In an effort to keep pollution from reaching a major source of Southern California's drinking water, the Pacific Gas & Electric Co. has begun pumping 20,000 gallons per day of groundwater out of the Mojave Desert.
Laced with chromium 6, the groundwater has migrated to within about 125 feet of the Colorado River, said Lisa Anderson, an environmental engineer with the Metropolitan Water District (MWD) of Southern California.
The MWD operates the Colorado River Aqueduct a major source of Los Angeles' drinking water and MWD officials say a plume of at least 108 million gallons of tainted water is on course to reach the river at a point 42 miles upstream from intakes for both the MWD's aqueduct and the Central Arizona Project an agricultural and urban water delivery system.
Water from that area also goes to California's Imperial Irrigation District, Palo Verde Irrigation District, and Coachella Valley Water District.
Although no traces of chromium 6 have been found in the river, officials with the state Department of Toxic Substances said it is crucial to begin heading off the plume now.
The plume is coming from PG&E's Topock natural gas compressor station, located on the Arizona line, south of Needles.
The utility has used chromium 6 to control corrosion and mold in water cooling towers at the station, which helps move natural gas along a pipeline from Texas to Los Angeles. From 1951 to 1969, PG&E dumped untreated wastewater in nearby percolation beds.
Chromium 6 is a known carcinogen when inhaled, but scientists disagree over what danger it may pose in drinking water.
Anderson said chromium 6 levels in a monitoring well near the river have ranged from nondetectable to more than 100 parts per billion over the past few weeks, reaching 12,000 parts per billion in the mass of the plume.
She said the maximum legal level for all types of chromium in drinking water is 50 parts per billion.
PG&E engineers began pumping the groundwater out today and are transporting it by truck to a toxic waste dump.
Meanwhile, MWD officials are pushing for the state Department of Toxic Substances to also require that the utility construct a 2,000-foot-long, 150-foot-deep underground barrier between the leading edge of the plume and the river.
PG&E said it is evaluating the proposal.