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Harms-Ringdahl, Peter (2007) Identifying possible sources of ammonium ions and arsenic in groundwater in the Nam Du area, Vietnam. Other thesis, SLU.

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Abstract

Concentrations of arsenic and ammonium ions well above drinking water standards have been detected in aquifers which supply Hanoi, the capital of Vietnam, with fresh water. Chemical analyses showed arsenic and ammonium ion concentrations varying between 27 – 264 μg/l in freshwater and 1.8 – 34.7 mg/l in groundwater. The current Vietnamese drinking water guidelines for arsenic and ammonium are set at 10 μg As/l and 1.5 mg NH4/l. The aim of this thesis is to investigate the fate of arsenic and nitrogen and the processes behind the mobilization of these elements in the Nam Du area, situated 10 km southeast of Hanoi. Two possible processes affecting the As concentration are investigated: (1) oxidation of arsenopyrite or (2) desorption of arsenite/arsenate bound to iron hydroxides due to iron reduction. Regarding ammonium ions, previous projects have identified two possible sources which were investigated further in this study: (a) mineralisation of organic matter in sedimentary peat layers and (b) ammonium ions derived from anthropogenic sources, such as fertilisers or leakage of waste water. The methods used in this study were: analyses of N-, O- and H-isotopes, the ionic composition and redox potential in the groundwater, combined with a conceptual model of the groundwater flow in the area. There were large spatial variations in ammonium and arsenic levels within the Nam Du area. According to tritium (3H) analyses the water was quite young, indicating recent mobilisation processes. The redox potential, pH and sulphate concentration in the water indicate that arsenic is mobilised by reduction of iron hydroxides rather than oxidation of arsenopyrite. Furthermore, the isotopic composition of N combined with increasing levels of ammonium ions over time, the C/N ratios and the age of the water, suggest that mineralisation of peat is not occurring to such an extent that this process could explain the currently high ammonium levels. The isotopic composition of the ammonium ions and the rising concentrations in the aquifers rather imply that the ammonium ions derive from one or several sources with a high 15N/14N ratio, such as waste water, organic fertilisers or urea.

Item Type: Thesis (Other)
Keywords: Groundwater, contamination sources, ammonium ions, arsenic species, redox potential, stable isotopes, Hanoi, Vietnam
Subject (faculty): Faculty of Veterinary Medicine and Animal Science > Dept. of Soil Sciences
Divisions: SLU > Faculty of Natural Resources and Agricultural Sciences
Depositing User: Anne Olsson
Date Deposited: 11 Jul 2007
Last Modified: 18 Aug 2015 09:52
URI: http://ex-epsilon.slu.se/id/eprint/1791

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