Leaching Processes
Leaching: Chemicals that remain in the soil profile are subject to processes tending to move them downward to groundwater. Several of these processes are introduced here.
Mass Flow: Water moving through soil pores carries dissolved chemicals with it. This process, called mass flow, is the dominant mechanism for leaching in most soil systems. It is capable of moving chemicals large distances. Soil properties and management decisions that influence water movement in soils will also affect chemical leaching. See the sections on water movement for more details on those processes.
The section on water movement illustrates how water enters a soil. When water is applied to a dry soil, a distinct wet front is created with a higher water content above the wet front than below it. The depth of the wet front depends upon the amount of water added, the initial wetness of the soil, and the volume and size of pores in the soil. It depends to a lesser extent upon the rate at which the water is applied. After water is no longer added, water continues to move but the rate of movement slows down rather quickly.
If the water entering a dry soil contains dissolved chemicals that are not adsorbed on the soil solids, those chemicals are transported to the same depth as the entering water. The chemicals will be moved further into the profile by subsequent rainfall or irrigation events provided those events are of sufficient magnitude to move water past the current chemical depth.
For chemicals that are adsorbed on soil solids, the process is similar to
that outlined above for non-adsorbed chemicals but the depth to which the
adsorbed chemical moves is reduced or the rate of movement is retarded. The
retardation factor, R, is related to the partition coefficient,
Kd, as shown here
where r is the dry bulk density of the soil and q is the volumetric water content of the soil. The retardation factor varies from a low of 1 for non-adsorbed chemicals to values in excess of 100 for some soil-chemical systems. Thus the depth to which the chemical is leached is highly dependent upon the extent to which it is adsorbed.
The situation for soil systems that are not initially dry is similar to that above with one exception. The location of the wet front in an initially dry soil corresponds to the leading edge of infiltrating water. If the soil was initially moist, the wet front may not correspond to the leading edge of the infiltrating water. In some soils, it appears that water entering a moist soil displaces water already in the profile and "pushes" that water ahead of it. In that case a non-adsorbed chemical would move only to the depth of the infiltrating water, not all the way to the wet front. Other studies indicate that only a portion of the pores take part in the infiltration process so some water at the wet front may represent new water. Most likely the situation is a combination of these two systems. This leads to uncertainty in the real location of the chemical. It also indicates that the location of a chemical is not likely to be a sharp pulse but will be more spread out or dispersed.
Dispersion: Water moving through soil pores travels at different speeds due to different pore sizes and the tortuous paths taken by the water. Therefore a pulse of chemical travelling with the water has its leading and trailing edges spread out. This spreading is called dispersion.
Diffusion: Chemicals also move from a region of higher concentration to one of lower concentration due to molecular motion. This is called diffusion. Diffusion is usually much less important than mass flow.
Interactive applets illustrating these processes: Several interactive programs are available to illustrate these processes and factors that influence them. Feel free to use them to gain insight into these complex processes.
Diffusion: In this model, a chemical is mixed uniformly in a soil to a specified depth. The user can specify soil and chemical properties for the system. This applet calculates and displays the concentration distribution resulting from only diffusion of the chemical. The mathematical equations used in the model are also presented. Some observations you may make from this applet include the following:
- Diffusion simply spreads the chemical out in the soil.
- The depth to which the chemical moves is relatively small.
- Diffusion cannot cause a pulse of chemical to move down the soil profile since movement only occurs from regions of higher concentration to regions of lower concentration.
- The spreading process occurs more slowly when the chemical is adsorbed on the soil surfaces (organic carbon partition coefficient is greater than zero and the organic carbon content is greater than zero).
Convective-Dispersive Transport: This model incorporates mass flow, dispersion, sorption, and degradation to predict the movement of a chemical through a soil. It is limited in that it assumes that water moves through the soil at a constant rate for all times. Special experiments may be set up to approximate this condition, but it will not occur under natural conditions in the field. In spite of this, the model is helpful to grasp the impact of different parameters and boundary conditions upon the predicted fate of the chemical. Partial differential equations and the solutions used in the model are provided for completeness.
CMIS Educational Model: The model predicts movement of pesticides through soils using weather and irrigation provided on a daily basis. The applet displays two soils side by side so comparisons can be made between different soils, chemicals, irrigation systems, and several other parameters. Soil and chemical properties along with cumulative infiltration amounts can be viewed on the output screen. The simplified model incorporates mass flow, dispersion, sorption, and degradation.
