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Environmental fate & pathways

Biodegradation in soil

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Description of key information

Degradation of cloquintocet-mexyl in soil under aerobic conditions is very fast. The half-life (DT50lab) periods are calculated to be between 0.4 days and 2.4 days. The geometric mean DT50 in aerobic soil at 20°C is 0.7 days. Converting to the EU average outdoor temperature of 12°C (in accordance with ECHA R.16 guidance) gives a normalised DT50 of 1.33 days. In sterile and anaerobic conditions the degradation rate is considerably slower; >1 year in sterile conditions. However, a degradation rate has not been determined in cloquintocet-mexyl in soils under these conditions as they are not representative of typical agricultural situations.
Degradation of a metabolite, CGA153433, was also investigated in three of the studies; this also degraded quickly in most soils (DT50 2.0 – 9.7 days) except in Mosimannacker soil where a DT50 of 72 days was determined. The geometric mean DT50 for CGA153433 in aerobic soil at 20°C is 7.46 days. Converting to the EU average outdoor temperature of 12°C (in accordance with ECHA R.16 guidance) gives a normalised DT50 of 14.14 days.

Key value for chemical safety assessment

Half-life in soil:
0.7 d
at the temperature of:
20 °C

Additional information

The rate of degradation of cloquintocet-mexyl was investigated in five GLP compliant studies, three of which also investigate degradation of the metabolite CGA153433.The experimental parameters selected were 10°C and 20°C, viable and sterile soils, as well as aerobic and anaerobic conditions. Soils used were as follows.

 

Table A: Details of soils used in laboratory degradation studies

Soil type

Name

Organic Carbon (%)

pH

microbial biomass

Reference

sandy loam

Mosimannacker

2.3

6.9

84 mg/100 g dry soil

Keller1991 (89AK05)

loamy sand

German standard soil 2.2

2.7

6.0

28 mg/100 g dry soil

sandy loam

Stein

1.26

7.0

49.7 mg microbial C./100 g dry soil

Keller 1992 (91AK01)

sand

Collombey

2.3

7.3

339 mg microbial C./100 g soil

Keller 1992 (90AK01)

loam

Les Evouettes

3.2

6.5

67 mg/100 g dry soil

loam/silt loam

Gartenacker

2.35-2.6

7.1-7.2

65.3 mg/100 g dry soil

Ellgehausen 2001 (00EH01)

loamy sand

Pappelacker

1.0

7.5-7.6

29.4 mg/100 g dry soil

 

The degradation rates derived in the studies are presented in Table B. It should be noted that the data obtained in the Keller (1991, 89AK05) and Keller (1992, 90AK01) studies have been re-evaluated in order to calculate the degradation kinetics of the main metabolite CGA153433.

 

Table B: Rates of degradation of cloquintocet-mexyl and CGA153433 in soil under various laboratory conditions

Soil types, Origin

Application rate

Temperature

Soil moisture

Cloquintocet-mexyl

Cloquintocet-mexyl

CGA153433

CGA153433

Reference

 

[mg/kg]

[°C]

[%]

DT50[days]

DT90[days]

DT50[days]

DT90[days]

 

Aerobic conditions:

 

 

 

 

 

 

 

 

Sandy loam, Mosimannacker, CH

1.0

20

40 MWC

1.1

3.6

72

240

Keller1991 (89AK05)

Loamy sand, German Standard, D

1.0

20

40 MWC

2.4

8.1

n. d.

n. d.

Keller1991 (89AK05)

Sandy loam, Stein, CH

10

20

60 FC

1.5

4.8

n. d.

n. d.

Keller 1992 (91AK01)

Sandy loam, Stein, CH

1.0

20

38 FC

1.5

5.0

n. d.

n. d.

Keller 1992 (91AK01)

Sandy loam, Stein, CH

1.0

10

60 FC

1.6 (0.72 at 20°C)

5.2 (2.34 at 20°C)

n. d

n. d

Keller 1992 (91AK01)

Sand, Collombey, CH

1.0

20

75 FC

0.6

1.9

3.0

10

Keller 1992 (90AK01)

Loam, Les Evouettes, CH

1.0

20

75 FC

0.4

1.3

2.0

6.6

Keller 1992 (90AK01)

Loam/silt loam, Gartenacker, CH

0.1

20

40 MWC

0.2

1

9.7

32

Ellgehausen 2001 (00EH01)

Loamy sand, Pappelacker, CH

0.1

20

40 MWC

0.2

1

5.5

18

Ellgehausen 2001 (00EH01)

Geometric mean (aerobic cond.)

 

 

 

0.7

2.5

7.46

24.66

 

Aerobic sterile conditions:

 

 

 

 

 

 

 

 

Loam, Les Evouettes, CH

1.0

20

75 FC

>1 year / -

n. d.

n. d.

n. d.

Keller 1992 (90AK01)

MWC = maximum water capacity          FC = Field capacity          n. d. = not determined

 

When incubated under aerobic conditions, cloquintocet-mexyl is degraded very rapidly under concurrent formation of the free acid CGA153433, irrespective of soil moisture, temperature and analyte concentration. The half-life periods of the parent compound is found to be between 0.2 days and 2.4 days, the DT90-values between 1 day and 8 days. The relatively low rate of degradation found in German standard soil 2.2 resulting in a half-life of 2.4 days is due to the low microbial activity of this soil (remark: the microbial biomass of the German standard soil was only 28 mg/100 g dry soil compared to that of the other soils used in the experiment which contained at least 50 mg microbial C/100 g dry; this soil released only small amounts of carbon dioxide).

 

Degradation of the acid metabolite CGA153433 is observed in all soils tested. The DT50- and DT90-values in the soil from Collombey and Les Evouettes are very short (2 - 3 days and 6.6 - 10 days, respectively), the values obtained in the other soils are between 5.5 - 72 days and 18 - 240 days, respectively (for details see Table B). Only small quantities of further degradates of unknown structure are detected (always <5% of applied).

 

Shortly after incubation, the non-extractable residues increased and reached the maximum amounts in the range of 77% to 87% between day 28 and day 84. The non-extractable components started to decrease slowly during further incubation and reached final values between 59% and 84% after 329 and 360 days, respectively. At the same time, the formation of14C-carbon dioxide increased in parallel. Until study termination, mineralisation amounted to 6 - 13% after 56 days incubation in the Pappelacker loamy sand and Gartenacker loam/silt loam, 6 - 9% after 329 days in Stein sandy loam, 18% after 336 days in the Mosimannacker sandy loam, 20% after 350 days in the Collombey sand and 28% after 360 days in the Les Evouettes loam.

 

The results of these studies give a consistent picture of the processes occurring in soil. The most important steps are the degradation of cloquintocet-mexyl yielding the intermediate metabolite CGA153433, as well as the formation of non-extractable components which are strongly bound to soil organic matter. However, traces of these residues can be partly released and become extractable again before they are finally mineralised to14C-carbon dioxide in viable soils.

 

The dissipation of cloquintocet-mexyl was also investigated in a number of field dissipation studies in wide range of soil types at sites in Germany, France, Italy, Spain and Switzerland. After application of up to 150 g/ha to bare soil, the residues of cloquintocet-mexyl in the 0 - 10 cm soil layer disappear completely within 1 - 3 months. The residues of CGA153433 decline to 0.01 mg/kg or less within 3 - 5 months. No residues above 0.5 μg/kg were detected below 10 cm soil depth. Therefore, it is concluded that accumulation in soil into the next growing season can be completely excluded. Additionally, the potential of the active ingredients and its metabolites to penetrate deeper soil layers and contaminate the shallow groundwater is negligible. The field studies regarding the dissipation of cloquintocet-mexyl are not included in the IUCLID dossier (since sufficient information regarding degradation is already provided by the laboratory studies). However, further details can be provided, if necessary.