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Sources and distribution of ammonium in the Red River delta aquifers by isotope studies
Arsenic concentrations in groundwater have been found in several aquifers in SE Asia. In recent years, arsenic concentrations high above the WHO drinking water standards have also been measured in the aquifers of the Red River delta in the Hanoi area, Vietnam. The contamination may pose a serious health threat since groundwater is the main drinking water supply for the population of the Red River delta. High and increasing ammonium levels have also been registered in the same aquifers and there is a great demand for identifying the origin of the contamination, both by arsenic and ammonium.
Possible sources of arsenic and ammonium are either of geogenic or anthropogenic origin, where the release in soluble form is governed by the redox conditions in the aquifers. The Red River delta is filled with sediments of recent to Pleistocene origin. Two main aquifers can be distinguished in and around Hanoi, partly separated by aquicludes. In the strata peat layers of up to 10 m thickness are encountered. High abstraction of groundwater (approximately 700,000 m3/day) has resulted in pronounced depression cones and the content of ammonium in the groundwater has increased. In particular, in well fields south of central Hanoi, high ammonium and arsenic concentrations have been reported.
The Swedish Geotechnical Institute (SGI) proposed a study to SAREC aimed at identifying the sources of arsenic and ammonium, and to map the spatial and temporal distribution of the contamination, mainly by groundwater sample analysis and isotope studies. The proposal was granted in November 2004 and will be ongoing during 2005 ¿ 2007. To perform some of the field campaigns and investigations, we are currently looking for Master¿s students who are willing to make their fieldwork in Hanoi, Vietnam. The study should be financed by MFS-grants, which covers all travel and living expenses to and in Hanoi.
The objective of the proposed MFS-study is to identify the source of ammonium in the groundwater in Hanoi by doing stable isotope analyses (15N, 33S) in sediments and in water. Further, starting up sampling-series for arsenic in the groundwater is requested for use in later parts of the SAREC-project.
Different isotopes of elements are fractionated by physical processes such as evaporation and discriminated by microbial enzymatic processes. The latter is applicable in the present case where the nitrogen is subject to microbial oxidation and reduction. Denitrification gives a clear isotopic signal to the residual nitrate. Thus comparing the 15N/14N ratios in the source materials, the effluents/surface near groundwater respectively the peat and in the product, the ammonium, should give a good possibility to discern the major reaction pathways. Also sulphur isotope ratios in the sulphate may be useful. Some of the sulphate reducing bacteria, i. e. Vibrio species, are also active dissimilatory nitrate reducers.
Redox potential (Eh) and pH are the most important factors controlling arsenic speciation. Redox potentials are difficult to measure as the equilibration may take time, especially in an anaerobic environment, as is the most common situation in the aquifers in the Hanoi area. Still, redox potential measurements have been done on groundwater in the arsenic ridden areas in West Bengal and were proven to be useful. The peat mineralisation and the dissimilatory nitrate reduction should be active under rather different redox conditions so that measurements should rather well give a clear indication of the prevailing process.
For the field campaign, the newly established well field Nam Du along the Red River, South of the Hanoi city will be utilised and compared with a site north of Hanoi, where conditions are more pristine. The site north of Hanoi is the field site developed for a larger research project funded by the Danish international development agency (Danida). It is expected that some exchange of materiala
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