Where are nitrates commonly found that can affect infants?

Nitrates are commonly found in well water, particularly in rural areas where agricultural practices are prevalent. Fertilizers, animal waste, and septic systems can lead to nitrate contamination as these substances seep into the groundwater. Infants are especially vulnerable to high levels of nitrates because they can lead to methemoglobinemia, or "blue baby syndrome," which interferes with the ability of blood to carry oxygen.

In contrast, while river water may contain nitrates, it is not typically a primary concern for infants. Ocean water generally has low levels of nitrates due to dilution and different ecological dynamics. Tap water varies in its source but is often treated to reduce contaminants, including nitrates, especially in urban areas where the water is frequently tested and regulated. The specific concern for infants primarily arises from well water sources, making that the correct answer in this context.