SAN FRANCISCO (Map, News) - The reportedly slow response to cleaning up one of San Francisco’s last remaining natural lakes in the idyllic Presidio is drawing criticism by a supervisor who now wants signs installed alerting passers-by to the dangers.

A 2000 study by students on pollen levels in the area led to the discovery of “high concentrations of lead” and pesticides in Mountain Lake, according to Doug Kern, who sits on the Presidio Restoration Advisory Board.

A hearing on the lake’s toxicity before the Board of Supervisors Land Use Committee was called by Supervisor Michela Alioto-Pier to pressure state and federal officials to resolve clean it up, she said.

“I am worried about the fact that children and animals are still playing in the water,” said Alioto-Pier, who represents the neighborhoods surrounding the lake.

The District 2 official said she wants signs put up along the shoreline “saying that this is a toxic site” and closing it off to swimmers and pets, and said she would introduce legislation for the signs at today’s meeting of the full board.

Presidio Trust Executive Director Craig Middleton said that while there “there is significant lead in the lake” it’s not posing a “human health problem.”

“This is not a dire situation at the moment. It’s something we should clean up. It’s got ecological implications and some implications to animals and people who might get into the sediment and stir it up a little bit,” Middleton told the supervisors Monday.

Middleton said that he suspected the lake was contaminated prior to 1999, when the Army handed over the Presidio to the Trust; the Army, however, determined then that the lake was clean, he said.

Middleton said that run-off from Highway 1, which flows into the lake, likely caused the lead contamination, from when cars ran on lead-based gasoline.

The Trust has been in talks with the Army and Caltrans about funding mitigation and cleanup efforts, Middleton said. To prevent road runoff, it would cost about $2.5 million and cleaning up the lake would cost about $8 million, Kern said.

Alioto-Pier said she would conduct meetings with interested parties to ensure resources and plans are in place to assure the lake’s cleanup. Caltrans has agreed to be a part of those meetings, Chief Deputy District Director Dan McElhinney told board members.

“It really needs to be a political solution now. There is no doubt that there’s contamination,” Kern said. “We just have state and federal agencies going back and forth.”

jsabatini@examiner.com