A group of neighbors from the surrounding area have been pressuring Dartmouth to expand the cleanup.

Residents will have a chance to ask questions and hear more about the water treatment plan on Tuesday at 4 p.m., when college officials and their environmental contractor have scheduled a visit to the site.

Rob Wolfe can be reached at rwolfe@vnews.com or 603-727-3242.

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