OAKLAND — The East Bay’s largest water supplier has agreed to pay $426,183 in penalties for accidentally dumping 12 truckloads of concrete into a creek last year.

The East Bay Municipal Utility District reaffirmed Monday that it’s sorry for the embarrassing toxic spill April 8, 2015, when a district employee on a construction project accidentally left open a pipe valve. Concrete spilled for about two hours into the Rockridge branch of Glen Echo Creek, smothering small organisms in water and sediment for about a mile.

“It was painful for us. We place a high priority on protecting natural resources,” said EBMUD spokeswoman Andrea Pook. “We have made changes in our procedures to see this doesn’t happen again.”

EBMUD’s total cost over the spill may approach $2 million, the district estimates.

In addition to the proposed $426,000 penalty, the district already has spent some $1 million responding to and cleaning up the spill. EBMUD may spend as much as $500,000 more to restore and monitor the creek over the long term.

The penalty is called for in the proposed settlement between the water district and two state environmental agencies — the San Francisco Regional Water Quality Board and the state Department of Fish and Wildlife.

In the 17-page settlement deal, the water district promises to waive its rights to a hearing to contest the civil allegations of damaging the creek with 170 cubic yards of concrete.

Penalty agreements are typically approved by the regional water board after a 30-day waiting period unless someone protests the deal.

The EBMUD penalty is substantial but not among the highest levied by the regional board in the past few years, said Tom Mumley, the regional board’s executive officer.

Under the penalty pact, $100,000 of the money will be used to study polychlorinated biphenyl in San Leandro Bay in Oakland, and another $161,568 will be used for environmental restoration projects to benefit aquatic life and watershed lands in Alameda County.

When the accident happened, district contractors were filling an old 24-inch-diameter water pipeline with concrete so it could be sealed and left in place underground.

Contact Denis Cuff at 925-943-8267. Follow him at Twitter.com/deniscuff or facebook.com/denis.cuff.