Assessing the groundwater quality of the coastal aquifers of a vulnerable delta: A case study of the Sundarban Biosphere Reserve, India

The Sundarban Biosphere Reserve (SBR) of India lies in the southwestern part of the Ganges Brahmaputra delta. The SBR, traversed by tidal creeks and rivulets, suffers perennially from the scarcity of freshwater in spite of heavy precipitation in monsoon months. Rivers in Sundarbans are saline in nature with brackish groundwater in the subsurface. Fresh groundwater is mainly available from the confined aquifers and occurs at a depth range of 160-400 m below ground level. The 4.43 million population in the SBR depend fully on deep groundwater to meet their drinking water demand. It is, therefore, necessary to assess the quality of groundwater to fulfil the sustainable development goal. The water quality index (WQI) has been estimated for shallow and deep aquifers based on water quality parameters like pH, TDS, EC, Cl, SO4, HCO3, TH, Ca, Mg and Na. The dominant cations are found in the order Na > Mg > Ca, and dominant anions in the order HCO3>Cl > SO4. The groundwater in the SBR region belongs to NaHCO3, NaCl and mixed type hydro-geochemical facies. The Cl/HCO3 ratio indicates that the shallow aquifers are affected by the saline water intrusion. Following the limit set by the Bureau of Indian Standards, the shallow groundwater in the SBR is found to be unsuitable for drinking. Although the deep groundwater is within the permissible limit of safe drinking water, its quality is poor. The Coastal Regulation Zone notification of 2011 has identified the SBR region as 'Critically Vulnerable Coastal Area' and restricted the use of deep freshwater for drinking purpose only. The paper argues that to meet the goal of sustainable development and safe drinking water for all in the future, extensive rainwater harvesting and supply of treated surface water through pipeline and desalination of subsurface brackish water are some of the feasible options in the region.