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The photochemical behaviour of many aromatic pesticides (mainly herbicides) are compared and the main reactions are separated in three different classes: 1- reactions involving carbon-halogen bond; 2 - other reactions involving the aromatic ring; 3 - reactions of the aliphatic moiety. It appears that the nature of the substituents and their relative positions on the ring play a major role in the orientation of the reaction. The molecular and ionic forms of ionisable molecules may have different photochemical behaviour. A wavelength effect is observed with some compounds. The case of mecoprop [2-(4-chloro-2-methylphenoxy)-propanoic acid] is presented as an example.The irradiation of herbicides in aqueous solution may lead to the formation of photochemical intermediates more toxic to microorganisms than the initial substrate.

This study shows that, although soil microbiota can adapt to rapid mineralisation of 2,4-D, this microbial activity is strongly dependent on climatic parameters. It suggests that only limited pesticide biodegradation occurs under Mediterranean climate, and that arable Mediterranean soils are therefore fragile and likely to accumulate pesticide residues.