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Environmental fate & pathways

Adsorption / desorption

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Description of key information

The adsorption behaviour of cloquintocet-mexyl was investigated in a reliable study performed to OECD 106 guideline. Keller (1990) reported Koc values for five soils with a mean value of 12850, which indicates strong adsorption to soil. In addition, Hill (1994) investigated the adsorption behaviour of the metabolite CGA153433 in three soils and reported a mean Koc of 1772.

Key value for chemical safety assessment

Koc at 20 °C:
12 850

Additional information

A reliable GLP-compliant study (Keller 1990) investigated adsorption behaviour of cloquintocet-mexyl for five soils. This study was conducted to OECD 106 guideline and is considered to be reliable without restrictions. The Freundlich adsorption constants ranged from 62.6 to 1855 ml/g, the Koc values were between 6521 and 19314 and the mean Koc was 12850 ± 4991 ml/g. Results indicate strong adsorption of cloquintocet-mexyl to soil.

The adsorption behaviour of CGA153433, a metabolite of cloquintocet-mexyl, was studied by Hill (1994) in a reliable GLP-compliant study conducted to OECD 106 guideline. Results for three soils show that CGA153433 is a slightly mobile to immobile compound, with a Koc ranging from 1186 to 2870 µg/g (mean Koc 1772).

The corresponding constants for the desorption cycles were somewhat higher revealing that sorption was only partially reversible and become stronger with time. Both compounds were stable within the equilibrium period of the adsorption and desorption tests.

The low mobility of cloquintocet-mexyl and CGA153433 is confirmed by the finding of aged residue column leaching studies (Ellgehausen 2001, Keller 1991 (three reports). These studies are not included in the IUCLID dossier. However, further details can be provided, if necessary.). The studies indicate that the total radioactivity which is found in the percolation water amount to ≤0.08% of the applied dose (200 mm precipitation within 4 days). Even under worse case conditions of 508 mm of artificial rainfall within 40 days, no radiocarbon exceeding 0.7% can be detected in the leachate. On the other side, 85 - 108% are retained in the soil columns, mostly in the top 4 cm layers (79 - 92%).

A field leaching study performed under realistic worst case conditions at two sites in England (Fogg and Carter, 1998. This study is not included in the IUCLID dossier. However, further details can be provided, if necessary.) confirms these results. TOPIK 240 EC was applied at a field rate of 60 g/ha clodinafop-propargyl and 15 g/ha cloquintocet-mexyl on a sandy and a clay soil on 6 December 1996 (Both sites were located in Warwickshire, England, on land typically used for growing winter cereals; Sandy soil (0-26 cm): 11.28% clay, 26.49% silt, 62.22% sand, 2.3% organic carbon, pH 7.0, CEC 11.40 meq/100g; Clay soil (0-22 cm): 30.77% clay, 42.16% silt, 27.07% sand, 1.7% organic carbon, pH 7.3, CEC 17.6 meq/100g; Average annual rainfall: 620 mm; winter rainfall: 200-210 mm). No residues of cloquintocet-mexyl or CGA153433 could be detected in the soil water samples (<0.05 mg/l) collected at soil depths between 25 cm and 120 cm. Drain water collected at 80 cm and run-off water both collected from the clay field did not contain any residues.

Based on these results, the potential of cloquintocet-mexyl and its metabolites to contaminate the shallow groundwater is negligible. Even under the worst case conditions of application in winter, no risk can be perceived.