Air–Seawater Exchange of Organochlorine Pesticides along the Sediment Plume of a Large Contaminated River

Abstract

Gaseous exchange fluxes of organochlorine pesticides (OCPs) across the air−water interface of the coastal East China Sea were determined in order to assess whether the contaminated plume of the Yangtze River could be an important regional source of OCPs to the atmosphere. Hexachlorocyclohexanes (HCHs), chlordane compounds (CHLs), and dichlorodiphenyltrichloroethanes (DDTs) were the most frequently detected OCPs in air and water. Air−water exchange was mainly characterized by net volatilization for all measured OCPs. The net gaseous exchange flux ranged 10−240 ng/ (m 2 ·day) for γ-HCH, 60−370 ng/(m 2 ·day) for trans-CHL, 97−410 ng/(m 2 ·day) for cis-CHL, and ∼0 (e.g., equilibrium) to 490 ng/(m 2 · day) for p,p′-DDE. We found that the plume of the large contaminated river can serve as a significant regional secondary atmospheric source of legacy contaminants released in the catchment. In particular, the sediment plume represented the relevant source of DDT compounds (especially p,p′-DDE) sustaining net degassing when clean air masses from the open ocean reached the plume area. In contrast, a mass balance showed that, for HCHs, contaminated river discharge (water and sediment) plumes were capable of sustaining volatilization throughout the year. These results demonstrate the inconsistencies in the fate of HCHs and DDTs in this large estuarine system with declining primary sources.

Diffusive flux of PAHs across sediment–water and water–air interfaces at urban superfund sites

“…At the sediment-water interface, the greatest PAH release from sediment was determined at locations within the PHSS and MCBSS, with high individual PAH and location variability reflecting the heterogeneity of the sediment. Hydrophobic contaminants in the sediment have the ability to be a source to the overlying water and ultimately to the air, as shown by Larsson [32] and more recently proposed to be the case in the sediment plume of the Yangtze River in China [44]. Examination of individual PAH flux patterns in the present study showed several cases supporting the possibility of sediments being a source to the air.…”

Atmospheric deposition and air‐sea gas exchange fluxes of DDT and HCH in the Yangtze River Estuary, East China Sea

“…However, the relative roles of the two sources remain poorly understood. The most recent study indicated that the major source of HCH and DDT was volatilization from seawater and the large contaminated Yangtze River served as a continuous significant regional secondary source to the upper atmosphere [ Lin et al , ]; however, that study focused only on one season (summer) rather than all four seasons, and only the air‐sea gas exchange flux was estimated. Therefore, further studies are necessary to more fully elucidate the seasonal source‐sink relationship according to fluxes of HCH and DDT in the region for the whole year.…”

“…DDT residues in agricultural soil were transported by enhanced riverine runoff to sustain net volatilization in summer. Lin et al [] concluded that the river plume including the Yangtze and Qiantang Rivers was unable to sustain the volatilization flux of DDT over a full year based on samples that had been collected only in summer. They assumed that DDT can show a deposition trend at other times to maintain the annual balance between riverine input and volatilization.…”

“…Based on the values of the dry and wet deposition fluxes in this study, the atmospheric deposition fluxes of HCH and DDT were about 0.05 and 0.06 t/yr, respectively. The atmospheric deposition fluxes of HCH and DDT were slightly higher than the results of Lin et al [] in the coastal ECS, who estimated atmospheric deposition fluxes of 0.03 t/yr for HCH and 0.15 t/yr for DDT using a surface area of 100,000 km 2 . This suggests that atmospheric deposition acted as a relatively effective removal mechanism and deposition fluxes decreased with continental outflow.…”

“…Based on the air‐sea gas exchange fluxes of HCH and DDT during the sampling period, the net deposition of HCH into the study area was estimated to be 0.02 t/yr, and the net volatilization of DDT from the study area was estimated to be 0.31 t/yr (with an area of 20,000 km 2 ). The air‐sea gas exchange fluxes of HCH and DDT in this study were significantly lower than those of Lin et al [] in the coastal ECS. In the latter study, the air‐sea gas exchange fluxes of HCH and DDT from the seawater to the atmosphere were approximately 3.5 and 11 t/yr (with an area of 100,000 km 2 ), respectively.…”