Nonconservative behavior

India (Borole et al. 1982) and the Forth estuary in the UK (Toole et al. 1987), nonconservative behavior of uranium was also demonstrated. In the Amazon estuary, uranium showed elevated concentrations compared to simple mixing (McKee et al. 1987). Release of uranium from bottom sediments on the shelf was suggested to be a source of dissolved (<0.4 im) uranium. However, subsequent studies in the Amazon also demonstrated that U removal (Fig. 3) occurred at salinities <12 (Swarzenski et al. 1995, Swarzenski et al. 2003). Overall, it was established that the behavior of U is highly variable examples have been found of conservative behavior as well as both additions and removal of U by interaction with sediments. 

In the Delaware and Chesapeake estuaries (USA), uranium shows distinctly nonconservative behavior at salinities <5 (Sarin and Church 1994 Church et al. 1996). This was suggested to be due to sedimentary redox processes in the extensive salt marshes in the Delaware and Chesapeake bays. From mass balance calculations it was concluded that almost two-thirds of the uranium in the tidal waters were retained in the sediments. It was also suggested that, extrapolated globally, uranium removal in salt marshes and marine wetlands, including mangroves, are important sinks for U that may responsible for up to 50% of the total marine removal (Church et al. 1996). Removal of U is also observed within the Baltic Sea, related to the association of U with colloids (see Section 2.5). 

As demonstrated by the polynomials in the equation of state of seawater, density is not linearly related to temperature or salinity and does not exhibit conservative behavior. One of the interesting consequences of this nonconservative behavior is that an... 

The nonconservative behavior of seawater density and compressibility is caused partly by H bonding and partly by the electrostatic attractions exerted by the salt ions on their neighboring water molecules. The effect of these attractions can be estimated by trying to compute the density of seawater as a simple sum of the volumes of water and salt present in 1 kg of seawater (5 = 35%o and t = 4°C). As shown in Table 3.6, the actual density, as tabulated in the online appendix on the companion website (ct, = 27.81, so p = 1.02781 g/cm ), is about 1% higher than that predicted from summing the volumes of salt and water (1.0192 g/cm ). 

In Chapter 4, we saw how conservative chemicals are used to trace the pathway and rates of water motion in the ocean. True conservative behavior is exhibited by a relatively small number of chemicals, such as the major ions and, hence, salinity. In contrast, most of the minor and trace elements display nonconservative behavior because they readily undergo chemical reactions under the environmental conditions found in seawater. The rates of these reactions are enhanced by the involvement of marine organisms, particularly microorganisms, as their enzymes serve as catalysts. Rates are also enhanced at particle interfaces for several reasons. First, microbes tend to have higher growth rates on particle surfaces. Second, the solution in direct contact with the particles tends to be highly enriched in reactants, thereby increasing reaction probabilities. Third, adsorption of solutes onto particle surfaces can create fevorable spatial orientations between reactants that also increases reaction probabilities. 

The easiest technique fiar establishing the nonconservative behavior of an element, or one of its chemical species, is to compare its concentration to that of a conservative tracer. Salinity is typically used because it is a standard measurement performed on most seawater samples. 

Possible salinity relationships resulting from conservative and nonconservative behavior when (a) the concentration of a chemical A in the lower salinity end member is lower than that of the higher salinity end member and (b) the concentration of a chemical B in the lower salinity end member is greater than that of the higher salinity end member. 

Most deviations from NAECs are the result of nonconservative behavior involving chemical processes that remove or supply the gas fester than the water mass can reequilibrate with the atmosphere. As shown in Figure 6.2, large areas of the surfece ocean... 

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.)...

The three isotopes of uranium (238U, 235U, 234U) found in nature have longer half-lives (>103 years) than the oceanic mixing time (ca. 103 years). The distribution and concentration of U in rivers, estuaries, and coastal regions are extremely variable and not well understood. More work is clearly needed to understand further the complex interactions of active and carrier phases (Fe and Mn oxides), redox transformations, direct and indirect microbial transformations, and colloidal complexation that may be involved in the nonconservative behavior of U in estuaries. 

Ertel et al., 1986 Lisitzin, 1995), inputs from porewaters during resuspension events (Burdige and Homstead, 1994 Middelburg et al., 1997), and atmospheric inputs (Velinsky et al., 1986) can all contribute to nonconservative behavior in estuaries. 

Sorption-desorption from suspended particulates and sediment fluxes play a large part in controlling the nonconservative behavior of dissolved concentrations of Fe and Mn in estuaries. 

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). 

---