By Dr. Mohamed Isa Abdul Majid
The Sun, March 1, 1997
THE WIDESPREAD USE AND DISPOSAL of pesticides by farmers, large plantations and the general public causes environmental contamination. It is estimated that the 68% of such contamination is a result of agricultural uses, followed by industrial and commercial uses (17%) and home and garden application (15%).following release into the environment, pesticides may give rise to different consequences. Pesticides which are sprayed can become airborne and may eventually end up in soil or water. Pesticides applied directly to the soil may be washed off the soil into water or may percolate through the soil to lower soil layers and groundwater.
The application of pesticides directly to bodies of water for weed control or indirectly as a result of run-off from soil or other routes, may lead not only to the build-up pesticides in water, but may also cause pollution to the air through evaporation.
As these pesticides may be broken down, or degraded, by the action of sunlight, water, or other chemicals or microorganisms. This degradation process usually leads to the formation of less harmful residue but in some instances can produce more toxic products.
The second possibility is that the pesticides become resistant to degradation by any means and thus remam in unchanged in the environment for long periods of time. The ones that are most rapidly broken down have the shortest time to move or will otherwise have adverse effects on humans or other organisms. The ones which last the longest, the so-called persistent pesticides, can build up in the environment, leading to greater potential for adverse effects to occur.
In addition to resistance to degradation, there are a number of other properties of pesticides which determine their behaviour. Among these properties, the most important is the volatility - the rate of evaporation - of the pesticide. The ones that are most volatile have the greatest potential to go into the atmosphere and travel far if aided by the wind.
Another important property is solubility in water. If a pesticide is very soluble in water, it seeps more easily through the soil and is a potential groundwater contaminant.
In addition, the water-soluble pesticide is more likely to stay in the water where it can have adverse effects on fish and other organisms. If the pesticide is insoluble in water, it usually tends to stick to the soil and will settle to the bottoms of bodies of water, making it less available to organisms.
Table 1 lists some of the more commonly used pesticides with an estimate of their persistence in soil. A standard approach to measure this parameter is the time taken for 75% to 100% of the material to disappear from the site of application. Using this classification, the grouping of the major classes of pesticides can be divided into non-persistent pesticides in the range of one to 12 weeks, moderately persistent pesticide (one to 18 months) and persistent pesticides (two to five years).
| PESTICIDE PERSISTENCE IN SOILS | ||
|---|---|---|
| Low persistence | Moderate persistence | High persistence |
| Captan | Atrazine | Picloram |
| Dalapon | Carbaryl | Bromacil |
| Dicamba | Carbofuran | Chlordane |
| Malathion | Diazinon | Lindane |
| 2, 4-D | Glyphosate | Paraquat |
| Dalapon | Carbaryl | Bromacil |
If more persistent pesticides such as organochlorines (for examples, DDT, dieldrin and BHC) are released into the environment, the potential hazard created to both land and acquatic ecosystems would be higher because the organisms would be exposed for long periods of time after a single application.
A non-persistent pesticide, on the other hand, would not have a chance to move far in the environment or exert its effects for a longer period of time. However, if it is also very soluble in water and the condition are right, it can move rapidly through certain soils. As it moves away from the surface, it moves away from the agents such as sunlight and bacteria which are degrading it. As it gets deeper into the soil, it degrades more slowly and thus has a chance to get into groundwater.
The dawnward movement of non-persistent pesticides is not an unlikely scenario and several pesticides with short half-lives, such as aldicarb, have been widely found in groundwater.
In contrast, very persistent pesticides may have other properties which limit their potential for movement throughout the environment. many of the chlorinated hydrocarbon pesticides are very persistent and slow to break down but are also water-soluble and tend not to move down through the soil into groundwater.
They can, however, pose problems in other ways since they remain on the surface for a long time where they may be subject to run-off and possible evaporation. Even if they are not very volatile, the tremendously long time they persist can lead, over time, to measurable concentrations moving through the atmosphere and accumulating in remote areas.
Living organisms may also play a significant role in the movement of pesticides in the environment. This is particularly important for pesticides which can accumulate in living creatures.
An example to illustrate this accumulation is the uptake of a very water-soluble pesticide by a creature living in water.
Since this pesticide is stored in the organism, the pesticide accumulates and levels increase over time. If this organism is eaten by a higher organism which can also store this pesticide, levels can reach higher values in the higher organism than is rpesent in the water in which it lives.
Levels in fish, for example, can ne tens to hundreds of thousands of times greater than ambient water levels of the same pesticide. This type of accumulation is called bioaccumulation, which is a phenomenon by which the concentration of a chemical is increased in successive steps of a food chain, with the danger that some species, especially the ones highest in the series, will be injured by the compound.
A classical example of this injury is the population decline of fish-eating birds cause by reproductive failure as we as the thinning of the eggshells of these birds caused by organochlorine insecticides.
Similarly, it should be remembered that humans are at the top of the food chain and may be exposed to these high levels when we eat animal-based foods which have bioaccumulated pesticides and other organic chemicals. Apart from fish, domestic animals too can be accumulators of pesticides. Therefore, caremust be taken in the use of pesticides in agricultural activities.
On a final note, the release of pesticides into the environment may be followed by a complex series of events which can transport the pesticide through air or water, into the ground or even living organism. The most important route of distribution and the extent of distribution are different for each pesticide. It will depend on the formulation of the pesticide and how and when it is released.
Despite this complexity, it is possible to identify situations that can pose concern and to try to minimise them. However, there are significant gaps in the knowledge of pesticide movement in the environment. So it is best to minimise unnecessary release of pesticide into the environment. The fewer pesticides that are unnecessarily released, the safer our environment will be.
The writer is a pharmacist and head of the Toxicology Laboratory at the Officer at the National Poison Centre, Universiti Sains Malaysia, Penang.
