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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Description of key information

A study of biodegradation in water sediment systems performed according to OECD guideline 308 reported maximum DT50 values of 3 days at 20 °C for the water phase and for the whole system. Converting to the EU average outdoor temperature of 12 °C (in accordance with ECHA R.16 guidance) gives a normalised half life of 5.7 days.

Key value for chemical safety assessment

Half-life in freshwater:
3 d
at the temperature of:
20 °C
Half-life in freshwater sediment:
3 d
at the temperature of:
20 °C

Additional information

Minet (1994) performed a laboratory water: sediment study in natural pond and river water.14C-labelled benoxacor was applied to the surface of the test systems corresponding to a field rate of 80 g a. i. /ha. The pond system was incubated under aerobic and anaerobic conditions at 20 and 9 °C and also under sterile conditions. The River system was incubated aerobically at 20 °C.

The results indicate that Benoxacor dissipates rapidly from the water phase (DT50 of 2-3 days at 20 °C). Even at lower temperatures (9 °C) dissipation in the water phase is fast (4-6 days). The DT50 values of benoxacor are in the range of 2-3 days for aerobic and anaerobic conditions (20 °C). At lower temperatures (9 °C) the rate of degradation in the whole system decreases only slightly (DT50 5-7 days). 

Sediment bound residues are the major degradation products of the parent compound under anaerobic and aerobic conditions. Their concentration increases to about 70% after 2-3 months. Thereafter the mineralisation increases. An accumulation in the sediment is therefore unlikely to occur. Mineralisation is a significant process in the dissipation of benoxacor. Up to 20% of the applied dose is mineralised to CO2 under aerobic conditions within one year. Anaerobic and aerobic sterile systems produce only minor amounts of volatiles. This demonstrates that biological activity of the sediment is a prerequisite for the degradation of bound residues.

Any metabolites formed were <6% of the applied dose, except for two fractions (M3 and M8) where maximum concentrations were 10.5% (anaerobic pond system at 20 °C) and 17% (aerobic Rhine system at 20 °C), respectively. At the end of the incubation (day 382) the concentration of both metabolite fractions M3 and M8 dropped to 2.3 and 7.6 %, respectively, demonstrating their transient nature.

Based on the results of this study, the key value selected for the chemical safety assessment is a DT50 of 3 days at 20 °C (the upper limit of the range), for both the water phase and the whole system.