Estuarine mixing

Figure 1. Schematic cartoon for idealized estuarine mixing of a dissolved component versus salinity, which serves as a conservative measure of the degree of mixing between freshwater and seawater. Redrawn after Berner and Berner (1987).

River inputs. The riverine endmember is most often highly variable. Fluctuations of the chemical signature of river water discharging into an estuary are clearly critical to determine the effects of estuarine mixing. The characteristics of U- and Th-series nuclides in rivers are reviewed most recently by Chabaux et al. (2003). Important factors include the major element composition, the characteristics and concentrations of particular constituents that can complex or adsorb U- and Th-series nuclides, such as organic ligands, particles or colloids. River flow rates clearly will also have an effect on the rates and patterns of mixing in the estuary (Ponter et al. 1990 Shiller and Boyle 1991). 

Figure 3. The concentration of nranium (nM) versns salinity on the Amazon Shelf with an ideal dilntion line drawn throngh the riverine and seawater end members. Removal of dissolved U is evident at salinities that range from 0 to 16. The bottom illustration shows as a function of salinity for the same waters on the Amazon Shelf. A seawater value (144 0.2) is rapidly reached at a salinity of 4 during estuarine mixing. The high turbidity zone of the water colurtm is defined by the greatest suspended particulate concentrations. Data from Swarzenski et al. (2003).

Atchafalaya and Mississippi Rivers. Florida Bay waters, which overlie U-rich sediments, contain much higher ( Ra/ " Ra) activity ratios than other estuaries. The increased ( Ra/ " Ra) values observed at high salinities in the Mississippi/Atchafalaya systems indicate preferential decay of the shorter-lived ""Ra over Ra during estuarine mixing. 

Kuma, K., Tanaka, J. and Matsunaga, K. (1999). Effect of natural and synthetic organic-Fe(III) complexes in estuarine mixing model on iron uptake and growth of coastal marine diatom Chaetoceros sociale, Mar. Biol., 134, 761-769. 

Calculated equilibrium speciation of (a) mercury and (b) copper during estuarine mixing of hypothetical river water with seawater. Hum, humic substance. Note logarithmic scale on y-axis. Source. From Mantoura, R. F. C., et al. (1978). Estuarine and Coastal Marine Science 6, 387 08. 

Mayer, L.M. (1982) Aggregation of colloidal iron during estuarine mixing Kinetics, mechanism, and seasonality. Geochim. Cosmochim. Acta 46 2527-2535... 

Moore, R.M., Burton, 3.D., Williams, P.3., Le B. and Young, M.L., 1979. The behaviour of dissolved organic material, iron and manganese in estuarine mixing. Geochim. Cosmochim. Acta, 43 919-926. 

Detailed investigations were carried out in the estuaries of some of the European rivers such as the Elbe, Weser and the Ems (Fig. 12) in order to understand the nature of these processes. The major emphasis during these studies was the behaviour of organic matter during estuarine mixing. 

Bewers, 3.M. and Yeats, P.A., 1981. Behaviour of trace metals during estuarine mixing. In 3.M. Martin, 3.D. Burton and D. Eisma (eds), River Inputs to Ocean Systems. UNEP and UNESCO, Switzerland, pp. 103-115. 

Fox, L.E. (1983) The removal of dissolved humic acid during estuarine mixing. Estuar. Coastal Shelf Sci. 16, 413 140. 

Liss, P.S. (1976) Conservative and non-conservative behavior of dissolved constituents during estuarine mixing. In Estuarine Chemistry (Burton, J.D., and Liss, P.S., eds.), pp. 93-130, Academic Press, London. 

Sholkovitz, E.R., Boyle, E.A., and Price, N.B. (1978) The removal of dissolved humic acids and iron during estuarine mixing. Earth Planet. Sci. Lett. 40, 130-136. 

Some studies have reported conservative behavior during estuarine mixing. In the unpolluted Krka Esmary of Yugoslavia, Seyler and Martin (1991) observed a linear increase in total arsenic with increasing salinity, ranging from 0.13 xgL in freshwaters to 1.8 JLgL offshore. Other studies however, have observed nonconservative behavior in estuaries due to processes such as diffusion from sediment pore waters, co-precipitation with iron oxides, or anthropogenic inputs (M. O. Andreae and T. W. Andreae, 1989 Andreae et al., 1983). The flocculation of iron oxides at the freshwater-saline interface as a result of increase in pH and salinity can lead to major decrease in the arsenic flux to the oceans (Cullen and Reimer, 1989). 

Li Y.-H., Burkhardt L., and Teraoka H. (1984) Desorption and coagulation of trace elements during estuarine mixing. Geochim. Cosmochim. Acta 1879-1884. 

Fig. 6.3 Idealized plots of estuarine mixing illustrating conservative and non-conservative mixing. Cp and Csare the concentrations of the ions in river and seawater respectively. After Burton and Liss (1976), with permission from Elsevier Science.

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