For decades, San Diegans have known that when it rains, they should avoid the beaches near the border. That's because when it rains, millions of gallons of runoff containing raw sewage overwhelms Tijuana's wastewater system. It spills into the Tijuana River, which dumps it into an estuary and the beaches of Imperial Beach.

In the 1990s, the federal government finally moved forward with a solution, constructing the South Bay International Wastewater Treatment Plant in San Ysidro, capable of treating an average of 25 million gallons a day. But because the plant was never fully completed, it only treats the sewage to “advanced primary” before releasing it into the Pacific through an underwater pipeline. This is a problem, because the Clean Water Act requires “secondary” treatment.

After years of debate, lawsuits and congressional legislation about how best to comply with the law, we are now out of time. A federal judge has ordered the plant operators to comply with the Clean Water Act by Sept. 30 of this year.

This week, a report by the Government Accountability Office tried to answer key questions about two options on the table. This report convinces me that the best option is to expand the San Ysidro plant. The other option – a project called Bajagua that would be built in Tijuana – has less certainty and more cost.

Consider two key findings of the GAO report:

• Over 20 years, the Bajagua project is estimated to cost taxpayers $539 million, compared with $331 million to upgrade the San Ysidro plant. So Bajagua would cost taxpayers $208 million extra.
• The Bajagua project has significantly more uncertainties, including major environmental and engineering challenges. These include the daunting task of pumping millions of gallons of wastewater uphill into Tijuana for advanced treatment.

Neither option will meet the judge's deadline. But I believe that expanding the San Ysidro plant is the best option available. This plant began operations in 1997. But lawsuits, funding issues and the push to explore other alternatives prevented the facility from being completed.

The International Boundary Water Commission has considered numerous alternatives over the years. In the face of a federal court order, the time to act is now. Bajagua simply has too many unknowns, and too heavy a cost.

The 66-page GAO report made it clear: “There are a greater number of uncertainties associated with the Bajagua plant that could affect schedule and cost.” Among the uncertainties: The fact that before it can be built and begin operations, Bajagua still must secure more than 30 permits, environmental approvals and concessions from both U.S. and Mexican authorities. “Delay in any of these items,” the GAO report said, “could cause the timeline to be put on hold.”

Bajagua's backers have failed to meet several previous deadlines. And so I'm skeptical of claims that it can somehow meet deadlines in the future.

Some have suggested that the GAO should conduct a more comprehensive analysis. In fact, that is what I originally requested last summer. The GAO estimates additional studies could take up to 16 months, further delaying construction without providing any more clarity. That is unacceptable.

Last December, Congress and the president agreed to spend $66 million to improve treatment of wastewater flowing into the United States from Tijuana. Congress and the president also agreed not to commit these funds until the GAO report was completed. That report is now done. We have enough information before us to make a decision.

Time is not on our side. The Clean Water Act is being violated, and a federal judge set a deadline that will not be met. Expanding the San Ysidro plant is the best option given the circumstances. It would cost less, and it has far fewer question marks than the alternative. It's time to move forward with this option.