Sensors flagged high sewer gas readings near Tijuana River weeks before citation

Raymond Johnson recalls the stench near his home was especially bad early this summer.

“It makes you feel like you want to throw up, it makes you feel sick inside,” said Johnson, who bought his San Ysidro home next to the Tijuana River in 2008, unaware that a smell of sewage would increasingly plague the neighborhood in the coming years.

“A couple years after this problem started it should have been fixed,” he said.

As officials grapple with hundreds of complaints from residents about the pungent odor emanating through their neighborhoods caused by the binational sewage crisis, data shows that hydrogen sulfide readings in the South Bay have exceeded state limits on 15 days in the past year.

Nine of those days were recorded following the breakdown of the Hollister Wastewater Pump Station on June 16, when some 300,000 gallons of raw sewage dumped into the river. But it wasn’t until nearly the end of July, after receiving the residents’ complaints, that the San Diego County Air Pollution Control District cited the plant for the first time.

Infrastructure problems stemming from both countries have caused sewage spills along the U.S.-Mexico border for decades, and it’s been especially bad in recent years due to mechanical failures at wastewater treatment plants. Researchers have estimated that more than 100 billion gallons of toxic waste, among it untreated sewage, have flowed through the Tijuana River Valley over the last five years.

The pollution has cut off access for predominantly Latino beach communities — the main stretch of Imperial Beach, for example, has been closed for more than two years. A new wastewater treatment plant under construction in Baja California is expected to help reopen beaches.

One byproduct of high levels of sewage in the river is hydrogen sulfide, a colorless gas that smells of rotten eggs and is also known as sewer gas. Its health effects depend on how much of the gas someone breathes and for how long, ranging from headaches, nausea and loss of smell at lower concentrations to rapid unconsciousness and death at high concentrations.

The California Air Resources Board sets the nuisance limit at 30 parts per billion, the level where the odor is so foul it significantly impacts quality of life, but not health. The Occupational Safety and Health Administration sets the level that most people can smell at 10 parts per billion, but some people can smell it at much lower levels.

In the South Bay, readings have reached as high as 58 parts per billion this summer.

But the district is capturing readings in a small area, only collecting data at a fire station on San Ysidro Boulevard since September. Officials have yet to move three other sensors to their planned sites throughout the South Bay.

Before June, the sensors recorded just five days with levels exceeding the CARB limit: four in October, and one at the end of April, according to data obtained by a public records request.

One month before citing the Hollister pump station — and in the middle of the week with the most exceedance days recorded by the sensors — a district official discussed the high readings in an email to local officials, saying it didn’t have the authority to affect the root cause of the odor.

“The unfortunate side of ambient air monitoring is that it can be very passive — collect data over (usually) multiple years to help inform future strategies, health effects of pollution, etc.,” Community Air Monitoring Supervisor Kevin Bradley wrote in his June 27 email.

That’s because the exceedances alone are not sufficient to take action.

The air pollution district, which received some 150 odor complaints last month, is still investigating the breakdown of the Hollister plant. Officials confirmed they started the investigation due to the large number of residents’ complaints, not the high hydrogen sulfide readings.

“The state law essentially says those odors or that public nuisance has to affect a significant number of people,” said Jeff Lindberg, manager of the district support section in CARB’s enforcement division. “In enforcement we have to tie it back to people. We can’t tie it back to a number coming from a monitor.”

Lindberg said the sensors can still serve as additional evidence to support what the public is saying, but that their real value is as a diagnostic tool.

The sensors being used by the district are equipped to pick up meteorology measurements like wind speed, wind direction and humidity. Once all sensors are installed throughout the South Bay, the district can use them to determine the source of the gas.

Regardless of what data is collected, Lindberg said it is very hard to establish a nuisance without public complaints.

“When you start getting these spades of several hundred complaints, now you’ve really clearly, easily demonstrated that that’s affecting a significant number of people and that’s sort of what the district is looking for,” he said. “That’s why complaints are so important in the process.”

David Soderman, the San Diego air pollution district’s chief of monitoring, said the data is too complex to determine whether the high gas readings were caused by the plant breakdown. The district has yet to conduct a deeper analysis of the data.

“We’re still really trying to make sure that we get the sites up and running, and we have a limited number of staff available,” he said. “It’s resource-intensive to look at and actually do that type of analysis.”

While the stench of hydrogen sulfide may be disgusting, experts say there is no evidence anything close to the levels recorded by the sensors is toxic to human health.

“In enforcement we have to tie it back to people. We can’t tie it back to a number coming from a monitor.”

Jeff Lindberg, CARB

The state’s 30-parts-per-billion standard was set in the 1980s to determine when the smell of the gas is so bad it begins to impact quality of life, not health. Dr. Tee Guidotti, a consultant and former professor in environment and occupational health, said there is no credible scientific evidence that the levels recorded by the sensors pose a threat to human health in acute or chronic exposures, putting the level of hydrogen sulfide that is strongly supported to be a health hazard at around 5,000 parts per billion, a level that creates an acute toxic reaction very quickly.

The highest level recorded by the sensors was 58 ppb. Health effects from chronic low level exposure to hydrogen sulfide, such as the levels that the South Bay sensors recorded, are not supported.

Still, he said the levels recorded create an “intolerable” stench that makes it “hard to do anything to live a normal life.”

“The distress from having something like this in the community, it’s not neutral, it’s not a figment of imagination, it’s a psychological reaction to a real issue. Because noxious odor, like other noxious stimuli, provokes anxiety, and that provokes a response of fight or flight,” Guidotti said. “But that’s where the evidence ends.”

Limited data

Even as residents across the region complain about the odor, the air pollution district has delayed additional monitoring across the South Bay.

The agency used a $100,000 federal grant to buy six solar-powered sensors. Until recently, all of them sat at the same San Ysidro location.

Officials have cited a series of problems, from paperwork delays to safety reviews.

Sibi Sutty has lived in the northeast corner of Imperial Beach since August 2023 and said the smell has gotten much worse since moving in. He has been following the data closely since the sensors came online.

“Whenever I noticed the odor near my home, I would look at the data and I don’t really see much of a correlation there,” Sutty said. “The sensor would be reading zero but you can obviously smell it outside my home.”

One sensor was installed on Aug. 7 near his home but as of last week, the data was not yet online. District officials said the sensor is having technical difficulties, needs to be calibrated and they are not sure when it will begin transmitting data.

Three additional sensors are also waiting to be moved to a new location. A land use agreement with San Diego County is being finalized to install a sensor at the Tijuana River Valley Campground. The district previously said in a statement that it should soon be operational, as should an additional sensor at the South Bay Wastewater Treatment Plant.

A sensor to be placed on the roof of the Imperial Beach Fire Department has been reviewed by safety advisors at the air pollution district who are waiting on safety equipment to be installed. The sensor is expected to be operational in a couple months.

The district has also obtained a reference grade monitor, which is more accurate but very costly. It can be used to improve the accuracy of the sensors and allow for comparisons between single sensors installed in different locations. Officials said they are not sure when it will be operational as it still needs parts.

Despite the delays and the intensity of the smell this year, Sutty is optimistic about the future. He is looking forward to seeing the data near his home and hopes it can be used to tell regulators and residents whether the upcoming improvements are reducing hydrogen sulfide exposure for residents.

“It would really help us understand how low does the flow need to be for this not to be a problem anymore,” Sutty said. “I’m not hopeful that this will be 100% fixed. What I am hopeful for is that the significant degradation that we have seen in the last couple of years will return back to what it was, you know, maybe three or four years ago.”

Federal officials announced earlier this month that they awarded a contract to expand the South Bay Wastewater Treatment Plant, with construction expected to begin this year. The expansion, in combination with infrastructure upgrades in Mexico, is intended to eliminate up to 90% of untreated wastewater reaching the coast.