A new source of fresh groundwater is found in the south western Bangladesh, according to a study using isotopic techniques.
The study, supported by the International Atomic Energy Agency (IAEA), found that deeper groundwaters in the Barguna and Patuakhali districts are free of arsenic and sea water intrusion which means they are viable additional sources of drinking water for a region made up of over two million people.
Bangladesh, a country of 165 million people, is facing acute limitations of potable water resources due to increased contamination, sea water intrusion and arsenic pollution of aquifers, reports IAEA.
"The government of Bangladesh has given high priority to domestic water supply, including arsenic mitigation and sustainable groundwater management," said Nasir Ahmed, Isotope Hydrologist at the Bangladesh Atomic Energy Commission (BAEC), which led the study. "The isotopic data collected during the study will be used to further guide the optimisation of sustainable groundwater management policies."
Surface waters are saline in most of Bangladesh's coastal rivers, and groundwater is the main source of safe drinking water. However, shallow groundwater in the country has high levels of arsenic - a toxic metalloid present in the local geology in contact with groundwater. Contaminated water used for drinking, food preparation and irrigation of food crops poses the greatest threat to public health from arsenic, according to the World Health Organisation, and long-term exposure to arsenic from drinking water and food can cause cancer and skin lesions.
With IAEA support, BAEC conducted the study from 2016 to 2019 to characterize groundwater recharge – the hydrologic process involving the movement of surface water to subsurface groundwater – in arsenic-impacted areas and salinization in the coastal aquifers of the country. Isotopic techniques (see The Science) enable scientists to understand the components of the water cycle, helping them better assess the quantity, quality and sustainability of water. In the water cycle, groundwater is the least understood component. Isotopes are the most direct and powerful tools available to estimate the age, vulnerability and sustainability of water resources.
The isotope data from the study revealed the existence of long-term high quality drinking water in the deeper 300-meter aquifers in the Barguna and Patuakhali districts.
"Isotopic techniques are a valuable tool for evaluating arsenic mobility in aquifers and the groundwater recharge mechanism," said Umaya Doss Saravana Kumar, Isotope Hydrologist at the IAEA. "Sustainable groundwater resource management requires understanding of recharge processes, their source and origin, as well as the geochemical evolution. Isotope techniques help in all these areas."
The availability of groundwater to cover human needs depends on its quantity, quality and rate of replenishment. Information on the source and age of groundwater is required to properly assess the sustainability of the resource, particularly in view of existing or planned withdrawal activities. The deeper groundwaters found in the Barguna and Patuakhali aquifers showed no sea water contribution or arsenic and apparent groundwater ages of about 9800 to 31,700 years BP, indicating the presence of old recharged water, meaning that the water flow is slow and that the aquifer can take a long time to replenish, but that it is also less affected by pollution and changing climatic conditions.
"This isotopic study has provided a scientific basis in delineating the present and future status of deep aquifers of Barguna and Patuakhali districts," said Ahmed.
The Department of Public Health Engineering of Bangladesh has now installed wells 300 meters deep where the aquifer is found free of arsenic contamination and high salinity, to supply safe drinking water to the rural population of the south-western Bangladesh.
In hydrology, some naturally occurring radioactive isotopes present in water, such as tritium (3H), carbon-14 (14C) and noble gas radioisotopes, are used to estimate groundwater age. Water samples have unique isotopic signatures, or fingerprints, that provide clues about their origin and history. Water samples from surface water and groundwater at various depths are collected at several representative sites for various hydrochemical analyses, including arsenic and isotopic analysis, the results of which reveal information about the source, renewability and recharge mechanism of groundwater. The arsenic concentrations in groundwater samples at different depths, combined with the hydrogeology and isotopic fingerprints, enable better conceptualization of the groundwater flow and the arsenic mobility in the groundwater.