Aspen Has a Hand in Urban Lake Restoration

Mountain Lake

Daniel Burgett of Clear Creek Systems watches over the lake’s filtering units.
Photo copyright Michael Macor, The Chronicle

Mountain Lake, one of the few natural lakes remaining in San Francisco, is located on the Presidio of San Francisco, adjacent to a popular City park.  The lake formed in dunes above the Pacific Ocean and, based on radiocarbon dating, is believed to be almost 2,000 years old.  Over the past 200 years the lake has undergone substantial change.

Aspen Environmental Group, under contract to The Presidio Trust, prepared the California Environmental Quality Act (CEQA) documentation needed for the Department of Toxic Substance Control (DTSC) to approve the Trust’s Remedial Action Plan (RAP) for cleaning up the lake.

Native species of frogs and turtles were long ago depleted from the lake, taken as meals for gold-rush era restaurant patrons.  Any that survived were overrun by non-native competitors introduced over the years.  Part of the lake was filled in the 1930s to build a road to the new Golden Gate Bridge.  Over decades, runoff from the busy roadway and a nearby golf course contributed sediment and contaminants to the lake bottom.  Neglect and the introduction of exotic species took their toll, leaving the lake and surroundings in a sorry state.  But a transformation is occurring.

Hydraulic dredging of lake sediment is underway in a phased operation expected to take five months to complete.  Removing 15,600 cubic yards of runoff- contaminated sediment containing lead, oil, and other compounds is a critical step in restoring the popular lake to its former state as a clean, healthy, and natural water body.   The effort has been described by the San Francisco Chronicle as “an experiment in urban landscape engineering the likes of which the city has never before seen.”

The sediment-water slurry is pumped to an upland site where the mixture is injected with chemicals to separate the water from the silt before being pumped into four 230-foot-long sack-like containers. These porous containers retain the sediment but allow water to leak out and collect in a pool, from where it is pumped into six 20,000-gallon filtering tanks. After going through sand filters and clay- and carbon-filled tanks, the clean water is returned to the lake.  When sufficiently dry, the sediment will be trucked to an approved landfill.

The ultimate objective is to restore the ancient lake ecosystem and reestablish as much of the area’s original ecology as possible. In removing the sediment, the 4-acre lake also will be deepened, reducing water temperature and enhancing aquatic habitat. After non-native fish and turtles are removed, native species will be reintroduced and the shallows and adjacent uplands replanted.