Deposition fluxes of PCDD/Fs in a reservoir system in northern Taiwan

In this study, polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) concentrations and depositions in ambient air, water column and sediment were measured at a coupled reservoir-watershed system in northern Taiwan. The atmospheric PCDD/F concentration measured in the vicinity of the reservoir ranged from 4.9 to 39 fg I-TEQ m−3 and the Asian dust storm in February accounted for the peak value, which corresponded to a total suspended particle concentration of 128 μg m−3. The atmospheric PCDD/F deposition ranged from 1.4 to 19 pg I-TEQ m−2 d−1, with higher deposition occurring during winter and spring (long-range transport events). During summer, when atmospheric deposition is lower, consecutive tropical cyclones (typhoons) bring heavy rainfall that enhances soil erosion and creates turbidity-driven intermediate flow. This results in significantly higher PCDD/F deposition in water column of the reservoir at 70 m water depth (179 pg I-TEQ m−2 d−1) than at 20 m (21 pg I-TEQ m−2 d−1) during typhoon event. The accumulation rate of PCDD/Fs (9.1 ng I-TEQ m−2 y−1) in the reservoir sediments (depth: 0–2 cm) was consistent with PCDD/F deposition obtained from water column (6.1 and 8.3 ng I-TEQ m−2 y−1); however, it is significantly higher when compared to the atmospheric deposition (2.0 ng I-TEQ m−2 y−1). Based on the mass balance between the measurements of atmospheric deposition and sinking particles in water column, around 54–74% of PCDD/F inputs into the reservoir were contributed by the catchment erosion during normal period. However, the PCDD/F input contributed by the enhanced catchment erosion significantly increased to 90% during intensive typhoon events.

► Atmospheric settling and sedimentary deposition was conducted in the reservoir investigated. ► The higher atmospheric PCDD/F deposition was observed during the long-range transport events. ► PCDD/F accumulation rate in the sediments was significantly higher to atmospheric deposition. ► 90% PCDD/Fs input of reservoir were contributed by catchment erosion during intensive typhoon periods.