Austin Water officials say zebra mussels on a raw water pipeline at the Ullrich Water Treatment Plant were behind foul-smelling tap water in Central and South Austin on Thursday.

The pipeline was put back in service on Wednesday after being out of service for repairs for two weeks. Some zebra mussels in the pipeline likely died in the midst of construction, and as the zebra mussels decayed, they released compounds into the water, said Rick Coronado, Austin Water’s assistant director for operations.

Although customers across the city reported fishy-smelling tap water, Austin Water officials said the supply was safe to drink.

“We still meet all the drinking water standards. ... There’s no indication the water isn’t safe,” Coronado said.

Austin Water fed powdered activated carbon into its supplies Thursday to help with the odor. The substance is commonly used by treatment plants to help absorb taste and odor compounds, according to the Environmental Protection Agency.

The Ullrich plant, near Redbud Trail, was also the source of trouble last year when high amounts of particles in the water prompted officials to ask residents to boil water and reduce water use.

Additionally, zebra mussels — an invasive species of mollusk that latches onto rocks, boats and pipes — have caused major infrastructure problems in the northern United States and have been steadily spreading across Texas for several years.

Clusters of zebra mussels were found clinging to filtering screens in the city’s water utility intake pipes on Lake Travis last year. Divers made the discovery in September while inspecting the screens at the Handcox Water Treatment Plant. The mussels also have been spotted in Lake Austin and Lady Bird Lake.

“We have a consultant we’re working with to identify ways to mitigate zebra mussels,” Coronado said. “We’ll be looking at other tools and other chemicals to either slow their growth or remove the zebra mussels.”