Contents and risks of potentially toxic elements in wastewater-fed food production systems in Southeast Asia

Non-treated wastewater is used for irrigation of aquatic food production systems in the peri-urbanareas of the major cities in Southeast Asia. Therefore concern is that potentially toxic elements(PTEs) may accumulate in foods produced. Hanoi and Phnom Penh, the capitals of Vietnam andCambodia, respectively are examples of cities where aquatic food production systems are fed withuntreated wastewater.In Hanoi, domestic and industrial wastewater is discharged to small natural rivers which flow todistricts south of the city where wastewater is pumped from the rivers into canals and is distributedto aquatic vegetable and fish production systems. Fish is produced in ponds whereas aquaticvegetables are produced in flooded fields. The use of wastewater for aquatic food production is lessorganized in Phnom Penh. Here, domestic and industrial wastewater is discharged to canals whichflow out of the city where it is pumped into the 3,200 ha Cheung Ek Lake located south of the city.The lake serves as site for natural treatment before the wastewater leaves the lake through a streamand ends up in Basak River. Many settlers have squatted at the banks of Cheung Ek Lake. Theymake a livelihood from producing floating aquatic vegetables and from fishing in the lake. Waterspinach (Ipomoea aquatica Forssk.) is the most important vegetable produced by weight in thewastewater exposed systems in Hanoi and Phnom Penh.One of the objectives of the study was to determine PTE concentrations in water spinach, fish, soiland sediment from wastewater-fed production systems in Hanoi and Phnom Penh. Another aim wasto assess the food safety risks of water spinach and fish with respect to PTEs. The third aim was toassess PTE retention capacity of river sediment in Hanoi and the governing retention mechanisms,that may prevent PTEs from entering the food production systems.In Hanoi, water spinach and soil samples were collected in five production systems with differentdegree of wastewater exposure; none, low and high. In Phnom Penh, water spinach and sedimentsamples were collected along transects from the wastewater inlets in Cheung Ek Lake to a distanceof 600 m. Furthermore, samples were collected at the lake outlet and at a control site not exposed towastewater. Fish samples of the species common carp (Cyprinus carpio), silver carp(Hypophthalmichthys molitrix) and tilapia (Orechromis niloticus) were collected in two wastewaterfedponds in Hanoi. In Phnom Penh, blackskin catfish (Clarias meladerma), snakehead (Channa striatus) and swamp eel (Monopterus albus) were collected in Cheung Ek Lake. Sediment samples) and swamp eel () were collected in Cheung Ek Lake. Sediment samples were further collected in the wastewater receiving rivers in Hanoi and pore water was extractedfrom the sediment under anaerobic conditions. Total concentrations of about 34 elements weredetermined in water spinach, soil, sediment and pore water. Arsenic, Cd and Pb concentrations weredetermined in liver, skin and muscle tissue of fish.The maximum observed PTE concentrations in water spinach produced in wastewater-fed systemsin Hanoi and Phnom Penh were for As: 0.19; Cd: 0.032; Cu: 2.95; Ni: 0.412; Pb: 0.206 and Zn:9.08 mg kg-1 f.w. The observed PTE water spinach concentrations in Hanoi and Phnom Penh werewithin or only slightly above the concentration range observed for water spinach grown atagricultural soil not exposed to wastewater in Malaysia. Water spinach grown at sites of high andlow wastewater exposure did not show a significantly higher accumulation of PTEs compared tosites without wastewater exposure. In Phnom Penh, water spinach from Cheung Ek Lake hadelevated Be, Cd, Fe, Ni, La, Pb, Sb and Zn concentrations near the wastewater inlets compared toconcentrations at the lake outlet and at the control site. Water spinach consumption in Hanoi andPhnom Penh resulted in an estimated intake of As, Cd, Pb and Zn amounting to less than 11% of thetolerable intake for each element. The PTE content of water spinach in Hanoi and Cheung Ek Lakein Phnom Penh constituted low food safety risks for consumers.Arsenic, Cd and Pb concentrations in muscle, skin and liver of fish grown in wastewater-fedsystems in Hanoi and Phnom were low and in most cases below the detection limit. However, meanCd and Pb concentrations in liver of tilapia were 0.366 and 0.31 mg kg-1 f.w., respectively. Arsenicconcentrations in skin of tilapia had a mean of 0.15 mg kg-1 f.w. Mean concentrations of As, Cd andPb in skin of blackskin catfish caught in Phnom Penh had averages of 17.3, 0.456 and 0.41 mg kg-1f.w., respectively. Consumption of muscle tissue from fish produced in wastewater-fed systems inHanoi and Phnom Penh resulted in an estimated As, Cd and Pb intake amounting to less than 9% ofthe tolerable intake for each element. The As, Cd and Pb concentrations in fish from Hanoi andCheung Ek Lake in Phnom Penh constituted low food safety risks for consumers.In Hanoi, soils from sites of none, low and high wastewater exposure showed no difference in PTEconcentrations. In contrast were PTE concentrations of Ba, Cd, Cu, Ni, Pb, Sb and Zn elevated atthe wastewater inlets in Cheung Ek Lake in Phnom Penh compared to concentrations at the lakeoutlet and at the control site. Especially the Ba and Zn concentrations were high with maximumconcentrations of 688 and 857 mg kg-1 d.w., respectively. Sediment in the wastewater exposedrivers of Hanoi was strongly polluted with As, Ba, Cd, Cu, Ni and Zn. The sediment concentrationsof Pb and Sb were also high. Of most concern was the observed Cd concentration in To Lich Riverwhich has a maximum of 427 mg kg-1 d.w.The Hanoi river sediment had a high retention capacity for PTEs. Logarithm to the maximumobserved distribution coefficients (log10Kd,obs) were for Cd: 5.70; Cr: 5.90; Cu: >6.60; Mo: 4.70; Pb:7.01 and Zn: 6.62. Precipitation of PTEs as sulphides was likely to be the main retentionmechanism. The high retention capacity of the river sediment prevents major PTE transport to thewastewater-fed aquatic food production systems in Hanoi.