Pesticides in Drinking-Water
Session Objectives
· To demonstrate the
conflict in pesticide use between agricultural and public health needs.
· To describe the two principal
methods of pesticide classification.
· To describe the GVs set for
pesticides and their by-products.
Pesticides are used for agricultural as well as public health
purposes. Often a choice has to be made between their detrimental effects on the
environment and their use for disease vector control, as for example, for
malaria or schistosomiasis control. The adverse environmental effects of
pesticides used in public health can often be mitigated through proper selection
and application procedures. Equally, many pesticides have both beneficial and
harmful health effects - their use may reduce the presence of particular
vectors, although they may be toxic is consumed through water. In these
circumstances, the relative benefits and dis-benefits should be evaluated.
With all pesticides, whether they have harmful health effects or
not, the application should be well focused both in terms of application
technique, quantity used and timing of application. As a general rule, the
minimum of pesticide should be applied by the most efficient method at the most
suitable time to achive the required goal. Over-application and/or application
at times when the action is less likely to be effective should be avoided.
Pesticides can be classified according to chemical class (e.g.
organochlorine, carbamate, organophosphorus, chlorophenoxy compounds) or
according to their intended use (e.g. fungicide, herbicide, fumigant). It is
important to know both since the chemical structure of the pesticide and its use
often determine its behaviour in the environment, occurrence in drinking-water
and toxicity to humans. Table 1 indicates the chemical class and use of the
pesticides evaluated in the Guidelines.
Of the 36 pesticides evaluated, 28 contain chlorine.
Organophosphorus pesticides were not evaluated although their use has increased
as replacement for organochlorine pesticides. However, the organophosphorus
pesticides are readily hydrolysed in water, adsorbed on sediments, or readily
degraded in soil. As a result, they are seldom if ever found in drinking-water.
Many of the pesticides evaluated are herbicides. Because of
their frequent use near waterbodies they have often been found in surface water.
Furthermore many of these herbicides are fairly mobile in soil and readily
migrate into groundwater.
While the use of organochlorine pesticides has declined in
industrialized countries, their use continues in developing countries for public
health as well as for agricultural purposes. For this reason, several
organochlorine pesticides were evaluated in the Guidelines.
The toxicological basis of the guideline values and exposure
assumptions made, as reflected in the percentage allocation of the TDI to
drinking-water, are summarized in Tables 2 and 3.
For organochlorine pesticides such as aldrin/dieldrin,
chlordane, DDT, heptachlor, and hexachlorobenzene only 1% of the TDI was
allocated to drinking-water since it is known that these pesticides are highly
persistent, have a high bioaccumulation potential, and are often found in food
(Table 2).
In the majority of cases limited information was available on
the contribution of drinking-water to the total exposure. Therefore a default
value of 10% of the TDI was used (Table 3).
While considerable information is available on the toxicity of
metabolites of pesticides formed in mammalian systems, the nature and toxicity
of the environmental degradation products of pesticides are largely unknown and
have not been taken into consideration in the Guidelines.
Alachlor, 1,2-dibromo-3-chloropropane, 1,3-dichloropropene and
hexachlorobenzene were considered to be carcinogenic. The linearized multistage
extrapolation model was therefore used to derive guideline values corresponding
to an upper-bound estimate of an excess lifetime cancer risk of 1 per 100,000 of
the population exposed.
Because limited information was available on the toxicity of
1,3-dichloropropane, ethylene dibromide and MCPB, no guideline values were
derived for these pesticides.
Not all pesticides that have been found in water have been
evaluated in the Guidelines. However, over 240 pesticides have been
evaluated by the Joint FAO/WHO Meeting on Pesticide Residues (JMPR). Such
evaluations could be used by countries wishing to establish standards or
guidelines for pesticides of national concern.
In many circumatances, it may not be the principla component of
the pesticide which is of concern, but impurities and by-products. It may be
more effective to control the release of toxic substances into the aqautic
environment through proper product quality control that by establishing
standards for drinking-water. It may be more appropriate therefore, to ensure
that product quality standards and their enforcement are in place than
drinking-water quality standards.
References
International Programme on Chemical Safety (IPCS). Summary of
Toxicological Evaluations Performed by the Joint FAO/WHO Meeting on Pesticide
Residues, 1996.
Table 1: Chemical family and use of pesticides evaluated in
the Guidelines
