Southern Californians who reside near the mountains, hillsides or canyons live with the risk of a wildfire burning down their homes.

But rarely do they think about a fire contaminating their drinking water. Perhaps they should.

Just ask residents of Ventura, Santa Paula and Ojai, who must boil their tap water before drinking it, cooking with it or brushing their teeth, the result of the powerful Thomas fire that knocked out portions of their water system Tuesday and grew to 96,000 acres by Thursday.

Related: What to do if you are under a boil water advisory near the Thomas fire in Ventura

The order affects more than 120,000 people who could become infected by bacteria causing intestinal illnesses if they drink tap water without boiling it first. The boil-water alert went into effect Tuesday and may last about a week or less, said Ventura Water spokesman Craig Jones.

Fire’s new threat: water supply

Possible contamination to the water supply in California communities slammed by wildfires is a growing concern. The breakdown of water systems — mostly occurring in smaller cities and among isolated, mutual water districts — is happening today and may become more common in the future as infrastructure ages and wildfires intensify due to global climate change.

“It speaks to the reality we live in, in Southern California. You have to be prepared for the worst,” said Bob Muir, spokesman for the Metropolitan Water District, which supplies about 45 percent of the water to Southern California.

While MWD has not had any problems with its pipes, pumps and local reservoirs during the recent firestorms, the same cannot be said of some smaller water providers.

A fire-related power outage shut off the pumps that moved water uphill to reservoirs in Ventura County. At least one reservoir went dry, causing a severe drop in water pressure. That created negative pressure, allowing the possibility of contaminants to be sucked into the system.

“Any time you depressurize the water system, there are contamination concerns,” said Jeff Densmore, district engineer for the Division of Drinking Water of the State Water Resources Control Board.

This can create a siphon-effect, whereby water from a garden hose attached to a dirty pool can reverse and flow into the city’s drinking water pipes, he explained.

The lack of pressure was made worse by firefighters pulling thousands of gallons per minute from fire hydrants, faster than the pumps could keep up, he said. “With no water pressure, we assume the worst. We tell customers to boil their water,” he said.

Water treatment plants affected

Things quickly got worse when the fire spread to edge of the Casitas Municipal Water District water treatment facility that serves the city. The staff were forced to evacuate, Densmore said. That meant chlorination of the water could not continue. Keeping treatment turned on could have caused a chlorine gas leak due to intense heat or flames, a disaster scenario of the worst kind.

“They didn’t feel comfortable leaving chlorine gas on line so they shut that down,” he said. “That left partially treated water.”

At a water pumping station near Ojai, another community threatened by the Santa Paula Fire, workers brought in generators after losing power. But the generator got so hot from the heat of the fire they stopped working, Densmore said.

Boil tap water

The state ordered a boil-water alert for a portion of Ventura early Tuesday, then broadened it citywide a few hours later, joining portions of Ojai and Santa Paula. On Wednesday, the state issued another boil-water alert to the beach communities of Rincoln, La Conchita and Seacliff, he said.

Jones said the city water department in cooperation with the local water districts were working on pumping enough water into their reservoirs. The workers were able to return to the plant. The city has brought in pumps powered by diesel generators to get water moving again, Jones said.

“We want the tanks (reservoirs) to come up to normal levels. We want to normalize chlorine levels. We are talking about microbiological contaminants,” Jones said on Wednesday afternoon.

E. Coli poses a threat

After there’s enough water stored, the city will take samples and send them to an independent laboratory that will test for Coliform bacteria, such as E. Coli, which can cause diarrhea, vomiting and stomach cramps and in severe cases, pneumonia and urinary tract infections. E. Coli is exclusively associated with fecal material.

Sampling can take two to three days. The samples must be clear of all Coliform bacteria and approved by the State Water Resources Control Board, Densmore said.

There have been no indications of E. Coli in the water in Ventura, he said, adding that the state is acting conservatively. “There is potential for that to happen, but they have to prove that didn’t happen,” Densmore said.

Backup power systems are key

How can water operators keep the flow going and the faucets turned on during a wildfire or after an earthquake knocks out power to its pumps?

“Our recommendation is to have standby power,” Densmore said. “You need to have a standby generation unit at your critical pump stations to keep water in your system.”

Los Angeles Department of Water and Power has not experienced any power outages as a result of the Skirball Fire, burning 475 acres in the community of Bel-Air, nor in the communities affected by the Creek Fire, burning through 12,605 acres as of Thursday morning. A power pole fell in Tujunga due to high winds but the blackout lasted only 30 seconds, said Marty Adams, chief operating officer.

Its water pumps are running normally on grid power, he said.

LADWP has multiple back-up systems at the ready, including internal combustion pumps that burn fuel and diesel generators that produce power. As a third backup, they can use the a Los Angeles City Fire Department pumper engine to close a breach in its water delivery system, he said.

Before the Santa Ana fires unleashed hell, Adams ordered city reservoirs filled to the top. “We intentionally topped of the tanks by manually overriding the system.” Adams said.

All reservoirs maintain a reserve for fighting fires, especially those in higher elevations where wildfires occur more frequently.

“It is a belt and suspenders approach,” Adams said.

Water agencies can run into problems when they rely on electrical power to pump most of their water, instead of gravity feed. But lower elevation communities that must pump water to uphill reservoirs may be more vulnerable to power outages due to natural disasters, Adams said.

Jones said the issue is not a lack of water infrastructure, as the city is doing everything it can to bring back clean water to the 113,000 Ventura residents.

“In this case the wind was so wild; it happened so quickly that they didn’t have time to react,” Densmore said.