Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 317

access. Both mines and mine spoil dumps are potential sources of leachable con-
tamination. When AMD/ARD enters streams and rivers, the change in pH causes
the iron to precipitate as ferric hydroxide. This is deposited on streambeds as an
orange sludge, a process which also depletes the water of its oxygen, both of which
have an impact on fisheries and insect life;

- Fish farming can have a variety of effects on the marine environment, through the

discharge of nutrients, solid waste, medicines and antifoulants;

- Road runoff and urban storm water discharges are often contaminated with sed-

iment, litter, oil and petrol, and with toxic metals from motor vehicles. Water
carrying these contaminants is washed off into drains and directly into nearby
watercourses. Most surface water drains are connected to watercourses and not
to sewage treatment works, hence any spillage of chemicals will tend to be washed
into rivers.

In terms of its nature pollution is often described as point source or diffuse source
(or nonpoint) pollution. Liquid, solid and airborne discharges from point sources as
well as pollutants from nonpoint sources may go either into surface water or into the
ground. Airborne pollutants can be assimilated into rainwater and can affect water
quality: acid rain is an example. The ability of these pollutants to reach surface water
or groundwater is enhanced by the amount of water available from precipitation (rain)
or irrigation.
The transport of contaminants within the soil shows some differences compared
to the transport in air and surface waters. While transport in the atmosphere is mainly
determined by weather conditions (wind, temperature, humidity and rain) and in
waters by the flows, currents and other water movements, transport in soil is more
complex due to the equally dominant three physical phases and countless physical
forms of material (soil gas, soil moisture, pore water, groundwater and soil solid). In
this complex matrix the contaminants are transported by advection, diffusion, and
partition at the same time, and particularly intensive interactions take place between
the contaminants, the constituents of the soil matrix and the soil biota.

2.2 Origin of point and diffuse pollution
Despite the common theoretical bases, efficient environmental risk management meth-
ods of point and diffuse contamination are different. We can use direct measurements
or predictive models with numerical input data and calculated output concentrations
for any point or time for the management of pollution from point sources. To look
into diffuse atmospheric, watershed-scale, regional or global pollutions, we have to
use system models which consider the combination of environmental conditions and
multiple interactions. Diffuse pollution originating from abandoned point sources calls
for both point source and diffuse source management.

2.2.1 Point sources and point-source pollution

Point source is a single, identifiable and localizable point of discharge due to an activity
or accident. Transport from a point source in air, water and soil may lead to human or
ecosystem exposure. A common/typical point source contamination scenario is shown
in Figure 10.1.