McKay, Donald. "Front matter" Multimedia Environmental Models ...

Figure 8.2 Chemical transport and transformation processes in a surface soil.
into the amounts present in the air and water phases. It also shows the extent to
which organic matter dominates the sorptive capacity of the soil.
Three loss processes are considered: degrading reactions, volatilization, and
leaching, each rate being characterized by a D value.
An overall reaction half-life t(h) is specified from which an overall rate constant
k R (h –1 ) is deduced as 0.693/t. The reaction D value D R is then calculated from the
total soil volume and the bulk Z value as k RV TZ TS. In principle, if a rate constant k i
is known for a specific phase in the soil, the phase-specific D value can be deduced
as k iV iZ i, but normal practice is to report an overall rate constant applicable to the
total amount of chemical in the entire soil matrix. If no reaction occurs, an arbitrarily
large value for the half-life, such as 10 10 hours, should be input.
A water leaching rate is specified in units of mm/day. This may represent rainfall
(which is typically 1 to 2 mm/day) or irrigation. This rate is converted into a total
water flow rate G L (m 3 /h), which is combined with the water Z value to give the
advection leaching D value D L as G LZ W. This assumes that the concentration of
chemical in the water leaving the soil is equal to that in the water in the soil; i.e.,
local equilibrium has become established, and no bypassing or “short circuiting”
occurs. The “solubilizing” effect of dissolved or colloidal organic matter in the soil
water is ignored, but it could be included by increasing the Z value of the water to
account for this extra capacity.
©2001 CRC Press LLC