SS3.05 Land-use, Groundwater and Lotic Ecosystems
Date: Monday, June 10, 2002
Time: 10:45:00 AM
Location: Colwood
 
ColmanJA, U.S. Geological Survey, Northborough, USA, jacolman@usgs.gov
Masterson, J, P, U.S. Geological Survey, Northborough, USA, jpmaster@usgs.gov
Portnoy, J, W, National Park Service, Wellfleet, USA, Portnoy@nps.gov
Lee, K, , National Park Service, Wellfleet, USA, Krista_Lee@nps.gov
 
USE OF 3-D SOLUTE TRANSPORT MODELING TO ESTIMATE GROUND-WATER NITROGEN LOADS TO A COASTAL EMBAYMENT OF THE CAPE COD NATIONAL SEASHORE, USA
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A subregional, steady-state 3-D MODFLOW model encompassing the 7.5 km2 ground-water contributing area of Nauset Bay, a coastal embayment, was used with transient MT3D/RT3D reactive solute-transport modeling to estimate nitrogen loads from ground water to the Bay. Among nitrogen reactivity models tested were double Monod kinetics for denitrification with nitrate and organic carbon half-saturation constants. Nitrogen loading to the aquifer was determined from current and historical land use. Carbon loading estimates used previous results relating concentration to vadose zone thickness at recharge and to path length of saturated-zone transport. Model-predicted nitrogen concentrations were confirmed by measurements from multi-level samplers at 23 sampling points (depths up to 27 m) near the Nauset Bay shore. Nitrogen transport was decreased by denitrification during passage through an upgradient freshwater pond and increased by septic-system leach fields near the shore. Ground-water flow and nitrogen loading were focused by the geometry of Salt Pond, a drowned 0.1-km2 kettle hole that intrudes landward 0.6 km from the Nauset Bay shoreline. Nitrogen loading was an order of magnitute greater (water-body-surface-area basis) to Salt Pond compared to Nauset Bay.