Biodiffusion

TABLE 6. Spermicidal/Cervical Mucus Blocking/Cervical Mucus Biodiffusion Activity of Various Formuiations. 

Typical DB values for Long Island Sound estuary (USA) and Chesapeake Bay have been estimated to range from 0.5 to 110 cm2 y-1 (Aller and Cochran, 1976 Aller et al., 1980) and 6 to >172 cm2 y 1 (Dellapenna et al., 1998), respectively. Since mixing is clearly not a one-dimensional process, estimated biodiffusivity values may seriously underestimate mass transport when mixing is affected by horizontal advection (Wheatcroft et al., 1991). 

Wheatcroft R. A., Jumars P. A., Smith C. R., and Nowell R. M. (1990) A mechanistic view of the particulate biodiffusion coefficient step lengths, rest periods and transport directions. J. Mar. Res. 48, 177-207. 

Fig. 3.28 Biodiffusion coefficients (particle mixing) in relation to their empirically determined dependence on the rate of sedimentation (after Tromp et al. 1995).

The above restrictions for the calculations of the sedimentation rate and the biodiffusion coefficient imply the use of radioactive isotopes with different half-lifes (t = 0.693 A, ) for different purposes and depositional environments. The higher the sedimentation rate, the shorter should be the half-life of the radioactive isotope. The more intense bioturbation in the surface layer, the shorter should be the half-life of the applied radioactive isotope be. For coastal and shelf sediments sedimentation rates of several decimeters to few meters per 1000 years are typical and can be determined by Pb (t,j =... 

The intensity of bioturbation is expressed as biodiffusivity (Dg) and the effect of bioturbation versus the molecular diffusion is described as the ratio between Dg and Dj. A vast amount of literature is available on the role of macrobenthos on the exchange of solutes across sediment-water interface in freshwater and marine sediments. It should be noted that in wetlands, rooting of vegetation in soils creates additional complexity on exchange of solutes across soil-floodwater interface. The macrobenthos can influence the vertical distribution of sediments and POM, and the... 

In general, the total solute flux from sediments to overlying water column is the result of the sum of several mixing processes, which include molecular diffusion (Dg), biodiffusion (Dg), diffusion due to irrigation (D,), and mixing due to wave and current mixing The sum of all these effects... 

Enhanced chemical transport in the upper sediment layer by the process called bioturbation is termed biodiffusion. It moves both the particle bound and soluble chemical fractions due primarily to the activities of macroinvertabrates and is not dependent on a chemical concentration gradient. The parameters Ep and Ed in Equations 2 and 4 are termed the particle and pore water biodiffusion coefficients respectively (L t). 

Including the particle resuspenion velocity, the details of five individual mass-transfer coefficients were presented above. These appear in Table I normalized to the chemical concentration on solid particles in the bed. Although the particle resuspension and particle biodiffusion transport coefficients remain unchanged the ones defined with solute concentrations require the msKo term for conversions to the equivalent particle concentration form. The Kd is the particle-to-porewater chemical partition coefficient (L /M). In doing so all can be compared on the same numerical basis. 

The LEA relates the chemical particle and porewater concentrations using Kd, the partition coefficient. All terms have been defined previously. It is seen that the particle and porewater biodiffusion processes terms are in parallel. They are added directly as conductances while the BBL coefficient P enters as an added resistance term. 

Physical-chemical data representative of the site appear in Table II. A particular PCB congener, 2, 4, 2 , 4 -tetrachlorobiphehyl was used to specify a molecular diffusivity, and value of the biodiffusion coefficient representative of Lake Huron sediments was used in the absence of one for the Fox River. The latter was corrected for temperature a factor of 2 increase for each 10 C increase was assumed (14). A correlation developed for stream-beds was used to estimate p as a function of flow rate (9). The results of these computations appear in Table II. 

This is the defining equation for the turbulent diffusivity in m /s, called the eddy diffusivity. Within a single fluid, it is highly variable and depends on position, direction, and the nature of the flow field. Nevertheless, by selecting appropriate, average numerical values, it enjoys some use in chemodynamic model dispersion applications such as horizontal dispersion of chemicals in surface waters, vertical dispersion in the water column, dispersion in the atmosphere and in groundwater and biodiffusions in soils and sediment (see Chapters 13, 15, and 20). 

This flux equation finds application across finite fluid layers and porous media layers. It has found use for chemical transport in stratified fluid bodies where the compartments are characterized by average eddy diffusion coefficients, and in biodiffusion of... 

SA13 Bioturbation driven biodiffusion coefficient for sorbed-phase chemicals in upper soil layers... 

The applicable mathematical form of the flux equation is model dependent. For example, the first type model consists of differential equations (DEs). They are developed to yield concentration profiles in the sediment layers as well as the flux. These DEs typically use Equation 4.1 as a boundary condition. The solutions to these DEs require one or more of the following boundary condition categories the Dirichlet condition, the Neuman condition, or a third condition. The first two types are the most common these require mathematical functions containing gradients of the dependent variable (i.e., Cw) as well as functions of the dependent variable itself. For these diffusive-type fluxes, the transport parameter is a diffusion coefficient such as Dg. Several other transport parameters are commonly used and represent diffusion in air and the biodiffusion or bioturbation of soil/sediment particles. 

This balance is Equation 4.17 with an additional term. An advective flux accompanies the diffusive flux on the air-side. Vapor diffusion and particle biodiffusion processes represent the fluxes on the soil side. The appropriate phase equilibrium relationships are... 

In addition to air and water, there are many other environmentally relevant phases in which diffusion and diffusion-related processes are important for understanding and forecasting chemical transport. Many of these processes will be covered in depth in subsequent chapters (diffusion in sediment beds, Chapter 12 biodiffusion, Chapter 13 dispersion in groundwater. Chapter 15) and will only be touched on here. Some of the media considered here are natural materials (soil, sediment, ice, snow), while others are of anthropogenic origin (oil slicks, organic liquids accumulated in soils and sediment). 

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. 

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