Geoscience Reference
In-Depth Information
rainfall after application. In addition to the chemical properties of the pesticides and
the weather conditions, soil properties have a significant effect on pesticide leaching
and surface run-off.
112
The most important processes to be taken into account when evaluating the
effects of pesticides on aquatic ecosystems are leaching, degradation, accumulation,
and toxicity. Pesticides with low solubility in water and which are tightly adsorbed
to soil particles can be washed out only by surface run-off. However, many pesticides
with high water solubility are currently widely applied. These pesticides can also
be leached from the soil by subsurface drainage systems and can infiltrate to ground-
water.
112
Pesticides are a chemically diverse group of compounds. Some are produced
naturally by certain species of plants, but chemists have synthesized the great
majority of organic pesticides. Prominent organic pesticides include natural
organic compounds (alkaloid nicotine, pyrethrum), synthetic organometallic
chemicals (organomercurials), and phenols (tri-, tetra-, and penta-cholorophenol).
Chlorinated hydrocarbons are a diverse subset of synthetic pesticides; prominent
examples are DDT and lindane. Other pesticides classes include cyclodienes,
organic phosphorus compounds, carbamates, triazine herbicides, and synthetic
pyrethroids.
Detailed information on pesticides is given in Montgomery,
113
Edit et al.,
114
Clark
et al.,
110
and Line et al.
115
BCPC,
116
DPR,
117
EXTOXNET,
118
Watts and Moore,
119
Jorgensen and Gromiec,
120
Miyamoto et al.,
121
Bennett,
122
Larson et al.,
123
Rao et al.,
124
and Hemond and Fechner-Levy
125
discuss fate and transport mechanisms of pesticides
in the aquatic environment.
4.2.3
F
ATE
OF
O
RGANIC
C
HEMICALS
IN
A
QUATIC
E
NVIRONMENTS
After a chemical substance enters the water, a number of processes will act to
decrease its concentration. Part of the substance can be transferred to the atmosphere
via volatilization, and the rest will distribute itself in water, suspended solids, and
living biota where chemical and microbiological transformations can occur. Particles,
with their associated chemicals, can sink through the water column and be incorpo-
rated into the sediment. Macrobiota can metabolize a chemical or accumulate it
within their tissues. The latter, however, results in a very minor loss. The same
processes will also govern the fate of the products of chemical and biological
transformation. In some cases, the products of such reactions may be of greater
concern than the precursor compound, but in others, the ultimate result of these
transformations may be that the compound is completely mineralized. These trans-
formations are caused by indirect and direct photolysis, oxidation, hydrolysis, and
microbial biotransformation.
119
Sorption can occur in both suspended particles and
bottom sediment; the former maintains the pollutant in the water column, while the
latter effectively immobilizes it. There can also be some association with dissolved
organic material present in the water body, effectively increasing the solubility of
the pollutant in water.
In coastal waters including lagoons, salinity-related changes in chemical reaction
rates are important, and are generated both by mixing-controlled changes in the
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