Which chemical compound is notable for causing gastrointestinal illnesses in contaminated water?

Giardia is a protozoan parasite that causes gastrointestinal illnesses, particularly giardiasis, which is characterized by diarrhea, cramping, and nausea. It is well-known for being transmitted through contaminated water sources, often affecting individuals who consume untreated or improperly filtered water. Giardia cysts are resilient and can survive in water for long periods, making it a significant concern in terms of water quality and public health.

While other substances listed can pose serious health risks, Giardia specifically targets the gastrointestinal system and is prevalent in waterborne outbreaks, particularly in areas lacking proper sanitation or clean drinking water. PCB, arsenic, and nitrates are primarily associated with other types of health issues and contamination sources, but they do not specifically cause gastrointestinal illnesses. Thus, Giardia stands out as the primary compound linked to gastrointestinal illnesses from contaminated water.