Twin Cities Seminar
Estimates of Annual Loads of Total Nitrogen Contributed to Streams by Groundwater, Landscape, and Point Sources in the Chesapeake Bay Watershed
In many areas across the country, elevated the nitrogen concentration of discharging groundwater impacts the water quality of streams. In the Chesapeake Bay watershed, catchment-scale nitrate concentrations in groundwater were estimated by a statistical model based on nitrogen concentrations in small streams at base flow, land use, and subsurface geology. The groundwater concentration, combined with estimates of baseflow volume, were used were used to calculate the annual loads of nitrogen contributed by groundwater to each catchment. The groundwater loads were combined with annual stream total nitrogen loads, derived by an existing watershed model, to apportion the stream loads into groundwater, landscape, and point sources. The annual total nitrogen stream loads were apportioned to seven sources: point sourced, landscape sourced from developed, agricultural and undeveloped landscapes, and groundwater sources from these three land uses. From a water quality management perspective, only the point-sourced and nonpoint-sourced landscape loads can be readily be affected. Any effect of landscape management on nitrogen sourced from groundwater will be delayed by the subsurface transit time. The apportionment of total stream TN loads to various sources can help set realistic expectations for the effectiveness and 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.
Paul Capel is environmental chemist and research team leader of the USGS National Water Quality Assessment Program’s (NAWQA) Integrated Watersheds Studies Team. He also holds an appointments as a adjunct associate professor at the University of Minnesota Department of Civil, Environmental, and Geo- Engineering and with the Water Resources Science Graduate Program. He received his PhD from the Department of Civil Engineering, University of Minnesota in 1986 and has spent almost 30 years conducting and leading environmental research. During his tenure with the USGS, he has been a research chemist with the Office of Water Quality, the NAWQA Pesticide National Synthesis Project, and the research leader of the NAWQA Agricultural Chemicals Team. His major research interests have focused on the behavior and fate of chemicals in the environment, particularly pesticides and nitrogen, and the importance of hydrologic flowpaths on chemical transport. Currently, he is working on the methods to quantify and model inputs of nitrogen from groundwater to streams.