Epsilon Open Archive

Abstract

This study investigates the impacts of wastewater and biosolids application on cultivated Vietnamese Fluvisols and Acrisols. Chemical impacts, i.e. enrichment of macronutrients (N, P, K) and trace metals (mainly Cu, Zn, Cd, Pb), and solubility and mobility of trace metals were examined. Experiments were carried out in peri-urban vegetable and/or rice-dominated farming systems of Hanoi, Ha Tay, Vinh Phuc and Nam Dinh cities. Batch studies were performed in order to measure metal solubility in soil solution, and multi-surface models including the soil sorption surfaces, i.e. iron hydroxides (DLM), organic matter (SHM and NICA-Donnan model), and phyllosilicate clay (Gaines-Thomas) were tested for describing metal binding. A multi-surface model was used to run various scenarios including (i) changing metal loading (Cd, Cu, Zn) up to the present Vietnamese maximum acceptable soil concentration (MAC), and (ii) varying soil organic matter (SOM) content and (iii) soil pH within the ranges found in the national soil inventory of Acrisols and Fluvisols, respectively. These soil types represent 73% of agricultural soils in Vietnam. Field balance calculations revealed macronutrient and trace metal (Cu, Zn) accumulation in intensive peri-urban vegetable systems in Hanoi City. The N surplus ranged from 85-882 kg ha-1 yr-1, while P and K surpluses were 109-196 and 20-306 kg ha-1 yr-1, respectively. For Cu and Zn surpluses varied from 0.2-2.7 and 0.6-7.7 kg ha-1 yr-1, respectively, wastewater and biosolids being the major sources accounting for 75-98 and 62-94% of the total Cu and Zn input. Application of biosolids significantly increased the ‘total’ (reverse aqua regia-extractable) soil Zn concentration, whereas concentrations of Cd and Pb were not significantly different from the control. However, there were positive relationships between EDTA-extractable Cd, Cu and Zn and biosolids application rates. For total dissolved Cd, Cu and Zn the multi-surface models provided very good model fits for all soils and thus these can be used to describe their binding and solubility in tropical Fluvisols and Acrisols. Scenario simulations identified the potential risk of metal losses to the environment (i.e. Cd and Zn), as influenced by loading up to MAC, soil acidification and decreasing soil organic matter content, especially for Acrisols. Assuming the same metal input rate as at present, it was roughly estimated to take 20-325 and 40-450 years to reach the MAC for Cu and Zn, respectively. It was concluded that wastewater and biosolids can be major nutrient sources in peri-urban agricultural production systems. However, they can also contribute to substantial trace metal inputs and represent a potential threat to soil quality, water environment and human health. Soil organic matter is a major sorbent for Cd, Cu, and Zn in tropical Acrisols and Fluvisols, and pH is the main factor controlling the solubility of metals in these soils. Regarding Pb, modelling was not successful and further refinements of the models will be required.