Environmental behaviour of polychlorinated biphenyls in a paddy field: Impact factors and canopy effects

Paddy fields play an important role in the transport of persistent organic pollutants (POPs) due to the filter effects of canopy and their wide distribution. Thus, most studies have been focusing on the filter effects of canopy for POPs. However, shielding effects of canopy might also influence transport and portion of POPs between top and bottom. To investigate these two important processes, our study involved 30 polychlorinated biphenyls (PCBs) in a paddy field. Samples of bulk depositions, surface water, and air were taken to investigate the occurrence and the behaviour of PCBs. We found that rice canopy has potentially crucial effects on the transport of PCBs. The results showed slightly higher abundances for most of high‑chlorine PCBs (81.0%) at the top of the canopy, indicating that the high‑chlorine PCBs were intercepted by the rice leaves. Moreover, our study showed that the PCBs in surface water and soil tended to escape into air according to air, water, and soil fugacity. And we found higher atmospheric PCB levels (103 pg m^-3) at the bottom of the canopy than top (88.9 pg m^-3), indicating canopy shielding effects on escaped PCBs. In addition, the study showed that the PCBs intercepted by the rice canopy may occur in surface water and soil due to air movement and precipitation. These results suggest that paddy fields can enrich POPs, and effects of the environmental factors on POPs transport need to be investigated further.