Environmental Engineering Reference
In-Depth Information
of metals in runoff are metal-based pesticides, which are tightly bound to solid
particles. In urban runoff metals are also most often associated with particulate
material, while metals in ground water might be in dissolved form due to soil
acidification
• Dissolved phase - these are chemicals, which are dissolved in runoff. This group
includesmainly in-organic nitrogen and soluble phosphorus. Inorganic nitrogen in the
drainage is mostly nitrate-nitrogen, and this ion does not adsorb to soil particles.
Phosphorus is a special case. Most phosphorus in runoff is solid-phase, but dissolved
phosphorus is directly available to plants and algae and hence cannot be neglected in
eutrophication studies.
• Distributed phase - chemicals, which are transported in both solid phase and dissolved
forms. Most organic pesticides could be included in this group.
3.1
Atmospheric depositions
Diffuse pollution from atmospheric depositions is associated with polluted precipitation
(rainfall) and is also known as “acid rain”. It is caused by polluted atmosphere, where
rain droplets and atmospheric moisture absorb and dissolve oxides of atmospheric carbon
dioxide, sulfur (emitted from burning fuels and specific industrial processes) and nitrogen
(emitted from heavy traffic). These constituents make the precipitation acidic and
damages man-made structures and the environment - vegetation cover, soils and water
resources. The phenomenon of acid rain occurs most often in heavily industrialized and
densely populated areas or in the neighborhood of heavy-industry enterprises.
3.2
Sources of diffuse pollution in urban areas
The most common sources of pollution within urban population centers are associated
with polluted runoff from roofs, streets, highways, and open developed areas, but other
sources could be mentioned as well. A brief description of such sources and the
associated process of generation are discussed briefly.
3.2.1
Urban drainage
Storm water runoff in urban areas, known also as urban drainage, is usually much higher
in volume and quantity, than rural runoff due to the distribution of impervious surfaces.
In the case of separate sewers, main pollution loads are associated with conventional
pollutants (SS, BOD
5
, N, P), metals and organic compounds. They are attached to the
sediments, carried with the runoff, and collected by urban drainage systems. In the case
of combined sewer systems, any overflow during rains contributes a pollution load to the
surface water, which has the characteristics of a weak sewage. Road runoff is an
important source of contaminants, including oils, tar products, dioxins, metals, phenols
and other materials mainly accumulated in sediments, which could have a toxic effect to
several fresh water organisms. The BOD
5
values of urban runoff vary from 10 to 285
mg/l in USA, and from 100 to 350 mg/l in UK (Ellis 1989).
One of the most problematic areas, related to urban drainage pollution, is the presence
of a wide variety of trace metals. Pb, Cd, Cu and Zn are the most frequently occurring
