The study focuses on defining the flow of groundwater through the Dire Dawa upperrnsandstone and limestone aquifer system and migration of high concentration nitratern(NO3-) plume to the Sabian well field which is the main source of water for the townrnwater supply. Groundwater extracted from the aquifer under research concern is thernonly available source of water in the area. Within the study, the hydrogeology hasrnbeen investigated, conceptual groundwater flow model has been developed andrnnumerical groundwater flow and solute transport model produced from this.rnExamination of boreholes lithology logs confirms that the upper sandstone andrnlimestone aquifer beds have complex orientation in space with varying thicknessrnfrom place to place, especially the upper sandstone. The major cause of this is thernrift forming normal fault displacement and volcanic plugs through it. In somernboreholes thickness of sandstone penetrated is as small as 10m while in othersrnthickness of 100 to 150m is penetrated before reaching the lower limestone.rnThe groundwater system in Dire Dawa town and surrounding area has beenrncategorized in to two, (WWDSE-BECEOM, 2004). i.e the Escarpment and the Riftrnfloor. The rift forming fault system divided the area in to two topographic pheatures,rnthe up thrown (Escarpment) and down thrown (Rift floor) plain. Dire Dawa town isrnlocated in the rift floor. In the escarpment outcrops, basement rocks of graniticrncomposition, limestone and sandstone. Limestone and sandstone on the escarpmentrnare fractured and the limestone especially karestified forming good potential area forrngroundwater recharge from direct percolation of the rainfall.rnIn the rift floor, the upper sandstone and limestone (major aquifer) are overlaid byrntertiary volcanic (basalt and andesite) and quaternary alluvial deposits. In Dire Dwarntown, the major aquifer, upper sandstone and limestone is overlaid by course tornmedium grained alluvial sediment dominantly of sand along Dechatu and Legeharernseasonal rivers. These sediments originate from the escarpment where, outcropsrnbasement rock of granitic composition.rnTwo hydrostratigraphic units identified: the alluvial unconfined aquifer alongrnLegehare and Dechatu seasonal rivers, and the upper sandstone-limestone complexrnaquifer (major aquifer). The upper sandstone and limestone even though, haverndifferent composition form one complex aquifer system due to hydraulic connectionrnthrough their fractures.rnPoor design of wells during their construction and improperly abandoned wells letrngroundwater contaminants enter to the underlying confined major aquifer. This isrneither through forming hydraulic connections between the upper unconfined alluvialrnaquifer which is vulnerable to pollution and the lower confined major aquifer, orrnbeing direct source of contaminants to the major aquifer in the case of improperlyrnabandoned wells.rnAddis Ababa University, 2013 Zelalem Desta Page viiirnA single layer groundwater flow and solute transport model using MODFLOW, MT3Drnand PM path was developed for the upper sandstone and limestone (major aquifer).rnThe model was calibrated using automatic model calibration technique under steadyrnstate condition. Simulated and observed head has been compared having goodrnagreement with correlation coefficient of 0.91. This showed the aquifer parametersrnwere estimated honoring the measured (observed) head within the given boundaryrnconditions.rnPredictive simulations with various annual abstraction scenarios prevails that a totalrnof 1773 l/s of groundwater enters the rift floor aquifer of upper sandstone andrnlimestone as a subsurface, mountain front groundwater recharge assigned a fluxrn(general head) boundary at the southern edge of the rift.rnConcentration of nitrate (NO3-) within boreholes in Sabian well field was simulatedrnfor the future 10 years. The concentration-time curve showed currently water fromrnmany boreholes do not meet WHO standard limit for their NO3- content. Andrngroundwater from some of other boreholes will fail to meet the upper limit of NO3-rncontent (45 mg/l) set by WHO within few years after now unless proper remedy berntaken there off.