Which element is primarily responsible for the negative health effects noted in water supplies due to agricultural practices?

Nitrogen is primarily responsible for the negative health effects observed in water supplies due to agricultural practices, particularly as a result of the use of fertilizers. When nitrogen-based fertilizers are applied to crops, excess nitrogen can leach into groundwater or run off into surface water bodies. This influx of nitrogen, particularly in the form of nitrates, can lead to several health problems.

One significant concern is methemoglobinemia, or "blue baby syndrome," which affects infants. It occurs when high levels of nitrates convert hemoglobin in the blood to methemoglobin, reducing the blood's ability to carry oxygen. This can lead to serious health issues in young children.

Additionally, elevated nitrogen levels can contribute to the eutrophication of water bodies, fostering excessive algal blooms. When these blooms die and decompose, they can deplete oxygen in the water, harming aquatic life and further disrupting ecosystems. Thus, the dominant role of nitrogen in agricultural runoff and its consequential health impacts make it a critical focus in environmental health discussions related to water quality.