Introduction A recent analysis of population distribution has con- cluded that nearly 75% of the United States population resides within 80 km of coastal waters (National Research Council 2000). Population growth and urbanization in these nearshore areas contribute to eutrophication of coastal waters by increased nutrient (nitrogen and phosphorus) loading from disposal of domestic and municipal wastes. This problem is of particular concern on Cape Cod, Mass- achusetts, where the U.S. Census Bureau reports that the population has increased by > 130% over the past 30 yrs. In marine waters, nitrogen is the nutrient of greatest concern because it is often the limiting nutrient for primary produc- tion (Valiela et al. 1997a). A substantial component of nitrogen transport to the shore is by way of coastal aquifers. Direct discharge of waste to coastal surface water or streams is becoming uncommon because of the consequent eutrophication hazard (or expense of constructing and maintaining tertiary treatment plants) and out-of-basin transport issues (Jantrania 2000). In addition to nitrogen from waste water and solid waste that is disposed of on land, nitrogen also enters aquifers through atmospheric deposition and through fertilizer use (Valiela et al. 1997b). Waste disposal on land decreases coastal nitrogen load- ing by comparison with direct discharge to surface waters, as significant quantities of nitrogen can be lost during trans- port through the soil (Christensen et al. 1990; Groffman and Tiedje 1989; Parkin 1987), unsaturated zone (Valiela et al. 1997b; DeSimone and Howes 1998), and aquifer (Korom 1992; Pabich 2000; Pabich et al. in press; Valiela et al. 1992; Valiela et al. 2000; Westgate et al. 2000) before discharge to surface waters. The degree and manner in which nitrogen is Abstract Effects of aquifer travel time on nitrogen reaction and loading to Popponesset Bay, a eutrophic coastal embay- ment on western Cape Cod, Massachusetts, are evaluated through hydrologic analysis of flow and transport. Approx- imately 10% of the total nitrogen load to the embayment is intercepted by fresh water ponds and delivered to the coast by connecting streams. For the nitrogen load not intercepted by ponds, we compare two steady-state methods of ana- lyzing nitrogen loss in the aquifer, one using a constant-loss factor and the other time-dependent loss rates. The con- stant-loss method, which assumes that all similar land uses have the same per unit area loading rate to surface water regardless of location within the watershed, predicts that 42% of the nonpond watershed nitrogen load originated within the zero to 2 yr time-of-travel zone, which is 40% of the contributing area. The time-of-travel loss method cal- culates loss rates based on aquifer travel times and denitrification reaction kinetics, evaluated separately for carbon- unlimited and carbon-limited cases. Time-of-travel loss calculations for percent of nonpond load that originated within the area of < 2 yr aquifer residence time are 64% when carbon is not limiting, but only 49% when carbon lim- itation is included, not greatly different from the constant-loss method. A feature of the kinetics used is that carbon (and the denitrified nitrogen) is lost rather quickly in the aquifer travel path, after which carbon limitation stops den- itrification altogether. Carbon limitation causes the time-of-travel loss model to approximate the constant-loss model such that in most of the watershed, a nearly constant fraction of the nitrogen input is lost in both models. 1069 Effects of Aquifer Travel Time on Nitrogen Transport to a Coastal Embayment by John A. Colman 1 , John P. Masterson 2 , Wendy J. Pabich 3 , and Donald A. Walter 4 1 Hydrologist, U.S. Geological Survey, Northborough, MA 01532; (508) 490–5028; jacolman@usgs.gov 2 Hydrologist, U.S. Geological Survey, Northborough, MA 01532; (508) 490–5027; jpmaster@usgs.gov 3 Tetra Tech EM Inc., 125 Cambridge Park Dr., Cambridge, MA 02140; (617) 576–1221, ext. 29; wendy.pabich@ttemi.com 4 Hydrologist, U.S. Geological Survey, Northborough, MA 01532; (508) 490–5050; dawalter@usgs.gov Copyright © 2004 by the National Ground Water Association. Vol. 42, No. 7—GROUND WATER—Oceans Issue 2004 (pages 1069–1078)