DEL MAR — In mid-May, when the city purchased a 5.3-acre lot from Del Mar Union School District, it did so with a $5 million down payment and $3.5 million promissory note due within a year.

Having raised $5 million in about 12 months, the community group funding the purchase felt confident that given another year, it could solicit donations to pay the balance on the site that was once home to Del Mar Shores Elementary School.

But a basic lesson in grade-school math reveals the Campaign for Del Mar Shores has its work cut out. The first of seven $500,000 monthly payments is due in November, and only $50,000 has been raised in the two months since escrow closed.

“Multiply that out to May (2009) and it doesn’t get there,” Councilman Richard Earnest said. “It doesn’t even come close.”

“We are on a short leash here,” Councilwoman Crystal Crawford said, noting the city has options if fundraising efforts fall short. They include interim financing, selling a portion of the Shores property or swapping the water fund’s Balboa property.

“Planning for worst-case scenarios is just prudent,” Councilman Carl Hilliard said. “That’s what we should be doing in any event. The community shouldn’t think that the council is going to step up to the plate in terms of committing public resources, which are scarce.”

Barbara Mandel Pache, campaign coordinator for Campaign for Del Mar Shores, said the group is trying to get that message out.

“We take very seriously the commitment … to raise the additional amount,” Mandel Pache said. “We continue our efforts to generate awareness, interest and participation in the community.

“We need to continually explain that the city doesn’t have the resources with which to make those final payments,” she said. “For the community to have raised over $5 million within a year is phenomenal. We should celebrate that success … and not move into a fear factor of, ‘Oh my gosh, we’re going to fail.’

If the city is unable to make the payments on the note, the property — bounded by Camino del Mar, Ninth Street and Stratford Court — would revert back to the school district and the money paid at escrow would not be refunded.

“This would be a fabulous resource for the community and it would be a terrible, terrible shame if we didn’t have participation throughout the community.”

Representatives from Campaign for Del Mar Shores will provide monthly updates to City Council. To date, about 230 people have donated almost $5.3 million. Mandel Pache said the goal is 1,200 donors, or about half the households in Del Mar. If that goal is reached, participants would need to donate about $3,600 each to cover the outstanding debt.

As an incentive, Mandel Pache said the group is offering large donors naming rights for the park.