Chemical weathering influxes to hudson basin stream chemistry

For the other four stream stations, the contribution of wet and dry deposition is much less important to stream chemistry than for Esopus Creek. The Mohawk River at Cohoes has TDS (median = 196 ppm) about a factor of six greater than Esopus Creek (Table 7.6). Here, chemical weathering of carbonates andpossiblyevaporite minerals (plus anthropogenic inputs) appear to dominate the inputs of major ions (Fisher, Isachsen, and Rickard, 1971; Garvey, 1990). Although subject to biological regulation via diatom growth and shell dissolution (Clark et al., 1992), relatively low dissolved silica levels at all five of the gauging points indicate that chemical weathering of primary silicate minerals provides only amodest contribution to stream chemistry in the Hudson basin.

Hudson River median TDS at Green Island (128 ppm) and Poughkeepsie (134 ppm) are similar, and shouldbe reasonably representative of mean freshwater discharge composition of the Hudson at the

Battery, exclusive of influxes of large amounts of wastewater in the NYC area. The main stem of the Hudson has a median composition similar to that calculated for the global flux-weighted "average" of all rivers (Meybeck, 1979; Berner and Berner, 1996). Compared to the global average river (TDS = 110 ppm), the Hudson River has about 20 percent greater TDS, with higher values for HCO3-, SO42-, Cl- and Ca2+, and significantly lower concentrations of SiO2. These data are consistent with chemical weathering of carbonates (and possibly evaporites) slightly greater per unit volume of surface water discharge than would be representative for global weathering of all continental areas, but appreciably less than average continental weathering of primary silicate minerals. Thus the Hudson River, draining a relatively small area (3.46 x 104 km2), has a distribution of rock types available for chemical weathering that results in stream chemistry in the main stem of the tidal Hudson which is surprisingly similar for most ion concentrations to the flux-weighted global river average.

The chemical flux of dissolved ions from the Hudson River basin is approximately 70 T (km2)-1 yr-1, compared to a global average estimated for natural conditions of 42 T (km2)-1 yr-1 (Berner and Berner, 1996). There is a large range of riverine chemical flux rates per unit area, depending upon the dominant types of rocks within a particular basin. At the low end are crystalline igneous and metamorphic rocks, and silica rich sediments [18 T (km2)-1 yr-1 ], while carbonates (100 T (km2)-1 yr-1) and evaporites (420 T (km2)-1 yr-1) generate much higher chemical weathering fluxes by rivers (Meybeck, 1987). Thus, relatively small areas of the latter two types of rocks can have significant influence on the chemical composition of a given river.

The global mean flux per unit area of riverine suspended particles is 226 T (km2)-1 yr-1 (Berner and Berner, 1996), a factor of 5.4 greater than the chemical flux. The total downstream transport of fine suspended particles to the Hudson estuary appears to be about one million tons, similar to the long-term annual average of dredging of fine-grained sediments from NY Harbor (Olsen, 1979; Olsen et al., 1984-1985; Chillrud, 1996). This translates into a net fine particle flux of about

29 T (km2)-1 yr-1, about a factor of eight less than the global average for all rivers. The ratio of chemical flux to particle flux by the Hudson River is about 2.4, while for the global average riverine flux this ratio is about 0.2. Relatively low suspended particle transport by the Hudson probably reflects the large proportion of forested basin area (Phillips and Hanchar, 1996), moderate topography, and uniform average monthly precipitation rate through the year. From the viewpoint of surface water management in the basin, low riverine transport of suspended particles is a major advantage, resulting in relatively low rates of silt accumulation in water-storage reservoirs and harbors, compared to high suspended particle flux rivers. Low suspended particle levels also reduce water treatment complexity and costs for municipal water supplies. Surface waters in the Hudson basin, especially in forested upland areas such as the Catskill Mountains, generally represent high quality raw water sources for municipal supplies.

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