Sources of nitrogen in wet deposition to the Chesapeake Bay region

Nitrogen-containing compounds in wet deposition can provide significant sources of nutrients to phytoplankton and potentially contribute to eutrophication in estuaries and coastal waters. Quantifying both inorganic and organic forms of nitrogen in wet deposition as well as determining their sources is important for understanding how to control eutrophication. Stable nitrogen isotope data can provide information regarding what source processes produced nitrogen in precipitation and air mass trajectories can predict where the air mass which produced the precipitation was geographically located before the event occurred. In this study, the wet deposition concentrations, fluxes, and δ15N values of ammonium, nitrate, and dissolved organic nitrogen were determined for 60 precipitation events collected from May, 1993 to December 1994 at a site near the Chesapeake Bay, an estuary currently experiencing eutrophication. Grouping the concentration data according to season showed a peak in ammonium coupled with depleted δ15N values in the spring which were indicative of agricultural emissions. A peak in nitrate in the spring seemed to indicate greater soil emissions at that time, but concentrations were also high at other times of the year. No trend was observed for the dissolved organic nitrogen with season. Back trajectories were calculated for each precipitation event and grouped into five major transport patterns. Combining the flux and isotopic composition data with the air flow history revealed that dominant sources of ammonium in precipitation to the region are probably fertilizers, soil, and animal excreta emissions which have the highest fluxes in air masses originating from the southwest and west. The dominant source of nitrate to the region is probably fossil-fuel combustion and the highest fluxes originate from the northwest and west. Speculation on the dissolved organic nitrogen sources is probably premature, but its flux pattern is similar to the nitrate pattern, suggesting that their sources may be similar.