Cases and solutions
1163 Environmental Geology 39 10) September ´ Springer-Verlag
Dissolved fluoride in the Lower
Ganges-Brahmaputra-Meghna River
system in the Bengal Basin,
Bangladesh
D.K. Datta ´ L.P. Gupta ´ V. Subramanian
Abstract The dissolved fluoride F
±
) in the Lower
Ganges-Brahmaputra-Meghna GBM) river system,
Bengal basin, Bangladesh, was studied during
1991±1993 to determine its distribution and source
in the basin, and its annual flux to the Bay of Ben-
gal. The concentration of dissolved F
±
varied
between 2 and 11 mmol l
±1
with statistically signifi-
cant variations both spatially and temporally in the
basin. Such variations are attributable to the geol-
ogy of the individual subbasins Ganges, Brahma-
putra and Meghna), dilution by rainwater during
monsoon and groundwater contribution to the
river systems during dry season. Correlation coeffi-
cients among F
±
and major cations and anions
suggest diverse inorganic processes responsible for
regulating the concentration of F
±
in these river
systems. However, fluorite seems to be one of the
major sources of dissolved F
±
. The concentration
of F
±
in the Lower GBM river system is low com-
pared to the rivers draining Deccan Plateau and
arid regions of the subcontinent, for example,
Yamuna and its tributaries. However, it is within
the range of most of the other Peninsular and
Himalayan rivers. The GBM system contributes
about 115 10
3
tonnes year
±1
of dissolved F
±
into
the Bay of Bengal, and thus accounts for about 3%
of the global F
±
flux to the oceans annually.
Key words Dissolved fluoride ´ GBM river system ´
Bengal basin ´ Bangladesh
Introduction
Fluorine is physiologically important, and it is the most
reactive element in nature. It is relatively abundant in the
earth©s crust relative abundance 13). The dominant
source of fluoride in the aquatic system is mineral flu-
orite CaF
2
). Other minerals such as topaz, fluorapatite,
micas, cryolite, etc., having fluorine as an essential com-
ponent, are accessory minerals, and are sparingly soluble
in water; therefore fluoride contribution from them is
minimal. Fluoride ions F
±
) in drinking water about
1 ppm or 52.6 mmol l
±1
) greatly reduce dental caries. It
makes tooth enamel much harder by converting hydrox-
yapatite [3Ca
3
PO
4
)
2
Ca OH)
2
] into fluorapatite on the
tooth surface. However, F
±
concentration above 2 ppm
causes discoloration of teeth and even higher concentra-
tions are harmful and cause fluorosis ± making bones
fragile Lee 1991).
The Bengal basin, Bangladesh is situated at the con-
fluence of the Ganges-Brahmaputra-Meghna GBM) river
system ± the largest sediment-dispersal system in the
world Kuehl and others 1989), and offers the passage of
an estimated amount of 1330 km
3
of water to the Bay of
Bengal annually Milliman and others 1995). More than
86% of the basin is composed of recent floodplain depos-
its of medium to fine sand, silt and clay composed domi-
nantly of quartz, feldspars, illite and kaolinite Datta and
Subramanian 1997a). The population density of the basin
is one of the highest in the world on average 755 people
per km
2
in 1991; Anonymous 1998), and ranges from 400
to 1200 people per km
2
Milliman and others 1989).
Industrialization is very low, but extensive agricultural
activities are peculiar to this region.
The GBM system constitutes one of the major pathways
transferring dissolved solute to the world oceans
152 10
6
tonnes year
±1
; representing about 5% of the
total solute flux from land to oceans globally; Datta and
Subramanian 1997b). The major ion chemistry of the
GBM river system has been updated from time to time
Received: 19 May 1999 ´ Accepted: 11 October 1999
D.K. Datta
Environmental Science Discipline, Khulna University,
Khulna 9208, Bangladesh
L.P. Gupta
)
)
Marine Geology Department, Geological Survey of Japan,
1-1-3 Higashi, Tsukuba, 305±8567 Japan
e-mail: gupta@gsj.go.jp
Tel.: +81-298-613765
Fax: +81-298-613765
V. Subramanian
School of Environmental Science, Jawaharlal Nehru University,
New Delhi 110 067, India