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A botched construction job from three decades ago will finally be fixed when a new power line that stretches across Boston Harbor to the Deer Island sewage treatment plant delivers its first jolt of electricity on Friday.

What a saga it has been to get to this point — one that landed Eversource, the Massachusetts Water Resources Authority, Massport, and the US Army Corps of Engineers in federal court to sort out the mess. The end result: a $115 million power line, funded by MWRA sewer customers. The new, 4.3-mile connection replaces an old line that ended up in the wrong place, at the wrong time.

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The first power connection was made to Deer Island about 30 years ago, to power up a plant whose main purpose was no less ambitious than the cleanup of the polluted harbor. A contractor working on behalf of Eversource predecessor Boston Edison buried the old power line roughly a dozen feet from the seabed as it traversed the Reserved Channel, along an industrial area of South Boston.

The problem? The federal permit required the line to be at least 25 feet under the sea floor, all the way to Deer Island. The crew apparently ran into bedrock in the channel, making progress difficult. Maybe they thought no one would notice.

Flash forward nearly 30 years. It turns out plenty of people noticed, eventually. This misplaced power line became a big deal. Here’s why: It’s in the way of a $350 million dredging project that would allow bigger container ships to dock in the Reserved Channel, at Massport’s Conley Terminal.

As you might imagine, litigation ensued. The Army Corps sued Eversource and the MWRA, Deer Island’s operator, in 2016 over the violated permit. Massport eventually joined in as a co-plaintiff.

They settled their differences the following year under a judge’s watchful eye: Eversource would build a new power line at the appropriate depths. MWRA ratepayers would pick up the tab. The new line avoids the bedrock at the bottom of the Reserved Channel by running on a parallel path underground in South Boston, in part through the Conley Terminal property, before going underwater just north of Castle Island and heading across the harbor’s main shipping channel.

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The MWRA agreed to pay $48.75 million up front, and another $48.75 million over 30 years. Eversource credited the MWRA $17.5 million, to represent the theoretical remaining life of the old cable to be decommissioned. Once financing costs are included, the entire expense equals $158 million.

The costs will be spread among 900,000 customers in 43 communities. A spokeswoman for the MWRA says the typical household will see an additional $2.11 a year on their bill for the remaining costs. Ratepayers have already been billed for the upfront half over the last five years.

Fred Laskey, the executive director of the water resources authority, points out that the new power line will provide an important dual function: It brings a backup fiber-optic line to Deer Island, to be used if the existing service through Winthrop gets interrupted.

Of course, a project of this scope didn’t get pulled off without a hitch.

The MWRA Advisory Board has been raising a stink for several months about the $8 million that Massport wanted for an easement through its shipping terminal. Executive director Joe Favaloro accused Massport of extortion, saying the port authority shouldn’t be requiring such a payment: It is both a public agency and a main beneficiary of the cable relocation. Favaloro says he plans to lodge a protest this fall with the Department of Public Utilities, which has authority over the payments, and will reach out to state lawmakers for support after Labor Day. (Massport, meanwhile, says the price was the result of a good-faith negotiation, and reflects the fair value of the easement.)

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Then there’s the minor glitch this week. Deer Island has been running on backup power since Aug. 5, at an extra cost of about $30,000 a day, as crews put the final touches on the new cable and test it out. The power line was supposed to go live Monday, but an equipment issue delayed the big switch until Friday.

One major infrastructure project is all but complete. Another one is well under way. The harbor dredging is nearly halfway done, and three bigger cranes are coming next year to Conley for the larger ships. Let’s hope no hidden surprises get left behind this time.


Jon Chesto can be reached at jon.chesto@globe.com. Follow him on Twitter @jonchesto.