Nitrapyrin

Biodegradation in water and sediment: simulation tests

Description of key information

DT50 (total system ) = 0.9 days; DT50 (water) = 0.8 days; DT50 (sediment) = 1000 days (worst case default for FOCUS modelling); EPA OPP 162-4, PMRA DACO 8.2.3.5.4, Laughlin et al (2013)

Key value for chemical safety assessment

Half-life in freshwater:

0.8 d

at the temperature of:

25 °C

Half-life in freshwater sediment:

1 000 d

at the temperature of:

25 °C

Additional information

The aerobic aquatic degradation of the test material in water and sediment was investigated in a study which was conducted under GLP conditions and in accordance with the standardised guidelines US EPA Subdivision N. Section 162-4 and Canada PMRA DACO Number 8.2.3.5.4. The study was assigned a reliability score of 1 in line with the criteria of Klimisch et al. (1997).

During the study 14C-nitrapyrin was evaluated in a sediment/water system (water pH 6.7, 8 ppm total suspended solids, sediment texture sand, pH 6.0, organic carbon 0.2%) from Suffolk, Virginia, USA for 30 days in the dark at 25 °C. The test material was applied at the rate of 0.4 mg a.i./L. The sediment/water ratio used was 1:2. The test system consisted of two-chambered biometer flasks: one chamber for the sediment and water and the other chamber containing 0.2 N NaOH for the collection of CO2. Polyurethane foam was placed in the bridge between the two chambers for collection of organic volatiles. Samples were analysed at 0, 0.25, 1, 3, 7, 10, 15, 21, and 30 days after treatment (DAT). Aliquots of the water were directly analysed by LSC and HPLC. The sediment samples were extracted on a horizontal shaker at high speed with 90:10 methanol:2.0 N NaOH. Test material residues were analysed by LSC and HPLC. Identification of the transformation products was performed by co-chromatography with authentic standards and LC/MS.

Material balance was 96.7 ± 3.0% (range = 91.3% to 101.4%) of the applied amount. The concentration of test material in water decreased from 97% of the applied amount at 0 DAT to below 1% of the applied amount at 7 DAT. The concentration of test material in sediment increased from 0% of the applied amount at 0 DAT to a maximum of 5% of the applied amount at 0.25 DAT. The concentration fell below 1% of applied at 7 DAT.

The major transformation product (>10% of applied amount) detected in the water was 6-CPA with a maximum concentration of 29% of the applied amount, observed at 21 DAT of incubation. The corresponding concentration in water at the end of the study was an average of 23% of the applied amount. The major transformation product detected in the sediment was also 6-CPA, with a maximum concentration of 16% of the applied amount, observed at 10 DAT of incubation. The corresponding concentration in sediment at the end of the study was 16% of the applied amount. The minor transformation product (<10% of applied amount) detected in both the water and sediment layers was DCMP, formed at a maximum of 8% and 4% of the applied amount, respectively. The maximum total system concentrations for 6-CPA and DCMP were 45% and 11% of the applied amount, respectively. The unidentified 14C ranged from 1 to 11 % of the applied amount and was composed of multiple low-level degradates that individually did not comprise more than 5% of the applied radioactivity.

Approximately 10% of applied radioactivity was recovered in a polyurethane foam trap throughout the study. This organic volatile was identified as parent test material.

Extractable 14C-residues in sediment increased from 0% at 0 DAT to approximately 25% of applied at 3 DAT. The extractable residues then remained at 25% of the applied amount through the remainder of the study. Non-extractable 14C-residues in sediment increased from 0% at 0 DAT to an average of 37% of the applied amount at study termination. At the end of the study, less than 1% of the recovered radioactivity was present in the caustic trap.

The best-fit DT50 values of the test material in the water column and in the entire system were 0.8 and 0.9 days, respectively.

The test material can therefore be concluded to dissipate from the water/sediment test system through volatilisation to the atmosphere, hydrolysis to the 6-chloropicolinic acid metabolite, and degradation to the DCMP metabolite. The DCMP metabolite further degrades to 6-CPA and non-extractable residues.