Info

Figure7.4. Hudson River basin stream gauge water [Ca2+]vs [HCO3-] concentrations: (A) Cohoes; (B) Waterford; (C) Esopus Creek.

Figure7.4. Hudson River basin stream gauge water [Ca2+]vs [HCO3-] concentrations: (A) Cohoes; (B) Waterford; (C) Esopus Creek.

in |Eq l-1 reveal several general features of the major ion chemistry. Absolute concentrations decrease from Cohoes (Fig. 7.4A) to Waterford (Fig. 7.4B) to Esopus (Fig. 7.4C). This observation indicates that stream chemistry in the Catskills is less buffered by soil and bedrock carbonate minerals against acid precipitation impacts than the Mohawk and downstream por tions of the upper Hudson. The correlation of [Ca2+] with [HCO3-] at Cohoes (Fig. 7.4A) was quite high (r2 = 0.92), consistent with weathering of carbonate minerals being the primary source of Ca2+.

Deviations of the slope from unity provide a measure of the relative importance of carbonate and silicate weathering inputs of the other major

Cl(mEq/L)

Figure 7.5. Hudson River basin stream gauge median [Na+] vs [Cl-] concentrations: (A) Cohoes; (B) Waterford.

Figure 7.5. Hudson River basin stream gauge median [Na+] vs [Cl-] concentrations: (A) Cohoes; (B) Waterford.

cations. The other major cations were most significant at Esopus (slope = 0.52) and least important at Cohoes (slope = 0.87). The positive intercepts reflect the importance of other major anions, especially SO42-. Although dissolution of gypsum cannot be ruled out, the lack of identifiable gypsum deposits in the Hudson-Mohawk basin focuses attention on two other sources of sulfate. Atmospheric deposition and oxidative weathering of iron sulfide minerals (pyrite) deliver sulfate as sul-furic acid. Whether the strong acid reacts with bicarbonate in solution or chemically weathers carbonates or silicates, the net effect on the water chemistry is the same - an increase in the relative importance of SO42- as amajor anion. Atmospheric deposition of sulfate appears to dominate inputs to Esopus (see below). Iron sulfide mineral weathering as a source of sulfate has been described in detail for groundwaters of the Mohawk basin (Bator, 1997).

The correlation of [Na+] with [Cl-] at Cohoes (Fig.7.5A) was quite high (r2 = 0.93), consistent with the dominant fraction of Cl- influx to the Mohawk being derived from halite dissolution, including both natural and anthropogenic sources (Phillips, 1994). The slope of [Na+ ] vs [Cl-] at Waterford (Fig. 7.5B)is appreciably lower than at Cohoes, with a moderate degree of overlap in the scatter plot distributions. This trend is also evident for Esopus Creek, with far lower concentrations of both ions than observed for Waterford, suggesting that other processes influence Cl- concentrations in this Catskills catchment. Atmospheric sources related to coal combustion couldplausibly provide a significant fraction of Cl- transported in streams of this area.

Stream concentrations of major ions at Cohoes (Fig. 7.6A), Waterford (Fig. 7.6B) and Esopus Creek (Fig. 7.6C), all are systematically lower at higher discharge rates (Q), but the proportional decrease at higher Q was greater at the Catskills site than for the Mohawk and upper Hudson. (Note the large range of observed concentrations at relatively low stream discharge rates for each site. Aggregating all chemical monitoring data as a function of only stream discharge rate is likely to obscure some contributing processes to the observed range of values, and should be viewed with caution.) A scatter plot of [Ca2+] vs Q-daily values for Cohoes (Fig. 7.6A) indicates that a relatively large fraction of the total pool of samples were collected when discharge rates were quite low. The distribution of samples as a function of discharge was significantly different for Waterford (Fig. 7.6B).

Was this article helpful?

0 0

Post a comment