In recent years many waterways in Slovenia have been subjected to an increased loading with pesticides due to intensification of agriculture. The most widely used herbicides are atrazine and alachlor and they were detected in some rivers and even in ground water. Therefore the effects of atrazine and alachlor on selfpurification processes were investigated. The basic selfpurification processes studied were biodegradation of organic substances and photosynthesis and growth of algae. The inhibiting effect of pesticides on the process of biodegradation of organic pollutants was evaluated by the use of laboratory river model and mathematical modelling. The harmful impacts of pesticides on aquatic autotrophic organisms were assessed by measurement of net assimilation inhibition (24-h acute toxicity test) as well as growth inhibition - chlorophyll- a content (72-h chronic toxicity test) of algae Scenedesmus subspicatus. The results obtained demonstrate that atrazine and alachlor in concentrations found in our rivers have practically no effect on biodegrading heterotrophic organisms, while their adverse effect on algae is quite considerable.

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