where N is the DIN source and |j. is a loss/uptake rate. The same value of ja was used for all three 
nitrogen sources and the value of p was determined by fitting total modeled DIN 
(Nocean+Nriver+Nwwtj) to observations of DIN within the estuary. The best fit to observations was 
found with p = 0.1 d' 1 . Simulations were also performed with no uptake (p = 0) which is 
equivalent to conservative transport of the sources. The results from the transport model were 
used to mix the three nitrogen sources using the following equation 
$m ~ //A + fo$o + fw$w 
Ir +fw + fo=\ 
where fR,fw, and fo are the fractions of riverine, wastewater treatment facility, and oceanic DIN, 
respectively, and 5r, 5w, and 5r are the isotopic end members for riverine, wastewater treatment 
facility effluent, and oceanic sources, respectively. Estimates of the oceanic and riverine end 
members were obtained by examination of the observed isotope ratios at the stations located near 
the mouth of the estuary and in the riverine portion of the estuary and comparison to the 
literature. The initial estimate for the WWTF end member (8w = 15-22%o) was determined from 
the literature. To arrive at the final end member isotope ratios, model simulations were 
performed varying each end member over the range estimated from the data and literature. The 
final isotope ratio of end members for the three sources (5r=2%o, 5w = 20%o, and 5o=8.4%o) was 
determined from the best fit (minimum root mean square error, RMSE) between predicted and 
observed isotope ratio at the five isotope sampling stations during 2003 and 2004. The final 
oceanic end member selected is consistent with marine end members for the west coast of the 
United States (Fry et al. 2001). While the riverine end member is consistent with the isotope 
ratio expected for nitrogen associated with red alder (leaf tissue ranges between -3 and -0.5% o ; 
Hobbie et al. 2000; Tjepkema et al. 2000; Cloem et al. 2002). 
List of Quality Assurance Project Plans (QAPPs) Used in This Study 
QAPP98.04. Evaluation of the Susceptibility of Eelgrass Beds in Oregon Estuaries to Changes in 
Watershed Uses. R. J. Ozretich, EPA, 1998. 
QAPP 2000.01. Changes in the Abundance and Distribution of Estuarine Keystone Species in 
Response to Multiple Abiotic Stressors. T.H. DeWitt, EPA, 2000. 
QAPP 01.02. Modeling of Landscape Change Effects on Estuarine Trophodynamics: an 
Optimization Approach Using Inverse and Forward Modeling. P. Eldridge, EPA, 2001. 
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