[en] Groundwater is the main source of drinking water supply for over one hundred million inhabitants in Bangladesh. It is severely contaminated with arsenic, resulting in a major public health crisis for millions of people. It is now widely believed that the source of arsenic is geological in origin, not anthropogenic. But the actual release mechanisms are yet to be known. The young (Holocene) alluvial and deltaic deposits are most affected, whereas the older alluvial sediments in the north-west and the Pleistocene sediments of the uplifted Madhupur and Barind Tracts normally provide low arsenic water. Environmental isotopes like ²H, ¹⁸O, ¹³C, ³H and ¹⁴C are the most suitable tools for investigating a series of problems linked with the management of water resources in the alluvial and deltaic sediments of Bangladesh. Isotope Hydrology of Groundwater in Bangladesh: Implications for Characterisation and Mitigation of Arsenic in Groundwater (BGD/8/016), a Technical Cooperation Project sponsored by IAEA, carried out in 1999-2000. Total 56 nos. water samples from shallow and deep tubewells, ranging in depth 10 to 335 meters, located mostly in south-east, southwest and north-west of the country were collected for hydro-chemical and isotopic analyses. Results of isotope techniques have provided adequate information on recharge conditions and age of groundwater in the basin, that is very important and open up prospects for further investigations using isotope techniques. Shallow groundwaters (<70 m) have oxygen, hydrogen, and carbon isotope patterns that are distinct from those of deep groundwaters. These isotopic patterns indicate that shallow groundwater is being continually recharged with a residence time of 10s of years. Deep groundwater may be either recharged on a long time scale of 100s or 1000s of years (Faridpur and Burir Char), or nor recharged at all with 20000-30000 years old water (Barisal). Arsenic contamination is found to be present mostly in shallow groundwater to depths of less than 70 meters. Groundwater samples from deep wells containing elevated arsenic concentrations are found to contain water mostly from shallow aquifers and do not indicate arsenic contamination of deeper aquifers. However, depth itself is not a criterion that can be reliably or easily used to find arsenic-free, safe drinking water. The groundwaters sampled in this study have stable oxygen and hydrogen isotope ratios ranging from -2.4 to -7.5 per mille and -11 to -51 per mille, respectively. All samples plot on or slightly below the meteoric water line, indicating an origin from local rain and rivers, with or without some evaporation before infiltration. The large range and depth trends of isotopic values indicate that groundwater at different depths has been impacted by one or more of the following processes: mixing with seawater, direct recharge from local rivers, and recharge under different climatic conditions. Most of the shallow samples (<70 m) have stable oxygen isotope values between -3.0 and -5.5 per mille. This range of values is similar to that expected for recharge from present day rain and flood waters in Bangladesh. BGS and DPHE studied the stable isotope technique in Chapai Nawabganj, Faridpur and Lakshmipur areas; and apparently found similar compositions to the findings of this TC project. The δ¹³C of As-bearing shallow waters range mostly from -3.0 per mille to -15.0 per mille. Higher arsenic concentrations are associated with higher carbon isotopic values, indicating that organic matter oxidation is not likely to play a role in arsenic mobilization in the aquifer. The carbon isotopic data indicate that the most likely process of arsenic mobilization may involve desorption from the sediments as a result of the relatively rapid and continuing (natural) renewal of shallow aquifers with arsenic free water. But according to the study of BGS and DPHE, the large δ¹³C variations observed in Bangladesh groundwaters reflect derivation of the DIC from these multiple sources; such as soil zone CO₂, oxidation of organic matter, potential oxidation of CH₄, dissolution of minerals and in places mixing with seawater. It is concluded that depth itself is not the best criterion for considering a deep tube well to be arsenic-free now or in the future. The isotopic composition of the groundwater provides a robust criterion. As a consequence of challenging results from the previous study, BAEC has the on-going TC project entitled 'Isotope Techniques for Mitigating Arsenic Contamination in Groundwater (BGD/8/018)' approved by IAEA implemented in the year 2001-2002. The detailed field campaigns were accomplished during August (monsoon season) 2001 in the 8 nos. nested wells at the IDE site, Savar and during November (dry season) 2002 in the 16 nos. shallow and deep wells in Chandpur area. The water samples for environmental isotopic analyses of ²H, He-³H, ¹⁸O, ¹³C, ¹⁴C and ³⁴S were taken