Work is starting on an 8-mile-long tunnel under the south side of Indianapolis that is the first major part of a $1.6 billion project aimed at reducing the release of raw sewage into the city's rivers.

The plan calls for crews to bore the 18-foot diameter tunnel about 250 feet underground between a sewage treatment plant on the city's far south side to a location near the White River near downtown. The work is expected to take five years and be followed by four shorter tunnels that will contain water from the city's combined storm and sanitary sewers after storms until it can be treated.

"Any time any significant rainfall occurs, even a quarter of an inch, we're putting raw sewage in the White River and Fall Creek — conditions that we can't tolerate," Carey Lykins, president and CEO of Citizens Energy Group, parent company of the city water utility, told WISH-TV.

Crews have started prep work on digging the shaft for the tunnel and drilling work is expected to begin by late fall after gargantuan boring equipment arrives on 40 semitrailers for assembly, officials announced Wednesday.

The project is required under a 2006 agreement between the city and federal and state environmental agencies to reduce sewage releases into the waterways by 2025.

Los Angeles-based Aecom Technology Corp. was chosen last September for a $25 million contract to manage construction of the sewage overflow tunnel and pump.

When complete, city officials say, the 25 miles of tunnels will be able to store 250 million gallons during and after rainstorms and reduce untreated sewage overflow by at least 95 percent.

Kevin Hardie, executive director of the Friends of the White River preservation group, called the project a significant step.

Money for the project is coming from recent annual hikes in city sewer rates, including a 10.8-percent increase this year and a similar increase planned for 2013.

The drilling work deep underground isn't expected to draw much attention as officials say slight vibrations lasting a few hours or a day are the only thing residents might notice immediately above any work area.

Mayor Greg Ballard said he believed the completed project would make the city's rivers much cleaner and more attractive for recreation and development.

"It's going to put a lot more activities at or near the waterways," Ballard said. "That's the intent so that we can really use it as an economic development asset."