New York State authorities expressed strong reservations yesterday about a $15 million federal loan offer that could help them buy the homes of residents of the Love Canal toxic waste dump.

However, all sides remained optimistic for a settlement to allow the affected 700 families to leave the Niagara Falls neighborhood permanently.

"It's fantastic," said Barbara Quimby, an area resident, "I'm still in shock."

She said joy reigned at the Love Canal Homeowners Association headquarters when news of the loan offer arrived Thursday evening and persisted yesterday despite the state's initial cool response.

"We figure we've got another one or two weeks of fighting to go through," she said, "but we don't think we'll ever be back to where it's altogether unacceptable. We really have hopes this time that they're going to get us out of here."

The Carter administration offered the state a $15 million loan at 8.25 percent to be repaid over 30 years in equal annual installments. The state had wanted a $20 million interest-free loan that would be repaid only in the event of a favorable settelment of damage suits against the Hooker Chemical Co., which disposed of chemical wastes in the old Love Canal during the 1940s and 1950s.

"The terms are onerous," said William C. Hennessy, chief of the state Disaster Preparedness Commission. "We can't handle a loan of that nature . . . we can do better at a local bank."

But he said that the offer was still "a breakthrough . . . a very, very positive step" and said he remained hopeful of an eventual agreement.

The money would be used to buy about 500 homes that have plummeted in value since the discovery of the toxic waste dumpsite under them in 1978, preventing the residents from seelling them and moving out.

The residents, who had suffered since the 1960s from a high incidence of birth defects, miscarriages and chronic illness, were finally given federal help to move into hotels in May when the latest in a series of studies found that some of them had suffered chromosome damage.

But no one had regarded that as a solution. The state has spent about $35 million in capping the old dump, draining the area and buying 200 houses, Hennessy said, and had proposed making a landscaped park out of the area once it was cleaned up. The state's proposal June 18 to the Federal Emergency Management Administration (FEMA) would finance the park and the purchase of the remaining 500 homes as well, he said.

In response, however, FEMA offered the $15 million loan and suggested that the rest of the needed amount be obtained from a combination of the state's standing $5 million grant offer and $7 million lying unspent in Niagara Falls' federal Community Development Block Grant program. The total available thus would be $27 million.

"I don't really understand the offer," said Hennessy. "If they're saying we have to pay them $500,000 a year beginning Jan. 1, they're not really giving us anything . . . If it's negotiable and the repayment can be attached to a Hooker settlement, if it can be used in a productive way, then we can do business."

He said little of substance would be known until the middle of the coming week, "but I have great confidence we'll be able to get together now that the principle of federal participation has been established."

Officials at FEMA declined to comment on New York's initial response, pending official notification.