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Monday, April 17, 2006 | One day last November, enough copper got swept into Tijuana’s overwhelmed sewer system to have minted about 7,700 pennies. And that’s just what got collected in a city where thousands lack basic sewers.

It isn’t unusual. The metal comes from scraping brake pads, from junkyards, gutters and industry. It wasn’t the only metal coursing through the city’s sewers that day, according to monthly monitoring reports. Almost seven pounds of lead were caught. So were nine pounds of nickel – enough to mint 800 of the five-cent pieces.

Despite the public health debate over Tijuana’s storm-water runoff, few people have actually quantified what’s in the frothy brew that gets swept into the Pacific Ocean. But a look into Tijuana’s sewers gives some insight, as the runoff that’s collected by the sewer system is likely similar to the runoff that makes its way into the ocean.

Metals are in the mix. The chemical components of gasoline can be found. So can pesticides, fecal material, bacteria and viruses such as Hepatitis A.

A voiceofsandiego.org review of the components of Mexican sewage treated by a sewage plant opened a window into revealing what escapes treatment and often winds up in the Pacific. Estimates vary, but somewhere between 15 percent and 50 percent of Tijuana’s residents lack sewers. Much of their waste – fecal material, bath water, dish detergents – gets dumped in the streets, destined for the ocean. Leaks and sewage overflows complicate the problem.

But broad, sweeping data about the runoff’s ingredients is hard to find and has to be pieced together. Aside from the work of San Diego State University environmental health professor Richard Gersberg, his students, and San Diego County officials, it appears little has been done to comprehensively quantify everything that’s in the mess.

Gersberg, who teaches at SDSU’s School of Public Health, said Mexican runoff can have disease levels twice as high as in San Diego, making pathogens – not metals or other pollutants – the contaminant that poses the greatest threat to human health. Gersberg questions the common assertion that Mexican runoff is rifer with metals and industrial pollutants than in the United States.

“I think a lot of it is just anecdotal information or what people fear,” he said. The common refrain that Mexican industries and maquiladoras are simply dumping chemicals down their drains “is not based on any data,” he said. “I’m sure it happens, but I haven’t seen any hard data that says either Mexican sewage or runoff is that much more contaminated than in the U.S.”

The runoff affects public health in San Diego, forcing frequent beach closures from the U.S.-Mexico border north to Imperial Beach and Coronado. It has kept water access closed at Tijuana Slough National Wildlife Refuge since Feb. 21.

During a two-month span last year, Mexican sewage had twice as much nickel and lead than a similar plant in San Diego, according to records kept by the San Diego Regional Water Quality Control Board. It had an equal amount of silver and comparable levels of copper.

While lead and mercury can cause neurological disorders, copper, nickel and zinc aren’t harmful to human health.

Gersberg said that data makes sense. Mexican sewage is typically more concentrated than in the United States, he said, because Mexicans use about one-fifth the water per capita.

Metals aren’t the only contaminant. Though the county monitors for bacteria levels in the water, the real threat comes from viruses such as Hepatitis A, Gersberg said, which can linger even once bacteria levels return to normal.

Its presence makes water-goers more susceptible to contracting the virus.

County agriculture officials have also found the region’s greatest concentrations of the pesticide diazinon in the Tijuana River basin, according to data kept between 2003 and 2004. The chemical’s sale was banned by the U.S. Environmental Protection Agency in Dec. 2004, though its use is still allowed.

Chlorpyrifos and malathion, two other widely used pesticides, are also found in the nearby waters. None of the pesticides are a human health risk, Gersberg said, but can pose serious risks to fish and other aquatic life in the Pacific and nearby Tijuana Estuary.

Gersberg, whose students have studied sources of diazinon pollution, suspects it comes from south of the border, but isn’t completely sure. County agriculture officials haven’t pinpointed the pesticides’ sources, either.

“It’s a matter of creating awareness of the issue,” said Paul Davy, supervising inspector in the San Diego County Agricultural Water Quality Program. “Not that there’s not awareness. But it’s usually characterized with two words – raw sewage – and it’s much more complex than that.”

Gersberg, one of the few academics to have studied the issue in-depth, said because contaminants come from a complex array of sources it’s hard to pinpoint where each originates – or exactly what’s in the runoff.

Please contact Rob Davis directly at

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