Groundwater quality data taken in August and December 2000 revealed that the groundwaters in Hanoi are contaminated by arsenic (up to 110 μg/l), iron (up to 32 mg Fe/l), organic matter (DOC up to 12 mg/l), and ammonia (up to 29 mg-N/l). Data on trace metals, as well as conventional water quality parameters such as pH, ORP, major cations and anions, were used to analyse groundwater flow and the mechanisms of groundwater contamination using multivariate data analysis methods. The results of Cluster Analysis (CA) demonstrated that the groundwater flow and characteristics were separated into three different zones, namely the Riverain zone, the Northwest zone, and the South zone where the highest levels of arsenic, iron, organic matter and ammonia were detected. Principal Component Analysis (PCA) showed strong correlation among two groups of trace metals: the first group includes Cd, Co, Mg, Mn, and Sr and these metals are considered to be leached out from rock minerals under moderately oxidizing conditions. The second group contains As, B, Ba, Cr, Cs, FeII, Mo, P, Pb, Rb, Ti, DOC and bicarbonate, and these are negatively correlated with SO42- and ORP, which indicates that these elements are eluted from organic matter and/or iron oxyhydroxide under anaerobic conditions. Further analyses of data obtained in the contaminated groundwaters by PCA and correlation between As and DOC suggest that there may be two possible mechanisms of arsenic release: reduction of iron oxyhydroxide and anaerobic degradation of organic matter in the peat layers.

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