Eutrophication of coastal waters due to non-point source land-derived nitrogen loads is a worldwide phenomenon and perhaps the greatest agent of change altering their ecology. Species composition of nitrogen loads to receiving waters is an important aspect of the coupling between watershed and receiving water because extent and rate of reactivity of the species differ. In this dissertation, I investigated processes controlling magnitude and species composition of groundwater-transported nitrogen exports to coastal waters from glacial outwash plain watersheds.

First, I developed two novel methods to measure groundwater-borne nitrogen loads to estuaries from wastewater treatment plants, and quantify nitrogen losses from wastewater effluent during transport through watersheds. I estimated treatment plant load to receiving water as the difference between total measured nitrogen load from the watershed and modeled load from all other major nitrogen sources. To corroborate, I quantified the total wastewater nitrogen load based on nitrogen stable isotopic signatures of primary producers in the receiving waters. Estimated nitrogen losses in wastewater plumes during transport through watersheds were 67 and 82%.

To identify controls on groundwater transported watershed exports of organic nitrogen, nitrate and ammonium, I used differences, within a set of 10 watersheds, in land uses and geological characteristics to examine relationships between watershed variables and species composition of groundwater nitrogen. Organic nitrogen and nitrate concentrations were primarily influenced by human population density, but substantial variability was produced by oxidation of organic nitrogen to nitrate during transport through watersheds. Species composition of nitrogen exports was primarily controlled by transformations during transport with secondary influence due to population density.

Lastly, to test the results of the correlation approach, I developed a land use-based mechanistic model to predict dissolved organic nitrogen yields. Major sources considered were wastewater, fertilizer, and atmospheric deposition. Coefficients for rate of organic nitrogen supply and of loss in watersheds were based on published literature. Results indicated that atmospheric deposition was commonly the dominant organic nitrogen source, but due to differences in losses in watersheds among the three sources, watershed dissolved organic nitrogen yield from wastewater was often as large as yield from atmospheric source.