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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Description of key information

The degradation of cloquintocet-mexyl was investigated in two studies conducted to a guideline similar to OECD TG 308. The degradation of cloquintocet-mexyl in non-sterile conditions was rapid (DT50 0.4 to 2 days in the whole system). The DT50 of 0.2 days for the water phase and 0.7 for the sediment phase under aerobic conditions at 20 °C (Keller, 1993) is selected for the chemical safety assessment. Converting to the EU average outdoor temperature of 12°C (in accordance with ECHA R.16 guidance) gives normalised half lives of 0.38 and 1.33 days for the water and sediment compartments, respectively. The metabolite (CGA153433) would disappear from the water phase in a few days but persist in the whole system (realistic worst case DT50 of 56 days for whole system under aerobic conditions at 20 °C).

Key value for chemical safety assessment

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

Additional information

The degradation of cloquintocet-mexyl was investigated in two GLP compliant studies conducted to a guideline similar to OECD TG 308 (Biologische Bundesanstalt für Land- und und Forstwirtschaft, Deutschland. Richtlinien für die Prüfung von Pflanzenschutzmitteln, Teil IV,5 - 1; Abbaubarkeit und Verbleib von Pflanzenschutzmitteln im Wasser/Sediment-System. December 1990).

The first study (Keller, 1993) investigated the degradation of cloquintocet-mexyl in water sediment systems at different temperatures and incubation conditions.Cloquintocet-mexyl dissipated very fast under all conditions. Metabolite, CGA153433 was formed in up to 37% yield within the first week of incubation. This metabolite was also degraded under aerobic and anaerobic conditions and finally reached 3-8% after 280-358 days. At the beginning of the study, CGA153433 was found mainly in the aquatic phase, but became adsorbed by the sediment upon prolonged incubation.

Non-extractable residues were detected in the sediment layers under all conditions. They increased to about 84% of the initial dose within 112 days of aerobic incubation at 20°C, within 358 days of aerobic incubation at 10°C, within 238 days of anaerobic incubation at 20°C, as well as to 60% after 99 days under sterile anaerobic conditions. As a result of microbial degradation, the bound residues were slowly mineralised. At lower temperature, as well as under anaerobic and sterile conditions, however, the mineralisation was found to be marginal. The formation of 14C-carbon dioxide accounted for 6% in the aerobic experiment at 20 °C, but <0.4% in the other tests.

The second study (Reischmann, 1996) investgated degradation of cloquintocet-mexyl in a natural water - sediment system collected from the Rhine river and incubated under aerobic conditions at 20°C.

The level of radiocarbon in the aquatic phase decreased to 2% of applied within 125 days. Within the first 7 days, the extractable radioactivity in the sediment increased to 46% and then dropped to 23% at study termination, whereas the non-extractable portions finally reached 63%. 

The disappearance of cloquintocet-mexyl from the system was very fast. The initial quantities accounted for 95% and were distributed between the aquatic phase (75%) and sediment layer (20%). Within 2 days, its amounts in the water declined to 3% and disappeared completely after 14 days. Only one major metabolite was observed, i.e. CGA153433, which reached its highest value of 65% after 2 days, 38% being found in the water and 27% in the sediment. It decreased to reach 25% in the whole system, 2% in water and 23% in sediment after 125 days. A further metabolite of unknown structure was always <1.4%.

In order to assess the degradation rates of the primary metabolites, the data from the present studies (Keller 1993b, Reischmann 1996) were compiled and re-calculated using the ModelMaker software (Ellgehausen 1998). The dissipation half-lives are summarised in the following table.

Experimental conditions and degradation rates of cloquintocet-mexyl and its metabolite CGA153433 in water and sediment systems

System Incubation conditions Temp. [°C] Dissipation from cloquintocet-mexyl [days]    CGA153433 [days]    Ref.
        Half-life DT90 Half-life DT90  
Pond Aerobic 20 Whole system 0.6 2.0 56 187 Keller 1993 (91AK02)
Water phase 0.2 0.7 5.6 ≈20 Keller 1993 (91AK02)
Sediment 0.7 2.2 n. d. n. d. Keller 1993 (91AK02)
10 Whole system ≈1 ≈4 ≈50 ≈160 Keller 1993 (91AK02)
Water phase <1 <1 7.8 ≈26 Keller 1993 (91AK02)
Sediment <1 ≈3 n. d. n. d. Keller 1993 (91AK02)
Anaerobic 20 Whole system ≈2 ≈8 ≈80 n. d. Keller 1993 (91AK02)
Sterile / aerobic 20 Whole system 33 110 n. d. n. d. Keller 1993 (91AK02)
 Rhine river Aerobic 20 Whole system 0.4 1.4 36 120 Reischmann 1996 (95RF02)
Water phase 0.2 1.5 n. d. n. d. Reischmann 1996 (95RF02)

From the table above, it can be seen that cloquintocet-mexyl was found to rapidly degrade in aerobic and anaerobic systems (DT50= 0.6 to 2 days in the whole system), but persist under sterile conditions (DT50 = 33 days in whole system), indicating that the most important pathway of dissipation in a viable aquatic environment is microbial degradation. Sterile and anaerobic systems are not considered representative of normal environmental conditions. Therefore, it is proposed that study results for aerobic systems should be used in the Chemical Safety Assessment. Accordingly, the DT50 of 0.2 days for the water phase and 0.7 for the sediment phase under aerobic conditions at 20°C is selected for the chemical safety assessment. The metabolite (CGA153433) would disappear from the water phase in a few days but persist in the whole system (realistic worst case DT50 of 56 days for whole system under aerobic conditions at 20 °C).