49
Properties of Pesticides and Contaminants
where C o is the concentration in the organism and C w, a is the concentration
in water or air at equilibrium. Chemical contaminants can enter an organism through tissue penetration, ingestion, and by respiration. Penetration of
tissue (e.g., human and animal skin; plant leaves and stems) by a chemical
depends on its polarity (lipophilic or hydrophilic), molecular weight, charge,
concentration, the area and duration of exposure, and species differences
among plants and animals.
3.4.2.1 Biota and Water
Chemical contaminants in water are often in contact with plants, other organisms, and sediment. Use is made of the affnity of chemical contaminants in
water to be absorbed by plants and adsorbed onto clays to create managed
wetlands for water decontamination. Contaminated water is allowed to fow
into the wetlands, where contaminants are removed and relatively clean
water is released for more productive uses, such as fsheries or migratory
bird sanctuaries. This principle has been suggested or put into practice in
areas like the Chesapeake Bay, where surrounding intense farming or livestock production practices release undesirably high loads of nutrients to the
Bay, encouraging algal blooms or contamination levels unsafe for fsh and
other aquatic species. Use of vegetative barriers or managed wetlands may
be the most practical solution in such cases.
3.4.2.2 Root Zone Absorption
In general, water-soluble neutral organic molecules do not adsorb tightly
to soil—the organic fraction or the clay mineral fraction. Examples include
glyphosate, fuorinated detergents, TFA, phenoxy acetic acids, other watersoluble xenobiotics including heavy metals, and ionized or ionizable organics.
This means that water-soluble insecticides will be available for absorption
by plant roots and translocated, acting as systemic insecticides. Seiber et al.
(1986) studied the absorption of root zone-applied systemic insecticides in
rice paddies (Seiber et al., 1986) (see Chapter 10). They found that, after translocation, some of the insecticides were excreted in the guttation fuid and,
once on the foliage surface, were available for volatilization. This is a mechanism whereby water-soluble systemics of low Henry’s constant get into the
air, rather than by direct water-to-air transfer, and become transportable airborne contaminants. Other soil constituents can be absorbed by plant roots
and translocated. This may explain recent reports of arsenic, cadmium, and
other heavy metals in rice, grapes, and other food products. Also, root zone
absorption can be used to remediate/clean up contaminated soil. An example is the remediation of soil contaminated with radionuclides that resulted
from past atomic testing (Entry et al., 1996).
Properties of Pesticides and Contaminants
where C o is the concentration in the organism and C w, a is the concentration
in water or air at equilibrium. Chemical contaminants can enter an organism through tissue penetration, ingestion, and by respiration. Penetration of
tissue (e.g., human and animal skin; plant leaves and stems) by a chemical
depends on its polarity (lipophilic or hydrophilic), molecular weight, charge,
concentration, the area and duration of exposure, and species differences
among plants and animals.
3.4.2.1 Biota and Water
Chemical contaminants in water are often in contact with plants, other organisms, and sediment. Use is made of the affnity of chemical contaminants in
water to be absorbed by plants and adsorbed onto clays to create managed
wetlands for water decontamination. Contaminated water is allowed to fow
into the wetlands, where contaminants are removed and relatively clean
water is released for more productive uses, such as fsheries or migratory
bird sanctuaries. This principle has been suggested or put into practice in
areas like the Chesapeake Bay, where surrounding intense farming or livestock production practices release undesirably high loads of nutrients to the
Bay, encouraging algal blooms or contamination levels unsafe for fsh and
other aquatic species. Use of vegetative barriers or managed wetlands may
be the most practical solution in such cases.
3.4.2.2 Root Zone Absorption
In general, water-soluble neutral organic molecules do not adsorb tightly
to soil—the organic fraction or the clay mineral fraction. Examples include
glyphosate, fuorinated detergents, TFA, phenoxy acetic acids, other watersoluble xenobiotics including heavy metals, and ionized or ionizable organics.
This means that water-soluble insecticides will be available for absorption
by plant roots and translocated, acting as systemic insecticides. Seiber et al.
(1986) studied the absorption of root zone-applied systemic insecticides in
rice paddies (Seiber et al., 1986) (see Chapter 10). They found that, after translocation, some of the insecticides were excreted in the guttation fuid and,
once on the foliage surface, were available for volatilization. This is a mechanism whereby water-soluble systemics of low Henry’s constant get into the
air, rather than by direct water-to-air transfer, and become transportable airborne contaminants. Other soil constituents can be absorbed by plant roots
and translocated. This may explain recent reports of arsenic, cadmium, and
other heavy metals in rice, grapes, and other food products. Also, root zone
absorption can be used to remediate/clean up contaminated soil. An example is the remediation of soil contaminated with radionuclides that resulted
from past atomic testing (Entry et al., 1996).