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Surface runoff from the Hudson River basin (Fig. 7.1) is currently diverted in large amounts for public water supplies, including about half of that supplying the New York City (NYC) municipal system, the largest in the nation. Many aspects of water quality in the Hudson basin are quite favorable for public water supplies. A major fraction of the NYC metropolitan area population resides in or near the downstream end of the basin. Population densities upstream of NYC are relatively low. Much of the uplands are forested and have very low population densities, especially in the Adirondack and Catskill Mountains, with the latter (including runoff from both the East and West Branch of the Delaware River) providing most of the municipal supply for NYC. The main stem of the tidal freshwater Hudson River, although of lower quality than upland streams in the Catskills, remains a valuable resource for public water supplies. This source is usedfor smaller cities such as Poughkeepsie, as well as the primary drought emergency supply for NYC through pumping from a station at Chelsea, New York (103 km upstream of the Battery).

Figure 7.1. Hudson River drainage basin with selected discharge gauging (triangles) and precipitation chemistry (X) monitoring locations indicated.

The major ion content of surface waters is largely determined by inputs provided by precipitation, dry deposition from the atmosphere chemical weathering of rock and soil minerals, and anthropogenic loadings, and then also by biogeochem-ical reactions that take place within the system. Examination of major ion composition in precipitation and surface waters can provide information on these sources and biogeochemical reactions occurring within the basin. Furthermore, it allows us to understand basin-wide variations in major ion content, which can be thought of as one simple proxy of gross drinking water quality.

Precipitation chemistry in the Hudson River basin is similar to that found in much of the northeastern (NE) United States (Sisterson et al., 1991). Regional combustion of fossil fuels generates large quantities of acidic gases, resulting in rain and snow being, to the first approximation, dilute solutions of sulfuric and nitric acids. A few aspects of Hudson

River basin precipitation chemistry are outlined here, based on monitoring data from a National Atmospheric Deposition Program (NADP) station at West Point, New York.

Some general features of Hudson River basin water chemistry are briefly summarized here, based on monitoring data collected by the U.S. Geological Survey (USGS) at five surface water stations (Fig. 7.1) over the past four decades, as well as on observations obtained during several university research projects. Major element chemistry data are described as primarily indicative of chemical weathering processes in the basin. Dissolved organic carbon is discussed as a source of chlorina-tion byproducts generated during treatment of raw water supplies from several regional sources.

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