Khai et al. (2007) presented a Cu and Zn balance for a wastewater-fed peri-urban
farming system in the Hanoi area. The flows of Cu and Zn were quantified over a 1-year
period for intensive small-scale aquatic and terrestrial vegetable systems situated in two
peri-urban areas of Hanoi. The surpluses of Cu and Zn in the system were up to 2.7 and
7.7 kg ha−1 year−1, respectively. This indicated a high risk for contamination with toxic
elements and associated transfers through the food chain (Khai et al. 2007).