Which organism might be found in contaminated well water?

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

Answer: (B)

Contaminated water can host tiny organisms that cause illness, especially when fecal material enters the water supply. Giardia lamblia is a protozoan parasite that forms cysts capable of surviving in water for long periods. If well water becomes contaminated, these cysts can be ingested when drinking, leading to giardiasis. The cyst form is hardy and can resist some disinfection, which is why Giardia is a common organism associated with contaminated well water.

Lactobacillus is a harmless gut bacterium not typically linked to well-water contamination. Escherichia coli can indicate fecal contamination but is a bacterial indicator rather than a waterborne parasite. Cryptosporidium is another waterborne protozoan that can contaminate water as well, but Giardia is the classic example of a parasite commonly found in contaminated wells.