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For the past thirty years or more, elevated levels of dissolved inorganic nitrogen delivered to the Gulf of Mexico by the Mississippi River have contributed to the seasonal occurrence of an extended hypoxic zone an area where marine life is threatened by oxygen depletion. These elevated nitrogen levels are attributed to increased point and nonpoint sources, and to alterations in the landscape and river channel that promote delivery instead of uptake and sequestration. In this paper, we examine estimates of nitrogen loads in the Mississipi River Basin, look at some simplistic load reduction scenarios, and discuss the difficulties in balancing different controls to address hypoxia.

Load estimates are taken from the National Water Pollution Control Assessment Model (NWPCAM), a policy support tool being developed for EPA by Research Triangle Institute. NWPCAM combines point source discharge information from the Permit Compliance System database with nonpoint source loading models for urban and rural areas to provide end-of-pipe and edge-of-field source estimates for conventional pollutant loadings. Load estimates for current conditions and for hypothetical control scenarios (e.g. point source nitrogen limits, feedlot regulation) will be compared in order to suggest potential benefits.

Although point source and agricultural controls both have tangible benefits in load reductions, there is considerable debate as to whether one approach is more effective and efficient than the other. This uncertainty reflects both the difficulties inherent in the large-scale analysis presented here and the realities of the regulatory climate. Point and agricultural sources and their controls are administered under different programs and authorities, and understanding and appreciation of the similarities and differences in these programs is vital to effective partnering between point source and nonpoint source stakeholders.