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The first results in Portland’s yearlong water sampling program in the Bull Run watershed provided the city with good news. The Portland Water Bureau announced Tuesday that lab samples found no evidence of the parasite cryptosporidium in the Bull Run reservoirs.

Portland officials are trying to make the case to the Environmental Protection Agency that the city’s water source has very little risk of cryptosporidium contamination. An EPA rule would otherwise require Portland to build a new treatment plant, estimated to cost $100 million.

Portland has tested for cryptosporidium in the past, said Yone Akagi, the bureau’s regulatory compliance manager, but only at the water intake point. The new round of testing also takes water samples from six potential “hot spots” in the reservoirs. Those are areas, such as tributaries, where cryptosporidium might be expected to be found.

Cryptosporidium, which is transferred by animal feces, forms hardy cysts called oocysts. The first round of samples to come back from the laboratory found no oocysts at either the water intake or the hot spots. The samples were taken in December and January.

Portland must meet a tough standard to get a variance from the EPA rule: the water from Bull Run has to be as free of cryptosporidium as water from agencies that actively treat for the parasite. If one oocyst turns up in the water intake, Akagi said, the city could cross that threshold.

Portland officials don’t want any cryptosporidium at the hot spots either, Akagi said, but finding an oocyst there doesn’t mean the parasite will make its way into the city’s water. “There’s no threshold at the hot spots,” she said. “(The EPA will) take that and use it to evaluate how risky our watershed is.”

If the EPA found Portland’s water needed additional cryptosporidium treatment, the city would need to have a new system up and running by 2014. That doesn’t leave much time for such a big project.

As a result, Water Bureau officials are taking two parallel paths: they’re designing a treatment system while also pursuing the water testing program that they hope will prove that more treatment is unnecessary.