Particle, chemical transport aquatic systems

The distribution of metals between dissolved and particulate phases in aquatic systems is governed by a competition between precipitation and adsorption (and transport as particles) versus dissolution and formation of soluble complexes (and transport in the solution phase). A great deal is known about the thermodynamics of these reactions, and in many cases it is possible to explain or predict semi-quantita-tively the equilibrium speciation of a metal in an environmental system. Predictions of complete speciation of the metal are often limited by inadequate information on chemical composition, equilibrium constants, and reaction rates. 

As illustrated by the results presented in Figure 2 and in Table 2 at high ionic strength and high Ca2 + for favorable particle-particle interactions (e.g., in the deposition of non-Brownian particles, F = F%Taviiy + Fdrag +FlVDW Fchem = 0), transport models based on physical and hydrodynamic characteristics of a system can predict the initial kinetics of aggregation and deposition processes in aquatic systems quantitatively. In the presence of repulsive chemical interactions, however, quantitative theoretical predictions of such kinetics are very inaccurate and even many qualitative predictions are not observed. The determination of Fchem in aquatic systems merits study and development,- it is necessary for the quantitative prediction of the kinetics of colloid chemical processes in these systems. 

The subject of advective porewater flux and associated chemical transport is covered in Chapter 11 in the context of aquatic bed-sediment systems, which include surface soil-derived particle layers on the bottom of streams, rivers, lakes, estuaries, and the near-shore marine environment. This soil system is a saturated porous medium and therefore the fundamentals of the transport processes and related parameters within this system are identical to that of surface soils. A brief review of advective transport in the subsurface follows. 

As mentioned previously, the bed-sediment of aquatic systems is a saturated porous medium and is similar to soil in many respects. The basic process of chemical transport by diffusion is comparable in these two systems. Section 12.5 contains equations to estimate the effective diffusion coefficients for several chemicals in bed sediment. See specifically Equation 12.18, Archie s law, as well as Eigures 12.2 and 12.3. Information on typical porosities in natural sedimentary materials appears in Table 12.7 and Figure 12.4. This section also contains information on pore structure and particle distributions. Tables 12.9 and 12.10 contain compilations of data on this transport parameter in porewaters. Generally speaking, the porous material types, particle sizes, porosities, and structures are comparable to surface soils so that the information and data provided is very relevant to the user. 

Information on several other transport processes and MTCs at the sediment-water interface appear elsewhere in this handbook. Due to special nature of these subjects and the complexity of the processes, individual chapters are devoted to three of them. Chapter 10 is concerned with particle resuspension and deposition as it affects chemical transport in flowing water streams. Chapter 11 is concerned with water advection processes that contribute to enhanced chemical transport in the various aquatic-sediment bed systems. Chapter 13 is concerned with chemical biodiffusion processes in the sediment bed as a consequence of the presence of macrofauna in the surface layers (i.e., bioturbation). These three processes do not fit nicely into a chapter devoted to the more conventional diffusive process the contents of this chapter are as follows. 

Distribution pathways are summarized in Figure 1.1. First of all, there can be movement within a compartment for example, any chemical introduced into an aquatic compartment can move to the extent that the water moves, whether or not the chemical is in solution or sorbed on a particle. This movement would be defined by the appropriate hydrological parameters. A chemical may find its way into the atmosphere where it may be transported in atmospheric currents In this situation the appropriate meteorological phenomena will determine the rate and direction of movement. Distribution in a plant or animal wifi be controlled by the transport mechanisms in that organism either the vascular system in an animal or the phloem in a plant. In a much broader context, the transport of a chemical in an ecosystem must have some relation to the overall mass flow in the system since the chemical moves with the food constituents of the various components in the ecosystem. 

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