Final answer:
Arsenic is the natural contaminant that has contaminated millions of wells in Bangladesh, leading to health issues and increased risk of diseases.
Step-by-step explanation:
Arsenic is the natural contaminant that has contaminated millions of wells in Bangladesh. The pyrite bedrock underneath much of western Bangladesh contains large amounts of arsenic, which contaminates the groundwater. As a result, millions of people in Bangladesh drink water contaminated with arsenic on a daily basis, leading to various health issues and an increased risk of diseases.