(Portland, OR)  --  Officials are approaching the possibility of a water-treatment plant for the Bull Run Watershed, after traces of a parasite called cryptosporidium were found.  Officials say levels of the microorganism don't pose a risk to public health, but the treatment plant is being discussed.  About five years ago, Portland received a ten-year exemption from treating its water for cryptosporidium because the parasite hadn't shown up in tests for a decade.  KOIN-TV reports water samples have tested positive for cryptosporidium four times within the past five weeks.