Page 69 - Soil and water contamination, 2nd edition

56 Soil and Water Contamination

pores between the grains or particles are filled with water, in the unsaturated zone the water
occurs as a thin film on the surface of the particles. The sizes of soil pores can range from
more than 1 cm in diameter to less than 0.001 mm in diameter. Pores larger than 0.1 mm
in diameter are often referred to as macropores , those between 0.1 and 0.01 mm in size as
mesopores, and those smaller than 0.01 mm as micropores (Beven and Germann, 1982). The
zone below the water table , which is defined as the depth at which pore water pressure is
equal to atmospheric pressure, belongs definitely to the saturated zone. In coarse-textured
materials (sand or gravel) the transition between the saturated zone and the unsaturated
zone is approximately the water table. In fine porous materials (clay, loam, and silt), capillary
rise also causes complete saturation in a zone just above the water table. Although some
organic liquid pollutants may move independently through soil, movement of water and
its dissolved constituents is the most important vector of transport of pollutants in soil.
In the vadose zone, the movement of water only occurs via the water-filled pores and is
driven by gravitational force and gradients in pore water pressure (or suction because the
pore water pressure in the unsaturated zone is negative due to the capillary forces between
water and the soil grains). These gradients in pore water pressure result from both vertical
and lateral variations in soil moisture content, which may be very large. The effect of gravity
and evaporation on soil water transport is dominant, so the water moves mainly vertically
through soil. The soil moisture is replenished by infiltrating rainwater and snowmelt water
and by capillary rise from groundwater, and is depleted by evaporation from the soil surface
and by plants transpiring water. If the soil moisture content in the unsaturated zone becomes
sufficiently large, the excess water percolates downward to the groundwater.
Traditional studies assumed that pore water flow in the vadose zone was homogeneous
in the horizontal dimension of space. However, the soil properties that govern soil water
flow are often heterogeneous in nature, due to, for example, the presence of soil horizons,
cracks, and root channels. Such heterogeneities may give rise to preferential flow , which
means that the percolating water does not tend to move as a horizontal wetting front. In
general, the occurrence of preferential flow path s in soil is attributed two major phenomena,
namely macropore flow (Beven and Germann, 1982) and finger flow (Hillel and Baker,
1988). Because of their large size, macropores generally do not participate in unsaturated
flow of water in soil. Macropores start to transmit when they are completely filled with
water. This only happens when the soil near the macropore becomes saturated or when
ponded water on the soil surface can flow into open macropores. Finger flow occurs when
infiltrating water breaks through the wetting front at isolated points. The rapid infiltration
at these isolated points causes the formation of narrow fingers of wetted soil, while the bulk
of soil remains dry. Because the fingers are narrow, the infiltrating water and the solutes
it carries can potentially move rapidly to great depths. Preferential flow can thus be an
important mechanism that causes contaminants to leach rapidly from the soil surface into
the groundwater. Finger flow tends to be transient and the fingers usually disappear as soon
as infiltration at the soil surface ceases.

3.2.4 Soil erosion

Precipitation that does not infiltrate into the soil is stored temporarily in puddles or as snow
or ice cover, or runs off the land surface via overland flow . Overland flow may give rise to soil
erosion if the shear stress aroused by the runoff water becomes sufficiently large to detach
the topsoil particles. The direct impact of raindrops (splash) also detaches soil particles and
contributes to soil erosion . Soil erosion is greatly enhanced by agricultural activities and is
a major land degradation process in many regions in the world. The amount of soil erosion
increases with increasing volumes of overland flow (which depends on the infiltration
capacity of the soil and the upstream catchment area) and with increasing slope gradient.

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