Alamitos Bay, closed nearly two weeks because of contamination, was reopened Tuesday to complaints from beach users that the water still smelled and looked foul.
The Long Beach Department of Health and Human Services traced the problem to a sewer construction project at Bennett Avenue and Division Street, which had pumped about 10 million gallons of ground water into the bay during the last week of October.
City officials said that when they approved the project last summer, nobody anticipated that the discharged water would pick up bacteria as it passed through the city's storm drain system.
"That would have happened had we had a rain," Deputy City Manager Henry Taboada said. "The difference is, when it's raining, people aren't using Alamitos Bay."
Taboada said that in the future, the city will redivert such waste water to the county sanitation system. "It's a lesson learned," he said.