Do nutrient concentrations typically exceed naturally occurring (or “background”) concentrations in streams and groundwater?
All five nutrients—including nitrate, ammonia, total nitrogen, orthophosphate, and total phosphorus—exceeded background concentrations at more than 90 percent of 190 sampled streams draining agriculture and urban watersheds. Nitrate exceeded background concentrations in 64 percent of shallow wells (less than 100 feet below land surface) in agricultural and urban areas. Concentrations of the other nutrients in groundwater were not significantly above background concentrations, in large part because these nutrients are generally not mobile or persistent in soil and groundwater.
