What causes beach pollution near the San Diego River? Blame sewer leaks, study finds


Bacterial outbreaks in the San Diego River that force local beaches to be frequently closed are primarily caused by leaks in aging sewer lines, a new study by scientists at San Diego State University reveals. not by homeless encampments or failing septic tanks.

Environmentalists are calling the study a wake-up call for local sewage agencies and political leaders, saying rapid and significant financial investment is needed to upgrade many local sewage lines that are well over 50 years old.

The cost of replacing the pipes, which could reach billions of dollars, would be passed on to sewer and water ratepayers by the sewer agencies funding the upgrades.

Fecal bacteria in the San Diego River is a major problem as the river empties into the ocean, potentially contaminating the water and forcing the closure of beaches that attract tourists and are frequently used by many local residents for swimming , surfing and other activities.

Recent beach closures in Coronado and Imperial Beach are unrelated to bacterial outbreaks in the San Diego River. These closures are the result of sewage discharges south of the international border.

The study comes about two years before a more comprehensive analysis of the sources of bacteria in the river is completed.

The San Diego Regional Water Quality Board ordered the analysis in 2019 when it found the river had consistently high levels of bacteria, ordering the study to be done by sewer agencies from San Diego, El Cajon, La Mesa, Santee and adjoining areas in the lower reaches of the river. watershed.

“I predict the SDSU study is a precursor to what the larger study will show,” said Matt O’Malley, executive director of San Diego Coastkeeper. “This study confirms a lot of what we thought. Everything indicates that the wastewater treatment infrastructure throughout the region is aging and leaking. »

The SDSU study attempted to identify the source of feces in the river by testing for the presence of two substances used only by humans – caffeine and artificial sweetener – and a virus and bacteria commonly found in the materials. human faeces.

“We were interested to know where it came from,” said Natalie Mladenov, professor of environmental engineering at SDSU, lead author of the study.

Local officials have previously said the most likely sources of feces in the river are leaky sewer lines, illegal dumps, broken septic tanks and homeless encampments where there are no bathroom.

The two-year study analyzed how long caffeine, the artificial sweetener sucralose, the bacteria HF183 and the RNA virus PMMoV had been present in the river water – and at what concentrations they were present.

For example, caffeine leaves wastewater faster than most other chemicals, so you wouldn’t expect it to stay in high concentrations in the river for very long. Because the study found it to be in high concentration in the river, the caffeine must come from a relatively fresh source.

Based on this, the researchers conclude that homeless encampments and septic tanks, where the caffeine is expected to dissipate fairly quickly as pollution enters the river, are unlikely to be sources of feces in the river.

And therefore, the study concludes that the most likely sources of feces are leaky sewer lines.

The study analyzed the sites of 13 homeless encampments along the river. There are also around 17,000 septic tanks located in the river’s catchment area.

One thing the SDSU study didn’t analyze is whether the sewer line leaks came from municipal lines or “private laterals” – private lines that connect homes to a municipal sewer line. .

O’Malley said it would be a key decision going forward. If the problem is primarily with municipal lines, sewer services should replace the leaking pipes. If the problem is primarily with private lines connecting homes to the system, agencies may need to encourage homeowners to upgrade those lines.

It’s a relatively new idea that leaky pipes can cause beach closures.

Local swimmers have for many years been told to avoid beaches for 72 hours after rain as polluted urban runoff washes into rivers and then into the ocean.

Conventional wisdom holds that heavy rain makes ocean water dangerous because the rain picks up chemicals and bacteria as the water rushes over concrete, pavement and other harsh landscapes. But that could be misleading if the main source of bacteria during torrential rains ends up being leaky sewer pipes.

Some scientists believe that the untreated sewage that slowly flows out of the pipes mostly stays put when it’s not raining. But during a rainstorm, this leaking sewage is quickly released into local waterways in large quantities.

San Diego officials, including Mayor Todd Gloria, frequently acknowledge that the city’s aging infrastructure — including sewer lines — is in dire need of major upgrades and repairs.

In February, the city’s infrastructure backlog topped $4 billion for the first time.

“There were so many more assets built in the 1950s and 1960s than before that time, and those assets are now maturing — they’re reaching the end of their useful life,” said James Nagelvoort, a public works longtime in the city. official. “So we should be anticipating a greater amount of stuff to deal with as we look to the future.”

On Friday, a city spokesperson did not comment directly on the SDSU study, but said the city’s sewage spill rate was 0.04 spills per 100 miles of pipeline. This rate does not take into account any additional leaks in the pipes.

Spokesperson Arian Collins also said the city has identified several pipe segments as potential candidates to figure out if sewer leaks are affecting the river.

Mladenov, lead author of the SDSU study, said she hopes the research will spur action. The study will be published in ES&T Water, a journal of the American Chemical Society.

“We want it to be relevant, not just sitting on a shelf,” said Mladenov, who came to the university eight years ago from Colorado.


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