Advective flux processes

Advective flux refers to the bulk flow of solids or pore water relative to an adopted frame of reference such as the soil-water interface of wetlands (Berner, 1980). Advection is associated with the flow of material with either the velocity of its own or the velocity of medium (water) through which the material is transported (Lerman, 1979). During advective transport, solutes are typically transported at the same velocity as water or air. Advective fluxes can include sediments and solutes carried in surface water flows, or solutes in groundwater and pore water flow. The flow of material of a given density p(M L ) with velocity f/(L T ) results in the advective sediment flux J as described in the following equation  

Similarly, the flux of solute (7J with a concentration C(M L ) transported in water at a velocity U is given as 

The flow velocity U is considered as the driving force for the flux of material. 

If nitrate concentration in inflow water of a constructed wetland is 10 mg and the average water velocity is 10 cm s , what is the average flux of the nitrate in the outflow (assume no biological transformations or downward flux into soil)  

Convert concentration units to stay consistent with velocity 

---

An estimate of the advective fluxes (processes 1, 2, and 3) requires knowledge of the concentration of the species in solutions and in the solid... 

An estimate of the advective fluxes (processes 1,2, and 3) requires knowledge of the concentration of the species in solution and in the solid particles as well as of the rates of sedimentation and pore water flow. The diffusive type processes 4 and 5 depend on vertical gradients of the concentrations of the species as well as on the diffusivities. In regions where bioturbation occurs, the effective diffusivity in the uppermost centimeters of the sediments can be more than that due to molecular diffusion in the pore water alone (cf. Fig. 4-12). 

Advection (or convection) is the process by which chemicals are transported by the average (or bulk) water velocity. Thus, the advective flux, is described simply by... 

The conservation of the mass of a fluid in a velocity field v(x, t) is expressed by Eq. (1.1) for the fluid density (mass per unit of volume) field p(x, t), with 5 = 0. The only process moving the fluid mass is transport by the velocity v, giving the advective flux J = pv. Thus, Eq. (1.1) becomes the continuity equation... 

At flow rates of about 10 cm s over an uneven sediment surface (mounds up to 1 cm high), the oxygen measured by means of microelectrodes had penetrated to a maximum depth of 40 mm, whereas a penetration depth of only 4 mm was measured under comparable conditions when the sediment surface was even (Fig 3.27). Huettel et al. (1996) were able to show in similar flume experiments that not only solutes, but, in the uppermost centimeters, even fine particulate matter was likewise transported into the pore water of coarsely grained sediments. Similar processes with marked advective fluxes are, however, not to be expected in the finely grained sediments predominant in the deep sea. 

The rate of transfer of solutes between soil and overlying water column and from one physical or chemical state to another is defined as flux. The dimensions of flux are M T where M is the mass of material transferred by flux, L is the distance or length, and T is the time. The processes associated with flux are advection, diffusion, and dispersion. Diffusive and advective flux between soil and overlying water and elemental uptake by rooted wetland vegetation are the major transport... 

Advective flux of solutes in wetlands can result from pressure gradients that force pore water from soil pores to overlying water, carrying solutes and line particulate matter with it across the soil-water interfaces. The flux is influenced by hydraulic gradients, associated water, and adjacent upland area (Figure 14.3). Pore water movement and its transport could be significant in sandy, permeable soils and sediments. In low-permeability soils such as those with high silt and clay contents, molecular diffusion (see Section 14.3) and bioturbation (see Section 14.4) can be the major transport processes. 

Here Npe > 1 means that transport in the chemical isolation layer is dominated by advection while Wpe < 1 implies that transport is dominated by diffusion. Advection and diffusion in either the cap isolation layer or bioturbation layer are not independent because advection tends to reduce diffusion gradients and diffusion tends to reduce the advective flux. In the cap isolation layer, a reasonable approximation is to assume that the flux is well-estimated by the dominant flux (either advection or diffusion). Solutions to the steady-state transport equations considering both diffusion and advection with and without reaction are feasible, but are algebraically more complicated and deviate significantly from solutions assuming only the dominant process in the relatively narrow range of approximately 0.3 < Npe < 3. Even within this range, the dominant process correctly estimates the flux within a factor of 2. 

From these relationships and by analogy to Eqs. 19-22 in Appendix B of Palermo et al. [1], the concentration in the biotmbation layer can be estimated. hi Appendix B, however, only diffusion and bioturbation by particles were considered. Here the more compUcated case is necessary because of the additional operative processes. Rewriting Eq. 14, defining the steady-state flux as the maximum of the diffusive or advective flux in the chemical isolation layer... 

Processes S A2 and SA3 are the air-to-soil advective deposition processes identical to the air-to-water processes presented above. Process SA6-dust resuspension from the soil can be included as well, its flux equation is... 

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... 

A final note. The interface compartment concept has application for the next generation of multimedia compartment box models and other models as well. Presently several of these models use versions of Ohm s law as noted above as well as other procedures. The numerical results produced appear to be reasonable and provide good approximations apparently. However, without the additional mass balance provided by applying the interface compartment concept, the advective transport processes fail to impact the magnitude of the interface concentfation. This influences the flux and finally the media mass concentrations. Comparative model studies using the present-day approaches of combining interface fluxes and the IC model approach need to be performed and the results evaluated. Such studies may aid the development of a more realistic and appropriate approaches for connecting chemical flux between multimedia environmental compartments. 

Each flux has units of mg/m s. The individual processes flux equations may contain both diffusive and advective fluxes. Mimic the examples presented in Section 4.4.2 above. Flux expressions with concentration gradients and effective diffusion coefficients are normal for the soil and sediment side of these interfaces. For the air and water side of interfaces, concentration difference flux equations are normally used for the diffusive processes. Table... 

Chemical contaminants in the atmosphere can be deposited to surfaces in association with aerosol particles or falling rain and snow. These are advective transport processes, since the chemical moves in association with aerosol particles, raindrops, or snowflakes. This chapter describes methods for estimating chemical fluxes associated with deposition of aerosol particles and precipitation, and provides recommended values for mass transfer coefficients for a range of environmental conditions. In this chapter we do not consider transport of gaseous species in the atmosphere and adjacent surfaces. These convective transport processes, termed dry deposition of gases, are covered in Chapter 2, Section 2.5.6 and Chapter 7, Section 7.3. exchange between air and plants in Chapter 7, air and water in Chapter 9, and air and snow in Chapter 18. 

To assess the relative importance of the volatilisation removal process of APs from estuarine water, Van Ry et al. constructed a box model to estimate the input and removal fluxes for the Hudson estuary. Inputs of NPs to the bay are advection by the Hudson river and air-water exchange (atmospheric deposition, absorption). Removal processes are advection out, volatilisation, sedimentation and biodegradation. Most of these processes could be estimated only the biodegradation rate was obtained indirectly by closing the mass balance. The calculations reveal that volatilisation is the most important removal process from the estuary, accounting for 37% of the removal. Degradation and advection out of the estuary account for 24 and 29% of the total removal. However, the actual importance of degradation is quite uncertain, as no real environmental data were used to quantify this process. The residence time of NP in the Hudson estuary, as calculated from the box model, is 9 days, while the residence time of the water in the estuary is 35 days [16]. 

In practice, the concentration gradient is estimated from obserrational data by computing the concentration difference (C - Q ) between two samples collected from two different depths (z. - z ). Real concentration gradients are not usually linear because of the effects of other processes that concurrently affect solute distributions, such as advection and biological activity. Thus, estimating fluxes from the concentration differences of discrete samples is best done over small depth intervals. 

In seawater, physical processes that transport water can also cause mass fluxes and, hence, are another means by which the salinity of seawater can be conservatively altered. The physical processes responsible for water movement within the ocean are turbulent mixing and water-mass advection. Turbulent mixing has been observed to follow Pick s first law and, hence, is also known as eddy diffusion. The rate at which solutes are transported by turbulent mixing and advection is usually much faster than that of molecular diffusion. Exceptions to this occur in locations where water motion is relatively slow, such as the pore waters of marine sediments. The effects of advection and turbulent mixing on the transport of chemicals are discussed further in Chapter 4. 

The vertical trends in POM fluxes exhibit temporal and geographic variability. This was shown in Figure 23.3, in which seasonal shifts in surface productivity were seen to affect the subsurface particle fluxes even in deep waters. Other processes that can affect the sinking flux of POM include (1) in situ production by mid-water microbes or zooplankton and (2) lateral transport of POM via advective currents. Both can produce mid-water maxima in the sinking organic matter fluxes. Geographic variability in these fluxes is common. As illustrated in Figure 23.6 for the central equatorial Pacific Ocean,... 

Recall the distinction between advective and diffusive transport, which we made in Section 18.1 while traveling in the dining car through the Swiss Alps. We then introduced Fick s first law to describe the mass flux per unit area and time by diffusion or by any other random process (Eq. 18-6). Rewritten in terms of partial derivatives, the diffusive flux along the x-axis is ... 

Besides interacting with suspended particles, a chemical also undergoes direct exchange at the sediment surface by diffusion and advection into the hyporheic zone. Furthermore, resuspension followed by exchange between water and particles also adds to the sediment-water interaction. These processes have been extensively discussed in Chapter 23, especially in Box 23.2. There we concluded that the effect from the different mechanisms can be combined into a flux of the form (see Eq. 23-25) ... 

Hansen and Rattray (1966) introduced a general classification scheme for estuaries based on stratification/circulation that is divided into the following four estuarine types Type 1 estuaries well-mixed estuaries with mean flow in the seaward direction and the salt balance being maintained by diffusive processes—via tidal transport Type 2 estuaries partially mixed estuaries where the net flow reverses at depth and the salt flux is maintained by both diffusive and advective processes Type 3 estuaries these estuaries include fjords with two distinct layers and advection accounting for the majority of the salt flux Type 4 estuaries these are salt-wedge estuaries where freshwater flows out over a stable more dense bottom layer. 

---