Epsilon Archive for Student Projects

Abstract

The rapid urbanization and industrialisation in many South-East Asian countries has resulted in increasing amounts of untreated industrial and domestic waste water, which is discharged into rivers and lakes. The waste water is a highly complex and nutrient rich water solution used by the farmers for irrigation of their fields. It contains various potentially toxic elements including several trace metals. A better understanding of the irrigation systems used, and the distribution of the metals across arable fields is essential in order to reduce the environmental and health risks associated with the use of waste water.

The main objective of this thesis was to compare the concentrations and distributions of metals within waste water irrigated systems in Bang B village, Thanh Tri district, a peri-urban area downstream Hanoi City, Vietnam. The reverse aqua regia extractable (AqReg) and
exchangeable (Exch) concentrations of Cu, Cd, Zn and Pb in soil and surface sediment were
quantified within two irrigation systems along preselected distance gradients from a concrete irrigation canal. In a "from earth-canal to field" irrigation system, the pumped waste water flowed from an earth canal to the connected fields through openings along the side of the canal. In a "from field to field" irrigation system, the pumped water flowed from one field to another through openings in the earth walls surrounding each field.

Within water flow gradients there were no statistically significant differences in soil metal concentrations, which were associated with the distance from the concrete irrigation canal. However, there were significant differences within the sampled soil profiles, the values being higher in surface layers than in deeper layers. These results indicate an accumulation of trace metals in the soil due to anthropogenic sources, such as the waste water used for irrigation. In all soil and sediment samples the AqReg metal concentrations were below the international maximum permissible levels. AqReg metal concentrations (per kg dry soil) ranged from 15 to 27 mg Cu kg-1, 55 to 93 mg Zn kg-1, 0.1 to 0.4 mg Cd kg-1 and 13 to 32 mg Pb kg-1. The Exch metal concentrations in soil and sediment samples were generally low, which meant a similarly low Exch metal/AqReg metal ratio. Exchangeable metal concentrations (per kg dry soil) ranged from below the detection limit to 0.2 mg Cu kg-1, 0.008 to 0.9 mg Zn kg-1, 0.0003 to 0.03 mg Cd kg-1 and from below the detection limit to 0.02 mg Pb kg-1. Other studies of waste water irrigation in Vietnam and in other similar countries, reported higher concentrations of
metals accumulated in soil. The low concentrations found in Hanoi could probably be
explained by the early stage of industrial development in this city.