Amazon River water

We turn our attention to developing a chemical model of water from the Amazon River, using a chemical analysis reported by Hem (1985, p. 9). The procedure is 

TABLE 6.7 Calculated molalities (m), activity coefficients (y), and log activities ( ) of the most abundant species in Amazon River water 

The command precip = off tells the program to not let supersaturated minerals precipitate, since we are not especially interested in the fluid s true equilibrium state. 

The resulting species distribution (Table 6.7), as would be expected, differs sharply from that in seawater (Table 6.4). Species approach mmolal instead of molal concentrations and activity coefficients differ less from unity. In the Amazon River water, the most abundant cation and anion are Ca++ and HCO3 in seawater, in contrast, Na+ and Cl predominate. Seawater, clearly, is not simply concentrated river water. 

In the river water, as opposed to seawater, the neutral species O2(aq), CO2(aq), and SiO2(aq) are among the species present in greatest concentration. Complexing among species is of little consequence in the river water, so the major cations and anions are present almost entirely as free ions. 

It is tempting to place significance on the relative magnitudes of the saturation indices calculated for various minerals and then to relate these values to the amounts of minerals likely to precipitate from solution. The data in Table 6.6, however, suggest no such relationship. Thirteen minerals are supersaturated in the initial fluid, but the phase rule limits to ten the number of minerals that can form only two (dolomite and quartz) appear in the final phase assemblage. 

For a number of reasons, using saturation indices as measures of the mineral masses to be formed as a fluid approaches equilibrium is a futile (if commonly undertaken) exercise. First, a mineral s saturation index depends on the choice of its formula unit. If we were to write the formula for quartz as Si2C 4 instead of Si02, we would double its saturation index. Large formula units have been chosen for many of the clay and zeolite minerals listed in the llnl database, and this explains why these minerals appear frequently at the top of the supersaturation list. 

Second, at a given saturation index, supersaturated minerals with high solubilities have the potential to precipitate in greater mass than do less soluble ones. Consider a solution equally supersaturated with respect to halite (NaCl) and gypsum (CaS04-2H20). Of the two minerals, halite is the more soluble and hence more of it must precipitate for the fluid to approach equilibrium. 

for minerals with binary or higher order reactions, there is no assurance that the reactants are available in stoichiometric proportions. We could prepare solutions equally supersaturated with respect to gypsum by using differing Ca++ to SO4 ratios. A solution containing these components in equal amounts would precipitate the most gypsum. Solutions rich in Ca++ but depleted in SO4 , or rich in SO4 but depleted in Ca++, would produce lesser amounts of gypsum. 

Finally, common ion effects link many mineral precipitation reactions, so the reactions do not operate independently. In the seawater example, dolomite precipitation consumed magnesium and produced hydrogen ions, significantly altering the saturation states of the other supersaturated minerals. 

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Fig. 6.3. Saturation indices of Amazon River water with respect to various minerals (left) calculated directly from a chemical analysis, and (right) computed assuming that equilibrium with kaolinite and hematite controls the fluid s aluminum and iron content.

To construct an alternative model of Amazon River water, we assume that equilibrium with kaolinite (a clay mineral, Al2Si205 (OH)4) and hematite (ferric oxide, Fe203) controls the aluminum and iron concentrations ... 

The species distribution (Table 6.9) calculated for the brine differs from that of seawater and Amazon River water in the large molalities predicted and the predominance of ion pairs such as NaCl, CaCl+, and MgCl+. The complex species make up a considerable portion of the brine s dissolved load. 

Carbon is transported into the Amazon River/ocean mixing zone in numerous forms including dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). The average DIC, DOC, and POC concentrations in Amazon River water are 460, 275, and 180 pmol L-1 (Richey et al. 1991), which indicates that DOC supply exceeds POC supply and that DIC is the most abundant form of carbon in the river. Sholkovitz et al. (1978) noted that most riverine dissolved... 

Modeling efforts must be conducted with these various geochemical studies. The cycling and export of chemical compounds within the river/ocean mixing zone are dependent on estuarine and shelf circulation. Characterizing the end member concentrations in Amazon River water, surface ocean water, upwelled offshore water, and the northward flowing North Brazilian... 

It is interesting to compare the effects of complexing in the three waters we have studied so far. As shown in Table 6.10, the complexed fraction of each of the major dissolved components increases with salinity. Whereas complex species are of minor importance in the Amazon River water, they are abundant in seawater and account for about three-fourths of the calcium and sulfate and more than half of the magnesium in the Red Sea brine. 

Several studies have examined the partitioning of U on particles and colloids. Results from detailed sampling and particle separation in the Amazon estuary shows that most of the uranium at the Amazon River mouth is associated with particles (>0.4 im) and that >90% of the uranium in filtered water (<0.4 im) is transported in a colloidal phases (from a nominal molecular weight of 10 000 MW up to 0.4 im) (Swarzenski et al. 1995 Moore et al. 1996). Mixing diagrams for uranium in different size fractions in the Amazon estuary reveal that uranium in all size fractions clearly display both removal and substantial input during mixing. 

In more recent studies, Feng et al. (1999) calculated a Th water column residence time of 2 to 12 days in the Hudson River estuary. McKee et al (1986b) determined that " Th was removed on a time scale of a day or less in the very particle-rich environment of the Yangtze River estuary. In the Amazon River estuary, another particle-rich environment, McKee et al. (1986a) determined that the residence time of dissolved " Th ranged from 2 to 4 days. McKee et al. (1986a) also calculated apparent distribution... 

Composed of beautiful coral reefe, the Caribbean Sea and the Gulf of Mexico have provided unique marine natural products. However, the active area of the Caribbean is small compared to the Indo-Pacific the Brazilian coast is made inhospitable to coral reefe because of the fresh waters brought in by the Amazon river. Therefore, the natural product diversity of the Caribbean is second to the Indo-Pacific. 

The concentrations of DOC in major rivers typically range from 250 to 750 pM, and concentrations in the surface ocean range from 60 to 90 J.M (Table I). Most of the river data compiled in Table I are from the Amazon River system (Hedges et al., 1994, 2000), the Parana River system (Depetris and Kempe, 1993), and the Mississippi River (Benner and Opsahl, 2001). The seawater data are from surface water samples collected in the Pacific and Atlantic Oceans (see Table I for references). Total hydrolyzable neutral sugars (glucose, galactose, mannose, xylose, fucose, rhamnose, and arabinose) account for about 1-2% of river DOC and 2-6% of ocean DOC, indicating... 

Figure 7.7a Surface waters in the Amazon River (in March and June 1990) showing nonconservative behavior with decreasing sahnity the U removal at salinities less than 15 implies that hydrous metal oxides are likely responsible for adsorptive removal during flocculation and coagulation processes. (Modified from Swarzenski and McKee, 1998.)...

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