EVALUATION AND DEVELOPMENT OF GROUND WATER RESOURCES OF ADA'A

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EVALUATION AND DEVELOPMENT OF GROUND WATER
RESOURCES OF ADA'ABECHO PLAINS FOR IRRIGATION
PROJECT

• Engida ZA, Tilahun A, Dr. Seifu K and Dr.
  Yilma

Client Ministry of Water Resources -
Consultant Water Works Design Enterprise (WWDSE)
WATER WORKS DESIGN AND SUPERVISION ENTERPRISE
P.O.BOX 2561 Tel [phone] Fax [phone]
email w.w.d.s.e_at_ethionet.et
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TABLE OF CONTENT

• Location
• Objective and scope of the study
• Methodology
• Geology and structures, Hydrology, Geophysics,
  Drilling of mapping wells
• Isotope and water quality, Water quality
  monitoring, Hydrogeology
• Main study output of the project
• Developing conceptual models (Inception phase)
• Preparing hydrogeological map
• Preparing groundwater flow system of AB Plains

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TABLE OF CONTENT

• Evaluation of groundwater potential of Ada'a and
  Becho plains
• Interaction of Debre Zeit Lakes with the
  groundwater system
• Interaction of Awash river with the groundwater
  system

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1. LOCATION OF THE PROJECT AREA

• Central Ethiopia, in Oromia Regional State in
  Upper Awash river Basin
• Adaa plain is found between elevation 1600 and
  1900 masl and approximately between Co-ordinates
  38050E - 39015E and 8030N - 8053N (T
  area-1641 km2 ,223km2M Net area of the plain1418
  km2
• Becho plain is found between elevation 2040 and
  2120 masl and approximately between coordinates
  38008E - 38036E and 8038N - 9000N (T area
  1414km2 seasonally inundated area117km2 net area
  1297 km2)

Fig1
S6
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Fig1 LOCATION MAP OF THE PROJECT AREA
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2. OBJECTIVE AND SCOPE OF THE STUDY

• Previous studies have indicated that there is a
  possible high groundwater potential, currently
  agriculture consists mainly of rain fed annual
  cropping. Crop yields are generally good in the
  project area, the high population growth has made
  the available land decreasing from year to year.
• Unless remedial measures are taken, the standard
  of living of the farmers will further decline,
  especially in view of the current rapid
  population growth rate of about 2.9 per year.
  Since the rural population is expected to double
  in about 25 years, it is increasingly important
  to deal with these issues as soon as possible by
  implementing irrigation in the area.

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2. OBJECTIVE AND SCOPE

• Determine the hydrogeological condition of the
  Ada'a-Becho plains groundwater system
• Qualitative and quantitative evaluation of the
  groundwater resources of the plain and select
  potential well fields
• Carry out analytical and/or numerical model of
  the main aquifer of the area to recommended
  appropriate groundwater management
• Develop strategy of groundwater resources
  development together with cost estimates

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3. METHODOLOGY

• Data collection review of previous works.
• Geological geomorphologic mapping
• Hydrological survey and analysis
• Hydrogeological investigation (water point
  inventory, water quality survey, water quality
  monitoring, mapping well drilling testing)
• Geophysical investigation (Resistivity and TEM)
• Isotope hydrogeology
• Detail qualitative quantitative groundwater
  potential evaluation
• Prioritization of potential prospective area for
  Pilot well field development
• Analytical Modelling

SN
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3. MAIN OUTPUTS OF THE PROJECT

• A. Developing conceptual models (In. phase) The
  hydrogeological and isotope data from Akaki
  catchment shows the following conditions
• Most of the deep wells drilled in Addis Ababa and
  adjacent areas are highly confined. Confining
  acidic volcanic and the aquifer basic rocks
  (basalt of different comp. and ages)
• The upper part of Akaki catchment is steep and
  composed of dominantly acidic volcanic rocks
  (which are practically impervious) it is
  difficult to assume that a recharge that occurred
  at the upper part of the catchment has formed a
  confined aquifer in the basalts at the central,
  eastern and western AA. The upper part of Akaki
  river catchment is merely runoff generator.

Fig 2
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3. MAIN OUTPUTS OF THE PROJECT

• A. Developing conceptual models (In. phase) The
  hydrogeological and isotope data from Akaki
  catchment shows the following conditions
• The tritium value from different sources Yves M.
  Travi, Kebede S. (2000) shows Except few wells
  (probably shallow wells) the other wells both
  cold and thermal wells are depleted in tritium
  value in general
• Reconnaissance surveys at Muger and Abbay rivers
  gorges showed that the Mesozoic sedimentary
  formation is most likely dipping in SSE
  direction, which highly favors the Abay plateau
  groundwater flow to SSE direction into Awash
  River basin
• These observations led to conceptualize that the
  groundwater of Addis Ababa and in general Becho
  and Ada'a plains groundwater is recharged mainly
  from outside their surface catchments. The basalt
  plateau of Abay river basin that is found
  adjacent to Akaki and Awash rivers catchments
  could be the main recharge zone of Ada'a plain
  (as shown the figs 3and 4 below)

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Fig 2 CONCEPTUAL MODEL DEV.
FIG 3
FIG 4
Main output
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FIG 3 CONCEPTUAL X-SECTION AP-AA-ADAA
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FIG 4 CONCEPTUAL X-SECTION AP-BECHO-ADAA
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3. MAIN OUTPUTS OF THE PROJECT

• B. Preparation of Hydrogeological map and
  cross-section Phase I Inv. and analysis
• Hydrogeological conditions of the area is a
  function of geomorphology and tectonics, on the
  Plateau older Tertiary volcanic outcrops to the
  south the younger geological formation over lies
  the older volcanic unit
• The recharge condition, groundwater flow and
  aquifer parameters in the Adaa-Becho plains is
  highly governed by the general bedding of the
  sedimentary formation underlying the volcanic
  unit, the tectonic condition and the hydraulic
  properties of the different volcanic units that
  outcrops in the basin
• There are two basaltic aquifers in the Becho
  plain and along west-east direction from
  Becho-Legedadi section which are separated by
  ignimbrites and welded tuffs

HG-Map
BECHO
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3. MAIN OUTPUTS OF THE PROJECT

• B. Preparation of Hydrogeological map and
  cross-section Phase I Inv. and analysis
• Upper Basalt Aquifer
• composed of Quaternary of Weliso-Ambo basalts,
  Akaki basalts scoria and spatter cones, and
  Tertiary-Neogenes basalt of Addis Ababa basalts
  and Tulu ries basalts.
• one aquifer system in Adaa plain (south of
  Dukem)
• distributed in Upper Awash, where it outcrops at
  Central Addis Ababa, Weliso and Akaki and it also
  overlain by ignimbrites and tuffs at Becho
  plaains, Legedadi area, in general where the
  ignimbrites and tuff outcrops
• Varies from more than 200 to 50 m thickness TDS
  400-700 mg/l
• by water balance method and chloride balance
  method the recharge of this aquifer is estimated
  to be 390 Mm3/year

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3. MAIN OUTPUTS OF THE PROJECT

• B. Preparation of Hydrogeological map and
  cross-section Phase I Inv. and analysis
• Lower Basalt Aquifer
• composed of tertiary Tarmaber basalt composed of
  dominantly scoraceous basalt and Amba Aiba
  basalt. These tertiary basalt formations occupy
  more than 75 of Abay plateau about 7,000 km2. .
• mapping wells drilling at Becho Plain, Holota,
  Melka Kunture, Legedadi areas penetrated at
  different depth this aquifer. The total confining
  head more than 220 meters
• The yield of the well was progressively
  increasing when the depth of penetration is
  increased in this aquifer. Water level varies
  from artesian condition to a depth of 67.5 meters
• The total annually recharge of the lower aquifer
  is estimated by different method is about
  700Mm3/year.

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Fig 5 Hydrogeological Map of ADAA-BECHO PLAINS
Fig 10
Fig 7
Fig 9
Fig 8
Fig 6
Fig11
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Fig 6 HYDROGEOLOGICAL X-SECTION ALONG BECHO
PLAIN
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Fig 7 Hydrogeological x-section along Kachise
Becho-Adaa line
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Fig 8 Muger-Holota-Becho
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Fig 9 SEGNO GEBYA-LEGEDADI
Well log
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Fig 10 Hydrogeological x-section along Plateau
-Legedadi - Akaki section line
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Fig 11 Hydrogeological x-section in Adaa Plain
(Southwest Northeast direction)
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3. MAIN OUTPUTS OF THE PROJECT

• B. Preparation of Groundwater flow system of AB
  Plains Phase I Inv. and analysis
• Groundwater level elevation contour map was
  constructed for the regional basalt aquifer
  (lower aquifer) applying the data generated from
  drilling of mapping wells and existing bore holes
  identified to be representing the regional
  aquifer water level
• On the groundwater elevation contour map the
  following understanding is obtained
• Entoto and Wechecha furi and Bedegebaba ridges
  acts as local barrier
• The recharge at Abay plateau flows to Upper
  Awash through two main direction i.e. through
  Becho plain and along Legedadi areas
• Almost 100 of Abay plateau is the recharge
  area of the lower basalt aquifer in Upper Awash
  basin
• Bore holes drilled along flow paths have yield
  and artesian if drilled at lower places

Fig12
Fig 12
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Fig 12 SIMPLIFIED GROUNDWATER FLOW PATHS IN AB
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Fig 13 Drilling at Holeta area
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Fig 14 DRILLING AT ASGORI
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Fig 15 Melka Kunture Pumping test Q36 l/s
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Fig 16 Artesian flow well drilled south of
legedadi
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3. MAIN OUTPUTS OF THE PROJECT

• C. Evaluation of groundwater potential of Adaa
  plains (ECGPAP)
• AQUIFER CHARACTERISTICS
• aquifer composed of scoria, scoraceous Basalt and
  fractured basalt unconfined extensive with high
  transmissivity
• groundwater recharge to Adaa plain groundwater
  system is from three direction i.e.i) between
  Mountains Yerer and Guji, ii) between Bede Gebaba
  and Ziquala (Qgz) and from Mojo, Wedecha and
  Belebela catchment (Qmw).

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ADAA PLAIN
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Fig 11 Hydrogeological x-section in Adaa Plain
(Southwest Northeast direction)
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3. MAIN OUTPUTS OF THE PROJECT C. ECGPAP
ESTIMATED RECHARGE AP
Recharge Direction Weighted transmissivity (Tav), m2/day Width of flow (L), m Average flow gradient (I) Recharge estimated in MCM/year Remark
Yerer Guji Zone 1200 10,800 0.02 94.6
Bede Gebaba Ziqual 10,200 13,460 0.01 532.0
ModjoWedecha Belebela 153.0 Vol. II chap 7.3
Total Total 779.6 MCM/Year
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3. MAIN OUTPUTS OF THE PROJECT ESTIMATED
EXPLOITABLE GWR OF AP Analytical method

• Excluding the recharge from Becho plain due to
  the maximum exploitation of the groundwater
  resource in the plain the net available
  exploitable ground water potential is estimated
  about 1,046MCM/year or 2.9MCM/day or 33 500 l/s

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3. MAIN OUTPUTS OF THE PROJECT PRIORITIZING
GROUNDWATER PROSPECTING SITES IN AP
Prospective site Prioritizing Criteria Prioritizing Criteria Prioritizing Criteria Prioritizing Criteria Ranking
Prospective site Area, ha GWD range, m T,Range, m2/day EC,range, µS/cm Ranking
A 23,425 Dominantly from 60-120 5,000-10,000 in some part gt10,000 Dominantly 500-750 2
B 19,760 lt 30 5,000-10,000 Dominantly 500-750 1
C 5,540 60-120 5,000-10,000 500-750 3
D 3,316 30-90 200-1,000 500-750 4
Fig 17
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3. MAIN OUTPUTS OF THE PROJECT C. ECGPAP Fig
17 Prospective groundwater potential sites
Table
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3. MAIN OUTPUTS OF THE PROJECT C. ECGPAP
Parameters considered

• The groundwater system as extensive unconfined
  aquifer.
• Average transmissivity of the confined aquifer
  Tavg 5000m2/day
• The specific yield of the aquifer is estimated to
  be about 0.25.
• Thickness of Aquifer 300 m
• 100 wells 1km apart in grid

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C. ECGPAP Fig 18 Proposed PW in prioritized sites
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C. ECGPAP Calculated Max DD in the WF for
different scenario of Abstraction rate and
duration of pumping
Scenario Well discharge Well discharge No. wells Total discharge Total discharge Calculated maximum drawdown at the center of the well field for different period of exploitation Calculated maximum drawdown at the center of the well field for different period of exploitation Calculated maximum drawdown at the center of the well field for different period of exploitation Calculated maximum drawdown at the center of the well field for different period of exploitation Calculated maximum drawdown at the center of the well field for different period of exploitation Calculated maximum drawdown at the center of the well field for different period of exploitation
Scenario l/s m3/ day No. wells l/s m3/day 500 days 1000 days 2000 days 5000 days 8000 days 10000 days
  l/s m3/ day No. wells l/s m3/day 1.4 2.7 5.5 13.7 21.9 27.4
1 20 1728 120 2400 207360 2.3 4.6 6.9 9.9 11.4 12.2
2 40 3456 120 4800 414720 4.6 9.1 13.7 19.8 22.9 24.4
3 50 4320 120 6000 518400 5.7 11.4 17.2 24.7 28.6 30.4
4 70 6048 120 8400 725760 8 16 24 34.6 40 42.6
5 100 8640 120 12000 1036800 11.4 22.9 34.3 49.4 57.2 60.9
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C. ECGPAP Estimated EGWP of prioritized area

• The estimated exploitable groundwater resource
  (safe yield) is about 378MCM /Year or 1,036,800
  m3/day or 8, 640 l/s by 100 wells each
  discharging 100 l/s.

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C. ECGPAP IMPACT ON THE LAKES
Lake Name Distance from the well field, m Discharge of the wells in m3/day and drawdown after 5000 days Discharge of the wells in m3/day and drawdown after 5000 days Discharge of the wells in m3/day and drawdown after 5000 days Discharge of the wells in m3/day and drawdown after 5000 days Discharge of the wells in m3/day and drawdown after 5000 days
Lake Name Distance from the well field, m 207360 414720 518400 725760 1036800
LakeBishoftu Guda 3025 2.68 5.36 6.69 9.37 13.39
Lake Hora 2663 0.89 1.79 2.23 3.12 4.46
Lake Bishoftu 3650 0.89 1.79 2.23 3.12 4.46
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C. ECGPAP IMPACT ON THE LAKES

• The result of analysis shows that the maximum
  drawdown created on the lakes is less than 14
  meters on the nearest lake Bishoftu Guda. Here it
  is found possible to shift further to the east
  the well field by two kilometers and the draw
  down on Lake Bishoftu Guda decrease similar to
  the other lakes.
• Further Numerical modeling shall be conducted to
  verify this analytical calculation in the final
  report. The estimated exploitable resources is
  estimated about 378 MCM/year on site B.

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C. ECGPAP NEXT STAGE

• Drilling of 5 pilot wells each well discharge
  100 l/s
• Detail design of 500 ha Pressurized irrigation
• Ground water modelling
• Feasibility design of 10, 000 ha pressurized
  irrigation

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MAIN OUTPUTS OF THE PROJECT

• D. Interaction of Debre Zeit Lakes with the
  groundwater
• Hydrogeological method of analysis
• Water quality monitoring
• Monthly water balance of DZ lakes (hydrologic
  method

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MAIN OUTPUTS OF THE PROJECT

• D. Interaction of Debre Zeit Lakes with the
  groundwater
• Hydrogeological method of analysis
• Constructing a groundwater level elevation
  contour two groundwater flow regimes within
  Adaa plain caused by Debre Zeit lakes and
  Mountain Bede Gababa which they act as a
  groundwater divide and Barrier to the ground
  water system (fig 19)
• Water Quality monitoring (fig 20)
• Lake Water balance of DZ lakes (fig21)

F1
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Fig 19 Groundwater flow paths at DZ lakes area
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Fig 20 EC versus time monitored BH (WQM)
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Fig 21 Time series of groundwater Inflow-outflow
(1000 m3)
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MAIN OUTPUTS OF THE PROJECT

• D. Interaction of Debre Zeit Lakes with the
  groundwater
• All the methods (Hydrogeological, water quality
  monitoring and Water balance of DZ lakes showed
  method the lakes except Kurftu lake acts as
  groundwater divide (recharge boundary) and Bede
  gebaba ridge as a barrier.

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MAIN OUTPUTS OF THE PROJECT

• Interaction of Awash river with the groundwater
• Awash river monitored two stations (upstream at
  Melkakunture and down stream at Ombole river
  gauging stations) far apart more than 50kms. From
  July- September, the EC at the two stations are
  nearly similar (about 100µS/cm). During dry
  season the electrical conductivity value measured
  at the two stations shows larger gap (400µS/cm at
  Melka Kunture and 600-650µS/cm at Ombole
  respectively). The conductivity of the regional
  groundwater of the area between Melka Kunture and
  Ombole vary from 600 to 650 µS/cm. The higher EC
  value measured at Ombole site verifies the fact
  that the river is being recharged by regional
  groundwater and the base flow from November to
  June at this station is from the regional
  groundwater

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Fig 22 Interaction of Awash with the groundwater
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TEM Interpretation at Inchini
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Segno Gebeya
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Sululta
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Chancho
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THANK YOUFOR YOUR ATTENTION!!