Final answer:
The Walkerton tragedy in May 2000 was caused by contaminated water supply due to heavy rainfall and manure runoff. Option b, which states that the likely sites are now covered by water, did not contribute to the tragedy.
Step-by-step explanation:
The Walkerton tragedy in May 2000 was caused by contaminated water supply that led to an outbreak of E. coli bacteria. The geo-environmental condition that did not contribute to this tragedy is option b, where the likely sites are now covered by water. The Walkerton tragedy occurred due to a combination of heavy rainfall and manure runoff from a nearby farm contaminating the town's water source, not because the likely sites were covered by water.