Numerical Groundwater Flow and Solute Transport Modeling of Dire Dawa Groundwater Basin

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2013-06-06

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Addis Ababa University

Abstract

The study focuses on defining the flow of groundwater through the Dire Dawa upper sandstone and limestone aquifer system and migration of high concentration nitrate (NO3-) plume to the Sabian well field which is the main source of water for the town water supply. Groundwater extracted from the aquifer under research concern is the only available source of water in the area. Within the study, the hydrogeology has been investigated, conceptual groundwater flow model has been developed and numerical groundwater flow and solute transport model produced from this. Examination of boreholes lithology logs confirms that the upper sandstone and limestone aquifer beds have complex orientation in space with varying thickness from place to place, especially the upper sandstone. The major cause of this is the rift forming normal fault displacement and volcanic plugs through it. In some boreholes thickness of sandstone penetrated is as small as 10m while in others thickness of 100 to 150m is penetrated before reaching the lower limestone. The groundwater system in Dire Dawa town and surrounding area has been categorized in to two, (WWDSE-BECEOM, 2004). i.e the Escarpment and the Rift floor. The rift forming fault system divided the area in to two topographic pheatures, the up thrown (Escarpment) and down thrown (Rift floor) plain. Dire Dawa town is located in the rift floor. In the escarpment outcrops, basement rocks of granitic composition, limestone and sandstone. Limestone and sandstone on the escarpment are fractured and the limestone especially karestified forming good potential area for groundwater recharge from direct percolation of the rainfall. In the rift floor, the upper sandstone and limestone (major aquifer) are overlaid by tertiary volcanic (basalt and andesite) and quaternary alluvial deposits. In Dire Dwa town, the major aquifer, upper sandstone and limestone is overlaid by course to medium grained alluvial sediment dominantly of sand along Dechatu and Legehare seasonal rivers. These sediments originate from the escarpment where, outcrops basement rock of granitic composition. Two hydrostratigraphic units identified: the alluvial unconfined aquifer along Legehare and Dechatu seasonal rivers, and the upper sandstone-limestone complex aquifer (major aquifer). The upper sandstone and limestone even though, have different composition form one complex aquifer system due to hydraulic connection through their fractures. Poor design of wells during their construction and improperly abandoned wells let groundwater contaminants enter to the underlying confined major aquifer. This is either through forming hydraulic connections between the upper unconfined alluvial aquifer which is vulnerable to pollution and the lower confined major aquifer, or being direct source of contaminants to the major aquifer in the case of improperly abandoned wells. Addis Ababa University, 2013 Zelalem Desta Page viii A single layer groundwater flow and solute transport model using MODFLOW, MT3D and PM path was developed for the upper sandstone and limestone (major aquifer). The model was calibrated using automatic model calibration technique under steady state condition. Simulated and observed head has been compared having good agreement with correlation coefficient of 0.91. This showed the aquifer parameters were estimated honoring the measured (observed) head within the given boundary conditions. Predictive simulations with various annual abstraction scenarios prevails that a total of 1773 l/s of groundwater enters the rift floor aquifer of upper sandstone and limestone as a subsurface, mountain front groundwater recharge assigned a flux (general head) boundary at the southern edge of the rift. Concentration of nitrate (NO3-) within boreholes in Sabian well field was simulated for the future 10 years. The concentration-time curve showed currently water from many boreholes do not meet WHO standard limit for their NO3- content. And groundwater from some of other boreholes will fail to meet the upper limit of NO3- content (45 mg/l) set by WHO within few years after now unless proper remedy be taken there off.

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Keywords

Numerical, GroundWater Flow, Solute Transport, Modeling, Dire Dawa, Ground Water Basin

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