Reply:

1. The questions raised by Meer Hussain in his e-mails dated 17
and 25 Dec 2001 have been raised before by him.
Most of these points were clarified in my e-mail dated
13 Jul in [the] Setubanbhan [discussion group]. In discussion that followed
it appeared that it is his firm conviction rather than
logic that played dominant role and there was more
heat than light. Even then let me once again put
forward my views in this matter.

2. Mr Hussain regards that low SO4 in arsenic affected
groundwater is not a valid objection against the
oxidation model for release of arsenic to
groundwater. The main evidence for oxidation model is
common presence of pyrite and other arsenic-bearing
sulfides in affected aquifer that release arsenic on
their oxidation. But studies in Bangladesh by Mott
Macdonald(1999) and our study in West Bengal
demonstrate that pyrite of any other arsenic bearing
sulphide is very rare or absent from the affected
aquifer. Biogenic pyrite is present in rare cases and
these occur in association with organic rich
sediments. These are clearly formed in reducing
conditions and these would act as sinks and not source
of arsenic on their later oxidation if at all.

3. If "oxidation ruled" the release of arsenic, then
dugwell and pond waters were expected to be most
toxic. Observation is just the opposite.

4. Our study reveals that arsenic contaminated aquifer
are mainly confined to organic rich argillaceous
deltaic sedimentation in the Bengal Basin (Acharyya et
al., 2000, Env.Geol)

5. Mr Hussain questions why arsenic problem has
cropped up in recent time why it was not recognized
earlier? Hussain believes that is was triggered by
several dams built on major rivers of Bangladesh. But
he does not explain the logical connection. Nor there
are any other examples of such a relation. Why there
is no such correlation with other river valleys and
arsenic problem in groundwater.

6. Exploitation in large scale of groundwater for
irrigation and drinking purposes is a recent
phenomenon in the Bengal basin. And this appears to
have triggered arsenic problem. Arsenic affected
groundwater is from shallow aquifer and has typical
isotopic signature. The shallow aquifers are
continually getting recharged. The deep aquifers free
of arsenic are isotopically distinct. Some deep wells
that have become contaminated has water similar to
shallow aquifer. Thus these are leaked water from
contaminated shallow aquifer. Normal hydrologic
conditions are not recharging the deep aquifers.

7. The process of natural and partly enhanced recharge
of shallow aquifer, because of extensive groundwater
abstraction has enforced movement of groundwater along
with strongly reducing degraded organic products from
the host and overlying sediments. Movement of degraded
organic product enriched groundwater through the FeOOH
bearing sediments, would enhance reduction of
arsenic-bearing FeOOH and release arsenic to
groundwater (Acharyya, 2001).

8. A parallel case to that of Bengal basin is with the
arsenic contaminated groundwater reported from Red
River delta in Vietnam (Berg et al., 2001,
Env. Sci. Technol. ). In this case also it was found
that extensive application of groundwater for
irrigation and for drinking purposes were made since
about 5-7 years. But although, people are consuming
arsenic toxic water no cases arsenicosis has yet been
diagnosed. It takes 7-10 yr time before such
manifestation is identified.

S. K. Acharyya
15, Dr. Sarat Banerjee Road, Kolkata - 700029
Emeritus Scientist,CSIR,Dept Geological Sciences,
Jadavpur University, Kolkata 700032
Phone 465-1712