Integrated groundwater modeling study in Addis Ababa area: Towards developing decision support system for well head protection Tenalem Ayenew And Molla Demelie Department of Earth Sciences, Addis Ababa University

Outline of presentation Introduction Objective and Methodology Results/Discussion Conclusions and Recommendation

Justification Problem of Groundwater level decline & pollution as the Metropolis grows in size and population.

Study area

Akaki Basin

Objective To understand the groundwater flow system and its relation with surface water To indentify vulnerable areas to pollution Scenario analysis

Methodolgy Conventional Hydrogeological Mapping Hydro-meteorological data analysis Hydrochemistry Establishing interactive database in a GIS Numerical Groundwater Flow Modelling The groundwater model used for the study is the popular three-dimensional finite difference groundwater flow model called MODFLOW (MacDonald and Harbaugh, 1988).-

Geology

Aquifer Parameters

Recharge Zones

Aquifer Classification

Boundary conditions and calibration

Groundwater contours

Scenario Analysis

Flow system

Hydro Chemistry

Scatter plots

Pollution susceptible zones

Groundwater pollution vulnerability map

Akaki Well Field

DEM Akaki Wellfield

Geological map

Evolution of the water table May 1999- Jan 2002

Comparison of logs - Reclassified log of BH05A

Comparison of logs – original geologic log of BH05a

Distribution of Akaki boreholes EP04 BH06 BH20 BH11 EP7 BH05a? EP8 BH19 MW01b BH23 TW1 BH16 P1 BH12 EP6 BH09 BH18 BH14 BH10 BH1 BH21 BH17 BH13 BH08 MW04 BH26 BH05 BH25 BH22 BH04 BH02 BH07 BH01 MW01 BH03a MW03 Deep well = TW1 Abandoned well = BH05a & BH03h

N E W S SW SE NW NE EP04 BH06 BH20 BH11 EP7 BH05a BH03b EP8 BH19 MW01b TW1 BH16 P1 BH12 EP6 BH09 BH18 BH14 BH10 BH1 BH21 BH17 BH13 BH08 MW04 BH26 BH05 BH25 BH22 BH04 BH02 BH07 BH01 MW01 BH03a MW03 N E W S NE SW SE NW

West- East (2.2 Kms) BH No 16 BH No 07 BH No 12 BH No 20 BH No 23

North-South (2.2 Kms) BH No 16 BH No 17 BH No 18 BH No 04 BH No 20

Northwest-Southeast (1.8 Kms) BH No 12 BH No 08 BH No 16 BH No 19

Northeast-Southwest (3.4 Kms) BH No 05a(dry) BH No 12 BH No 09 BH No 17 BH No 26 BH No 22

Fence Diagram 1

Fence Diagram 2

TW1, 2003 MW04, 2022 EP6, 2018 EP4, 2026 BH13,2024 N

Conclusions Model simulations made under different pumping scenarios indicate that an increase in pumping rate results in substantial regional groundwater level decline, which will lead to the drying of springs, and shallow hand dug wells. This causes reversal of flow from contaminated rivers in to productive shallow aquifers close to highly polluted rivers draining through the city of Addis Ababa. Excessive pumping and luck of considerations on issues of wellhead (recharge area) protection will likely lead to large-scale groundwater pollution. The vulnerable areas are close to major aquifers. The study clearly indicates the importance of numerical groundwater flow models in identifying the most susceptible areas, groundwater and surface water interactions. This work is believed to have far-reaching impactions for many urbanized areas established on fractured aquifers.

Alemaya Catchment

DEM and Drainage

Groundwater level decline (mismangement!?)

Borehole locations

Groundwater flow

Cross-section showing pumping wells from the lake bed

Groundwater level monitoring

One year groundwater level trend

Dried chat plant as a result of water level decline