Precipitation
The retention of contaminants in the soil may often occur through the passing of
contaminants from a dissolved form to an insoluble form. Precipitation reactions are
controlled by acidbase equilibria and redox conditions. They are reversible and may lead to
the dissolution of formerly precipitated compounds, if conditions are changed.
4.2 Infiltration, diffusion and transport by soil solutions
4.2.1 Infiltration/rainfall
It is the most common mechanism of contamination of soil solutions in the vadose
zone, as well as, deeper regions of the saturated zones of the groundwater. As fluids move
downward under the influence of gravity, they dissolve materials to form leachates that
contain inorganic and organic constituents. As they reach the saturated zone of the
groundwater, the contaminants spread horizontally and vertically by joining the main cycles
of the geochemical flows. In the infiltration/rainfall, the colloidal size particles or elements
attached with mobile colloids would be transported to the groundwater through unsaturated
porous media. Several mechanisms are responsible for colloid transport in unsaturated zone
in addition to that of saturated zone, such as, liquid-gas interface capture, solid-liquid-gas
interface capture, liquid-film straining, and storage in immobile liquid zones.
The rate that water will infiltrate into the soil is dependent on textural characteristic of
the soil profile, the condition of the soil surface, and the initial moisture content within the
soil profile at the time of the infiltration event. The soil’s textural characteristics determine
the saturated hydraulic conductivity of the soil and the water retention or storage
characteristics of the soil. Naturally, soils that have a relatively coarse texture will tend to
have greater saturated hydraulic conductivity than fine–textured soils, and so these coarsetextured
soils will tend to have greater infiltration capacities. The condition of the soil surface
is important because effects such as compaction of the surface layers or deposition of fine
particulates onto the surface can reduce the hydraulic conductivity of the surface and thereby
reduce the infiltration capacity of the soil. The initial moisture content of the soil is an
important factor in determining infiltration capacity: the drier the soil is at the beginning of
an infiltration event, the more capacity the soil has to absorb the water available at the soil
surface.