Summary of PhD Thesis:
- Incidences of fluorosis, caused by consumption of groundwater with elevated fluoride (F–) content, have arisen amongst the population of Khanh Hoa province in central Vietnam over recent decades, after people turned from rivers to domestic wells and boreholes for water supply. Studies in the 1990s that revealed high F– in groundwater of some parts of the province, focused on the health issues. This study represents the first systematic, detailed investigation of the F– content and distribution in groundwater and other water supply within the province. It is also the first investigation of the possible source, or sources, of the fluoride.
- A total of 1250 water samples were collected from Ninh Hoa town and 27 communes in Ninh Hoa district, and adjacent Khanh Vinh district. Samples included groundwater, surface water and hot springs, as well as rainwater and rice paddy. All samples were analysed for F– in the field by the fluoride ion-selective electrode (FISE) method, while 834 samples were transported to the EIGG laboratory at Curtin University for F– analysis by high performance ion chromatography (HPIC) and major ions and trace metal analysis. The mean difference in F– concentration measured by FISE and HPIC was ~2.4% (r = 0.992), indicating that FISE provides a reliable, as well as cost effective, means of F– determination for the study of the fluorosis problem in Khanh Hoa.
- The results indicated that the average F– concentration in groundwater in Ninh Hoa district (n = 1178) is 2.13 mg/l with the highest value of 28.1 mg/l found in Ninh Sim commune. 42.5% of the samples exceeded the Vietnam drinking water standard for F– of 1 mg/l; whereas more than 34% of the samples had higher than the WHO guideline value of 1.5 mg/l. The mapped spatial distribution for F– revealed that 12 of 27 communes in Ninh Hoa and adjacent Khanh Vinh district have wells with F– content presenting a risk of fluorosis. F– concentration in surface waters (n = 43) was usually <1 mg/l, although some streams contained up to 7.84 mg/l as a result of the influx of hot spring water. Hot springs (n = 25) reported consistently elevated contents of F–, with an average of 7.65 mg/l.
- Total flouride in 30 igneous rocks, 20 sedimentary rocks and 13 soil samples was determined by the FISE method after fusion and acid dissolution. Average F– concentration was 489 ppm in granites, 630 ppm in granodiorites, 548 ppm in shales; 449 ppm in siltstones; 565 ppm in sandstones and 289 ppm in soils. These values are usually below those normally found in such rocks. Leaching experiments with distilled water (5:1 water:solid ratio) yielded an average of 1.79 mg/l F– in solution from igneous rocks, 5.04 mg/l F– from sedimentary rocks and only 1.05 mg/l from soils. However, an extremely high value (40.1 mg/l F– was obtained in the case of leachate from soil collected in the vicinity of a small brick-making factory. Electron probe microanalysis revealed biotites and amphiboles to be major F– bearing minerals in the igneous rocks with average F– concentration of 0.39 and 0.35 wt %, respectively.
- PHREEQC modelling predicted flouride in the water supply of Ninh Hoa to be overwhelmingly in the form of the free F– ion. The modelling revealed significant correlation between the Saturation Index (SI) of fluorite and the activity of F– in groundwater and surface water, indicating that dissolved flouride is controlled mainly by fluorite solubility. Modelling of fluoride geochemistry of the Truong Xuan hot spring shows it to be of medium enthalpy with maximum reservoir temperature <150°C. It is considered that granite-water interaction at depth releases granite components into the thermal waters while initial minerals are transformed into new minerals more stable at the temperature of interaction. Water-rock interaction continues during heating and cooling of the waters, with fluoride concentration constrained by calcite solubility. The input of such thermal water is considered to be the predominant source of the elevated F– into the shallow groundwater of Ninh Hoa. Other possible factors contributing to F– in solution in groundwater include exchange of F– by OH– on clay surfaces prevailing high pH, precipitation of CaCO3, and cation exchange on clays (replacing of Ca2+ with Na+). Other minor factors include temperature, depth profile, and input of rainwater containing anthropologic flouride from activities such as coal burning and brick-making.
- Analytical results of this study have shown that more than one third of the wells had higher than 1.5 mg/l and present a medium to high fluorosis risk. If we presume that residents of Ninh Hoa town use treated tap-water, while all other communes derive water supply from either wells or surface water (including hot springs), then approximately 60,000 people within the study area are consuming water which may result in dental or skeletal fluorosis.
- The fluorosis risk map of Ninh Hoa shows that most parts of Buon M Dung and Nui Deo areas where the host rocks are granites of Deo Ca and Dinh Quan complexes are of medium to high fluorosis risk. In terms of administrative areas, most parts of Ninh Sim, Ninh Tay, Ninh Thuong and Ninh Xuan communes were found to present a risk of fluorosis.
- The fluorosis problem does not exist only in Ninh Hoa district but may extend to a broader scale. Other districts in Khanh Hoa and Phu Yen province (immediately north of Khanh Hoa) were reported with high F– values. Hence, a further study needs to be carried out to determine the possibility of a large scale of high F– distribution in Phu Yen and Khanh Hoa as well as coastal provinces in the Central part of Vietnam.
* Original online at http://trove.nla.gov.au/work/38317381?selectedversion=NBD46220165
- Tu, Vu Le.
- Curtin University of Technology. Division of Science and Engineering. Dept. of Applied Geology.
- 1 v. (various pagings) : col. ill., col. maps ; 31 cm.
- Dissertations, Academic — Doctorate — Western Australia.
- At head of title: Division of Science and Engineering, Department of Applied Geology.
- Thesis (Ph.D.)–Curtin University of Technology.
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