In many of the large Quaternary delta systems in Southeast Asia, groundwater in the shallow Holocene aquifer contains elevated concentrations of arsenic (As). In anoxic sediments, reduction of As bearing Fe-oxides by organic matter oxidization, releases As to the groundwater. Therefore, from 1990s, most of water supply to Hanoi, in Red River delta plain, has been abstracted in the deeper Pleistocene aquifer. There is hydraulic correlation between Holocene and Pleistocene aquifers, especially in the area that close to river, the pumping has generated a leakage of water from the river into the adjacent Holocene aquifer and further into the Pleistocene aquifer. The Nam Du well field is the one of the largest water plants in Hanoi, with a daily pumping of 60.000 m3. We applied a 3D groundwater flow modeling, coupled with advection, dispersion and reaction solute transport modeling, to simulate the As mobilization in this study area. The results show that sorption isotherm in represent of retardation factor is the one of importance mechanisms controlling As mobilization from Holocene to Pleistocene aquifer. The results also show that retardation factor of Pleistocene aquifer ranges from 2 to 20 and much lower than results of laboratory sorption experiment in sediments which were taken from this area as the state of sorption - desorption at the site may not be reached equilibrium as in the laboratory. In flooding season river water level is raised up enough for gravitational flow and it can be a source of recharge to groundwater. Therefore, a line of injection wells close to river bank which may be constructed as artificial recharge will reduce the As concentration at abstraction wells in coming years.
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