Per- and polyfluoroalkyl substances (PFASs) in water, soil and plants in wetlands and agricultural areas in Kampala, Uganda

Abstract

Occurrence and concentrations of 26 per- and polyfluoroalkyl substances (PFASs) were evaluated in wastewater, surface water, soil and crop plants (yam (Dioscorea spp.), maize (Zea mays) and sugarcane (Saccharum officinarum)) in Nakivubo wetland and Lake Victoria at Kampala, Uganda. ∑PFAS concentrations in effluent from Bugolobi wastewater treatment plant (WWTP) were higher (5.6-9.1ngL^-1) than in the corresponding influent (3.4-5.1ngL^-1), indicating poor removal of PFASs within the WWTP. ∑PFAS concentrations decreased by a factor of approximately five between Nakivubo channel (8.5-12ngL^-1) and Lake Victoria (1.0-2.5ngL^-1), due to dilution, sorption to sediment and uptake by plants in the wetland. ∑PFAS concentrations were within the range 1700-7900pgg^-1 dry weight (dw) in soil and 160pgg^-1 dw (maize cobs) to 380pgg^-1 dw (sugarcane stems) in plants. The dominant PFASs were perfluorohexanesulfonate (PFHxS) in wastewater, perfluorooctanoate (PFOA) in surface water, perfluorooctanesulfonate (PFOS) in soil and perfluoroheptanoate (PFHpA) and PFOA in different plant tissues, reflecting PFAS-specific partitioning behaviour in different matrices. Soil-water partitioning coefficient (log K_d) in wetland soil under yam was lowest for short-chain PFHxA (1.9-2.3Lkg^-1) and increased with increasing chain length to 2.8-3.1Lkg^-1 for perfluoroundecanoate (PFUnDA) and 2.8-3.1Lkg^-1 for perfluoroctanesulfonate (PFOS). The log K_oc values ranged between 2.2 and 3.6Lkg^-1, with the highest log K_oc estimated for long-chain perfluorocarbon PFASs (i.e. PFUnDA 3.2-3.5Lkg^-1 and PFOS 3.2-3.6Lkg^-1). The concentration ratio (CR) between plants and soil was <1 for all PFASs and plant species, with the highest CR estimated for PFHpA (0.65-0.67) in sugarcane stem and PFBS (0.53-0.59) in yam root. Overall, this investigation demonstrated PFASs entry into the terrestrial food chain and drinking water resources in Kampala, Uganda. Source identification, assessment of impacts on human health and the environment, and better wastewater treatment technologies are needed.

Keywords: Maize; PFAS; PFOS, PFOA; Plant uptake; Soil; Sugarcane; Wastewater; Yam.