Contrasting behaviour of chlorpyrifos and its primary metabolite, TCP (3,5,6-trichloro-2-pyridinol), with depth in soil profiles

Regulatory agencies and natural resource managers are increasingly recognising the role of pesticide metabolites in the overall risk to non-target organisms. However, the environment fate data on pesticide metabolites are relatively meagre. We report the sorption and degradation behaviour of chlorpyrifos and its primary metabolite, TCP (3,5,6-trichloro-2-pyridinol), with depth in 2 Australian soil profiles. Sorption isotherms were determined by batch equilibrium method and the degradation was studied under controlled incubation conditions. Sorption of chlorpyrifos ( K-d = 40.3 - 209.6 L/kg) was found to be about 100 times higher than that of TCP (K-d = 0.45 - 2.86 L/kg) in both the soil profiles. A significant correlation (r(2) = 0.88**) was found between sorption of chlorpyrifos and the soil organic carbon content, but not in the case of TCP. However, in the case of TCP a significant inverse relationship was observed with pH (r(2) = 0.81**). The rate of degradation of chlorpyrifos increased with depth in the soil profile, whereas the converse was true for TCP. The time for 50% loss of chlorpyrifos (DT50) was found to be 23 - 28 days in the surface soils ( acidic pH) and only 7 - 16 days in the subsurface ( alkaline pH). The DT50 values for TCP in the surface soils ranged from 42 to 49 days and in subsurface soils from 64 to 117 days. The contrasting behaviour of chlorpyrifos and TCP in soil profiles is clearly evident from the study. Due to its lower sorption and longer persistence, TCP has a much greater leaching potential than chlorpyrifos.