The Portland Water Bureau plans to resume using the Bull Run watershed as its drinking water source this week.
Related: Portland Switches To Its Backup Water Supply: What You Should Know
The bureau stopped using Bull Run water in February and switched to its backup source, water from the Columbia South Shore Well Field after it repeatedly detected small amounts of a single-celled parasite called cryptosporidium.
Tests over the past month have continued to detect cryptosporidium in the water, in low concentrations. Thirteen out of 47 samples have tested positive for cryptosporidium.
The most recent positive test, on March 8, detected a single cryptosporidium parasite in a 50-liter sample. The bureau posts its test results online.
The Water Bureau says public health officials have reported fewer than average cases of cryptosporidium illness so far in 2017.
That's led the bureau and its regulators at the Oregon Health Authority to conclude that Bull Run water is safe to drink, in spite of the ongoing cryptosporidium detections, according to a press release.
“Our ongoing surveillance for cryptosporidium illness has not detected any unexpected increase,” said Multnomah County and Tri-County Health Officer Dr. Paul Lewis. “At this time the general public does not need to take any additional precautions. As always, we recommend that people with severely compromised immune systems discuss their individual health needs with their physicians.”
Related: Health Experts: Parasite Cryptosporidium Not Established In Portland Water Supply
Some variants of the cryptosporidium parasite can infect humans and cause diarrhea and vomiting. The infections can be debilitating and fatal in people with compromised immune systems. The Centers for Disease Control and Prevention call cryptosporidium "a leading cause" of waterborne disease in the United States.
The Portland Water Bureau treats its drinking water with chlorine and ammonia to kill other pathogens, but those additives are not effective against cryptosporidium.
Filtration can be an effective method of removing crypto, but Portland does not filter its drinking water.
In 2006, the EPA issued new regulations requiring more aggressive treatment of drinking water for cryptosporidium.
It required some cities with unfiltered drinking water systems, including Seattle, to build ultraviolet light treatment systems.
In 2012, the Portland Water Bureau received a variance from state regulators that allowed the city to avoid building an UV treatment plant to comply with the EPA rule.
That variance was largely based on the fact that the Portland did not detect any cryptosporidium in more than 400 samples collected over a year.