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Environmental fate & pathways

Phototransformation in soil

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Description of key information

WoE: Half life, 34.6 days, EPA 161-3, Peters, 1998.
WoE: Half life, 33.6 days, EPA 161-3, Peters, 1999.

Key value for chemical safety assessment

Half-life in soil:
34.6 d

Additional information

The effect of phototransformation in soil has been determined in two studies, provided as a weight of evidence to address this endpoint.

Peters, 1998 and 1999 were both performed according to GLP, in line with the EPA Guideline 40 CFR 158, Subdivision 161-3 standardised guidelines, with a high standard of reporting and have thus been assigned a reliability score of 1 in line with the principles for assessing data quality set out by Klimisch (1997).

In both studies, the labelled test material was applied to bosket loam soil at an analytically confirmed concentration of 0.03-0.032 ppm and incubated for 30 days at 25ºC (Peters 1998 and Peters 1999 studies respectively). Samples were irradiated for 12 hours per day at an average daily light intensity of 176 W/m²; dark controls (non-irradiated samples) were run concurrently for comparison. Samples were collected and extracted for analysis by HPLC, TLC and mass spectrometry in order to identify and quantify the concentration of the parent compound present at each interval. Identification of any measured compound was performed by co-chromatography and comparison to reference substances.

In both studies, material balance of the applied radiocarbon indicated that photolysis had little effect on the distribution of radioactivity. Regardless of whether samples were irradiated or not the majority of radioactivity was found in the soil compartment.

 

The proposed pathway of the test material in bosket loam soil under photolytic conditions is initially the loss of sulphur dioxide by bridge contraction, followed by the loss of NH2CO. Further degradation involved bridge cleavage of the pyrimidinyl moiety and the amino-pyrimidinyl moiety.

 

Under the conditions of the test, the degradation of the test material by soil photolysis indicated first order kinetics. Photolysis was found to contribute to the degradation of the test material on soil, however photolysis does not appear to produce any significant new degradates that are not observed in non-irradiated samples.

 

Both studies report that photolytic degradation slightly increases the rate of degradation, no unique or significantly different degradates were observed in comparison to non-irradiated samples. The available data are considered to be complete and the half life has been reported based on the worst case, at 34.6 days.

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