N export from deposition
N export from agriculture
(% agricultural N fixation)
deposition of nitrogen is much greater closer to urban sources (Holland et al., 1999; NRC, 2000). On the other hand, a significant fraction of the N entering the freshwater Hudson is not exported to the saline estuary, due to the in-river processes of den-itrification and sedimentation (Lampman et al., 1999).
In the two decades between the 1970s and the 1990s, nutrient inputs from sewage decreased, due in part to improved sewage treatment (see Hetling et al., 2003 and Brosnan et al., Chapter 23, this volume, for further discussion). The improvements were designed to lower BOD loadings, and not nutrient levels, but nonetheless probably resulted in some reduction of nutrient loading. Population in the lower part of the Hudson watershed grew by only2percentbetween 1970 and 1990 (U.S. Census Bureau, 2002), and so total discharge of waste-water into the estuary likely remained almost unchanged over that time, at 3.4 x 106 m3 d-1. While a major new treatment plant, the North River plant, was built and came on line in the 1980s, the total wastewater flow into the saline estuary was not increased; the plant simply replaced the waste-water volume of raw sewage from fifty individual outlets from Manhattan to the Hudson estuary with the same volume of secondary-treated sewage (Clark etal., 1992;BrosnanandO'Shea, 1996).As of the early 1970s, 38 percent of the wastewater discharge into the Hudson estuary was raw sewage,
15 percent received primary treatment, and 47 percent received secondary treatment (calculated from data in Clark et al., 1992). Using the average concentration of nutrients in effluents from plants receiving those different levels of sewage treatment in the United States (Table 10.4; NRC, 1993), we estimate that nutrient loads from wastewater plants would have been 30 x 103 tons N y-1 in the early 1970s. Thus, the improved sewage treatment of the 1990s resulted in an estimated 25 percent decrease in N loadings to the saline estuary (Table 10.2). As noted above, our estimate for nitrogen loading from wastewater in the 1990s is in reasonable agreement with that derived from scaling down the estimates of Brosnan and O'Shea (1996), Hetling et al. (2003), and Brosnan et al. (Chapter 23, this volume). Similarly, scaling down the estimates of Hetling et al. (2003) and Brosnan et al. (Chapter 23, this volume) for the 1970s by the smaller effluent volume in the smaller watershed we are considering gives excellent agreement with our estimate. Considering all sources, we estimate that N loading to the Hudson River Estuary decreased by just over 10 percent, from 49 x 103 tonsNy-1,to43 x 103 tons N y-1, between the early 1970s and the mid 1990s (Table 10.2).
Phosphorus loadings to the Hudson estuary decreased substantially between the early 1970s and the mid 1990s not only because of some improvements in wastewater treatment but also because of
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