IUKWC Workshop Nov16: Developing Hydro-climatic Services for Water Security – Session 6.4 Surajit Chakraborty

1. Understanding migration of arsenic in the aquifer of North
Bengal Plain using numerical modelling: A case study of
English Bazar Block, Malda District, West Bengal, India
Surajit Chakraborty
Department of Environmental Management
IISWBM, Kolkata
Workshop on : Developing Hydro-Climatic Science, Information and Services for Water Management
Nov 29-Dec 3, 2016, IITM, Pune

2. • In this research numerical simulations of regional-
scale groundwater flow of North Bengal Plain have
been carried out with special emphasis on the
arsenic-rich alluvium filled gap between the
Rajmahal hills on the west and the Garo hills on the
east.
• The proposed concern of this modeling arose from
development that has led to large water table
declines in the urban area of English Bazar block,
Malda district, West Bengal and possible transport
of As in the near future from the adjacent As-
polluted aquifer.

3. Literature Review
Studies on migration of As within the aquifer system of the Bengal Basin are very
few:
1.Michael and Voss (2009a, b) carried out a quantitative, large-scale hydrogeologic
analysis and numerical simulation of the entire Bengal Basin, looking at the benefits
of water wells that pump from depths where the water is less contaminated as an
alternative to other solutions such as filters.
2.Mukherjee et al. (2007; 2011) developed groundwater flow model for the southern
part of West Bengal stretching from Murshidabad in the north to the Bay of Bengal
in the south, approximately 21,000 km2
in area, designed to better understand
shallow and deep large-scale flow patterns in the region.
3.Sikdar et al. (2013) have developed a heterogeneous anisotropic steady-state
groundwater flow model for the multi-aquifer system of a part of southern Bengal
Basin which shows that human intervention has changed the natural groundwater
flow system. They calculated that the As migration rates range between 0.21 and
6.3 m/year and 1.39 x 10-2
and 0.4 m/year in horizontal and vertical directions,
respectively.
Till date no comprehensive modeling study has been done for the North Bengal
Plain.

4. Study area - at a glance

5. STUDY AREA
 Total area - 266 sq. km.
 Latitude -24°50’N to 25°05’N;
 Longitude - 88° 00’E to 88° 10’E
 Municipal area -14 sq.km., 11,846
persons/sq km
 Non-municipal area -252 sq.km., 899
persons/sq km.
 The entire block has a topographic
elevation in the range of 22.4 m to 25 m.
 Tropical climate
Temp - 4o
C - 44o
C
Rainfall – 943 mm (average of 50 yrs)
MUNICIPALITY

6. Groundwater Concerns
 Development has lead to a number of groundwater supply
and groundwater quality concerns, including:
• Large water table declines near the pumping centres in
the municipal area.
• Arsenic contamination from the adjacent aquifer.
• The δ18
O and tritium values of groundwater are within the
range of monsoon precipitation composition which
indicates that groundwater is probably recharged primarily
from precipitation.
• Therefore, modern groundwater has invaded into the
deeper part of the aquifer.

7. Groundwater Concerns
• What would be the effect of
large scale pumping in the
municipal area on the head
and drawdown?
• Whether groundwater of
English Bazar Municipality
(which is at present
arsenic-free) will be
contaminated with arsenic
in near future?

8. Regional Geology
 The region forms the northeastern part of the Indo-
Gangetic alluvial plain.
 The study area is a part of a Rajmahal-Garo Gap or the
‘saddle zone’ to the north of the Ganga.
 The area is covered by Quaternary fluvial sediments of
Older Alluvium and Newer Alluvium.

11. High
arsenic
Low
arsenic
Fence diagram depicting the subsurface geology

12. East-West Panel DiagramsEast-West Panel Diagrams Municipal areaMunicipal areaRural areaRural area
High ArsenicHigh Arsenic Low ArsenicLow Arsenic

13. North-South Panel DiagramsNorth-South Panel Diagrams
Rural areaRural area
Municipal areaMunicipal area
Low arsenicLow arsenic
High ArsenicHigh Arsenic

14. To better understand the groundwater flow
system in regional and local scales of a large,
complex sedimentary aquifer system and the
potential for sustainable supply of arsenic-free
groundwater from wells from a highly stressed
aquifer adjacent to an arsenic-rich aquifer.
Objectives

15. Modelling steps

16. Model Design: STEADY STATE MODFLOW MODEL
Model Domain :
East = Brahmaputra River
West = Boundary of hard and
soft rocks
North = Himalayan foothills
South = Ganga River
Top = Land surface
Bottom = Clay/ Shale / Granite
Boundary Conditions :
Top: Prescribed Head
=Topography (90m space
shuttle radar data for the
ground surface)
Bottom and western sides:
No Flow
Rest : Prescribed Head
Stress :
No pumping or pumping

17. Parameter estimation
Stratigraphy observed
during drilling
• 6 lithologic layers (Clayey
silt, Fine sand, Medium
Sand, Coarse sand, Clayey
silt, Basement)
Calculation of K
• The values of hydraulic
conductivity for different
layers of were chosen based
on literature values specific
to the area, where available,
or more general estimates for
specific lithologies.
12 borehole logs12 borehole logs
Depth= 10 - 308 mDepth= 10 - 308 m

18. CONCEPTUAL
MODEL
Heterogeneous
anisotropic

19. MODFLOW MODEL

20. Vertical discretisation and unit thickness
Unit
Number
Depth range
(m)
Thickness
(m)
Number of
Vertical Cells
1 0-1 1 1
2 1-10 9 1
3 10-40 30 3
4 40-70 30 3
5 70-140 70 3
6 140 - basement 840 1

21. Groundwater Abstraction
Domestic and industrial pumping
• Districts - based on population (per capita consumption 0.05 m3
/day) =
1814x 103
m3
/ day m3 /day
• English Bazar block – based on GEC 1997 norms = 41 x 103
m3
/ day
Irrigation pumping
• Districts - based on total irrigated area = 53568 x 103
m3
/day
• English Bazar block – based on GEC 1997 norms =115 x 103
m3
/day
Total discharge at present = 55.5 x 106
m3
/day

22. Base Case Value
• Horizontal Conductivity (Kh)= 3 x 10-4
m/s
• Vertical Conductivity (Kv) = 1 x 10-7
m/s
• Anisotropy (Kh/Kv) = 3,000
• RMS error = 1.25

23. • More or less random
distribution
• Zoning of K may not
be required .
• The heterogeneous,
anisotropic porous
medium of the area
is simulated as
homogenous
anisotropic aquifer
for groundwater
modelling.
Distribution of residuals (observed head - simulated head)

24. Kv:Kh= 1:3000
Homogeneous and Anisotropic
Large-Scale Hydrogeology
(3 x10-4
m/s)
(1 x10-7
m/s)

25. Pre-Development Condition

26. Spatial distribution of head under pre-development conditions in (a) North
Bengal Plain and (b) English Bazar block at a depth range of 70-100 m. The
area where the basement depth is less than 100m has been indicated
‘inactive area’.

27. Pre-development pathlines to locations
at 100 m depth for anisotropy of
3000 in English Bazar block.
3-D view from south of the same,
vertical exaggeration is 50x

28. Current Development Condition

29. Spatial distribution of head under current abstraction. The area where the
basement depth is less than 100m has been indicated ‘inactive area’.
• Domestic pumping based on
population (per capita
consumption 50L/day)
• English Bazar municipal
pumping = 65 m3
/hr for 33
wells
• Irrigation pumping based on
total irrigated area
• Total discharge
= 55.5 x 106
m3
/day

30. Spatial distribution of head in the area of municipal wells under current abstraction
Groundwater trough in the
municipal area

31. Spatial distribution of drawdown in the area of
municipal wells under current abstraction.
• The drawdown contour
extend far beyond the
municipal boundary.
• Interference effect of wells
in rural area.
• Increases lowering of
water table in rural areas.
• May cause groundwater
quality problems in rural
areas.

32. Current-development pathlines to
locations at 100m depth for 65m3
/hr
discharge per well with base case
anisotropy in English Bazar block
GROUNDWATER FLOWPATHS
Base case,
Discharge = 65 m3
/hr

33. Current development pathlines at 40m depth
for 65m3
/hr discharge per well where most of
the irrigation wells are screened.
Outside the municipal area
Under current pumping conditions the
flowpaths are near-vertical with very short
horizontal path lengths.
This indicates that, where As is present or
released at shallow depths it will reach the
well depths within few tens of years and will
continue to occur in pumping wells
This corroborates with the findings of Mukherjee et
al. (2011) local-scale study site in Nadia district
where they showed that deep groundwater
abstraction can draw As-rich water from 50 m
below land surface to 150 m depth within a few
decade.

34. Advective Transport of Arsenic in Municipal area
Current-development pathlines to locations at 100m depth for different simulation time
at 65m3
/hr discharge per well with base case anisotropy (a) 25 yrs. (b) 50 yrs. (c) 100
yrs. These times represent three different times of travel to the wells, showing which
water will reach the wells after pumping for the indicated amount of time.
No particles from As zone 1 % particles from As zone 3% particles from As zone
25 yrs25 yrs 50 yrs50 yrs 100 yrs100 yrs

35. Hence, lowering of the domestic wells in the As-free zone may Not
ensure As-free water permanently.
Therefore, our findings in English Bazaar of the North Bengal Plain
differs from that of Michael and Voss (2008) based on regional
scale modelling of the entire Bengal Basin where they inferred that
with shallow high irrigation pumping and low scale deep pumping
for domestic purpose, the deeper part of the aquifer system may
provide a sustainable source of As-safe water. This may be due to
the limited thickness of the aquifer in English Bazar block.

36. • Change in discharge of municipal wells
– 30 m3
/hr per well
– 65 m3
/hr per well (Base Case)
– 100 m3
/hr per well
Simulations with Assumed Future Abstraction

37. Simulations with Assumed Future Abstraction
Base case,
Discharge = 30 m3
/hr

38. Pathlines to locations at 100m-130 m depth for different simulation time at 30 m3
/hr
discharge per well with base case anisotropy (a) 25 yrs. (b) 50 yrs. (c) 100 yrs.
No particles from As zone No particles from As zone 1% particles from As zone
Discharge at 30m3
/hr
25 yrs25 yrs 50 yrs50 yrs
100 yrs100 yrs

39. Pathlines to locations at 100m depth for different simulation time at 100 m3
/hr
discharge per well with base case anisotropy (a) 25 yrs. (b) 50 yrs. (c) 100 yrs.
No particles from As zone 2% particles from As zone 4% particles from As zone
Discharge at 100m3
/hr
25 yrs25 yrs 50 yrs50 yrs
100 yrs100 yrs

40. CONCLUSIONS
• The present withdrawal scenario of English Bazar
Municipality indicates that
– by 2025 majority of the water will come from the
arsenic free region of the aquifer, some wells will
receive water from the surface
– by 2050 the wells may draw some water from arsenic
rich area (0.05-0.1mg/l) and more water comes from
surface.
• If the abstraction rate is increased to 100m3
/hr
then within 50 years there is a possibility of the
aquifer getting contaminated but if the rate is
decreased to 30m3
/hr then the aquifer may remain
uncontaminated at least for the next 50 years.

41. • Due to pumping of municipal wells most of the
paths originate in the area north of the municipal
area where the groundwater is free from arsenic.
• At the current and increased pumping rates the
chance of arsenic arriving in the wells in the
municipal area after 50 years is in only a few
percent of the wells.
• Local heterogeneities and change in aquifer
parameters such as effective-porosity, hydraulic-
conductivity and anisotropy could make some flow-
paths faster or slower than that calculated by
MODPATH while geochemical and other processes
could slow arsenic migration.

42. • If these processes act simultaneously arsenic
migration may be significantly retarded than that of
the predicted rate by advective-transport. Hence
municipal-wells may have even more time to pump
arsenic-free water.
• Thus arsenic contamination in the groundwater of
the municipal area seems to be a minor concern
for the municipal authorities.
• But outside the municipal area the near vertical
nature of flow under current pumping conditions
indicates that where arsenic is present or released
at shallow depths, it will continue to occur in the
pumping wells.

43. ACKNOWLEDGEMENTS
• Department of Science and technology, Government of
India (DST File No. SR/S4/ES-56/2003) for funding the
project.
• Dr. Clifford Voss, USGS and Dr. Holly Michael, University
of Delware, USA for their help during the modelling effort.