The price to construct a new drinking water reservoir in Southwest Portland has ballooned more than 200% over the project’s lifetime, according to officials’ recent testimony to the City Council.

New expenses were added Wednesday when the mayor and commissioners approved $8 million in spending increases that officials said were necessary to keep the project going.

All told, the cost to build a 12.4 million gallon underground reservoir at Washington Park in order to satisfy federal regulations has soared far past officials’ original estimate of $67 million when the project launched in 2009. In 2015, engineers at the Water Bureau said the figure was actually closer to $170 million. Today, the cost is at least $205 million.

Additional expenses have arisen because of what the Portland Water Bureau has described as unforeseen soil problems encountered at the site.

The cost overruns anger skeptics who question officials’ ability to manage large infrastructure projects as Portland drinking water rates keep rising.

“We predicted this from the very beginning,” said Floy Jones, an activist with Friends of the Reservoirs, a group that has opposed the demolition of Portland’s historic water reservoirs.

Jones, who has filed suit against the Portland Water Bureau over what she views as illegal spending, said the expanding budget for the Washington Park project is “totally absurd” and the consequence of what she said was the project’s “poor design.”

Water Bureau spokeswoman Felicia Heaton said Thursday that the $67 million amount told to the City Council at the project’s start was “a very, very early estimate.”

After work began, engineers encountered unexpected costs because the soil at the Washington Park site was more unstable than previously thought, said Teresa Elliot, the bureau’s chief engineer. The complexity of the terrain made assessing underground conditions difficult to estimate, said another spokesman, Brian Balla.

Elliot said the City Council in 2015 increased the project budget to $170 million with an option for it to rise an additional 20%. She said the contracts approved Wednesday are not new spending because they fall under the range authorized in 2015.

Construction costs play a central role in rising water rates. This year, rates increased 7.4% for the average single-family residence, according to the Water Bureau. The typical monthly bill is $42.15 — an inflation-adjusted 60% increase from a decade ago, according to city figures. (Sewer and stormwater service bills rose an adjusted 23% in that time.)

Elliot, the Water Bureau engineer, said the Washington Park project is nevertheless necessary because it will help provide Portland with water in the event of an earthquake.

“It's important to note that — in additional to complying with federal regulations — this project is an important part of the city's work to become better prepared for earthquakes and other impacts on our infrastructure,” Elliot said.

The reservoir and other seismic retrofits, she said, are “extremely valuable investments in our community’s current and future health and safety.”

— Gordon R. Friedman

GFriedman@Oregonian.com

Correction: An earlier version of this story stated Portland’s drinking water rates were among the nation’s highest. According to date provided by the Portland Water Bureau, the rates are average.