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IUKWC Workshop Nov16: Developing Hydro-climatic Services for Water Security – Session 6 – Item 4 S_Chakraborty

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

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IUKWC Workshop Nov16: Developing Hydro-climatic Services for Water Security – Session 6 – Item 4 S_Chakraborty

  1. 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. 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. 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. 4. Study area - at a glance
  5. 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. 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. 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. 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.
  9. 9. High arsenic Low arsenic Fence diagram depicting the subsurface geology
  10. 10. East-West Panel DiagramsEast-West Panel Diagrams Municipal areaMunicipal areaRural areaRural area High ArsenicHigh Arsenic Low ArsenicLow Arsenic
  11. 11. North-South Panel DiagramsNorth-South Panel Diagrams Rural areaRural area Municipal areaMunicipal area Low arsenicLow arsenic High ArsenicHigh Arsenic
  12. 12. 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
  13. 13. Modelling steps
  14. 14. 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
  15. 15. 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
  16. 16. CONCEPTUAL MODEL Heterogeneous anisotropic
  17. 17. MODFLOW MODEL
  18. 18. 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
  19. 19. 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
  20. 20. 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
  21. 21. • 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)
  22. 22. Kv:Kh= 1:3000 Homogeneous and Anisotropic Large-Scale Hydrogeology (3 x10-4 m/s) (1 x10-7 m/s)
  23. 23. Pre-Development Condition
  24. 24. 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’.
  25. 25. 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
  26. 26. Current Development Condition
  27. 27. 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
  28. 28. Spatial distribution of head in the area of municipal wells under current abstraction Groundwater trough in the municipal area
  29. 29. 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.
  30. 30. 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
  31. 31. 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.
  32. 32. 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
  33. 33. 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.
  34. 34. • 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
  35. 35. Simulations with Assumed Future Abstraction Base case, Discharge = 30 m3 /hr
  36. 36. 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
  37. 37. 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
  38. 38. 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.
  39. 39. • 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.
  40. 40. • 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.
  41. 41. 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.

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