OR/15/047 Methods

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MacDonald A M, Bonsor H C, Taylor R, Shamsudduha M, Burgess W G, Ahmed K M, Mukherjee A, Zahid A, Lapworth D, Gopal K, Rao M S, Moench M, Bricker S H, Yadav S K, Satyal Y, Smith L, Dixit A, Bell R, van Steenbergen F, Basharat M, Gohar M S, Tucker J, Calow R C and Maurice L. 2015. Groundwater resources in the Indo‐Gangetic Basin: resilience to climate change and abstraction. British Geological Survey Internal Report, OR/15/047.

Developing the groundwater typologies

The groundwater typologies reflect differences in aquifer properties, hydrology and climate, which affect the available groundwater storage, recharge and groundwater chemistry in the IGB aquifer system. The typologies were developed from combining together the best available national, regional and local‐scale datasets and studies on the geology, sedimentology, aquifer properties, groundwater chemistry, hydrology and climate of the basin. Over 500 studies were reviewed in total — 56 of these focused on geological information; 415 hydrogeological studies and datasets; and 42 relating to climate and hydrological studies. The 80 studies which provided the highest quality systematic regional data form the key benchmark papers for the typologies.

Different processes of alluvial sediment deposition have operated across the basin through time and fundamentally determine the aquifer properties of the IGB. The characteristics of the sedimentology were mapped out for the top 200 m of the basin’s alluvium to form a basis for the hydrogeological typologies; this process incorporated a review of geological and sedimentological literature and parameterised with information on likely grain size and modes of deposition. Specific yield (the drainable porosity) was mapped across the basin using available grain size distribution for the top 200 m of alluvium, and validated with several key hydrogeological studies of specific yield undertaken in different parts of the basin. Differences in transmissivity were mapped using a combination of primary data from pumping tests (mainly in Pakistan and India) and a review of existing studies within the area. From this framework broad areas of similar hydrogeological properties could then be identified.

Groundwater chemistry for the IGB alluvial aquifer system was mapped by considering the distribution of elevated concentrations of salinity and arsenic in groundwater, the two most significant water quality issues within the basin. Groundwater salinity was mapped by compiling existing information from hydrogeological maps, with more specific local‐scale data studies and literature. In Pakistan, the published hydrogeological maps and drainage atlas were used (WAPDA 2001[1], IWASRI 2005[2]) in conjunction with specific information from additional studies and surveys. In India a survey of shallow groundwater quality by CGWB was used to estimate the extent of salinity (CGWB 2010[3]) and in Bangladesh a recent survey of specific electrical conductivity (Ravenscroft et al. 2009[4]) combined with a national survey of water chemistry (DPHE/BGHS 2001[5]). Arsenic concentrations across the basin were mapped within India using available data and maps by State Water Resources Agencies, the CGWB and available local datasets (e.g. Mahanta et al. 2012[6]). Within Bangladesh the DPHE/BGS (2001)[5] national hydrochemical survey data and other large‐scale studies by Ravenscroft 2007[7] and Amini et al. 2008[8].

Climate and hydrology play an important role in determining recharge, availability and use of groundwater resources in the basin. Rainfall for the basin was taken from the CRU datasets for the years 1950 to 2012 (Jones and Harris 2013[9]) and maps of average annual rainfall and number of rainy days were developed. The extent of rivers and canal networks were mapped on GIS using a variety of different sources, and validated on Google® Earth. These three different datasets were used to help develop an understanding of how groundwater recharge may systematically vary across the basin.

The final groundwater typologies for the IGB alluvial aquifer system were developed by combining the basin‐wide maps of rainfall, rivers and canals, groundwater salinity and arsenic concentrations with the map of physical hydrogeological properties.

Investigating resilience of the groundwater system to change

To investigate the resilience of the groundwater systems to change two approaches were taken: (1) mapping the volume and distribution of the available freshwater groundwater resource as an estimate of the capacity of the aquifer system to buffer changes in recharge or abstraction; and (2) mapping the current changes in groundwater storage across the IGB alluvial aquifer system and relating these changes to current abstraction and groundwater recharge, as an indicator of the impact of future pressures of climate and abstraction.

The volume of the available freshwater groundwater resource was estimated by integrating the specific yield across the top 200 m of the IGB alluvium (with the exception of the Bengal Basin where a depth of 350 m was used) and then attributed to different groundwater quality classes according to the salinity. For the second approach, changes in groundwater storage were calculated from annual changes in post monsoon groundwater level using available maps, databases and individual water level monitoring points collated and QA’d within the project. The annual change in water level could then combined with the maps of specific yield to estimate annual changes in groundwater storage. A basin wide map of groundwater abstraction was developed by combining data available at a district level for India from the CGWB (accessed online 2014)[10], with data for Bangladesh (Holly and Voss 2009, DWASA 2012[11]) and Pakistan (Ahmad 2002[12], Halcrow 2013, FAO 2013, Cheema et al 2014[13]). Data for the Nepal Terai were estimated from the global assessment from Seibert et al. 2010.

References

  1. WAPDA 2001. Hydrogeological map of Pakistan (scale 1:250 000). Directorate of Hydrogeology, WAPDA, Lahore.
  2. IWASRI 2005. Drainage Atlas of Pakistan, International Water Logging and Salinity Research Institute, Lahore.
  3. CGWB 2010. Groundwater quality in shallow aquifers of India. Central Groundwater Board, Ministry of Water Resources, Delhi.
  4. Ravenscroft P, Ahmed, K M and Samad M A. 2009. WORKING DOCUMENT NUMBER 9, Groundwater: Quantity and Quality Issues Affecting Water Supply. Sector Development Plan (FY 2011–25), Water Supply and Sanitation Sector in Bangladesh. Policy Support Unit, Local Government Division, Government of Bangladesh.
  5. 5.0 5.1 DPHE/BGS. 2001. Arsenic contamination of groundwater in Bangladesh. Kinniburgh DG & Smedley PL (eds). British Geological Survey Technical Report WC‐00‐19, BGS, Keyworth
  6. Mahanta et al. 2012. Mineralogy, grain size and sediment composition as factors controlling telease and mobilisation of Arsenic in an aquifer regime of the Brahmaputra floodplains, northeastern India, JNU Indo‐Australia Workshop 2013.
  7. Ravenscroft P. 2007. Predicting the global extent of Arsenic Pollution of Groundwater and its potential impact on Human Health, UNICEF report.
  8. Amini M. et al. 2008. Statistical Modelling of Global Geogenic Arsenic contamination in groundwater, Environmental Science and Technology, 42; 3669–3675
  9. Jones P D and Harris I. 2013. University of East Anglia Climatic Research Unit; CRU TS3.21: Climatic Research Unit (CRU) Time‐Series (TS) Version 3.21 of High Resolution Gridded Data of Month‐by‐month Variation in Climate (Jan. 1901‐ Dec. 2012). NCAS British Atmospheric Data Centre, 24th September 2013. doi:10.5285/D0E1585D‐3417‐485F‐87AE‐4FCECF10A992.
  10. CGWB. 2014. Ground Water Year Book 2012‐13 — India. CGWB. 100 pp
  11. DWASA 2012. Annual Report of 2011–12 Dhaka Water Supply & Sewerage Authority, Dhaka
  12. Ahmad, S, Mulk S and Amir M. 2002. Groundwater Management in Pakistan. First South Asia Water Forum Kathmandu Nepal. Printed by Pakistan Water Partnership
  13. Cheema MJM, Immerzeel WW and Bastiaanssen Wgm 2014. Spatial quantification of groundwater abstraction in the irrigated Indus Basin. Groundwater 52: 25–36