Groundwater
Arsenic in Bangladesh
POPULATION
DENSITY AND SURFACE
WATER
RESOURCES IN BANGLADESH
- Bangladesh has a large
population (year
2000 estimate = 128 million) living very close to sea level, mostly on
the flood plain of the Brahmaputra and Ganges Rivers. The population
density, relative to the total area of the country, is about 900
people per km2, almost triple that of India and seven times
that of China (Fig 1). (See also
the world population
map developed by CIESIN.)
- In the same units, the
mean
for the
global continental area is about 44 and for the USA about 30. Two
states
in the USA similar in total land area to Bangladesh are Wisconsin and
New
York, which have population densities of 30 and 150, respectively. The
population density of Egypt expressed in these units is only about 60
per
km2, due to the huge area of desert away from the Nile. Thus
population data based on the total area of a country can sometimes be
quite
misleading.
- The population
of Bangladesh (Fig
6)
increased from about 77 million in 1975 to about 120 million in 1995,
representing
an annual increase of about 2.3%. The population is currently
predominant
rural (about 80%), with urban areas growing much more rapidly than the
population of the country as a whole.
- Using total area of
croplands
(cultivated
land plus land in "permanent" crops such as orchards), the
ratio of population to area of cropland in Bangladesh is about
1300
per km2, similar to that for China, and more than double
that
of India (550 per km2) (Fig
2). Egypt has about 2400 people per km2 of total
cropland,
almost double that for Bangladesh. Comparable ratios of population to
total
cropland area for the USA and the global land area are 150 and 400,
respectively.
Clearly the amount of land in Bangladesh available for producing food
crops
is quite limited, with respect to the large resident population.
- Precipitation
in Bangladesh is dominated by the South Asian monsoon pattern of heavy
rain during the months of June through September (Fig
3),with very little rain during the winter. The monthly pattern of
river discharge is similar, with highest flows and most frequent
flooding
during July through October.
- If total potentially
available renewable
water resources are considered, however, Bangladesh appears to
have
very ample supplies (2400 km3/yr), similar to that estimated
for the USA (Fig 4).
Current
withdrawals of freshwater in Bangladesh (23 km3/yr) are only
about 1% of the estimates of renewable water resources. In contrast,
Eqypt
and Israel currently withdraw fresh water at rates very similar to the
estimated total renewable supplies. In the case of Egypt, the renewable
supply value quoted in The World's Water (Gleick) is based on the
"natural"
annual discharge of the Nile, assuming no diversions in upstream
countries
such as Sudan or Ethiopia, and no losses by evaporation in reservoir
storage
(Lake Nasser). This value clearly is considerably higher than the
amounts
actually available for irrigation and other uses in Egypt. Thus there
are
assumptions and other approximations used to derive water resource
estimates
that can sometimes be quite misleading. It is important to understand
any
conventions used to derive such estimates.
- Another example of such
issues
is the
very high percentage of "safe" water
reported
for Bangladesh (97%). This value is based on the population deriving
water
from either treated surface waters or groundwaters thought to be free
of
human pathogens. Comparable estimates for other developing countries,
such
as Pakistan, Egypt and Nigeria, are much lower, suggesting that
pathogen
contamination from drinking water in Bangladesh should now be much less
of a problem than a few decades earlier. Since the early 1970s, there
has
been an enormous effort in Bangladesh by UNICEF and other international
donor organizations to install tube wells throughout much of
Bangladesh,
to reduce exposure of the population to infectious disease transmission
via contaminanted surface waters. There are now about 10 million
shallow
wells, accessed by hand pumps, located in many regions of the country.
In some areas, which have saline water in the upper layers of
groundwater,
there are deeper wells that required much more extensive drilling
operations
to install.
- Per
capita
withdrawals
of freshwater for domestic demands
are very low in Bangladesh (6 m3/person/yr), reflecting hand
pumped supplies for most of the rural population (Fig
5). Domestic water withdrawals in the USA average about 250
m3/person/yr, reflecting extensive use for washing of
clothes,
dishes, showers, flushing of toilets plus watering of lawns and other
vegetation.
Irrigation withdrawals in Bangladesh (200 m3/person/yr) are
appreciably lower than in Egypt (900 m3/person/yr),
reflecting
reliance on rain as the primary source of water for most crops in
Bangladesh.
However, use of groundwater for irrigation has expanded rapidly in
Bangladesh
over the past two decades.
- The most unique aspect of
the
natural
geography of Bangladesh is that it lies on the flood plain of two of
the largest
discharge rivers in the world, the Brahmaputra and Ganges, each
of which is comparable to the Mississippi River in terms of annual
water
discharge. The combined annual discharge of the Ganges, Brahmaputra and
Meghna Rivers is comparable to that of the Zaire, the second largest
river
in the world, and suspended particle discharge is the highest in the
world
(Fig 7). The Brahmaputra and
Ganges each carry suspended particle loads in excess of 500 million
tons
per year, more than twice the current mean annual particle discharge
from
the Mississippi (200 million tons per year). This enormous quantity of
suspended sediments in deposited in the BG delta both above and below
sea
level and helps maintain soil fertility and maintain the level of the
soil
surface in opposition to the continued sinking of the delta due to the
great weight of sediments accumulating there.
- The drainage basin of the
Brahmaputra
(population=110 million) includes an appreciable area in China, Bhutan
and India, as well as Bangladesh, with a high fraction of the landscape
intensively modified for food cultivation (Fig
8, Fig 37). The headwaters
region includes large areas of high mountain plateau in the Himalaya.
- Most of the drainage basin
of
the Ganges
(population=450 million) lies in India, but also includes extensive
high
mountain plateau in Nepal (Fig
9). Except for Nepal, nearly all of the drainage basin of the
Ganges
has been extensively modified for food production. More than 70% of the
entire basin area is crop land, probably the highest for any major
river
in the world. The portion of crop land in the Mississippi basin is
about
35%. Total crop land fraction in the Yangtze basin is about 56%.
- Total dissolved solids (TDS)
in
the
Brahmaputra and Ganges Rivers (Fig
10) are about 100 ppm and 200 ppm, respectively, compared to about
270 ppm in the Mississippi. Thus both of these rivers entering
Bangladesh
have dissolved major element compositions quite similar to other major
rivers, and have calcium and bicarbonate as their highest abundance
ions.
From dissolved ion data in the major rivers reaching Bangladesh, there
would be no reason to expect any particular problems with groundwater
chemical
compositions.
HEALTH AND ECONOMIC INDICATORS IN
BANGLADESH.
- Countries
with
relatively high economic resources per capita tend to have much lower
rates
of water-borne and other infectious diseases. Many of the deaths of
young
children in developing countries are the result of lack of access to
adequate
quantities of clean water. This general pattern is evident in a scatter
plot of child mortality (deaths per 1000 children < 5 years of age)
vs GNP per capita (Fig 11). In
1995,
annual deaths per 1000 of children < 5 years of age in Bangladesh
was
about 115, comparable to India, and about an order of magnitude greater
than in the USA (10). China had appreciably lower childhood mortality
(47),
comparable to Egypt (51) in 1995. From the general trend of the scatter
plot, much of the gains in improved health for children appear to be
feasible
at relatively modest increases in GNP per capita.
- Countries with populations
>
20 million
in 1995 were then grouped in rank order of increasing GNP per capita (Fig
12), with 4 countries discussed having 42% of world population
(China,
India, Pakistan, Bangladesh).
- Mortality
data
for these 45 largest population countries illustrate very clearly the
much
lower rates of young child deaths in countries with higher GNP per
capita
(Fig 13). Bangladesh, India
and
Pakistan had comparable young child mortality rates (110 to 140 per
1000/yr),
while China had considerably lower young child death rates (about 50).
- Tabulations of access to
"Safe"
drinking
water were greater than 70% for all four of these countries, with
Bangladesh
being the highest (97%) of the ground of countries with relatively low
GNP per capita (Fig 14),
reflecting
the very high fraction of that population with access to groundwater
through
shallow tube wells installed since the early 1970s. Note that this
indicator
may be fairly difficulty to relate in a simple way to something as
complex
as young child mortality rates.
- Mortality
rates
for children under five years of age remain quite high (about 100 per
1000
live births), but have been reduced by more than a factor of two since
the 1970s (Fig 15). For
comparison,
most developed countries have a child mortality rate (under five years
of age) of less than 10 per 1000 live births. The primary reasons for
the
substantial improvement of child mortality risks over the past three
decades
in Bangladesh are not well defined, but probably include at least the
following:
introduction of shallow groundwater supplies, oral rehydration therapy
for rapid treatment of diarrheal diseases, and expanded child
innoculations.
The relative importance of shifting of water supplies from surface
waters
contaminated with microbial diseases to groundwaters is currently a
contentious
issue in Bangladesh.
DISSOLVED ARSENIC [As] IN
GROUNDWATERS
OF BANGLADESH.
- One of the great tragedies
of
the latter
half of this century is that the well-intentioned efforts of the
international
community and the Bangladesh government over a number of decades to
improve
health conditions through installation of shallow tube wells to access
groundwater has now resulted in a massive episode of poisoning through
dissolved arsenic [As] in drinking water (Fig
39). From evidence available up to now, the source of the dissolved
arsenic appears to be mostly natural, resulting in
mobilization
of As from mineral phases in the sediments of the Ganges-Brahmaputra
delta.
Similar environmental conditions exist in the state of West Bengal of
India
and As poisoning is now widespread in that area as well (Fig
42).
- International standards on
maximum permissible
levels (MPL) of dissolved As are currently under extensive review.
Standards
between various countries now range over a factor of seven (Fig
16), and the proposed new standard for the USA would be a factor of
five to ten lower than that currently in force. Thus there is
substantial
scrutiny as to the level of dissolved arsenic in drinking water which
does
not represent significant risk to human populations.
- The World Health
Organization
(WHO)
maximum level for As has been 10 parts per billion (ppb) since 1993,
while
the USEPA and Bangladesh MCL is 50 ppb, a standard that has been in
place
in the USA for more than half a century (Fig
17). Large number of shallow tube wells in Bangladesh have As
concentrations
that are in the range of 2 to 20 times the current Bangladesh and EPA
standard,
clearly far greater than what has been judged to be safe for continuous
human consumption.
- Over the next few years,
it
appears
quite likely that the "approved" level for drinking water [As] in the
USA
will be decreased substantially, probably converging towards the WHO
standard
or below, based on the level of 10 ppb recently proposed by EPA. Such a
change would only make the groundwater As situation in Bangladesh of
even
greater concern.
- Arsenic occupies a position
in
the periodic
table immediately below phosphorus, and has a valence state of +5 in
the
presence of dissolved oxygen concentrations typical of most surface
waters,
as does phosphorus. Arsenic is extremely toxic, while phosphorus is one
of the essential elements required by a large range of molecules in
living
systems. In acute doses, arsenic is lethal on the time scale of a few
hours.
Chronic low doses cause a range of serious health problems, including
skin
lesions, fatal skin cancer, gangrene, and a range of fatal organ
cancers
including those initiated in liver, kidney and lungs (Fig
23). Signs of As related disease were detected in the Bengal basin
only in the the mid 1980s (Fig 24).
- Data (through Jan 1999) for
dissolved
As in 30,000 wells from many areas of Bangladesh indicate that the
problem
is widespread, but not uniform in geographical distribution. A band of
several hundred kilometers width near the middle of the country (Fig
19) has more than 20% of the wells with As levels greater than 50
ppb.
Note that the field kits used to obtain this data do not have enough
sensitivity
to detect dissolved As below about 150 ppb. Thus it is likely that the
extent of the problem of dissolved As is appreciably greater than
indicated
from this distribution of monitoring data.
- Monitoring data for
dissolved As
using
other analytical methods, compiled by the British Geological Survey,
also
support the observation that elevated As is widespread in Bangladesh,
and
that deeper wells (> 200 meters below the surface) tend to have
considerably
lower As concentrations (Fig 38). The
degree of local variation in dissolved As is extreme, even on a very
local
scale (Fig 45). Thus two
wells within 10 meters of each other can have very different As
concentrations,
for reasons that are really not understood at the present time.
Monitoring
data from a group of scientists in India, working with Bangladesh
colleagues,
provided some of the earliest data linking As in drinking water to a
huge
increase in a number of health problems caused by As poisoning. These
data
also indicate that the problem is probably most severe in a band
running
east-west across the middle of the country, and also show a large range
of As concentrations, ranging from < 10 ppb to greater than 700 ppb
in a number of districts.
- The detailed causes of
elevated
As in
Bangladesh and West Bengal groundwaters remain obscure. The most
plausible
explanation currently under consideration is that arsenic from the
sediments
has become mobilized by strongly reducing conditions of the
groundwaters,
which are largely anoxic (ie no dissolved oxygen). Such conditions
would
lead oxidized iron [Fe(III)], which is very insoluble, to be reduced to
a much more soluble form of iron [Fe(II)]. Arsenic in a higher
oxidation
state [As(V)] has a strong tendency to be sorbed onto solid iron
mineral
phases. Once iron mineral phases go into solution in reducing
groundwaters,
As would also tend to go into solution. Thus the iron phases serve to
control
the amount of As in solution. The detailed processes involved in
Bangladesh
groundwaters are clearly more complicated than the simple outline
provided
here, but they do appear to involve primarily "natural" processes, and
are not the result of human pollution. Arsenic can be present in a
number
of chemical forms in natural waters, with considerable variation in
species
type and oxidation state depending upon the pH and Eh (oxidation
potential)
conditions in the water. The lower oxidation state As (III) is
generally
more soluble than higher oxidation state As(V), but people drinking As
in any inorganic chemical form are exposed to significant health risk
when
the amounts of soluble As appreciably exceed 10 ppb. The details of the
processes causing mobilization of As are not critical to define here,
but
are mentioned to provide some indication that the factors controlling
water
quality can sometimes be quite complicated and poorly understood (Fig
43).
- The land surface of
Bangladesh
&
adjacent state of India (West Bengal) is composed of alluvium deposited
over the last several million years. A major fraction has accumulated
since
the end of the last glacial period as sea level rose from about 130
meters
below current sea level. Note there are some areas of Bangladesh where
the surface sediments were deposited at least several hundred thousand
years ago. These deposits do not appear to have elevated arsenic
concentrations
in groundwaters. The original source of As was probably from pyrite
minerals
derived from the uplands in the Himalaya mountains, but major changes
in
the chemical environment during erosion, deposition and burial have
considerably
altered the mobility of arsenic as a function of geological environment.
- The alluvial deposits in
Bangladesh
were derived from three major river systems (Ganges, Brahmaputra and
Meghna
Rivers), all three of which appear have delivered sediments which can
readily
lead to high dissolved arsenic in groundwaters. The pattern of
accumulation
of sediments in the Bengal Basin has varied greatly over the past
20,000
years, as the channels of the major rivers have changed during that
period.
In general, very deep groundwaters tend to have relatively low arsenic
concentrations, but the most common depths of the shallow wells have a
very large range of concentrations.
The Columbia University's Arsenic
program
(An overview)
- There are a number of areas,
including
the following countries, which have experienced high dissolved As in
drinking
water which has impacted the health of human populations: Taiwan, West
Bengal, Bangladesh, Mongolia, Argentina, Chile and Mexico. Within one
region
of Bangladesh, there is now a multi-year research program
involving
a number of researchers at Columbia University and other institutions,
focussed on documenting the effects of arsenic poisoning on the
population
of that region, as well as the geology and hydrology factors which tend
to lead to high dissolved As and could potentially permit reduction in
human exposure (Fig 20). The
project
is located in an area about 20 km east of the capital city of
Dhaka
in Arahaizar Thana (Fig 26). This
effort involves faculty and staff from the School of Public Health,
Lamont-Doherty
Earth Observatory, Engineering School, School of International and
Public
Affairs, plus other units of the university (Fig
21) and partners in Bangaldesh (Fig
22).
- The following pictures
provide
some
visualization of the study area and the construction of the wells (Fig
27, Fig 28). Villages are
typically
built on slightly elevated artificial "islands" that protect
residents
from the big floods in the summer. Wells are being installed by using a
wet drilling technique (Fig 29).
Almost
all existing wells (~5000) in the area of investigation were sampled in
2001/02 (Fig 30, Fig
36). The village wells in this area, as in much of the country are
pumped by hand. Concentrations of As are quite
variable
as a function of location, which makes it especially difficult to
predict
the likely level of contamination prior to drilling a new well. As an
example
for the horizontal and also vertical distribution of As, one village in
Araihazar is shown (Fig 31). As
concentrations
show a very large spatial variability, but all of the wells beyond a
depth
of ~60ft seem to be low in As. There also appears to be a certain depth
interval (50-100 ft) where almost no wells exist. A sediment core was
eventually
collected at the site that showed a massive low permeability clay layer
separating the shallow from the deep aquifer (Fig
32). It looks like that at this site the aquifers are well
separated
and that if not too large quantities of water are pumped, the deper
aquifer
might provide an alternative source for low-As water. However, not
everywhere
[As] drops off that dramatically with depth. Other options (Fig
33) might have to be studied there.
- In areas with high spatial
varible As
concentrations, well switching might be a remediation option. Most
residents
would not have to walk very far to obtain access to a "safe" well (Fig
34), but major cultural boundaries complicate this scenario.
- A small number of
single-well
treatment
systems to remove arsenic have also been deployed (Fig
35). This particular system uses several plastic buckets, sand
filters,
and the addition of iron in tablet form to remove dissolved [As].
However
it appears difficult to create enough incentives for people to
consistently
use and maintain these systems.
- Certainly, the As problem
poses
major
challenges to the scientific community to better constrain the
processes
mobilizing As and the dose/response relationships of As, to engineers
to
develop feasible solutions, and the social scientists to help
implement
them (Fig 44).
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