One Sunday afternoon in mid-October, Daniel Whalen was helping a neighbor saw a limb off a tree when the neighbor said something disturbing: state environmental officials had told him to stop drinking water from his well, and to stop cooking and showering with it, too.
Alarmed, Mr. Whalen called the state's Department of Environmental Conservation. He discovered that his own well, as well as several others in the area, was contaminated with methyl tertiary butyl ether, or M.T.B.E., a gasoline additive that is a possible cause of cancer and other health problems. The source appeared to be old leaks from underground storage tanks at four gas stations just up the hill, on Route 9G, state officials said.
Two months later, the number of wells identified as being contaminated in Mr. Whalen's working-class neighborhood, which is known as Greenbush, has grown to 123 from a handful. That makes this one of the largest contaminations of its kind in the state, according to Walter Hang, the president of Toxics Targeting, an Ithaca company that has analyzed government data on similar spills.
The state agency has installed carbon filters in all the homes where water tested above allowable levels for M.T.B.E., and has provided fresh drinking water from a truck to anyone else in the neighborhood who wants it.
About 200 people have been using the water, including some of those who have had filters installed, said Joe Coppola, a technician for Verizon who helped to organize the neighborhood after he heard about the problem in October.
But many residents have grown angry at the agency's failure to act sooner. Most found out about the contamination through newspaper articles or from neighborhood gossip in October or November, though the agency became aware of it in January. In addition, there had been about a dozen earlier incidents of M.T.B.E. contamination in the neighborhood during the last 15 years, said Kevin Hale, an engineering geologist for the agency.
''To know that I've had a river of M.T.B.E. flowing under my house for a decade is absurd,'' Mr. Coppola said.
Levels of contamination vary from the single digits per billion parts of water to 940 per billion, state officials said. It is difficult to say what the effects on health would be at those levels, said Claire Pospisil, a spokeswoman for the State Department of Health. The state standard for safe drinking water was recently changed from 50 parts per billion to 10 parts per billion.
In addition to homes and a handful of small retail businesses, the affected area contains a Roman Catholic elementary and middle school with 300 students. The school has been supplied with its own water truck, and a filtration system is being installed, said Jennifer Meicht, a spokeswoman for the conservation department.
Methyl tertiary butyl ether has been added to gasoline in many states since the 1970's to increase the octane rating and make the fuel burn more cleanly. But it is being phased out in New York State because of its health effects and its tendency to contaminate water, and will not be allowed after 2004. Unlike other components of gasoline, M.T.B.E. is water soluble and spreads quickly underground.
On Wednesday evening, roughly 200 Hyde Park residents attended a meeting convened by state officials in a middle school auditorium.
They listened patiently for about an hour as engineers pointed to maps showing the locations of the gas stations and the affected homes.
But after an official from the State Health Department began to explain that M.T.B.E. has caused cancer and other illnesses in laboratory animals, Mr. Coppola stood up and said, ''Basically what you're telling us is that we're lab animals.''
The crowd cheered. ''Enough is enough!'' another man shouted.
Assemblyman Joel Miller, who attended the meeting, and Hyde Park's town supervisor, Yancy McArthur, endorsed the residents' demand that their homes be linked to a water main owned by the town of Poughkeepsie. It is not clear how long linking would take or how much it would cost.
Marc Moran, the regional director for the conservation agency, said the agency would have notified residents sooner if it had been aware of the extent of the contamination.
The agency first heard of the problem in January, after a leak from underground tanks was reported at a Cenco gas station on Route 9G, Mr. Moran said. The owners of the gas station were ordered to remove a pile of contaminated soil and to perform tests to see if wells had been contaminated. They were slow to provide information, and finally the agency was forced to step in, he added.
The owners of the Cenco station declined to comment. But at the meeting Wednesday, they distributed a leaflet that said a scientist they hired had determined that residual petroleum at the site had leaked before they had begun operations there.
There have been more than 1,500 spills in New York State resulting in water or soil contamination with M.T.B.E., according to Ms. Meicht. The largest concentration of spills has been on Long Island.
Several well owners in other parts of the state filed a suit against the gasoline industry in State Supreme Court in January, and that suit has since been consolidated with a national class action on behalf of well owners, said Lewis J. Saul, one of the lawyers handling the case. Mr. Saul attended last week's evening meeting in Hyde Park, and has held discussions with residents, he said.
The residents in Hyde Park were particularly concerned about evidence of earlier spills, which suggested that they had been exposed to the additive for many years.
Christine M. Molloy, who lives in Greenbush with her husband and three children, said her water began to smell like gasoline in May of 1998. She called the conservation agency, which sent engineers who confirmed that there was M.T.B.E. in the Molloys' water and installed a filtration system in the family's basement.
In 1985, state officials found the additive near the home of a neighborhood woman at the extraordinary level of 100,000 parts per billion, they said. The woman, Mary Curcio, has moved, and could not be reached for comment.
To several local people, the episode is a threat to property values as well as to their health.
''We just bought our house, paid over $100,000 for it,'' said John O'Connell, a groundskeeper for the Hyde Park school district who lives on West Dorsey Lane with his wife and four children. ''Now it's not worth spit because of the M.T.B.E. in the well.''