SAN FRANCISCO (AP) — While the weekend rain was a welcome sight in many parts of drought-hit Northern California, California transportation officials said the moisture reached a place it shouldn’t have — inside a new, $6.4 billion section of the San Francisco-Oakland Bay Bridge.

Part of the steel structure that was supposed to be watertight is leaking, the San Francisco Chronicle reports Caltrans told the newspaper that rainwater was found beneath the road deck on the marquee stretch of the new eastern span that opened last year, which features a sailboat-shaped tower with a pair of suspension cables.

The seepage might be coming from the steel guardrails that were used on that portion of the bridge to reduce weight, the Chronicle said. The rails were bolted into place through holes drilled into the steel that supports the roadway. But instead of applying caulk between the two surfaces, the project’s main contractor applied caulk outside the rails so they could be repositioned more easily. A coat of waterproof epoxy was put over the caulk, but the seal may not be performing as expected.

Another potential culprit could be electrical holes put in near the guardrails, the Chronicle said.

“You do the best you can, but water will always find its way in,” said Bill Casey, a Caltrans project engineer.

Transportation officials knew rain was getting into the bridge within months of its September opening, but it took a storm as large as the one that hit the Bay Area experienced this weekend for inspectors to get a good look at the problem, Casey said.

Bridge spokesman Andrew Gordon said the source of every leak has not been pinpointed but that the problem so far looks manageable.

“You don’t want any water inside,” Gordon told the Chronicle. “So far, it’s slowly dripping. But we are going to have teams of engineers and inspectors there this weekend to assess the problem.”

Independent engineers say the leaks could cause the bridge to corrode and significantly shorten its life span.

“That’s a problem, a big problem,” said Lisa Thomas, a metallurgical engineer who studies material failure at a laboratory in Berkeley. “They want it to last 150 years. But with water coming in, something is going to corrode until it’s too thin and weak.”

Concerns about the structural integrity of the seismic safety bolts used on the new span threatened to delay its opening on Labor Day. More than 30 of the 17-foot-long fasteners bolts that secure earthquake shock absorbers to the deck of the bridge cracked when they were tightened. Caltrans is conducting further tests on their long-term viability.