Portland may be forced to cover its historic reservoirs at Mt. Tabor and Washington parks, paying as much as $281 million, due to a U.S. Court of Appeals ruling Tuesday.

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One of Mt. Tabor Park's historic reservoirs.

A three-judge panel of the Washington, D.C. District Court of Appeals issued a unanimous decision Tuesday ruling against the City of Portland's challenge to a federal water treatment rule.

The Environmental Protection Agency has mandated that cities attempt to eliminate the cryptosporidium parasite, which can pose health problems for people with a severely weak immune system, from drinking water sources.

Also: Mt. Tabor reservoirs get historic listing

The parasite has only been detected at very low levels in Portland's drinking water.

"We are very disappointed in this ruling," said Water Bureau Director David Shaff, "Portland's water system has protected public health for more than 100 years. We challenged this rule because we do not believe the costly, one-size-fits-all approach promulgated by the EPA is necessary for Portland's pristine water system."

Portland's petition had challenged two requirements of the rule. One would force the city to provide additional treatment of its Bull Run drinking water source to either eliminate or inactivate cryptosporidium.

The second would require that the city either cover its open drinking water reservoirs at Mt. Tabor and Washington parks, provide treatment for cryptosporidium at the outlets of the reservoirs, or take the reservoirs out of service.

According to the Portland Water Bureau, in 2002, new treatment was estimated to cost from $55 to $204 million to construct and millions more to operate on an annual basis. The city estimated then that it would cost an additional $77 million to reach compliance with rule's open reservoir requirements of the rule.

"The city will evaluate its options for maintaining compliance with the Safe Drinking Water Act based on the decision," Shaff said. "Our immediate responsibility is to look at the options available to us and to include the community in analyzing and discussing the city's options."