Which organism might be found in contaminated well water?

• A. Giardia
• B. Escherichia coli
• C. Salmonella
• D. Cryptosporidium

Answer: (A)

Giardia lamblia is a waterborne parasite that commonly contaminates wells. It forms hardy cysts that can persist in groundwater and resist some disinfection, allowing it to survive long enough to be ingested with drinking water. When these cysts reach the human gut, they can cause giardiasis, a diarrheal illness often associated with exposed or untreated well water. This resilience in water and the association with well contamination make Giardia a classic organism found in contaminated well water. While other organisms can occur in such water, Giardia’s persistent cyst form and strong link to groundwater outbreaks explain why it’s the best choice here.