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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Sustainability and Failure of Groundwater Abstraction

from the Bengal Basin:

Scenarios from the JAM Aquifer, West Bengal

Peter Ravenscroft, John McArthur,

Dhiraj Banerjee, Anindya Sarker,

Rajaram Purohit, Rajesh Mishra

and Saikat Sengupta

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

The JAM Aquifer Study:Acknowledgements

• John McArthur, University College London

• Dhiraj Banerjee, Delhi University– Rajaram Purohit

– Rajesh Mishra :

• Anindya Sarker, IIT Karagpur– Saikat Sengupta

• Others:– Karen Hudson-Edwards, John Milsom & Sam Tonkin, UCL

– Aidan Cronin, Surrey Uni.

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Purpose and Scope

1. Explain the movement of groundwater in a contaminated aquifer system, and how this controls the migration of arsenic.

2. Present an idealised geological model of the contaminated aquifers

3. Explain the geology of the JAM site in its regional context

4. Illustrate the hydraulics of the JAM aquifer

5. Present a conceptual model of arsenic movement in the alluvial

aquifers of the Bengal Basin

6. Outline a framework for risk assessment of safe-wells in partially contaminated aquifers

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Arsenic in the Bengal Basin

Assumptions:

• Audience aware of As contamination in Bengal Basin:

– Tens of millions exposed

– Contamination of shallow alluvial aquifers

– As mobilised by reductive dissolution of iron oxides

• Geochemistry of JAM site described by Banerjee

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

• JAM – contiguous villages of:

Joypur, Ardivok &Moyna

• 3 km N of Barasat, and 15 km N of Kolkata

• Holocene floodplains of Ganges delta

• Hydrochemistry described by

– McArthur et al. (Applied Geochemistry, 2004)

– 3 piezometer nests

– Mobilised by reductive dissolution

– Hydrochemical mapping

The JAM Site

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

• JAM – contiguous villages of: Joypur, Ardivok &Moyna

• 3 km N of Barasat, and 15 km N of Kolkata

• Holocene floodplains of Ganges delta

• Hydrochemistry described by

– McArthur et al. (Applied Geochemistry, 2004)

– 3 piezometer nests

– Mobilised by reductive dissolution

– Hydrochemical mapping

The JAM Site

N

S

E

W

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

• Low sea levels at the Last Glacial Maximum induced river incision

• LGM terrain divided into interfluves and two types channel:

– Regional rivers (> 100m)

– Tributaries developed on interfluves (10’s of metres)

• Contrast modern and LGM conditions

• Note the depositional conditions A-B and C-D

Alluvial Aquifers of the Lower Bengal Basin

>=100m

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Alluvial Aquifers of the Lower Bengal Basin

JAM Scenario

>=100m

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

A general model for shallow aquifers in the Bengal Basin

Last Glacial Maximum (>=18 Ka)

Early Holocene transgression (8 - 11 Ka)

Mid-Holocene marine flooding and channel-fill (6 - 8 Ka)

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Generalised Hydrogeological Scenarios

Note: ‘AD’, ‘BD’ etc. refer to

combinations of A, B, C & D in Slide #6

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Holocene

Pleistocene

Holocene

Pleistocene

Hydrogeology of the JAM aquifer

grey coarse sand

grey fine-medium sand

?

<50 ppb As

>50 ppb As

brown medium-sand

stiff grey silty-clay

grey-green to black silt & peat

Stiff brown clay

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Holocene

Pleistocene

Holocene

Pleistocene?

<50 ppb As

>50 ppb As

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Holocene

Pleistocene

Holocene

Pleistocene?

<50 ppb As

>50 ppb As

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Hydraulic Properties

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

DP: Similar drawdowns

in aquifer:

DP: water levels rise in

aquitard above

palaeosol

:

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Hydrochemical Profiles: Sediment Chemistry

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Flow Directions: Vertical and Horizontal

Main

aquif

er

Shallo

w a

quita

rd PONDS

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Groundwater Abstraction

• Four sources:

– Shallow hand tubewells

– Deep hand tubewells

– Shallow irrigation tubewells (11)

– Deep irrigation tubewells (2, old)

60m

--- Village area --- - - - - -- Paddy Fields-- - - - -

Drinking water wells

>50 <50 <50

Irrigation wells

>50<50<50

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Conceptual Model of the JAM Aquifer

1

2

3

4

5

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Sustainability Framework (1 of 4)

AFAGroundwater flow

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Sustainability Framework (2 of 4)

Barrier to

vertical flow

As

DOC DOC

AFAGroundwater flow

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Sustainability Framework (3 of 4)

??

?

As

Barrier to

vertical flow

As

??

DOC DOC

AFAGroundwater flow

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Sustainability Framework (4 of 4)

An achievable water management scenario?

drinking

waterirrigation irrigationPresent

drinking

waterirrigationirrigationFuture?

Present situation

AFAGroundwater flow

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Attenuation in Deeper Strata:Solution or Delayed Destruction?

• Nature’s solution

• Policy: priority protection for drinking water

• Possible mitigation approach: draw down contaminated water and sequester As in brown sands

• Outcome: uncertain

• Current practice: 90% of mitigation from deep wells – it is already happening

• Conclusion: Talk about it or understand it and plan response?

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

Summary and Conclusions

1. The geology of the As-affected alluvial aquifers can be usefully divided into two lithological sub-types:

– Pleistocene and Holocene (pre- and post-LGM)

– Interfluve and Channel-fill sequences (Regional & Shelf valleys)

2. The idealised hydrogeological scenarios provide a framework fro assessing to wells

3. A regionally extensive, but discontinuous (palaeosol) aquitard separates Pt. and Hol. sediments in the interfluves, and is of fundamental importance in controlling the distribution and migration of As.

4. Channel-fill sequences form important vertical conduits for the movementof water and As – Pt. and Hol. Sediments are in continuity and wells are at high risk.

5. The Pt. palaeosol protects wells in the underlying aquifer from vertical migration of As. Water and As migrate horizontal, and risk is proportional to distance from the channel.

6. Migration of As away from channel-fill sequences will be slow, and offers potential for permanent sequestration of large amounts of arsenic.

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International Conference on Arsenic in the Tropics. Raipur, Chhattisgarh, India, 19-21/02/07

The End