Environmental Engineering Reference
In-Depth Information
developed areas, which discharge their effluents in the drainage system or directly on
streets.
• On-site sanitation (septic tanks and pit latrines) - they could be a significant source of
diffuse pollution only in the case of improper application practices, or when applied in
areas with high ground water levels, which would be directly polluted by leachate
from latrines or septic tank soak ways.
3.3
Sources of diffuse pollution in rural areas
The major cases of diffuse pollution in rural areas are associated with agricultural
practices and animal operations. The most common cases are as follows:
• Application of fertilizers, pesticides and insecticides - excessive use of fertilizers, when
application rates exceed the assimilative capacity of the crops, would lead to soil
enrichments with nutrients. They could find their way to water resources in the form
of polluted runoff, or in the form of dissolved fractions, could infiltrate to ground
water. The same applies to pesticides and insecticides. This type of pollution is of
higher concern, considering the fact that even low concentrations of SOCs have toxic
effects on humans and the aquatic environment.
• Irrigation return flow - this is the excess irrigation flow, usually characterized by
increased salts concentrations, compared to the original water used for irrigation. It
percolates to ground water and might form the base flow in rivers, downstream of the
irrigation fields. In cases where irrigation fields have drainage systems to collect and
convey excess flows, they would be discharged into surface water. Irrigation return
flows might be classified as diffuse pollution in specific cases of increased salt
concentrations. Usually, this would be the case of irrigation in arid regions, where the
evapotranspiration rates are very high. Another example is when the downstream users
of irrigation return flows, reuse this water for irrigation again, and in this case we have
a repeated cycle of salinization.
• Irrigation with wastewater/sludge - causes of pollution in such cases are associated with
improper practice and excessive application rates. Pollutant constituents consist of
organic matter, nutrients, mi-crobiological contamination and heavy metals. Increased
salinity of ground water is enhanced, because wastewater's salt concentrations, even
after treatment, are much higher compared to natural water. It should be noted that this
type of pollution source has been included under the rural areas classification, but it is
also applicable in many cases of urban or suburban developments, as it is illustrated by
the case studies presented in Chapters 8, 9 and 10.
• Animal operations - diffuse pollution from barnyards, where animals are concentrated
for milking or other operations might constitute a significant pollutant load through
runoff, if not cleaned and managed properly. Pollution is severe in terms of organic
matter, nutrients and pathogenic microorganisms.
• Mining operations - mine drainage and the wastewater from mineral processing could
be defined as point sources of pollution from such activities. However, there is a
significant amount of polluted water coming as a result of runoff from stockpiles, spoil
heaps and surface drainage, which are typical non-point sources of pollution. Open
mining activities are large contributors of diffuse pollution. Acid waters, together with
dissolved iron compounds usually associated with them, are amongst the principal
