INVITED KEYNOTE: GEOGENIC ARSENIC AND MANGANESE IN BENGAL DELTA GROUNDWATERS: SUSTAINABLE MITIGATION FOR SAFE DRINKING WATER SUPPLIES

Elevated levels of geogenic arsenic (As) in groundwater from the shallow Holocene sedimentary aquifers has undermined the success for achieving safe water and food security in Bangladesh. In order to increase the safe water access, local drillers target safe groundwater on the basis of the colour of the sediments. This study discusses about the correlation between aquifer sediment colours and groundwater chemistry, characteristic aqueous and solid phase geochemistry and dynamics of As and to assess the risk for cross-contamination of As between various hydrostratigraphic units in the Matlab region of Bangladesh. Drillings made to a depth of 60 m in Matlab, indicated two distinct hydrostratigraphic units, a strongly reducing aquifer unit with black to grey sediments overlies a patchy sequence of weathered and oxidised white, yellowish-grey to reddish-brown sediment separated by a unit of impervious clay. The reducing aquifer is characterized by high concentrations of dissolved As, DOC, Fe and PO₄^3-‑tot. On contrary, the off-white and red sediments contain relatively higher concentrations of Mn and SO₄^2- and low As. Geochemical investigations indicate that secondary mineral phases control dissolved concentrations of Mn, Fe and PO₄^3--tot. Dissolved As is influenced by the amount of Hfo, pH and PO₄^3--tot as a competing ion. Laboratory studies suggest that oxidised sediments have a higher capacity to absorb As. Monitored hydraulic heads and groundwater modelling illustrate a complex aquifer system with three aquifers to a depth of 250 m. Groundwater modelling illustrates two groundwater flow-systems: i) a deeper regional predominantly horizontal flow system, and ii) a number of shallow local flow systems. It was confirmed that groundwater irrigation, locally, affects the hydraulic heads at depths. The aquifer system is however fully recharged during the monsoon. Groundwater abstraction for drinking water purposes in rural areas poses little threat for cross-contamination. Installing irrigation- or high capacity drinking water supply wells at depths is however strongly discouraged and assessing sustainability of targeted low-As aquifers remain a main concern. The knowledge gained from this study, could be used for developing guidelines for installing safe wells in other affected regions in South Asia.