Page 194 - Soil Degradation, Conservation and Remediation

6.2 Sources of Soil Pollutants 183

Organisms (flora and fauna) absorb some of the pollutants from soil and concentrate

them into their tissues. The tendency of chemicals to concentrate in biota is expressed
as a bioconcentration factor (BCF), defined as the ratio of the chemical concentration

in biota to that in its environment at steady state (Hamelink 1977 ). The uptake
of hydrophobic or lipophilic organic pollutants by organisms results in the accumu-
lation of pollutants in the food chain. Plants are often used to measure BCF for soils.
Bioavailability of POPs plays key role, because only bioavailable fraction of POPs
can be accumulated in soil organisms. Bioconcentration is the process that causes an
increased chemical concentration in an organism, say a plant, compared to that in
soil, due to the uptake of chemical by absorption which can occur via the absorbing
surface. In addition, pollutants enter into biota along the food chain through dietary
uptake, which is referred to as biomagnification. Bioaccumulation, which is the slow

buildup of organic chemicals in bodies, occurs through bioconcentration. If the
concentration of a chemical in an organism is dependent on both the concentration in
the medium and in the food the compound is said to bioaccumulate. Transfer of a
pollutant from one trophic level to another leading to increased concentration is

referred to as biomagnification. If bioaccumulation occurs in a keystone species, it
can affect every other organism in its far- reaching niches. Bioaccumulation of pollutants
depends on a number of factors, including biological diversity and abundance, soil
characteristics including texture, pH, organic matter, CEC, and solubility, polarity,
hydrophobicity, and molecular structure of the pollutant (Reid et al. 2000 ; Doick
et al. 2005 ). Bioaccumulation of persistent organic pollutants (POPs) by earthworms
is a very important process occurring in contaminated soils. It results to toxic effects
for earthworms (while their ecological function is impaired) and to biomagnifi cation
of food chains with all related effects. Earthworms play crucial role in the soil envi-
ronment contributing to the decomposition of organic matter and improving soil
structure. They live in close contact with soil organic matter and are exposed to POPs
accumulated there. The most important pathway for humans to bioaccumulating
organic compounds in the terrestrial environment is represented by cattle through
dairy products and beef (McLachlan 1996 ). The importance of beef and dairy products
compared to fruits and vegetables can be attributed to the large quantity of herbage
that must be fed to cattle to produce a given quantity of milk and beef. Organic
chemicals which are lipophilic and persistent accumulate in the milk and beef fat. Grasses
bioaccumulate the pollutants, and cattle biomagnify them in milk and beef. There

is bioconcentration, bioaccumulation, and biomagnification along the grass–cow–
human food web. Biomagnifi cation occurs as pollutants move from a lower trophic
level to a higher trophic level.

6.2.11.3 Remediation of Organic Pollutants from Soil

Physical and Chemical Methods

Based on isolation, immobilization, toxicity reduction, physical separation, extraction,
and decomposition (Sheoran et al. 2008 ), the conventional methods of remediation

of organic pollutants include soil washing/flushing, thermal desorption, chemical