Tracing nitrate in watersheds
Plants need nitrogen to grow, and nitrate is a common fertilizer ingredient, but high levels of nitrate contamination in drinking water sources can cause health problems. It is generally known that nitrogen flows through watersheds from upslope areas down to streams, but the relationships between upslope soil solution or groundwater nitrate concentrations and stream water nitrate levels—and the ways in which land use changes may alter this relationship—are not fully understood.
Sudduth et al. analyzed published studies of 62 watersheds to see if a consistent relationship between upslope soil solution or groundwater nitrate concentrations and stream water nitrate concentrations exists and whether ecosystem disturbances such as fire or land use changes such as clearing forests for agriculture or urbanization affected the relationship between soil and stream water nitrate concentrations.
For 40 undisturbed forest watersheds and 10 disturbed forest watersheds, they find that stream water nitrate concentrations are typically about half that of soil solution nitrate concentrations, indicating that a significant amount of nitrate is removed as water passes through watersheds to streams. For the 12 watersheds in their study that had significant agricultural or urban development, there is less reduction in nitrate concentrations between soil solutions and stream waters, suggesting that in human-dominated landscapes, upland soils and riparian zones are less efficient at removing nitrate than in undisturbed ecosystems.
The study suggests that, in general, stream water nitrate concentrations can provide an indication of upland soil solution or groundwater nitrate levels. The authors conclude that undisturbed watersheds have a significant capacity to remove nitrate, but land use changes tend to diminish the efficiency of nitrate removal from watersheds or alter the flow paths of nitrate.