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a Dates forwhich USGS Water-Quality Monitoring Networks (WQN) data is available.

ions, and lesser concentrations of sulfate and nitrate. Mean annual concentrations in NY99 precipitation of both Na+ and Cl- were often greater during the 1990s than the 1980s (Fig. 7.2B). Mean annual concentrations of both ions tended to vary together (r2 = 0.93), consistent with their dominant sources being from marine aerosols. Increased frequency and/or intensity of marine air precipitation episodes during the latter half of the monitoringpe-riod (1984-99) couldhave also contributed to lower mean annual concentrations of sulfuric and nitric acids. Thus, trends in mean annual precipitation ionic concentrations over the period 1984-99 may reflect a higher proportion of marine-source precipitation episodes, as well as regional changes in acidic gas emission rates.

The molar ratio of Cl- to Na+ in amount-weighted precipitation for each year from 1984-99 (range: 1.25 -1.78) was always greater than the sea-water molar ratio (Cl-/Na+ = 1.17). Assuming marine aerosols are the only significant source of Na+ in these samples, then there must be significant source(s) of Cl- in addition to marine aerosols. Using mean equivalence values of Cl- and Na+ for this sixteen-year period (Table 7.1), the nonmarine aerosol source of Cl- represents about 26 percent of total Cl- in precipitation (Cl-/Na+ = 1.48; compared to Cl- /Na+ in seawater = 1.17). In an analysis of rainfall over the United States from July 1955 to June 1956 none of the more than 60 stations had average Cl- /Na+ ratios in excess of that in seawater (JungeandWerby, 1958). However, Junge (1963) cited studies of precipitation chemistry in Europe from that same period where excess Cl- was found in precipitation samples from urban and industrial areas. The release of HCl during combustion of coal was suggested as the probable source.

Precipitation for much of the Hudson basin would be expected to have somewhat lower concentrations of Cl- and Na+ than West Point, due to longer transport pathways for air from marine aerosol sources (JungeandWerby, 1958). Thus data from NY99 should provide an upper limit to likely contributions of wet deposition Cl- to surface waters in most of the basin. Dry deposition of aerosols and gases from the atmosphere can also deliver significant inputs of dissolved ions to surface waters in the Hudson basin.

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