River water activity coefficients

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

The resulting species distribution (Table 6.7), as would be expected, differs sharply from that in seawater (Table 6.4). Species approach millimolal 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 HCOJ in seawater, in contrast, Na+ and Cl- predominate. Seawater, clearly, is not simply concentrated river water. 

Table 1.4. Composition and activity coefficients in mean river water (I = 2.08 x 10-3, pH = 7.0).

The results of the river water and seawater test cases computed by the aqueous models listed in Table I are summarized in Tables IV-X. Tables IV and V compare selected major and minor species computed for the river water test case, and Tables VI and VII make a similar comparison for the seawater test case. Table VIII compares activity coefficients computed for the major species in seawater and Table IX and X tabulate saturation indices for selected minerals in the river water and seawater test... 

Ba, Sr, and B. Consistency between programs was evaluated by comparing the log of the molal concentrations of free ions and complexes for two test solutions a hypothetical seawater analysis and a hypothetical river water analysis. Comparison of the free major ion concentrations in the river water test case shows excellent agreement for the major species. In the seawater test case there is less agreement and for both test cases the minor species commonly show orders of magnitude differences in concentrations. These differences primarily reflect differences in the thermodynamic data base of each chemical model although other factors such as activity coefficient calculations, redox assumptions, temperature corrections, alkalinity corrections and the number of complexes used all have an affect on the output. 

One approach to determine the reliability of geochemical codes is to take well-defined input data and compare the output from several different codes. For comparison of speciation results, Nordstrom et al. (1979) compiled a seawater test case and a river-water test case, i.e., seawater and river-water analyses that were used as input to 14 different codes. TTie results were compared and contrasted, demonstrating that the thermodynamic databases, the number of ion pairs and complexes, the form of the activity coefficients, the assumptions made for redox species, and the assumptions made for equilibrium solubilities of mineral phases were prominent factors in the results. Additional arsenic, selenium, and uranium redox test cases were designed for testing of... 

Figure 9.28. The distribution of nonpolar organic substances between aquatic solids and water (as given by the distribution coefficient Kp) is dependent upon the lipophilicity of the compound and the organic C content of the adsorbing material (foe = weight fraction). The solid phases considered here are coastal sea and lake sediments, river sediments, solids from aquifers and biomass (activated sludge). The octanol/water distribution coefficients are, respectively 500, 2400, 11,200 and 52,000 for chlorobenzene, 1,4-dichlorobenzene, 1,2,4-trichlorobenzene and 1,2,4,5-tetrachlorobenzene. (Modified from Schwarzenbach and Westall, 1980).

The challenge of correcting concentrations to activities for natural waters is that the activity coefficients vary non-linearly, often in complex relations to bulk ion concentrations. For dilute electrol5fte solutions, such as some lake and river waters, it is practicable to estimate the activity coefficient of an individual ion theoretically based on that ion s charge and a general measure of the effective total ion concentration of the bulk solution. The latter measure is called the ionic... 

Experience gained during the last decade in the determination of CBs in off-shore surface and deep waters has shown that concentrations are extremely low, much lower than reported earlier (see Table 22-1). Concentrations reported for the Mediterranean Sea (Tolosa et al, 1997 Schulz-Bull et al., 1997), the North Sea Schulz-Bull et al., 1991) and the Baltic Sea Schulz-Bull et al., 1995) were well above those found in surface waters of the open ocean Iwata et al., 1993 Schulz-Bull et a/.,1998). In deep-ocean water Schulz et al., 1988 Petrick et al, 1996 Schulz-Bull et al, 1998) much lower concentrations were found than in surface waters. In North Atlantic Deep Water, values of individual CBs were found to be < 0.01 pg/L, yet concentrations in solution were higher than those in suspended material on an equal volume basis. It turns out that the distribution of CBs between solution and suspension is determined primarily by molecular properties (characterized by octanol/water distribution coefficients). However, biological processes disturb the establishment of equilibria. This phenomenon has been observed in river water, in estuarine and coastal waters and during biologically active periods in the surface layer of the open ocean. 

Under equiUbrium or near-equiUbrium conditions, the distribution of volatile species between gas and water phases can be described in terms of Henry s law. The rate of transfer of a compound across the water-gas phase boundary can be characterized by a mass-transfer coefficient and the activity gradient at the air—water interface. In addition, these substance-specific coefficients depend on the turbulence, interfacial area, and other conditions of the aquatic systems. They may be related to the exchange constant of oxygen as a reference substance for a system-independent parameter reaeration coefficients are often known for individual rivers and lakes. 

Artificial radionuclides in bottom sediments of the Don River Estuary-Azov Sea (Black Sea) were measured to examine the storage and migration of radionuclides within this inland sea and to estimate the annual dose received by individuals in the local population who regularly consume fish (Matishov et al. 2002). The specific activity of surface sediments was 0.5—100 Bq/kg dry wt. [mean = (33.8 25.9) Bq/kg dry wt., the number of samples = 57] for Cs, 0.2-5.7 Bq/kg dry wt. [mean= (2.1 1.4) Bq/kg dry wt., the number of samples = 34] for Sr and 0.31-0.51 Bq/kg dry wt. (the number of samples = 2) for Pu in 1997—1999. Cs specific activities increase with distance from the mouth of the Don River and correlate negatively with the sediment grain size (square of the correlation coefficient, / = 0.77, the number of data points, n = 21). The annual Cs-derived dose received by an individual through the tropic chain water—fish-humans (approximately 10 Sv/a) is well below regulatory recommended limits. 

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