and working backwards from the first emergence of symptoms in the community, investigators determined that the majority of infected community members had been exposed to the bacteria between May 13 and May 16 and the contamination had thus likely entered the infective well on or about May 12. In the spring of 2000, Walkerton's system typically drew most of its water from one well at a time, alternating unevenly between wells 5, 6, and 7. Well 7 was turned off between March 10 and May 2, ran alone from May 2 through May 9 before turning off again, and was then used again between May 15 and May 19. From May 9 to May 13, Well 5 provided the bulk of Walkerton's water, supplemented from May 10–13 by Well 6. Though Well 7 ran without chlorination between May 3 and May 9, expert evaluation of well 7's pumping schedule and lack of physical vulnerabilities to contamination concluded that it was highly unlikely to have been involved in the outbreak. Well 6, which records indicate was out of service beginning May 13, was similarly determined to have been unlikely to have been the source of contamination based on its location and characteristics. Source of contamination To the immediate south and west of Well 5 lay farmland used as a cattle-breeding operation, occupying four lots (18–21, west to east). On April 22, 2000, following heavy rainfall on April 20–21, manure from the cattle was used to fertilize crops growing on lot 20 of the property, the nearest edge of which lay approximately from Well 5. The manure was subsequently incorporated into the soil of lot 20 using a disc harrow. The April incorporation of E. coli-contaminated manure into cool, wet soil near Well 5 meant that the bacteria were in an ideally suited environment for longevity. Subsequent heavy rains totalling fell between May 8 and May 12, drawing the bacteria deeper into the soil near Well 5. During later investigations of the outbreak, experts on the Walkerton Commission Expert Review Panel determined that the manure-contaminated water had likely seeped into the aquifer that underlay Well 5, though it may have been possible that surface runoff, rather than contaminated groundwater, was the source of the bacteria. Cattle manure, even under best practices, typically contains fecal coliform bacteria, including E. coli. A subsequent government report concluded that the cattle operation had followed all safety