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Flowpaths and Travel Time of Groundwater from Arsenic Contaminated Zone to Deeper Aquifer Units Under Different Development Stresses

Anwar Zahid*
Ground Water Hydrology, Bangladesh Water Development Board, Dhaka-1205, Bangladesh,

Muhammad Qumrul Hassan
Department of Geology, University of Dhaka, Dhaka-1000, Bangladesh,

Abstract/ Executive Summary:

WHO ranked arsenic contamination in shallow groundwater of Bangladesh as “the largest poisoning of a population in history”. Increasing pore water velocity in shallow aquifer due to huge irrigation abstraction can increase arsenic mobility. Still about 62% of cultivable land in south-east Bangladesh has the potential for irrigation. Increased irrigation abstraction in future may enhance arsenic contamination in deeper aquifers. A three-dimensional modular finite difference model has been used to simulate groundwater flow of multi-layered aquifer system in southeastern Bangladesh focusing flow paths and travel time of groundwater mainly from arsenic contaminated zone to deeper aquifers under current, projected and alternate development stresses. The results may provide guideline of suitable depth zone in aquifers for future safe drinking and irrigation withdrawal of groundwater considering arsenic problem.

Keywords: Groundwater, Arsenic contamination, Flowpaths, Travel time, Anisotropy

Location:

Start Date:

End Date:

Program Area: Risk Management in Agriculture

Commodity: Arsenic

Objectives:

Increased irrigation abstraction of groundwater in future may enhance arsenic contamination in deeper aquifers too. Thus, overall situation needs for an urgent groundwater management for sustainable development. A three-dimensional modular finite difference model was used to simulate groundwater flow of multi-layered aquifer system in southeastern Bangladesh focusing flow paths and travel time mainly from arsenic contaminated zone to deeper aquifers under different development stresses and variations in values of geologic conditions. This would provide guideline of suitable depth zone for future safe drinking and irrigation withdrawal of groundwater.

Methodology:

Model description: Darcy’s law is the base for flow dynamics that allows to assess groundwater flow by means of graphical, analytical and numerical models and shows the relationship between flux, pressure gradient and the hydraulic conductivity of the porous material and the water.

However, Darcy’s law alone is not enough to describe groundwater flow, unless the head distribution within the entire flow system under review is measured. The general flow equation for saturated groundwater flow is derived in numerous text books (e.g. Bear 1972; Spitz and Moreno 1996). The general flow equation is formulated by applying the law of conservation of mass over a control volume of an aquifer situated in the flow field.

Boundary conditions: Highlighting Kachua sub-district for model simulations the entire model area (Figure 1a) of southeastern Bangladesh is characterized by heavy withdrawal of groundwater for agricultural and domestic water supplies. However, groundwater level in the monsoon is very close to the land surface in the model area and actual recharge is considerably less than potential recharge (BWDB-UNDP 1982) with low topographic relief. Considering this situation, a steady-state hydraulic head boundary condition is used at the top of the model where surface water bodies can interact freely with the aquifer. The assumption is that, hydraulic head cannot be reduced at the model surface due to pumping providing an unlimited source of water (Michael and Voss 2008). The vertical boundary extends to the depth of shale i.e. basement of about 3000m below land surface.

The eastern boundary is fixed as the Tripura hills formed by massive sandstone and hard shale and is considered as no-flow boundary. The low permeable strata of Tertiary formation of Tripura hills reach to the surface along the folds and generally do not allow horizontal flow across the model boundary (Michael and Voss, in press). The northern and western side is bounded by the rivers Titas and Meghna respectively. The groundwater flow in the areas of western boundary of the model is southward along the river Meghna and are closed to east-west movement irrespective of development stresses. Hence, northern and western boundary conditions are taken as closed and constant head boundaries. The Meghna river has a very large water supply and are suitably modeled as constant head boundary (Anderson and Woessner 1992). Southern part of the model bordered by the Bay of Bengal is also fixed as constant head boundary.

Source of Information: 2nd International Conference on Water & Flood Management (ICWFM-2009), BUET, Dhaka, Bangladesh

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Aquifer system in the model area is generally stable under current development stress. Anisotropy is the primary hydrogeologic control on the flowpath length and travel time of groundwater. Groundwater head in the shallow (1^st) and main (2^nd) aquifers varies in places due to local-scale lithologic variations, leakage, the impact of irrigation abstraction and partially by unconfined storage change in the aquifer. The water from the shallow arsenic contaminated aquifer generally has a higher head than water in the main aquifer, indicating that water from the shallow aquifer may move downward. In addition, huge abstraction of irrigation water by the shallow wells has direct influence to move water from arsenic contaminated layer above to irrigation zone. Excessive irrigation abstraction in future from the shallow aquifer may decrease vertical percolation of recharge water to deeper aquifers. Under current development stress (anisotropic condition) water in the lower part of shallow aquifer may start to contaminate after 20 years. For main and deep (3^rd) aquifers minimum travel times are 125-1250 and 1200 years i.e. these aquifers remain safe at least for these time periods. If the aquitards are discontinuous, time will be reduced. Where aquitards are weakly anisotropic, all aquifer units would be start to contaminate by 150 years, that is less than 10 and 25 years for lower part of shallow and main aquifers. Maintaining current trend of irrigation abstraction, if only domestic wells are shifted to main aquifer from shallow aquifer, main aquifer will remain safe for longer period. Where weakly anisotropic condition exists in the alluvial aquifers, groundwater from the arsenic contaminated layer will take very little time to reach in different aquifers.

Knowledge Management: Report/Proceedings