Minnesota’s biggest development project is being pushed back by as much as a year.

A pollution problem deep underground is forcing a delay in approvals for the 427-acre town center planned for the Twin Cities Army Ammunition Plant (TCAAP) site in Arden Hills, officials say.

They say the discovery of contaminants in a plume of water deep underground has forced a complicated new look at where the project’s infrastructure needs to be located without affecting the attractiveness of the site or the lives of future buyers.

“While we are changing the schedule,” Ramsey County planner Josh Olson said Wednesday, “this is still a once in a lifetime development opportunity in the Twin Cities. We want to make sure to complete all the due diligence, and that includes all the partners.”

Master developer Alatus stressed from the beginning last summer its keen desire for swift action while market conditions are favorable, noting that big teams from multiple firms had already spent months preparing.

But a deadline that had been set for May 31 for final approval of a master development agreement is being delayed until the end of the year. That means shovels may not break ground until 2018 and first occupancies could push to 2019. Dozens of interested parties already have asked about finding homes there.

Alatus said in a statement Wednesday that it is untroubled by the delay and is “pleased with and supportive of Ramsey County’s process.”

Arden Hills City Administrator Bill Joynes is warning members of his City Council that the new target dates are only estimates.

“Six agencies have to sign off on a plan for contaminated water, plus it needs the governor’s signature,” Joynes said. “It’s out of our hands.”

Last summer a clutch of public officials convened a news conference to announce, in the words of U.S. Rep. Betty McCollum, that site cleanup was done and construction could begin on Rice Creek Commons — the tentative new brand name for a project uniting more than 1,000 living units plus stores and offices on a walkable site laced by a restored creek.

But that cleanup applied only to soil. Officials say the water is a separate issue: a U.S. Army-led cleanup is still taking place.

Properly locating that effort is holding things up, Olson said. He declined to say how negotiations aimed at pricing the site and handing it over to developers is going.

One beneficiary of the holdup: Arden Hills, which has been pushed to consider some sort of civic facility on the site — possibly a community center — to make it more of a true town center.

The city’s timeline had been short for such a major decision, but the delay offers breathing room.