Cleaning up Mountain Lake’s toxic past

The Presidio Trust is moving forward in cleaning up San Francisco’s toxic Mountain Lake, and they’ve definitely got their work cut out for them.

Mountain Lake sits just east of where Park Presidio Boulevard — Highway 1 — enters the Presidio at Lake Street.

After testing samples taken from sediment layers under the 2,000-year-old lake, the Trust found high levels of toxic chemicals like lead, copper, and chromium. Researchers found that, in the 1930s after the Golden Gate Bridge was built, drains from the roadway connecting to the bridge were directed into the 4-acre lake.

Michael Boland, chief planning projects and programs officer for the Presidio Trust, told the Richmond Review:

“Every time there was a storm it would wash the heavy metals, from the lead in gasoline to chrome and copper from the tires, all were flowing right into the lake. They filled in about 40 percent of the lake.”

Boland believes the lake used to be around 30 feet deep, but now the deepest part of the lake is only a mere eight or nine feet.

As CalTrans finishes stabilizing nearby Highway 1 — expected to complete later this month — the Trust has been removing around 50 trees that currently surround the lake, which will eventually be replaced when the project is complete.

In December the Trust expects to begin dredging the lake. Genevieve Coyle, environmental remediation project manager at the Presidio Trust, told the Review that contaminated sediment will be pumped via pipeline from the lake to a “staging area” nearby, then chemically treated to purify it.

From there, the dirt will go to a landfill while the clean water will be returned to the lake. Currently, officials don’t know how long the process will take but estimate it may take as long as five months to complete.

Before the Trust can dredge the lake, though, they need to clear out invasive fish and turtles — mostly former pets — that now live in the toxic lake’s waters after being dumped there by their owners.

The goal is to restore native species of plants and animals to the lake and its immediate surroundings.

While researching the sediment layers, the Trust also uncovered the lake’s history and were able to identify when Spanish settlers arrived because of the introduction of non-native pollen in the stomachs of the cattle they brought with them.