Increased development in towns near Cambridge's two main reservoirs could threaten Boston's sorely depleted water supply by forcing Cambridge to draw on back-up reservoirs it shares with Boston, state officials said yesterday.

If a contamination hazard forces Cambridge to take water from the Quabbin reservoir, the Metropolitan District Commission (MDC) will be "hard-pressed" to supply water to all its customers, said M. Ilyas Bhatti, MDC's director of watershed management. Extremely low levels of rainfall this winter have already prompted state officials to declare a "water emergency" for Boston and 43 other communities served by the Quabbin.

Cambridge maintains independent reservoirs at Stony Brook and Hobbs Brook but has the right to use the Quabbin as a reserve source.

"If the Quabbin system continues to be stressed further because of the lack of precipitation, the MDC would find it very difficult to provide water to the partially supplied communities to bail them out in case of an emergency," said Bhatti. Most of greater Boston draws on the Quabbin and Wachusett, MDC's reservoirs.

The state Department of Environmental Quality Engineering (DEQE) and the Massachusetts Water Resources Authority (MWRA) declared a water emergency on February 16. Currently, the Quabbin reservoir is at 68 percent capacity, the lowest level in 15 years.

Several Cambridge officials have said they are concerned about the threat development in neighboring towns could pose to the city's water supply. Both reservoirs are located in other towns, and Cambridge has difficulty controlling development outside its jurisdiction.

At a talk sponsored by Harvard-Radcliffe Young Democrats last night, State Sen. Michael J. Barrett '70 (D-Cambridge) said that one of the primary health hazards is the high level of sodium in city water, caused by the runoff of salt-filled drainage from neighboring highways. City water currently contains 43 milligrams of sodium per liter, more than double the state-set limit.

A second problem is that developments in the nearby towns of Weston, Lincoln and Waltham are paving over much of the city's watershed, the area of land that drains into the reservoirs, Barrett said.

Barrett has proposed including Cambridge in the Watershed Protection Bill, which would set up a protected zone around water sources where development would be limited. Currently, the bill only protects MDC water.

"Five years from now, we will definitely have the worst drinking water in the surrounding area if we don't do something now," said Barrett.

In an interview after the meeting, Barrett said the shortage in the Quabbin reservoir demonstrates that protecting Cambridge water is in the interest of the entire state.

`Very Precious and Scarce'

"The drought is going to prove that Quabbin water is very precious and scarce," Barrett said.

But the author of the bill, State Rep. David B. Cohen (D-Newton) said that he does not approve of Barrett's amendments. Cohen said his bill was carefully designed for the MDC reservoirs, adding that he would probably support a separate bill to protect the Cambridge water system.

"I think the question is, `why does it belong with this bill,' which has gotten wide support from environmentalists," said Cohen. "Why does he want to get in on my bill?"