People worry about nitrate in drinking water for a variety of reasons.
Concentrations of nitrate in stream waters around the world are usually blamed on human activities such as atmospheric emissions, livestock feeding, agricultural runoff, tree cutting and effluent discharge. While monitoring water quality in the streams of the Mokelumne River watershed in the central Sierra Nevada, Dahlgren and colleagues noticed that the streams flowing certain rock types, especially sedimentary rocks, had much higher concentrations of nitrate than usual.