Why is the water in Niagara Cave not considered potable?

The water in Niagara Cave is not considered potable primarily because it contains traces of nitrates from pesticides. Nitrates can originate from agricultural runoff and can have harmful effects on human health, particularly when ingested in high concentrations. They can lead to a condition known as methemoglobinemia or "blue baby syndrome," which affects the ability of blood to carry oxygen in infants and young children. Therefore, the presence of nitrates in the water makes it unsuitable for drinking.

While other factors, such as temperature and mineral content, may affect the palatability or quality of water, the critical reason for the water's non-potability in this case is the contamination with nitrates, which poses a significant health risk.