Arsenic invasion
Almost every house in the
village has the same nightmare to
relate. All the 100-odd villagers suffer from skin lesions (known as melanosis — the
first stage of arsenicosis (arsenic poisoning). In some, skin of the palms and feet has
turned rough, dry and thick (keratosis) and few others suffer from breathlessness. And
that’s not all. Arsenic seems to have pervaded far and wide, with several villages in
the neighbourhood also reeling under similar alarming health consequences.
Dinanath was well aware of
the cause of his disease.
Senior dermatologist at Banaras Hindu University hospital— an apex postgraduate
medical research institute in eastern Uttar Pradesh, where he went for treatment before
coming to Delhi — had suspected that the water he drank might be contaminated
with arsenic. Thus, he believed that either the hand pump water or the well water in his
village was laced with arsenic. According to the villagers, the hand pumps were installed
in the early 1970s, after which they started suffering from skin diseases. But nobody
connected the two for a long time. It was only after Down To Earth (DTE) did an
exclusive story on the arsenic-affected regions of the Ballia district that the severity
of the problem came to light (see More
arsenic, DTE, Vol 13, No 8).
Safe drinking
water: a myth?
In India, up until now, the problem
has been recognised only in West Bengal and one district of Bihar. But arsenic is no
longer restricted to just these areas. Steady reports on the menace of this contaminant
are trickling in from several hitherto unaffected areas of the states of Assam, Uttar
Pradesh, Chattisgarh and Jharkhand and some of the neighbouring countries too (see table: Crisis
chronology).
The permissible limit of arsenic in drinking water — as given
in the World Health Organization (WHO) guideline and followed by Bureau of Indian Standard
— is 0.01 milligramme per litre (10 ppb).1,2 Water samples tested in the arsenic-affected villages of Ballia district and
surrounding regions had arsenic levels ranging from 47 to 129 ppb — about five to 13
times the permissible limit! Even the hair samples of the residents were found to contain
arsenic in very high concentrations — 2,480-6,310 ppb, as against 80-250 ppb in
normal healthy hair.
Unarguably, West Bengal is the most affected state in the country.
It is estimated that approximately 8.7 million people residing in nine districts of the
state are exposed to arsenic at levels above the permissible limit.3 About 4.9 per cent of
the water samples tested by United Nations Children’s Fund (UNICEF), in 2003-2004, in
these districts recorded arsenic concentration above 10 ppb.
| About 8.7 million people residing in nine districts
of West Bengal are exposed to arsenic levels above the permissible limit |
 |
Assam has turned out to be another
blip in the arsenic radar. Arsenic concentration in about 42 per cent of the water samples
tested in 56 villages of the state exceeded 10 ppb, as revealed by Jadavpur University,
West Bengal, in 2004. Moreover, about 19.1 and 2.1 per cent of the samples recorded
concentrations exceeding 50 and 300 ppb, respectively. In yet another study conducted by
Jadavpur University in 2004, 83 per cent of the samples tested in 17 villages of
Shahibganj district of Jharkhand reported arsenic levels above 10 ppb.
Approximately, 21.8 per cent of the screened
villagers manifested skin lesions. It is being feared that about 150,000 square kilometres
(km) of West Bengal and Bangladesh are arsenic contaminated and a population of 66 million
people residing in this area is continually exposed to the risk of arsenic poisoning.
Dipankar Chakraborti, professor of environmental sciences at Jadavpur University,
estimates that by 2010, about 450 million people living in the Ganga-Meghna-Brahmaputra
basin—encompassing the states of Uttar Pradesh, Bihar, West Bengal, Jharkhand, Assam
and Bangladesh — will be at risk of contracting arsenicrelated diseases. 4,5
Where does arsenic come from?
Arsenic occurs naturally in the Earth’s crust. Scientists are of the view that
arsenic originates in the Himalayan headwaters of the Ganga and Brahmaputra rivers. For
the past many centuries, these rivers have been depositing arsenic-laden sediments across
the Gangetic plains and the Padma-Meghna-Brahmaputra basin. It is the human intervention
in the form of groundwater abuse that has accelerated the leaching of arsenic into our
drinking water sources. Two theories have been put forth explaining this phenomenon —
oxidation and reduction theories (see Box: How arsenic enters groundwater?).
| How arsenic
enters groundwater? |
| Theories explaining how
arsenic leaches into the groundwater Oxidation
theory
Researchers believe that the oxidation (gaining oxygen) of iron releases arsenic into the
sediments. It is present in pyrites — iron-containing rocks — that are deposited
in the aquifer sediments. When the iron is exposed to oxygen, its capacity to adsorb
arsenic reduces and the toxic chemicals start leaching into the aquifers. Dipankar
Chakraborti, professor of environmental sciences at Jadavpur University, concurs that it
is the heavy groundwater withdrawal that facilitates the entry of oxygen into the aquifer,
leading to the oxidation of arsenic-rich iron sulphide that ultimately leads to the
leaching of arsenic into water. In fact, between 1970 and 1990, groundwater irrigation in
West Bengal increased by a staggering 575 per cent. Similar patterns were observed in
Bihar, Uttar Pradesh and Punjab, and neighbouring Bangladesh, pointing to the
indiscriminate use of groundwater in these regions. Quite predictably, these are the
regions that record a high prevalence of arsenic-related diseases.
Reduction theory
According to the reduction (loss of oxygen) theory, arsenic is adsorbed by iron oxides
that are a part of fine-grained sediments. These sediments are rapidly
‘reducing’ because the organic-rich matter (microbes in the sediments)
constantly consumes oxygen. This reduction of the sediments triggers a series of
geochemical reactions that lead to the release of arsenic into the groundwater. A 2004
paper in the British journal Nature reports that anaerobic metal-reducing bacteria can
also play a key role in the mobilisation of arsenic in the sediments. "The iron
oxides scavenge arsenic from soil solution and river water, building up a store of
adsorbed (trapped) arsenic", reports the British Geological Survey that has conducted
various studies on the arsenic problem of Bangladesh.
Further, to explain arsenic contamination in deep
aquifers, organic carbon reduction theory has been propounded — dissolved carbon in
the waters of some regions reacts with iron hydroxides rich in arsenic, thus discharging
arsenic into the groundwater.
Source: P Ravenscroft et al 2001,
Arsenic exposure and health effects, Elsevier Science Ltd. Oxford, pp 53-78. |
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