PORTLAND — Facing the largest boil-water order in the region’s history, Portlanders emptied stores’ supplies of bottled water, chucked potentially contaminated food and braced for a complicated Memorial Day weekend.About 670,000 people in the Portland area were told after lunch Friday to boil water used for drinking, cooking and tooth-brushing. Three routine tests this week showed the water system was contaminated by E. coli bacteria, usually associated with animal or human fecal matter.The samples that tested positive for bacteria were collected this week from two uncovered reservoirs at Mount Tabor. The Water Bureau said it collects about 240 bacterial samples per month throughout the system, and the test to determine the presence of bacteria takes 18 hours.“The chance of any health problems related to this water test result is low,” Tri-County Health Officer Dr. Paul Lewis said. “If any problems occur, we would expect diarrhea.”The Water Bureau is investigating the cause of the contamination. The city said contamination can occur when there is a loss of water pressure, a pipe breaks or when conditions expose drinking water to outside elements, such as animal waste.