Mass transport in the unsaturated part of the road construction (the sub-base and upper part of the subgrade) strongly depends on the soil moisture distribution inside the pores. Where the mass transport is principally by advection then the water movement direction will control the contaminant flux direction. As the principal fluxes in the vadose zone are those due to evaporation and percolation, it follows that the
direction of the mass transport will then be essentially vertical, upwards or towards the lower part of the subgrade.
Soil moisture travelling through the unsaturated part of the road construction moves at different velocities in different pores due to the fact that saturated pores through which the moisture moves have different-sized pore throats and different thickness of the water film on the mineral grains of the soil. The theory of mass transport inside of unsaturated soil is much more complicated than in saturated media. The processes are described in standard textbooks (e. g. Fetter, 1993; Hillel, 2004).
In general, the structure of the equations for mass transport in unsaturated soil is similar to the equations for saturated soil. They differ in that the diffusion and dispersion coefficients and flow velocities for unsaturated soil depend on the water content.
The rate of change of the total pollutant mass present inside the unsaturated part of the road construction must be equal to the difference between the pollutant flux going into the road pavement and that leaving it and going into the saturated subgrade. Due to the complex processes inside the unsaturated road layers, several sources and sinks of pollutants can exist. These processes can be associated with biological decay (for organic contaminants) as well as chemical transformations and precipitation.
If occurring in large quantities, organic compounds originating in petroleum products form a special case of great concern in connection with roads. Spillages of petroleum products from traffic accidents or from petrol-filling stations, often situated adjacent to roads, may result in large quantities of organic compounds entering the road surface of roadside soils. The different interaction of these organic fluids with the soil’s chemistry will frequently increase the effective permeability and enhance the flux of the contaminants through the soil — behaviour referred to as incompatibility. Such situations are largely undesirable from an environmental and from other points of view.
The steady-state diffusion of solute in soil moisture is given by
„ dC
F = -D* (в) (6.10)
dx
where F = mass flux of solute (units of M/L2T); D*(e) = soil diffusion coefficient which is a function of the water content, the tortuosity of the soil, and other factors related to the water film on grains (units of L2/T); C = the concentration of the contaminant (units of M/L3), x = the distance in the direction of travel (units of L) and dC/dx = the concentration gradient in the soil moisture.
The second-order diffusion equation for transient diffusion of solutes in soil water is defined as
In road aggregates that are between saturated and residual saturation, some advection can occur with higher saturation allowing advective transport to increase. In terms of contaminant destination, the influence of advection is important. Diffusion will occur evenly in all directions in which the differences in contaminant concentrations exist, while advection will transport contaminants wherever the water is draining, either into a fin drain, or down the vadose zone towards the phreatic surface.
In an unsaturated soil, some of the void space is filled with gas. Due to evaporation, some contaminants will pass from the liquid phase into the gas phase both by volatilisation and by transport in water vapour. Contaminants within the gas will then be transported by diffusion and advection within the gas phase. Exchange processes transferring contaminants between the gas and liquid phases in the road construction are very complex. The transport in the gas phase inside a road construction can be substantial, especially when incidental spills appear on the road surface. However, the extent to which these processes occur inside the road construction is not well known and may be small for most metals. Certainly, as a soil or aggregate becomes less saturated the opportunity for advective transport reduces markedly, particularly because of the substantial reduction in permeability as described in Chapter 2, Section 2.8.
Rather as in Eq. 6.7, the soil-moisture dispersion coefficient, D(e), is defined as the sum of the mechanical and diffusion mixing and is now expressed as:
|
D* (в) |
D (в) = £ M + D* (в)
where £ = an empirical dispersivity measurement (units L) that depends on the soil moisture and v = the average linear soil moisture velocity. This definition of D(e) may be contrasted with that for Dt given above for saturated conditions (see Eq. 6.7) which includes a soil tortuosity term, a, in place of the £ term which is also controlled by the water content.
In a road aggregate where pores are only partially saturated, especially during dry periods, the capillary suction of the aggregate can increase very significantly, moving the aggregate towards its residual saturation condition and hindering advective contaminant transport. In these conditions, contaminant transport will, therefore, be very slow and primarily occur by diffusion.
