Transcript Slide 1

AQUIFER VULNERABILITY ASSESSMENT IN AKAKI RIVER
CATCHMENT, ADDIS ABABA (FINFINNE): IMPLICATIONS FOR
LAND USE AND WATER QUALITY MANAGEMENT
by
Dereje Nigussa
Oromia Water Works Design and Supervision Enterprise
Tel.: +251-116630987(Office)
+251-91174 4842
E-mail: [email protected]
P.O.Box 870 code 1250 Addis Ababa (Finfinne), Ethiopia
An
Overview
the Study Area
Location
map
of the studyof
area
=1150mm
Figure1.1 Location map of Akaki River catchment
160c
F ig ure 1.1a . Ins e t M ap Sho w ing M ajo r R iv er B as in s o f E thio pia
MEREB G ASH
TEKEZE
480 000
490 000
39°00 '
500 000
DENAKIL
510 000
RED SEA
1020000
ABBAY
A
38°45 '
470 000
A
YI
SH
460 000
1020000
450 000
AW ASH
RI
FT
1010000
1010000
<
OGAD EN
GEN ALE DAW A
Figure 1.1b
LEGEN D
9°00'
9°00'
1000000
1000000
To Well ega
<
<
To Goja m
WABI SH EBELE
VA
LL
EY
BAR O AKOBO
To Well o
OM
OG
IB E
AKAKI
Air Po rt
i R iv
er
Add is Abe ba & S en daf a t ow n B ou nda ry
Ra ilw ay
Riv ers/ srea ms
A
L it tl
<
To Jima
ak
k aki
R i ve
r
k
A
990000
990000
All W ea the r R oad
ig
B
e
Wet la nd/ s wam p
Water b od y/ R es ervo ir
980000
980000
N
<
To Harar & S idam o
8°45'
970000
8°45'
970000
450 000
dulating landform
460 000
470 000
38°45 '
480 000
490 000
500 000
39°00 '
510 000
0
5
Km
LOCATION MAP OF AKAKI RIVER CATCHMENT
PROJECTION UTM ZON E 37
Geological Settings & Structures
46 00 0 0
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48 0 0 0 0
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Geological map of Akaki River Catchment
Figure 3.1
LE G EN D
QUA TE RNA RY SE DIM ENTS
SAL A LLUV IA L S OIL
LC LACUS TRINE CLAY S A ND SI LTS
1010000
1010000
GE NERALLY T HI CK 5 TO 50 M
YOUNG V OLCANI CS (P LIO CE NE - QUA TERNA RY)
RBS1 SCORIA
RBS2 BA SA LT P REDOM INNTLY
SCORIA CEOUS A ND V ES ICULAR
V1a FINE GRA INE D IG NI MB RITE
V1b TUFF
V2
TRACHYT E
V3
TRACHY B AS ALT
1000000
1000000
ADDIS A BA BA B A SA LT (UPP E R MI OCENE TO P LIOCE N)
A4a
A PHANNI TIC BA SA LT
A4b
P ORPHYRI TIC BA SA LT
A4c
P ORPHYRI TIC OLMNE B AS ALT
TARA MB ER B A SA LT (25 M A TO 5 M A )
T5 a
P ORPHYRI TIC BA SA LT
MIDDI LE MIO CE NE ACI D V OLCANICS
M6
COA RS E GRAI NED IGNI MB RI TE
ALA JI S ERI ES (LOWE R M IOCE NE)
Aj7 a RHYOLI TE
Aj7 b TRA CHY TE
Aj9 A PHANI TI C BA S ALT
990000
990000
Aj8 TUFF A ND AGG LOME RA TE
UNDI FFE RE NTI ATE D V OLCANI CS
PREDOM INA NTLY TRACHYT ES ,
EN TRACHYB AS ALT A ND RHYOLI TES
WI TH S UBORDI NATE I GNIM BRI TE
& TUFF S
EAA I GNI MB NITE S TUFF S AND
RHYOLI TE S WIT H S UB ORDINZT E
TRA CHY TE S & BA SA LTS
SWA I NCLUDES T RA CHYTE S,
RHYOLI TE S & B AS ALT
FA ULTS WI TH DOW N THROW
S IDE
980000
980000
LINE AM ENTS CE RTAI N
LINE AM ENTS P ROBA B LE
Ri ver s/ sreams
Reservoir s
wetl and/sw am p
N
970000
970000
5
46 00 0 0
47 0 0 0 0
48 0 0 0 0
49 00 0 0
5 0 00 0 0
5 10 0 0 0
0
5
Km
GEOLOGICAL MAP OF
AKAKI RIVER CATC HMENT
PR O J EC T I O N U T M Z O N E 37
Aquifer characteristics
 Multi-layer aquifers and with types (confined,
unconfined, semi-confined and perched).
 Inter-granular and fracture porosity type
aquifers
 Tectonized and fractured basalts and
ignimbrites have good permeabilities
Background:
 Need to protect groundwater resource base
 Rapid urban & industrial development without
proper waste management
 An indiscriminate municipal waste disposal
 The volcanic rocks are highly fractured
 Recent studies show both surface and groundwater

Background(…contd):
 The extent surface and groundwater pollution was
studied
 Water samples from streams/rivers, springs and
boreholes by Tamiru Alemayehu(2000) show:
 Heavy metal pollution
 Total coliform and pathogen pollution
 Nitrate pollution
 General consensus=>pollution of groundwater is
caused by infiltration of polluted water

=>The unconfined aquifer is vulnerable
Objective of Work:
1.
General:
Mapping of groundwater vulnerability to pollution,
and Preparation of vulnerability index maps.
2.
Specific:
Produce dynamic GIS spatial data base,


Provide map based information that assist
decision/policy makers and land use planners for
groundwater conservation and management, etc.
METHODOLOGY:
 To achieve the above objective, the empirical model known as
“DRASTIC” developed in USA by Aller et al. (1987) with GIS was
adopted.
 DRASTIC is the acronym derived from the initials of:
D= Depth to water,
R= Net Recharge,
A= Aquifer media,
S= Soil media,
T= Topographic slope,
I= Impact of vadose zone media, and
C= Hydraulic Conductivity.
Methodology(…contd):
 Data flow and mapping procedures
Static water level
Elevation
Base (DB)
Depth to water
Data
Aquifer Media
Geologic DB
Vadose
Media
Well log DB
Hydraulic
Conductivity
Hydrometeorological DB
Net Recharge
Land use DB
Soil DB
Topographic DB
Soil Media
Topographic
Slope
Zone
General
DRASTIC
Pesticide
DRASTIC
 Procedures of Data Base Construction & GIS Analysis
Data Base Design
Data Collection/Automation
Field Data
Data Entry
Data Verification
Vectorizing
Editing& Constructing Topology
Projection &Transformation
Inputting Attribute Values
Rasterization
GIS data analysis:
-Overlay, etc
Maps & Tabular
Data Outputs
Steps followed:
(1) Data collection
(2) Database construction
(3) Extraction of
hydrogeologic factors from
the database
(4) overlay analysis of
the factors
Methodology(…contd):
 DRASTIC Index (DI) was computed as an additive
overlay model by using the following equation:
DI =DrDw+ RrRw+ ArAw+ SrSw+ TrTw+ IrIw+ CrCw
7
DI = ∑ riwi ……………………… …..
(1.1)
i=1
Where: D, R, A, S, T, I and C are the seven parameters
stated before, and
r = rating value, w = weight and i = the seven DRASTIC
factors too.
Application of Results
 Provides the user with a measure of relative
groundwater aquifers vulnerability to pollution
 It is also used as/ for :
 a first phase of the actual site selection



/preliminary screening tool/;
used to identify areas where special attention, or
protection efforts are warranted;
used for selection of waste disposal sites as
preliminary screening tool, and land use planning
for groundwater protection as basic data.
Useful for sensitizing planners
Groundwater Vulnerability Mapping (GVM)
&
Assessment Concept
The concept of GV is based on the assumption that
the physical environment (soil-rock-groundwater
system) may provide some degree of protection to
groundwater system against human and natural
impacts =>‘self-purification’ or ‘natural attenuation’
 Vulnerability is defined as ‘‘an intrinsic property
of a groundwater system that depends on the
sensitivity of that system to human and /or natural
impacts’’ ( by IAH)
Approaches to Vulnerability Mapping and
Assessment
 Two basic approaches exist to vulnerability mapping:
general or intrinsic and specific or integrated
 Vulnerability map is commonly produced to evaluate
the upper most aquifer,
 In this study, general or intrinsic groundwater
vulnerability is adopted
Ranges, Ratings and Weights in DRASTIC Model
ranges for DRASTIC factor is assigned a
subjective rating, varying between 1 and 10.
 The
 “Variable” and “typical”
rating is assigned for
each ranges of “A” and “I” while the rest of the
DRASTIC factors are assigned one value per range.
 Two relative
weight strings (normal and
pesticide) varying from 1 to 5 are assigned to each
DRASTIC factor
 In this work the typical rating was applied
Inherent Assumptions and Limitations
 Important assumptions:
 The contaminant is introduced at the ground surface,
 The contaminant is flushed into the groundwater by
precipitation,
 No interaction b/n chemical pollutants and the
physical environment,
 The area evaluated should be 100 acres or larger, etc.
Limitations
 Ambiguity with regard to scale at which the model should be
used
 Insufficient representation of available data to analyze the
behavior of natural systems
 Little importance has given to the soil attenuation action
 Definition of variables and weights are subjective and based on
an incomplete knowledge of the physical processes involved
 Absence of a valid methodology in evaluation of net Recharge
MODELING & APPLICATION OF DRASTIC WITH GIS
ON THE AKAKI RIVER CATCHMENT
 Depth to water (D):
 Aquifer types
 a modified DRASTIC approach was applied
Depth to water map of the Akaki River Catchment
Net Recharge (R)
 High recharge => imply high vulnerability.
 The average recharge for the catchment ~
128.35mm per year
 Factors for spatial disparity of recharge to
groundwater

Recharge map of the Akaki River catchment
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46 00 0 0
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Fi gur e 8.2
1000000
1000000
990000
990000
980000
Le gend
980000
1 (32 mm )
3 (64 - 103 mm )
6 ( 128 - 150 m m)
Wetland/ sw am p
Reser vo ir s
Note: N um bers in b racketes are estim ated
an nual recharg e val ues
N
4
0
4
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970000
Km
RECHARG E MAP O F AKAKI RIVE R
CATCHMENT
PROJE CTION UTM ZONE 37
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5 0 00 0 0
Aquifer media
 Aquifer classifications were made basically for three
reasons:

for identification of aquifer types,

for the identification of the aquifer material,

to know the significant vadose zone media(s),
and

to extract vadose zone thickness.
The major unclear concepts of DRASTIC Model, are:
a) Vertical & horizontal lithologic variability, and
b) Problem of interpolation of point (well) data.
Aquifer media map of the Akaki River Catchment
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39°0 0 '
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5 10 0 0 0
Figure 8.3
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1010000
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38 °45 '
45 0 0 0 0
Rating
4
7
1000000
1000000
6
8
10
9°00'
9°00'
9
Res erv oir s
990000
990000
980000
Wet lan d /sw am p
980000
N
5
0
5
970000
970000
Km
AQUIFER MEDIA MAP OF
AKAKI RIVER CATCHMENT
8°45'
8°45'

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48 0 0 0 0
38 °45 '
49 00 0 0
5 0 00 0 0
39°0 0 '
5 10 0 0 0
PR O J EC T I O N U T M Z O N E 37
Soil media
 Soil is the upper portion (up to 2ms) of the
unsaturated zone.
 Act as a first defense-line of the hydrogeologic
system.
 Soil distribution in the study area
 soil texture classification according to US SCS (1951)

classification/rating => clay is dominant
Soil map of the Akaki River Catchment
38 °45'
47 0 0 0 0
39°0 0 '
48 0 0 0 0
49 00 0 0
50 00 0 0
510 0 0 0
1020000
46 00 0 0
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450 0 0 0
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Figure 8.4
Rating
4 (Silt loam )
1000000
1000000
3 (C lay l oa m )
6 (Fine sa n dy loam )
7 (C lay)
Wetlan d/sw am p
9°00'
9°00'
10 (Thin/ ab sent)
Res erv oir s
990000
990000
980000
Riv ers/ srea ms
980000
N
5
0
5
46 00 0 0
47 0 0 0 0
48 0 0 0 0
38 °45'
49 00 0 0
50 00 0 0
39°0 0 '
510 0 0 0
SOIL MAP OF AKAKI
RIVER CATCHMENT
8°45'
450 0 0 0
970000
970000
Km
8°45'

PR O J EC T ION U T M Z ON E 37

Slope map of Akaki River Catchment
ranges from 0 % to
>47%).
Impact of the vadose zone media
 Type of vadose zone media determines the
attenuation characteristics of the material
 Weighted mean was calculated by using the
thickness as a weighting criterion to handle vertical
lithologic variation
 To aerially interpolate point data same procedure as
in aquifer media above was adopted.
Impact of the vadose zone media map
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510 0 0 0
Figure 8.6
1010000
1010000
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1020000
38 °45'
450 0 0 0
Rating
1
2
3
1000000
1000000
4
5
6
8
990000
990000
980000
9°00'
9°00'
W etlan d/s wa mp
W ate r bod y/R ese rvo irs
980000
N
5
0
5
VADOSE ZONE MEDIA MAP OF
AKAKI RIVER CATCHMENT
8°45'
8°45'
970000
970000
Km
450 0 0 0
46 00 0 0
47 0 0 0 0
48 0 0 0 0
38 °45'
49 00 0 0
5 0 00 0 0
39°0 0 '
510 0 0 0
PR O J EC T I ON U T M Z ON E 37
Hydraulic conductivity map of the Akaki River Catchment
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510 0 0 0
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Figure 8.7
1000000
1000000
990000
Rating
990000
4
6
8
10
Reservoirs
Wetland /sw am p
980000
980000
N
1
0
1
2
Km
HYDRAULIC CONDUCTIVITY MAP OF
AKAKI RIVER CATCHMENT
45 0 0 0 0
970000
970000

46 00 0 0
47 0 0 0 0
48 0 0 0 0
49 00 0 0
5 0 00 0 0
510 0 0 0
PRO JE CTIO N UTM ZON E 37
RESULTS AND DISCUSSION
RESULTS:

The final DRASTIC result have been grouped
together into very low, low, moderate, high, and
very high pollution potential classes

Normal DRASTIC map of the Akaki River Catchment

Pesticide DRASTIC map of Akaki River Catchment
RESULTS & DISCUSSION
DISCUSSION:
1) Modified DRASTIC approach was followed to map depth to
groundwater
2)Highest net recharge values => high elevation areas.
3) Scoria and scoraceous rocks, and fractured basalts assigned
highest average rating value in rating aquifer media
While the lowest value was assigned for highly weathered and
decomposed volcanic rocks.
4)The presence of thick clayey sediments, paleosol and massive
volcanic rocks significantly affected the rating values of vadose
zone media.
DISCUSSION (…contd)
5)The impact of thin/absent soil affects the vadose zone rating
=>high vulnerability.
6) Topographic Slope is was assigned least weight for normal
DRASTIC computation.
 Generally, the northern part of the study area have very high
index values =>high vulnerability
 Very small areas found in east central parts were mapped as
very low vulnerability degree.
 Areas with moderate pollution potential cover significant part of
the catchment.
CONCLUSIONS AND RECOMMENDATIONS
CONCLUSIONS:
 Generally, vulnerability of groundwater increases from the
central part to the peripheral areas.
 The recharge areas are more vulnerable to groundwater
pollution as compared to other areas in the catchment
 The high index values also indicate the precautions that
should be taken for well head protection and general
groundwater resource management in the catchment
 Further detailed studies are requested in vulnerable areas
before we plan any development activities that have the
potential to pollute groundwater
CONCLUSIONS AND RECOMMENDATIONS(…contd)
RECOMMENDATIONS:
 Carefully plan future land use/development/ activities located in
the relatively sensitive/vulnerable zones of the catchment.
 Groundwater level measurements in boreholes for critical
periods.
 An objective oriented project of core-barrel drilling and pump
testing.
 Detailed net recharge estimation will be required by using
different methodologies/approaches.
 Inventory of potential contamination sources for the purposes of
groundwater protection.
RECOMMENDATIONS(…contd)
 The contaminant transport modeling with particular
emphasis on its rate and path length are important in order
to visualize the general scenarios of pollution movement
from a certain source to a particular area.
 Integrated Land use planning to solve conflicts between
land use and groundwater protection, as it takes, from the
beginning, all relevant aspects into consideration.
 Awareness creation on groundwater vulnerability to
pollution among the decision/policy makers and planners
to give an impulse to environmental thinking and public
concern.
Thank you!
“A picture speaks more than a thousand words and a map more than a
thousand pictures.”
Vrba and Zaporozec (1994)