Sustainable Water Management - Sustainable Practices in Geoenvironmental Engineering - page 76

3.2.1.1 Human Interference on Iniltration and Runoff
Human interference in the hydrological cycle is most signiicant in the processes of inil-
tration and runoff. The form of some of the major interferences on iniltration and their
impacts, some of which are shown in Figure 1.5 in Chapter 1, are as follows:
• Development, production, and construction of impermeable surface areas: These
impermeable surfaces include housing and similar structures, roads and pave-
ments, runways and aprons, parking lots, and other generally paved surfaces con-
structed in the urban environment. The effect of these impermeable surfaces is
to deny iniltration into the ground, hence denying recharge of any underlying
aquifer. Runoffs obtained on the impermeable surfaces are generally fed to storm
drains or other similar drainage discharge systems.
• Compacted surface layers: These are surfaces of natural soil compacted by
agricultural and construction machinery and other similar devices. The effect
of compacted surface layers is to reduce iniltration rate and iniltration capa-
bility in general. Surface runoff occurs when the rate of precipitation onto the
surface is greater than the iniltration rate. Unlike the paved impermeable sur-
faces, these runoffs are not generally fed to storm drains or other catchment
facilities.
• Soil porewater and aquifer contamination by iniltration: The term soil pore-
water is used to mean the water in the pores of the soil matrix. Normally, in
compact clay soils, this porewater is not easily or readily extractable. However,
in more granular materials such as silts and sands, this porewater can be har-
vested. Aquifers in general are seams or layers of primarily granular materials
that are full of water. We use the term groundwater to mean both soil porewa-
ter (above and below the water table) and water in the aquifers, but mainly to
denote water in the ground that is normally considered as a water resource.
When it is necessary to talk about soil porewater, the term porewater is generally
used. Contaminants on the land surfaces will be transported into the subsoil
via iniltration and other transport mechanisms such as dispersion and diffu-
sion. Pesticides, herbicides, insecticides, organic and animal wastes, ground
spills, other deliberate and inadvertent discharges of hazardous and noxious
substances on surfaces, etc., serve as candidates for transport into the subsoil.
Contamination of groundwater will occur when communication between the
contaminants and these water bodies is established, i.e., when the contami-
nated iniltration plume reaches the groundwater, meaning that contamination
of groundwater occurs the moment contaminants reach soils containing pore-
water. The result is impairment of groundwater quality. Ingestion of polluted
groundwater can be detrimental to human health.
As with iniltration processes, surface runoffs can occur on natural ground surfaces
devoid of much human contact. This occurs when rainfall rate exceeds iniltration rate,
and when natural surface cover (vegetation, plants, etc.) is so dense that it acts as a shield
or umbrella. As we have seen from the preceding, natural iniltration properties of soils
can be severely compromised by human activities resulting in runoffs. There are at least
two types of runoffs: (1) managed runoffs where the runoffs are channeled into drains and
sewers and (2) unmanaged runoffs where the runoffs take directions controlled by surface
topography and permeability properties of the surface cover material. In both cases, the