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Environmental fate & pathways

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Fate and behaviour in soil

Total mineralisation of propargite to CO2 is the major route of degradation with metabolism via initial hydrolysis of the propynyl sulphate side chain to form TBPC. This then undergoes aliphatic oxidation of a methyl group of the tert-butyl to form TBPC-acid. Very low levels of these intermediate degradation products were seen. TBPC reached 10 % at day 22-30 in one soil and then degraded. However, in four other soils the levels were always <2 %. TBPC-sulphate was found in one soil only at levels >5 % (6-8 %) at three consecutive timepoints and PTBP was identified in one soil at <0.5 %. A number of unknown peaks were quantified in various soils but only in one soil did one of these appear at >5 % (this was a very polar compound, probably consisting of a number of compounds).

In five soils (silt loam, loamy sand, sandy loam, sandy clay loam, silty clay loam), respective DT50 (and DT90) values of 90.74 (301.43), 55.53 (184.48), 39.5 (131), 53.3 (177.1) and 67.2 (223) days were calculated on the basis of aerobic degradation studies conducted in the laboratory giving mean DT50 and DT90 values for propargite of 61.3 and 203.4 days, respectively. A DT50 of 94.4 days was reported for propargite incubated in sandy clay loam soil under anaerobic conditions. In field dissipation studies undertaken in the North and South of the EU, respective mean DT50 and DT90 values for propargite were 55 days (range 28-90 days) and 183 days (94-298 days). These are consistent with the results of four US studies carried out in California and Florida where the range of DT50 values was 67-99 days. No movement of propargite below the top soil layer was observed in any field study. Concentrations of TBPC were generally low in field studies and there was no evidence of accumulation.

Based on the results from ten soils and sediments, mean adsorption Koc and 1/n values of 30,538 and 1.16, respectively, have been reported for propargite. These values indicate that the mobility of propargite in soil is very low. This conclusion is further supported by the results of aged leaching study and data from field dissipation trials where no leaching of propargite occurred.

Fate and behaviour in water

Groundwater

In the light of the limited mobility of propargite in soil, significant concentrations of this substance are not expected in groundwater.

Surface water

In water/sediment systems, propargite rapidly partitions into the sediment layer, as indicated by DT50 values for the water phase of 1.7-2.5 days (DT90 5.6-8.2 days). A maximum level of 58 % propargite was attained in one sediment eight days after application. An additional water/sediment system in which soil and water were vigorously mixed after application of propargite (leading to 98.3 % propargite in the sediment on day 0) showed that propargite did not readily desorb from the sediment once adsorbed. Degradation of propargite in sediment was seen in all studies with DT50 values of 19.9-38.4 days (DT90 66-127.8 days). Whole system DT50 and DT90 values were 18-38 days (mean 26.4 days) and 66-128 days (mean 87.7 days), respectively. TBPC was the major metabolite formed in water-sediment studies.

Propargite is stable to hydrolysis under acidic conditions but degrades in neutral/basic conditions, resulting in the formation of TBPC. At 25 ºC, the hydrolytic DT50 of propargite was 66.3 days at pH 7 and 1.1 day at pH 9.

Propargite rapidly degrades by photolysis to TBPC (maximum 41 % after 30 days continuous illumination) and PTBP (maximum 12 % after 10 days) as well as a number of other unidentified polar fragments. First-order DT50 values for propargite under experimental conditions were 3.9-6.4 days and this was extrapolated to 10.4 days summer sunlight at 40ºN. However, given the rapid removal of propargite from the water column by adsorption to suspended solids and sediments and the turbid nature of natural water bodies, photolysis is not expected to contribute significantly to total degradation.

Fate and behaviour in air

The low vapour pressure (<4.04 x 10 -5 Pa at 20 ºC) and Henry's Law constant (6.4 x 10-7 atm m3/mol) indicate that propargite will not partition into air at a significant extent. A theoretical half-life of 2.155 hours (12 hour day) has been calculated (Atkinson calculation) indicating that any compound reaching the air would be rapidly degraded.

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