Water Supply at Risk if Mammoth Erupts

Although no one knows exactly when it will happen or precisely how severe it will be, the volcanic eruption geologists believe is building in the Eastern Sierra near Mammoth Lakes could seriously jeopardize thirsty Los Angeles’ largest source of water.

Two-thirds of all the water used in the nation’s second-largest city comes from a far-flung network of streams, reservoirs and aqueducts that stretches for hundreds of miles down the backbone of the Sierra Nevada.

A recent swarm of earthquakes near Mammoth Lakes, believed to be caused by magma fracturing rock or pressurizing liquid four miles underground, has heightened concern that it is only a matter of time before the region experiences another violent episode in its long and tumultuous volcanic history. And that raises troubling questions about the potential impact of another eruption on the city’s chief source of water.

“We know it would be bad,” said Winston K. Wu, a geotechnical engineer with the Los Angeles Department of Water and Power. “Our water supply up there will not be usable depending on the nature of the eruption.”

But when and precisely where such an eruption will occur is uncertain. “The magma is still five to six kilometers below the surface,” Wu said. “You just don’t know. It could be years, decades, centuries away.”

Geothermal activity along Hot Creek south and east of Mammoth Lakes has been causing water quality problems for years by introducing arsenic and other chemicals into the water supply. Extensive treatment is required at several points downstream to reduce the level of the toxic element in the city’s drinking water.

Despite mounting evidence of seismic unrest and volcanic precursors in the area, including a more than 2-foot rise in the earth’s surface during the past 18 years, it was only last week that DWP geologists recommended forming a committee to update the department’s contingency plan “in regards to this ongoing threat of a volcanic eruption.”

In fact, Assistant General Manager James F. Wickser acknowledged that the DWP’s emergency plan for the Owens Valley anticipates a major earthquake, but not a volcanic event. He promised that the plan will be revised.

“Clearly the volcanic threat was not on the horizon in the early 1970s,” said Wickser, who is in charge of the city’s water system. “It could have dramatic effects, far more pronounced than an earthquake.”

After a magnitude 4.9 earthquake struck the Mammoth Lakes area Nov. 30--the strongest of thousands of quakes since the summer--U.S. Geological Survey scientists came close to declaring a “yellow volcano watch” for the region.

Such a watch is the second level on a four-step, color-coded system designed to alert the public and government agencies of changing conditions and hazards leading to a volcanic eruption.

Developing an Emergency Plan

The earthquake activity decreased dramatically in the past week to about 850 quakes, most small, so the condition remains “green” with no immediate risk of an eruption.

On Dec. 1, the day after the 4.9 quake, a DWP geologist asked the head of the U.S. Geological Survey team monitoring volcanic activity in the Eastern Sierra to alert the DWP of any changes that might indicate a serious threat of an eruption.

The head of the team, David Hill, said he would be too busy to call but asked for a fax number so the Geological Survey can send information to concerned agencies.

Asked whether the DWP should develop an emergency plan in the event of a volcanic eruption, Hill said: “It seems like a prudent thing to do.”

Using some of the most advanced technology available, Hill’s team of geologists, volcanologists and other scientists closely monitor changing conditions in and around the Long Valley Caldera, a huge crater-like basin formed 760,000 years ago by a violent volcanic eruption so huge that vast areas of the West and Midwest were buried in falling ash.

The Long Valley eruption spewed forth about 2,000 times more material than the Mt. St. Helens volcano did in 1980 and created the 20-mile-wide and 10-mile-long caldera. No one foresees a repetition of that catastrophic event.

Instead, Hill’s team expects future volcanic activity in the caldera to mirror the pattern found in “recent” geologic time.

There have been 20 mostly explosive volcanic eruptions in the area during the past 5,000 years, the most recent occurring on Paoha Island in Mono Lake about 250 years ago.

“The probability of an eruption in any given year is one in a few hundred,” Hill said. “In the long term, we can expect to see more small to moderate eruptions somewhere along the Inyo-Mono chain” of volcanoes that stretches from Mammoth Mountain to Mono Lake.

The latest period of increased volcanic activity began just before the area was rocked by four magnitude-6 earthquakes in 1980. Since then, sensitive instruments have recorded that magma pushing upward has caused a 28-inch rise, known as the Resurgent Dome, in the surface of the earth in part of the caldera.

Key Facilities Found in Caldera

Within the caldera are some of the most important facilities in the city’s water system. The Long Valley Reservoir, better known as Crowley Lake, is the largest storage point on the system that captures runoff from the Eastern Sierra.

An earth-filled dam holds back the water until it is sent down the Owens Valley and into twin aqueducts that serve Los Angeles.

On the caldera’s northern edge is the end of an 11-mile-long concrete tunnel built by DWP in the 1930s to divert water from streams feeding Mono Lake. The tunnel runs underneath the Mono Craters, where the last volcanic eruption occurred about 600 years ago. Water from the tunnel enters the Owens River and flows into Crowley Lake.

At the tunnel’s east portal, scientists have detected carbon dioxide associated with volcanic activity. They are planning an experiment nearby to test the effect on native vegetation of elevated carbon dioxide levels in the air, said U.S. Geological Survey hydrologist Chris Farrar.

High concentrations of the carbon dioxide in the soil have killed more than 100 acres of trees on Mammoth Mountain by interfering with root systems that take up nutrients and water.

Crisscrossing the caldera from west to east is Mammoth Creek, which flows from Mammoth Lakes, through an area of intense geothermal activity--boiling hot springs and steam vents--called Hot Creek before it joins the Owens River.

Farrar, who has extensively studied the caldera, said he has felt a number of earthquakes in the Hot Creek area. “There’s a lot of shaking going on there,” he said. “The earthquakes are being triggered by magma moving up into the crust.”

At Hot Creek, the mountain runoff picks up “arsenic and a whole range of chemicals commonly found in hydrothermal systems in volcanic areas,” Hill said.

Wickser said the area is “essentially the whole source” of arsenic in the city’s Eastern Sierra water supply. The presence of arsenic in the range of 25 ppb, half the drinking water standard, requires DWP to take steps to remove as much of it as possible before the water reaches customers in Los Angeles.

Advanced Filtration in San Fernando Valley

The water is treated at another reservoir down the Owens Valley and at the city’s advanced filtration plant in the San Fernando Valley, to lower the arsenic level to the range of 8 ppb, Wickser said. The DWP, other water agencies and the U.S. Environmental Protection Agency are jointly funding research into the potential health effects of low levels of arsenic in drinking water. Wickser believes the research could lead to a significantly tighter restriction on the amount of arsenic allowed to remain in drinking water.

“There is some concern about drinking water standards,” said Farrar, the hydrologist on the geological survey team.

The arsenic and other chemicals removed by DWP at the San Fernando Valley plant are so toxic that they must be trucked to a so-called Class 1 waste disposal site.

Altogether, about 30% of the 380,000 acre-feet to 400,000 acre-feet of water drawn by the city from the Eastern Sierra in a typical year passes through the Long Valley caldera.

It takes about 620,000 acre-feet of water in a typical year to meet the needs of the city’s residents, visitors and businesses. (An acre-foot is 326,000 gallons, enough to supply two families for a year).

In 1991, Santa Barbara geologist R. Forrest Hopson warned in a journal article that a volcanic eruption at Long Valley could have serious consequences for Los Angeles’ water supply.

Hopson said the extent of the damage to Los Angeles’ water supply will depend on the kind of eruption and the volume and type of material that is released.

Small to moderate volcanic eruptions generally send plumes of hot ash 30,000 to 50,000 feet into the air, where it is carried tens or hundreds of miles downwind. The abrasive, gritty ash is “a big nuisance” and can pose a health hazard to people with respiratory problems, Hill said.

“The impact depends a lot on where the vent tends to be, the wind direction and the time of year,” he said.

Ash Rain, Lava Flow to Streams

Volcanic eruptions can also produce pyroclastic flows, a mixture of ash and gas that travels at speeds of 100 mph or more. Hill said these tend to flow outward in narrow, tongue-like patterns from 2 to 5 miles from the volcano’s vent.

The Geological Survey’s chief scientist in the area said one of the greatest potential hazards would be an eruption in the winter that could produce a massive snowmelt and flow of water and volcanic debris, known as a lahar.

“An eruption anywhere in the caldera will tend to run down” to the caldera’s low point--Crowley Lake--Hill said.

That could pose a major problem for DWP. With enough advance warning of an eruption, Wickser said the department would immediately lower the level of the lake by sending water through the Crowley Lake Dam and down the Owens River.

“It would be real frightening if something happened to that dam,” he said. The lake has a capacity of 183,000 acre-feet of water.

Wickser said the impact on the city’s water supply would depend on the ash fall and the potential effect of any lava flow on water courses.

In the worst case, he said, the DWP would completely shut down the twin aqueducts to Los Angeles, spill water into Owens Lake and lose its largest and cheapest source of water.

The city would be forced to draw heavily on other sources, including more extensive ground water pumping in the San Fernando Valley, and use of far more costly water from the Colorado River and the State Water Project.

“Our basic contingency plan is to buy more water” from the Metropolitan Water District, said S. David Freeman, the DWP’s new general manager. “If you live in a Cadillac desert, you’re going to pay what it costs for water.”

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A Shaky Water Source

Though no one can predict when it will happen, a volcanic eruption in the Eastern Sierra near Mammoth Lakes would pose a serious threat to Los Angeles’ largest source of water. The Mammoth Lakes area, the most volcanically active region in California, has seen heightened seismic activity and a variety of volcanic precursors in the past 18 years.

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AQUEDUCT SYSTEM

A 338-mile network of streams, reservoirs and aqueducts carries runoff from Sierra Nevada. In a typical year, the Eastern Sierra supplies two-thirds of L.A.’s water, far more than is pumped from San Fernando Valley ground water, the Colorado River or Northern California through the State Water Project.

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LONG VALLEY CALDERA

The huge crater-like basin created by an enormous volcanic eruption 760,000 years ago.

EARTHQUAKES

Thousands of small earthquakes have occurred since early summer. Most have been caused by magma intensifying pressure on rocks far beneath the surface.

RESURGENT DOME

Magma pushing upward has lifted part of Long Valley caldera more than 2 feet since 1979.

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VOLCANIC CRATERS

The Inyo-Mono chain.

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11-MILE TUNNEL

Built by L.A. in 1930s inside Mono Craters to carry water south from streams that feed Mono Lake.

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HOT CREEK

Area of intense hydrothermal activity where steam and boiling water reach the surface. Chemicals commonly found in volcanic areas, including arsenic, enter city’s water supply here, requiring extensive treatment to reduce concentrations.

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CROWLEY LAKE

Largest storage point in L.A. water system; it’s also known as Long Valley Reservoir.

Sources: U.S. Geological Survey, Los Angeles Department of Water and Power.

Researched by JEFFREY L. RABIN / Los Angeles Times