Data on water quality parameters and daily mean water discharge from five USGS monitoring stations were obtained from web sites and from correspondence with USGS personnel. The stations examined (Table 7.2) were selected as representative of important components of the Hudson River basin. Stations at Waterford and Cohoes include discharge from the upper Hudson River (including the Adirondacks) and Mohawk River, respectively, together including runoff from about 60 percent of total basin area. Monitoring data at Green Island should yield the combined influx to the northern end of the tidal Hudson, which is about 250 kilometers upstream of the Battery. All three of these stations have reported daily mean water discharge for a significant fraction or all of the period of water quality monitoring data. Poughkeepsie, situated approximately midway through the tidal reach of the Hudson (116 km upstream of the Battery), has more than 85 percent of basin area upstream of the site. Direct freshwater discharge gauging data based on stage measurements at Poughkeepsie are not available due to the tidal nature of the Hudson at this site. Saline intrusions are not reflected in the monitoring data at Poughkeepsie discussed here; however, during major droughts the salt front has reached that far upstream. USGS monitoring during the recent drought watch (February 2002) placed the salt front near Wappingers Falls (15 km south of Poughkeepsie). A significant fraction of natural runoff from western tributaries of the tidal Hudson upstream of Poughkeepsie are stored in reservoirs and diverted through the Catskill and Delaware Aqueducts to NYC, and thus do not influence water chemistry at this station. Water from the NYC municipal supply is returned after use to the downstream estuarine portion of the Hudson via a number of large wastewater treatment facilities in the NYC metropolitan area. Finally, data for Esopus Creek are discussed as representative of surface water quality entering NYC municipal storage reservoirs in the Catskill Mountains.
Data from each of these five monitoring stations were compiled and pruned here to eliminate samples for which relatively complete major element chemistry data were not available. Periods of monitoring varied significantly among the stations, and there were often substantial intervals with little or no data. The total number of individual samples with relatively complete major element chemistry data (Table 7.3) ranged from 37 (Poughkeepsie) to 204 (Esopus Creek). During high discharge episodes, more frequent samples were sometimes collected. For purposes of comparison among stations, median values for each of the five monitoring stations are examined (Table 7.3). Median andmean concentrations were generally similar for most chemical species.
Median total dissolved solids (excluding SiO2) was lowest for Esopus Creek (33 mgl-1), and highest for the Mohawk River at Cohoes (192 mg l-1). Green Island (124 mg l-1) and Poughkeepsie (131 mg l-1) TDS values were similar to each other and intermediate between Cohoes and Waterford (88 mg l-1), as would be expected. Thus, the range of median TDS values for these five stations was about a factor of six. A similar, relatively large range of median concentrations was also observed for HCO3-, Cl-, Ca2+, Mg2+, and Na+. In contrast, the range of median SiO2 concentrations (2.6 to 4.6 mg l-1) was much smaller. The above data suggest that chemical weathering rates involving silicate minerals appear to be more uniformly distributed in the basin relative to weathering of formations with appreciable carbonate and evaporite minerals, which show much greater geographical variability.
[HCO3-] can be used as a first order index of inputs of major ions derived from carbonate and silicate weathering (Drever and Zobrist, 1993). At the five monitoring sites, [SO42-] increased in the same order as [HCO3-] (Fig. 7.3A; Esopus < Waterford < Green Island < Poughkeepsie < Cohoes), but the last four showed less of a range in [SO42-] than in [HCO3-]. Potential inputs of [SO42-] include wet and dry deposition, chemical weathering of sulfides and gypsum, as well as direct anthropogenic sources. The Esopus sampling point has low [HCO3-] and [SO42-], reflecting the lack of easily weathered carbonates, sulfides, or evaporite minerals in the central Catskills. Using the same approach comparing [Cl-] and [HCO3-] (Fig. 7.3B), the Cohoes median [Cl-] was substantially higher than for all of the other sites. These data suggest that the Mohawk River basin experiences greater chemical weathering of carbonates, and perhaps halite, than any other large area of the Hudson basin. Moreover, anthropogenic sources of [Cl-] probably contribute significantly to the Mohawk at Cohoes (Phillips, 1994). The importance of chloride influxes in the Mohawk basin compared to the other regions of the Hudson basin is further illustrated by the high ratio of [Cl-] to [SO42-] (Fig. 7.3C). The relatively
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