COI Funded Project: An Isotopic Tracer Investigation of Organic Nitrogen Cycling Within the Subterranean Estuary and Groundwater Seepage face


Project Funded: 2002

Proposed Research

Increases in the supply of land-derived nitrogen to receiving waters, on Cape Cod and on a global scale, has led to eutrophication of many coastal waterbodies. On Cape Cod, delivery of land-derived N to the coastal zone occurs primarily through groundwater transport. However, little information is currently available on the influence of nitrogen cycling processes in the mixing zone of fresh and saline groundwater or on processes in the sediments of the seepage zone on the transport of land-derived nitrogen to coastal waters. In addition, our past studies and those of others have shown that a large fraction of these nitrogen loads to coastal waters are often composed on organic nitrogen. To our knowledge there have been no field experiments on the reactivity of the organic nitrogen and on the role of land-derived organic nitrogen in the nitrogen cycle and eutrophication of coastal waters. The goal of the proposed research is to quantify nitrogen transformations, retention and losses in aquifers at the margin between watersheds and coastal waters, with a particular focus on the role of the organic nitrogen.

We propose to utilize anthropogenically generated variations in the isotopic signatures of N exported from the watersheds of Waquoit Bay to obtain isotopically labeled dissolved organic nitrogen of natural chemical composition. We will use ultrafiltration to concentrate the organic nitrogen from a sample of groundwater collected from an urbanized watershed, spike the sample with isotopically labeled inorganic nitrogen and a conservative tracer and inject the sample into the nearshore aquifer of a forested watershed. This is an innovative technique and, in contrast to past studies using additions of only inorganic nitrogen isotope tracers, will allow quantification of organic nitrogen cycling, in addition to the cycling of the other major forms of nitrogen.

This study will provide both basic and applied information on the influence of poorly studied segments of the aquatic cascade from land to sea on the magnitude and species composition of nitrogen loads transported to receiving waters. The results will improve management of coastal waters by updating nitrogen transport models and by providing a more accurate link between land-derived nitrogen loads and the eutrophication of receiving waters.