Estimates of annual loads of nitrogen contributed to streams by groundwater, landscape, and point sources

In many areas across the country, elevated nitrogen concentration of discharging groundwater impacts the water quality of streams. To better understand sources of nitrogen to streams in the Chesapeake Bay watershed, catchment-scale nitrate concentrations in groundwater were estimated using a statistical model based on three factors: nitrate concentrations in small streams at base flow, land use, and subsurface geology. The groundwater nitrate concentration, combined with estimates of baseflow volume, were used to calculate the annual loads of nitrogen contributed by groundwater to each catchment. The groundwater nitrogen loads were combined with annual stream total nitrogen loads, derived from an existing watershed model, to apportion the total stream loads into groundwater, landscape, and point sources. The annual total nitrogen stream loads were apportioned to seven sources: point sources, sources from developed, agricultural and undeveloped landscapes, and groundwater sources from the three land uses classes. From a water quality management perspective, only the point-source and nonpoint-source landscape loads can be readily managed. Any effect of landscape management on nitrogen sources from groundwater will be delayed by the subsurface transit time. The apportionment of total stream nitrogen loads to various sources can help set realistic expectations for the effectiveness of management decisions. This overall approach can be replicated for other areas of the country, as long as there are sufficient observations of nitrogen concentrations in streams at baseflow.