Pesticide Degradation in Water - Pesticides: Evaluation of Environmental Pollution - page 95

expressed as the sum of the parent compound and toxicologically relevant transformation
products, when it is derived from a toxicological risk assessment, but for monitoring pur-
pose, it is more convenient to set a limit for the parent compound or a marker residue in
order to take a simple residue definition for enforcement purpose. But it is also suggested
that in certain cases, when the degradation products are separated from the parent com-
pound in the environment because of different mobilities, separate limits should be set for
degradation products.
In the United States, the threshold values for pesticide residues in water (standards)
are legally enforceable by agencies of the US government, for example, Environmental
Protection Agency Maximum concentration levels (EPA MCLs) for drinking water. Federal
guidelines for these values are provided as advice or recommendation for the protection
of human health and/or aquatic organisms. Nowell and Resek (1994) provided a detailed
review of US standards for pesticide residues in water.
The overall legislation and guidelines for pesticide residues in water are derived from
various criteria in different countries, and the harmonization of criteria is a difficult task.
Although valuable progress has been achieved in the last few years to guarantee a good
quality of the environmental water and drinking water, it is still necessary to work harder
to improve the enforcement of legislation in view of the scientific progress.
4.5 Occurrence of Transformation Products in Environmental Waters
Both, surface waters and groundwater are the main sources for drinking water produc-
tion (Ibáñez et al. 2004). In many countries, drinking water is obtained from groundwater,
which undergoes only simple treatment, such as aeration and filtration. Consequently, the
numerous detections of pesticides in groundwater constitute a severe threat to the drink-
ing water supplies (Barceló and Hennion 1997; Tuxen et al. 2000). The growing concern
of preserving the quality of groundwater and surface water promoted the research on
the presence of pesticides in natural waters in the 1980s. However, until recently, little
research has been available on the residues of degradation products in waters, and moni-
toring studies have only focused on the parent compounds. This limited the understand-
ing of the contamination of waters by pesticides. The detection of some of the degradation
products of some pesticides in groundwater stimulated research on these xenobiotics as a
contaminant of water set aside for human consumption.
Agrochemical industries focus the synthesis of new pesticides toward degradable
compounds since the ban of recalcitrant pesticides such as organochlorine DDT and
lindane. Many of the replacement pesticides are organophosphorus, pyrethroid, or urea
compounds that, in general, degrade in the environment in a relatively short period of
time.
However, this tendency presents some disadvantages. Transformation products gener-
ated in the environment have, in general, little biological activity on target pests, but these-
products are often more mobile and can be more persistent than their parent compounds,
leading to concerns about water contamination. It is known that the most frequent reactions
in the environment increase the polarity of the molecule and, therefore, the tendency to
leach into groundwater or to be dissolved in the runoff water. This is particularly applicable
to some of the soil-applied herbicides.