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2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 6
Presentation Time: 9:00 AM-6:00 PM

GROUNDWATER FLOW MODELLING FOR ASSESSMENT OF TARGETING SHALLOW AND LOW ARSENIC AQUIFERS IN MATLAB, SOUTH-EASTERN BANGLADESH, AS A MITIGATION OPTION IN REGIONS WITH HIGH ARSENIC GROUNDWATER


VON BRÖMSSEN, Mattias, Ramböll Sweden AB, Box 4205, Stockholm, SE-102 65, Sweden, MARKUSSEN, Lars Møller, Rambøll Danmark A/S, Bredevej 2, Virum, DK-2380, Denmark, BHATTACHARYA, Prosun, KTH-International Groundwater Arsenic Research Group, Department of Land and Water Resources Engineering, Royal Institute of Technology (KTH), Teknikringen 76, Stockholm, SE-10044, Sweden, AHMED, K.M., Department of Geology, University of Dhaka, Ramna, Dhaka, 1000, Bangladesh, HASAN, M. Aziz, Department of Geology, University of Dhaka, Dhaka-1000, Bangladesh, Dhaka, 1000, Bangladesh, RAHMAN, M. Zillur, Department of Geology, University of Dhaka, Bangladesh, Dhaka, 1000, Bangladesh, ISLAM, M. Mainul, NGO Forum for Drinking Water Supply and Sanitation, 4/6, Block-E, Lalmatia, Dhaka, 1207, HOSSAIN, Mohammed, Department of Geology, University of Dhaka, Dhaka, 1000, Bangladesh and JACKS, Gunnar, KTH-International Groundwater Arsenic Research Group, Dept of Sustainable Development, Environmental Science and Engineering, KTH Royal Institute of Technology, Teknikringen 76, Stockholm, SE-10044, Sweden, mattias.bromssen@ramboll.se

Elevated levels of naturally occurring arsenic (As) in shallow Holocene aquifers in Bangladesh are a major concern for supplying the rural people with safe drinking water as they are using this resource for drinking water purposes. The objective of this study was to evaluate the risk for cross-contamination between reducing greyish aquifers containing high levels of dissolved As and oxidised off-white to brownish-/reddish aquifers with low As that is targeted by local drillers. This is one of the most important criteria for delineating the sustainability of this mitigation strategy. The three-dimensional finite-difference groundwater model MODFLOW was used to determine the hydraulic properties of the aquifer and to delineate the hydraulic processes that affect the sustainability of the low As aquifers; especially the vertical and horizontal hydraulic conductivity. Only saturated flows have been simulated. The model was calibrated to match measured drawdown in piezometers during the tests. The parameter estimation module, PEST, was used to calibrate the model. The results were compared with previous groundwater modelling on basin scale and analytical calculations for evaluation of the hydraulic tests. The vertical hydraulic conductivity (Kv) of the shallow aquifer was approximately 10-6 m/s and the horizontal hydraulic conductivity (Kh) 10-4 m/s. The ratio Kh/Kv of 100 is less than previously reported (100 – 10 000) but may differ from place to place. For further studies of the cross-contamination risk between high and low As groundwater in Bangladesh the conservative value should be used. The work has resulted in a better conceptual understanding of the aquifer system and the risk for cross-contamination. The risk for cross-contamination needs to be further studied but the conceptual understanding of the system corresponds to the low As concentrations (reported elsewhere) found in the brownish to reddish shallow and low As aquifers found in Matlab.

Session No. 174--Booth# 152

T23. Geochemistry of Arsenic and Other Toxic Elements and Assessment of Environmental Risks in Global Groundwater Systems (Posters)
Tuesday, 20 October 2009: 9:00 AM-6:00 PM
Hall A (Oregon Convention Center)
Geological Society of America Abstracts with Programs. Vol. 41, No. 7, p.462
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