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Steady-state groundwater flow and contaminant transport modelling of Akaki wellfield and its surrounding catchment (Addis Ababa, Ethiopia)

Tesfaye, Alema (2009) Steady-state groundwater flow and contaminant transport modelling of Akaki wellfield and its surrounding catchment (Addis Ababa, Ethiopia).

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Abstract:The study focuses on groundwater modelling of the Akaki catchment and the Akaki wellfield in particular under steady state flow conditions. Besides that, advective contaminant transport modelling was implemented in Akaki wellfield to delineate capture zone. The aquifer system was modelled using PMWIN (Chiang and Kinzelbach, 1998) as pre and post processor for MODFLOW (McDonald and Harbaugh, 1988). The aquifer was simulated under confined/unconfined condition and is represented by a single layer of 100m constant thickness. The grid cells of the model were taken 400x400m for Akaki catchment with the area of 1500 km2. A grid size of 400x200m was used to represent the wellfield which is approximated 197.3 km2 in extent. Model input parameters were determined based on analysis of pumping test data. The recharge condition in Akaki catchment is characterized by three sources of water: rain recharge, the river bed infiltration and the seepage from Aba Samuel and Legedadi lakes. In addition, the recharge conditions in Akaki wellfield include a fourth component which is inflow from the northern and north-western boundary. Outflow (discharge) conditions from the aquifer are characterized as base flow, springs, subsurface lateral flow and well abstractions. Boundary conditions were assigned to the model domain to simulate inflow and outflow terms of the model domain. The Chloride Mass Balance Method (CMB), semi-distributed water balance model and recursive digital filter methods were employed to estimate the recharge in the catchment. From the water balance as simulated by the catchment’s model, the total average base flow estimated is 92 MCM/yr or 62 mm/yr. From the water balance model described in chapter 3, it is found that a base flow and recharge value of 130 MCM/yr or 87 mm/yr is found. Using the digital filter method, the base flow and recharge values are roughly 65 MCM/yr (63mm/yr) indicating good agreement between the models. The trial and error method was used to calibrate the models using the observed and simulated hydraulic heads. The water budget of the area reached equilibrium conditions with recharge from precipitation 158 MCM/yr, well abstraction some 23 MCM/yr, 8 MCM/yr as out flow from the catchment at the catchment outlet, 130 MCM/yr seepage from aquifer to the surface water bodies and 11 MCM/yr from surface water bodies to the aquifer. Optimised parameters (hydraulic conductivity and recharge) are spatially distributed over the model area. Furthermore, PMPATH, has been used at the wellfield eventually to calculate path lines and travel times of contamination. This approach involved the introduction of particles at contaminant sources upstream of wells and at the wellfield itself, then identifying the path lines, and finally determining the spatial distribution of the contaminants through steady state flow field. The flow lines converge towards Akaki wellfield from all directions, implying that any contaminated water from the upper part of the aquifer will end in the wells, indicating a high risk of vulnerability of the wellfield to pollution. The following recommendations are helpful in curing the risks posed. Manufacturing activities having pollution potential must be limited in special areas sufficiently far from water supply wells; the chemical quality of groundwater must be monitored and an environmental policy must be implemented with particular emphasis on the protection zones around the wellfield. Key Words: Addis Ababa - Akaki - Modelling - Modflow - Volcanic aquifer - Groundwater - PMPATH
Item Type:Essay (Master)
Faculty:ITC: Faculty of Geo-information Science and Earth Observation
Programme:Geoinformation Science and Earth Observation MSc (75014)
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