The intense rainfall triggered the landslide, although pre-existing conditions such as a leaking water main and poor sewage flow likely contributed to the event.

LEAD: Pesticides, industrial chemicals and sewage flow into the harbors along America's coasts, often collecting and concentrating to pollute the sediment, according to a Government study.

But the sewage flow grew too much for the pits, leading to the installation of the pipes.

Besides the projected sewage flow, the size and characteristics of the watershed are the overriding design considerations for combined sewers.

The illegal connections add millions of gallons of rainwater to sewage flows, overwhelming treatment plants.

The required storage capacity of a holding tank depends upon two items: daily sewage flow, and available or optimal pumping service frequency.

"I want answers too, but it is important to recognize that sewage flow is not steady," Ms. Gelber said.

Raw sewage arriving from eastern Westchester join the combined sewage flow from the Yonkers system before it reaches the treatment plant, Mr. Zabinski said.

Insuring municipal compliance, they added, is neither easy nor cheap, and the degree to which illegal connections contribute to sewage flow is not clear.

In addition, many cities where storm water and sewage flow together to treatment plants would not face the same stringent controls that had been looming under the existing law.