Pesticide Residues in Man - Pesticides: Evaluation of Environmental Pollution - page 509

one of four reactions: oxidation, reduction, hydrolysis, and photolysis. Biological mecha-
nisms in soil and living organisms utilize oxidation, reduction, hydrolysis, and conjuga-
tion to degrade the chemicals. The process of degradation will largely be governed by the
compartment (water, soil, atmosphere, biota) in which the pesticide is distributed, and this
distribution is governed by the physical processes previously mentioned (Linde 1994).
Pesticides that have high solubility in water will remain in water and tend to not be
adsorbed onto soil and living organisms. Among the factors influencing the water solu-
bility and hydrolysis of the chemicals are polarity, molecular size, temperature, and pH.
Most pesticides are less polar than water, so they tend to accumulate in soil or living
organisms that contain organic matter; based on their octanol-water partition coefficient
(K ow ) (Linde 1994).
Adsorption of pesticides onto soils or sediments is a major factor in the transportation
and eventual degradation of chemicals. Chemicals that are nonpolar tend to be pushed out
of water and onto soils that contain nonpolar carbon material. Organic carbon content of
the soil, pesticide polarity, and soil water's pH and salinity are among the factors influenc-
ing pesticide adsorption onto soil. There are two major ways by which pesticides can reach
surface waters and groundwater: runoff and leaching. Runoff will occur if the chemical
does not adsorb onto soil. Leaching occurs when the chemical is weakly adsorbed by soil
and can easily move through the soil. Weak acid pesticides are bound weakly to soil, so
they can easily move downward to groundwater (Linde 1994).
Bioconcentration factor (BCF) is indicative of the accumulation of a chemical in living
organisms (biota) compared with the concentration in water. It is an indicator of how much
a chemical will accumulate in living organisms, such as fish. Once absorbed into an organ-
ism, chemicals can move through the food chain ending up in humans. BCF values are
unitless and generally range from one to a million. Bioconcentration is based on the law
of equilibrium. For example, if a fish exposed to atrazine has a BCF of 110, the chemical
would be absorbed according to the relation
BCF = concentration in fish/concentration in water = 110.
According to Linde (1994), BCF values for some pesticides are aldicarb (2), carbaryl (46),
simazine (100), atrazine (110), lindane (169), and DDT (29700).
Pesticides with high vapor pressures (P vp ) may become environmental problems because
they can volatilize and disperse over a large area. A chemical with a low vapor pressure
does not move into air, so there is a potential for accumulation in water if it is water-
soluble. If it is not water-soluble, the chemical may accumulate in soil or biota. Therefore,
volatilization is one of the main transport pathways by which pesticides move from water
and soil surfaces into the atmosphere. Volatilization from these surfaces is governed by
various physical, chemical, and environmental factors, which mean that the rates will be
different for each chemical depending on the environmental conditions (e.g., wind, tem-
perature, and location; Linde 1994).
Large quantities of pesticides are lost by volatilization into the atmosphere. In the
atmosphere, there are two major degradation pathways that occur (i.e., photochemical
reactions and free radical reactions). The products formed may or may not be more
toxic than the parent chemical. Once a pesticide has been degraded, a major removal
process for chemicals is to precipitate out of air and return to the Earth's surface.
Another removal process is for the products to be dissolved in rain and fall back to
Earth (Linde 1994).