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Conjugated use of chemistry and isotope data
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PresentationPresentationbyby
Prof. Dr. Mohamed Fahmy HusseinProf. Dr. Mohamed Fahmy Hussein
The 4The 4thth Conference on Recent Technologies in Agriculture Conference on Recent Technologies in Agriculture
Challenges of Agricultural Modernization Challenges of Agricultural Modernization
Faculty of Agriculture,Faculty of Agriculture,
Cairo UniversityCairo University
Tuesday, 3 November, 2009Tuesday, 3 November, 2009
Mohamed Fahmy Hussein *, Ali Islam **, Sawsan Gamal **,Moloto Caetan *** and Chantal Djebebe***
Isotope Hydrogeochemistry of Urban-Zone Isotope Hydrogeochemistry of Urban-Zone Groundwater , Central AfricaGroundwater , Central Africa
ررنظائنظائيوكيمياء اليوكيمياء الهيدروجهيدروجية لمنطقة عمرانية ية لمنطقة عمرانية جوفجوفبالمياه البالمياه ال
بوسط أفريقيابوسط أفريقيا
* Cairo Univ., Fac. of Agric., Soil & Water Dept., Egypt, and Bangui Univ., Fac. of Sci, Geo. Dept., RCA** Atomic Energy Authority, Nuclear -Safety Center, Cairo, Egypt*** Bangui Univ. , Fac. of Sci, Geo. Dept., RCA
Bangui, Location Map, Border of RCA and Congo (RDC)Bangui, Location Map, Border of RCA and Congo (RDC)
18o 2
8'
18o 2
9'
18o 3
0'
18o 3
1'
18o 3
2'
18o 3
3'
18o 3
4'
18o 3
5'
18o 3
6'
18o 3
7'
4o19'
4o20'
4o21'
4o23'
4o24'
4o25'
4o26'
4o27'
1 - UNICEF
2 - Gbangouma
3 - Ouango(nogopou)
4 - Ngarangba
5-EcoleSt. Jean
6 - Bimbo(Usaca)
7 - Bimbo(Soeurs)
8 - Guitangola(F)
9 - Guitangola(Puit)
10 - Boy Rabe
11 - Boy Rabe(Kaimba)
12 - PK10
13 - PK12(Ecole Begoua)
4.31
4.32
4.33
4.34
4.35
4.36
4.37
4.38
4.39
4.40
4.41
4.42
4.43
4.44
4.45
4.4618
.45
18.4
6
18.4
7
18.4
8
18.4
9
18.5
0
18.5
1
18.5
2
18.5
3
18.5
4
18.5
5
18.5
6
18.5
7
18.5
8
18.5
9
18.6
0
18.6
1
18.6
2
18.6
3
18.6
4
Latitude
Longitude
Decimal coordinates of wells, Bangui, (large deep red-colored spots = high ionic charge)
AIRPORT
longi 18° 27'
longi 18° 28'
longi 18° 29'
longi 18° 30'
longi 18° 31'
longi 18° 32'
longi 18° 33'
longi 18° 34'
longi 18° 35'
longi 18° 36'
longi 18° 37'
longi 18° 38'
lati 4° 19'
lati 4° 20'
lati 4° 21'
lati 4° 22'
lati 4° 23'
lati 4° 24'
lati 4° 25'
lati 4° 26'
lati 4° 27'
well location
NO3-
2km distance
Location Map, Well-field and Sampling SitesLocation Map, Well-field and Sampling Sites
Hydrochemical Hydrochemical and Isotope Data Isotope Data
Sample date T °C pH pH EC dis.O 2 Ca2+
Mg2+
Na+
K+
HCO3- Cl
-SO4
2-NO3
- d 18O d 2H S ++S -
no. field lab m S.cm -1 mg.l -1 meq l-1
11 28-Apr-2007 Boy Rabe (Kaimba) 26.6 5.11 8.07 19.5 2.53 0.03 0.01 0.12 0.03 0.09 0.05 0.02 0.03 -2.21 -7.75 0.359 28-Apr-2007 Guitangola 24.0 6.87 7.95 19.5 0.80 0.09 0.04 0.19 0.02 0.14 0.11 0.02 0.07 -1.74 -4.28 0.61
10 28-Apr-2007 Boy Rabe 26.7 5.45 7.90 59.6 2.40 0.10 0.04 0.30 0.08 0.07 0.15 0.03 0.26 -2.03 -3.32 0.763 28-Apr-2007 O uango (Nogopou) 27.7 5.77 7.70 59.0 2.31 0.23 0.21 0.25 0.02 0.64 0.05 0.05 0.003 -2.44 -8.62 1.452 28-Apr-2007 Gbangouma 28.6 5.63 7.68 64.5 2.20 0.24 0.18 0.33 0.02 0.68 0.06 0.05 0.02 -2.17 -3.38 1.564 28-Apr-2007 Ngarangba 28.2 5.32 7.61 183.0 3.40 0.37 0.38 0.46 0.13 0.49 0.73 0.09 0.65 -1.92 -2.53 2.221 28-Apr-2007 UNICEF 28.0 5.91 7.51 228.0 1.00 0.86 0.36 0.45 0.03 1.10 0.28 0.14 0.34 -0.67 5.59 3.21
12 28-Apr-2007 PK 10 25.0 6.34 7.46 290.0 1.88 1.31 0.62 0.78 0.04 2.81 0.07 0.02 0.005 -2.00 -4.50 5.658 28-Apr-2007 Guitangol (F) 25.7 6.18 7.41 332.0 3.40 3.16 0.06 0.11 0.02 3.45 0.08 0.10 0.03 -1.91 -1.52 6.99
13 28-Apr-2007 PK12 (Ecole Begoua) 26.3 6.28 7.46 283.0 2.10 2.30 0.80 0.77 0.02 2.94 0.13 0.03 0.02 -2.31 -5.70 6.997 28-Apr-2007 Bimbo (Soeurs) 25.1 6.45 7.38 522.0 1.83 2.99 2.10 0.23 0.03 5.76 0.04 0.06 0.01 -2.13 -6.27 11.205 28-Apr-2007 Ecole St. Jean (Lakouanga) 25.6 6.90 7.63 516.0 3.61 3.10 2.37 0.18 0.02 5.81 0.14 0.05 0.02 -2.13 -4.75 11.686 28-Apr-2007 Bimbo (Usaca) 26.0 6.72 7.43 644.0 2.57 4.18 1.99 0.20 0.02 6.27 0.64 0.06 0.004 -1.91 -5.25 13.36
19 26-Mar-2007 Rain water 7.61 26.0 0.31 0.01 0.11 0.03 0.28 0.04 0.09 0.004 1.92 28.25 0.8720 26-Mar-2007 " " 6.46 30.0 0.37 0.01 0.06 0.03 0.38 0.03 0.02 0.003 0.9118 23-Mar-2007 " " 7.81 38.0 0.36 0.02 0.23 0.03 0.43 0.05 0.06 0.03 -0.23 19.54 1.1915 18-Feb-2007 " " 7.34 86.0 0.62 0.08 0.22 0.08 0.62 0.12 0.18 0.0002 1.9216 20-Feb-2007 " " (evaporated ?) 7.71 114.0 0.35 0.07 0.49 0.16 0.84 0.10 0.05 0.01 7.96 45.15 2.0617 19-Mar-2007 " " 7.70 75.0 0.61 0.07 0.27 0.13 0.79 0.11 0.12 0.02 0.86 22.25 2.0914 28-Jan-2007 " " 6.01 119.0 0.36 0.08 0.46 0.21 0.78 0.33 0.09 0.02 2.91 30.89 2.31
run off 28-Apr-2007 Run-off water, at Boy-Rab -5.46 -39.5421 1-Apr-2007 River O ubangui 7.67 68.0 0.55 0.37 0.33 0.09 0.97 0.16 0.05 0.02 0.98 13.40 2.51
Site
meq.l-1
/SMOW% o
B a
n g
u i
C
i t
y ,
R C
A, C
e n
t r
a l
A
f r
i c
a
Prelude – page-1Prelude – page-1
1) The isotope hydrogeochemical approach is a modern technique used for studying GW.
2) Coupling the hydrochemical and isotope data (e.g. d18O and d2H) provides an interesting insight into GW origin and composition needed for the management of the available water resources.
3) Many GW aquifers are isotopically and chemically unknown, in particular in Africa south of the Sahara.
4) The studied aquifer is heavily used by the local population despite the nearby river flow and urban pollution.
Prelude – page-2Prelude – page-2
5) Despite the nearby river and abundant precipitation (~1500mm yr-1, subdivided into 7 months wet-season and 5 months dry-season) a major part of the urban-zone population depends on groundwater and is exposed to health hazard related to groundwater contamination.
6) The objective is to conjugate the geochemical and isotope data to understand the major processes governing GW composition and exploring aquifer conditions such as mineralogy, water-balance, flow and porosity.
0
50
100
150
200
250
month
mmstation 1
station 2
400600800
10001200
1400160018002000
1960 1970 1980 1990 2000 2010
year
mm.y-1
station 1
mean 1
station 2
mean 2
Local Precipitation RateLocal Precipitation Rate
11
9
1032
4
1
12
813 7
56
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
0 1 2 3 4 5 6 7
pH
HCO3, meq l-1
Rain, shippment. lab
Rain forced to PCO2 = -3.5
Pure rain equil.atm CO2
Groundwater
11
9
10
3
24
1
12
8
13 7
5
6
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
0 2 4 6 8 10 12 14
pH
S cations + S anions, meq l-1
Rain, shippment. lab
Rain forced to PCO2 = -3.5
Pure rain equil.atm CO2
Groundwater
11
9
10 32
4
112
813
75
6
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
1 10 100 1000
pH
PCO2, in groundwater (expressed as times its atmospheric level)
Groundwater
pH pH versusversus HCO HCO33 and Total Ions, and Total Ions, in meq.l-1, and PCO, and PCO22
Piper, GW Piper, GW and RainwaterRainwater
20
40
60
80
#19start
#21 riverend
0
100
0 100
PIPER CATIONSBangui Rainwater
% Mg
% Na+K
% Ca
80
60
40
20
#19start
#21 riverend
0
100
0 100
PIPER ANIONSBangui Rainwater
% Cl
% CO3+HCO3
% SO4
80
60
40
20#11start
#6end
0
100
0 100
PIPER ANIONSBangui GW
% Cl
% CO3+HCO3
% SO4
20
40
60
80
#11start
#6end
0
100
0 100
PIPER CATIONSBangui GW
% Mg
% Na+K
% Ca
Bar, GW Bar, GW and Rainwater Rainwater
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
1 2 3 4 5 6 7 8 9 10 11 12 13
meq l-1Bar Cations Bangui GW
Na+K
Mg
Ca
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
1 2 3 4 5 6 7 8 9 10 11 12 13
meq l-1Bar Anions Bangui GW
Alk
SO4
Cl
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
1 2 3 4 5 6 7 8
meq l-1Bar Cations Bangui Rainwater
Na+K
Mg
Ca
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
1 2 3 4 5 6 7 8
meq l-1Bar Anions Bangui Rainwater
Alk
SO4
Cl
Schoeler DiagramSchoeler Diagramss
0.01
0.1
1
10
1 2 3 4 5 6
meq l-1
Schoeler Diagram Bangui GW
6
5
7
13
8
12
1
4
2
3
10
9
11
CaMg Na Cl SO4HCO3
0.01
0.1
1
10
1 2 3 4 5 6
meq l-1
Schoeler Diagram Bangui Rain Water
21
14
17
16
15
18
20
19
CaMg SO4HCO3Mg Na Cl SO4
D’Amore DiagramsD’Amore Diagrams
-100
0
100
A B C D E F
GW # 11
-100
0
100
A B C D E F
GW # 9
-100
0
100
A B C D E F
GW # 10
-100
0
100
A B C D E F
GW # 3
-100
0
100
A B C D E F
GW # 2
-100
0
100
A B C D E F
GW # 4
-100
0
100
A B C D E F
GW # 1
-100
0
100
A B C D E F
GW # 12
-100
0
100
A B C D E F
GW # 8
-100
0
100
A B C D E F
GW # 13
-100
0
100
A B C D E F
GW # 7
-100
0
100
A B C D E F
GW # 5
-100
0
100
A B C D E F
GW # 6
-100
0
100
A B C D E F
Rain # 19
-100
0
100
A B C D E F
Rain # 20
-100
0
100
A B C D E F
Rain # 18
-100
0
100
A B C D E F
Rain # 15
-100
0
100
A B C D E F
Rain # 16
-100
0
100
A B C D E F
Rain # 17
-100
0
100
A B C D E F
Rain # 14
-100
0
100
A B C D E F
River # 21
A = [ ](HCO3- - SO4
2-)S-
* 100 B = [ ]( )SO42-
S- - ( )Na+
S+ * 100 C = [ ]( )Na+
S+ - ( )Cl-
S- * 100
D = [ ](Na+ - Mg2+)S+
* 100 E =
( )( )Ca2+ + Mg2+
S+ - ( )HCO3
-
S- * 100 F = [ ](Ca2+ - Na+ - K+)
S+ * 100
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0
HC
O3,
Ca,
Mg,
meq
.l-1
Sum of ions, meq.l-1
GW HCO3
GW Ca
GW Mg
Rain HCO3
Rain Ca
Rain Mg 0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0
HC
O3,
(C
a +
Mg)
, meq
.l-1
Sum of ions, meq.l-1
HCO3
Ca+Mg
0.0
0.1
0.2
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0S
O4,
meq
.l-1
Sum of ions, meq.l-1
GW
Rain
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0
Cl,
meq
.l-1
Sum of ions, meq.l-1
GW
Rain
0.0
0.1
0.2
0.3
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0
K, m
eq.l
-1
Sum of ions, meq.l-1
GW
Rain
0.00.10.20.30.40.50.60.70.80.9
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0
Na,
meq
.l-1
Sum of ions, meq.l-1
GW
Rain
Solutes, GW Solutes, GW & Rainwater Rainwater
dd1818OO – – EC RelationshipEC Relationship
Oubangui River
humid-season rain with assumed Na
concentration-7.0-6.0-5.0-4.0-3.0-2.0-1.00.01.02.03.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
d18O/SMOW
EC, mmhos/cm
Mineralisation shown byEC - d18O, rain water, and groundwater, Bangui
GW, Bangui
Rain, dry seasondissolution
Oubabngui River
humid-season rain with assumed Na
concentration-45
-35
-25
-15
-5
5
15
25
35
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
d2H/SMOW
EC, mmhos/cm
Mineralisation shown byEC - d2H , rain water and groundwater , Bangui
GW, Bangui
Series3dissolution
NONO33--, , in mg.l-1, Bangui GW, Bangui GW
# 10
# 4
# 1
0
10
20
30
40
50
0 100 200 300 400 500 600
NO
3, m
g.l-1
Sum of ions, mg.l-1
GW
Rain water
dd1818O O & dd22HH versus Solutes Solutes
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
d18O/SMOW
EC, mmho/cm
Mineralisation mechanisms as shown by
EC - d18O relationship, groundwater, Bangui
-10-8-6-4-202468
10
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
d2H/SMOW
EC, mmho/cm
Mineralisation mechanisms as shown by
EC - d2H relationship, groundwater, Bangui
dissolution
dissolution
-10-8-6-4-202468
10
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
d2H/SMOW
Individual dissolved ions, meq/L
Mineralisation mechanisms as shown by
Ion - d2H relationship, groundwater, Bangui
Ca
Mg
Na
K
HCO3
Cl
SO4
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
d18O/SMOW
Individual dissolved ions, meq/L
Mineralisation mechanisms as shown by
Ion - d18O relationship, groundwater, Bangui
Ca
Mg
Na
K
HCO3
Cl
SO4
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
d18O/SMOW
EC, mmhos/cm
Mineralisation mechanisms as shown by
EC - d18O, river water, and groundwater, Bangui and Cameron
GW, Bangui
River, Cameron
mixing ?
evaporation ?
dissolution
-20
-15
-10
-5
0
5
10
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
d2H/SMOW
EC, mmhos/cm
Mineralisation as shown by
EC - d2H , river water, and groundwater , Bangui and Cameron
GW, Bangui
River, Cameron
mixing ?
evaporation ?
dissolution
-10-8-6-4-202468
10
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
d2H/SMOW
(Ca+Mg) or HCO3, meq l-1
Mineralisation mechanisms as shown by
Ca+Mg (or HCO3) - d2H relationship, groundwater, Bangui
Ca+Mg
HCO3
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
d18O/SMOW
(Ca+Mg) or HCO3, meq l-1
Mineralisation mechanisms as shown by
Ca+Mg (or HCO3) - d18O relationship, groundwater, Bangui
Ca+Mg
HCO3
Saturation Indices,Saturation Indices, Calc, Hal Dolo Calc, Hal Dolo
GypGyp
12 3
4
56
7
8
9
10
11
12
13
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
4.5 5.5 6.5 7.5
log[
HC
O3- ]
pH
Bangui GW
Bangui Rain
Calcite
1
2
34
567
89
1011
1213
-9.5
-8.5
-7.5
-6.5
-5.5
-4.5
-3.5
-2.5
-9.5 -8.5 -7.5 -6.5 -5.5 -4.5 -3.5 -2.5
½ (l
og[C
a]+
log[
Mg]
)
log[CO3]
Bangui GW
Dolomite
-5
-4
-3
-2
-1
0
1
-5 -4 -3 -2 -1 0 1
log[
Cl]
log[Na]
BanguiGW
Halite
1:1 line
-6
-5
-4
-3
-2
-6 -5 -4 -3 -2
log[
SO
4]
log[Ca] + 2 log[H2O]
BanguiGW
Gypsum
1:1 line
lnK = - G°
RT , logK = - G°
2.3RT , pK = G°
2.3RT
Predominance DiagramPredominance DiagramDolomite
CaMg(CO3)2
CalciteCaCO3
BruciteMg(OH)2
Sea water
-8.0
-7.0
-6.0
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
-4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0
logPCO2
log of molality ratio (Ca/Mg)
Bangui GW
Boundary 1
Boundary 2
Boundary 3
Boundary 4
Boundary 5
Sea water
MagnesiteMgCO3
1
2
3
4
5
6
7 8
9
10
11
1213
CalciteCaCO3
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0 1.0 2.0
logPCO2
log of molality ratio (Ca/Mg)
Bangui GW
Boundary 2
Sea water
dd1818O O –– dd22HH, , Bangui & CameronBangui & Cameron
<--- Run-off, 28 April 2007(start of humid season)
<--- Oubangui River,1st of April 2007(end of dry season)
?
-45.0-40.0-35.0-30.0-25.0-20.0-15.0-10.0-5.00.05.0
10.015.020.025.030.035.040.045.050.0
-8 -6 -4 -2 0 2 4 6 8
d2H/SMOW%o
d18O/SMOW%o
Stable isotope composition of rain water, river water and groundwater, Bangui and Cameron
Rain, Dry Season, Bangui
GW, Bangui
Precipitation, Cam
River water, Cam
recycled Trans. (S=8, I=18)
Rozansky line
Craig line
evapo_1 (S=7.4, I=6.0)
evapo_2 (S=6.2, I=1.5)
evapo_3 (S=3.2, I=20)
V. cross-hair
H. cross-hair
-30.0
-25.0
-20.0
-15.0
-10.0
-5.0
0.0
5.0
10.0
15.0
20.0
25.0
30.0
-6 -5 -4 -3 -2 -1 0 1 2 3 4
d2H/SMOW%o
d18O/SMOW%o
Stable isotope composition of river water and rain water, Cameron
Precipitation, Cam
River water, Cam
recycled Trans. (S=8, I=18)
Rozansky line
Craig line
evapo_2 (S=6.2, I=1.5)
V. cross-hair
H. cross-hair
dd1818OO –– dd22HH, Bangui, Bangui
<--- Run-off, 28 April 2007(start of humid season)
<--- Oubangui River,1st of April 2007(end of dry season)
?
-45.0-40.0-35.0-30.0-25.0-20.0-15.0-10.0-5.00.05.0
10.015.020.025.030.035.040.045.050.0
-8 -6 -4 -2 0 2 4 6 8
d2H/SMOW%o
d18O/SMOW%o
Stable isotope composition of rain water, river water and groundwater, Bangui
Rain, Dry Season, Bangui
GW, Bangui
recycled Trans. (S=8, I=18)
Rozansky line
Craig line
evapo_1 (S=7.4, I=6.0)
evapo_3 (S=3.2, I=20)
V. cross-hair
H. cross-hair
d2H%o = 8.00 d18O%o + 10.0 (Craig line) (or MWL)
d2H%o = 8.17 d18O%o + 11.3 (Rosanky line)
dd1818O O & & dd22H H –– EC, Cameron EC, Cameron
-3.5
-3.0
-2.5
-2.0
-1.5
0.00 0.02 0.04 0.06 0.08 0.10 0.12
d18O/SMOW
EC, mmho/cm
Mineralisation mechanisms as shown byEC - d18O relationship, river water, Cameron
mixingevaporation?
dissolution
-20.0-18.0-16.0-14.0-12.0-10.0-8.0-6.0-4.0-2.00.0
0.00 0.02 0.04 0.06 0.08 0.10 0.12
d2H/SMOW
EC, mmho/cm
Mineralisation mechanisms as shown byEC - d18O relationship, river water, Cameron
mixingevaporation?
dissolution
dd1818O O –– T Taa°°CCmean air temperaturemean air temperature
-6
-5
-4
-3
-2
-1
0
1
2
3
10 12 14 16 18 20 22 24 26 28 30 32
d18 O
, %o
Dansgaard Temperature, Ta� C
Ta - 18O Dansgaard relation
Bangui mean 18O in GW
Bangui mean Ta
Rain, Bangui, Dry Season
Rain, Cameron
Cameron GW
Bangui GW
modified intercept -19.75
Ta modified intercept -19.75
mean d18O rain, Dry Season, Bangui
Ta corresponding to mean d18 rain, Dry Season, Bangui
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
30
35
10 12 14 16 18 20 22 24 26 28 30 32
d2H
, %o
Dansgaard Temperature, Ta� C
Ta - 2H Dansgaard relation
Bangui mean 2H in GW
Bangui mean Ta
Rain, Bangui, Dry Season
Rain, Cameron
Cameron GW
Bangui GW
modified intercept -146
Ta modified intercept -146
mean d2H rain, Dry Season, Bangui
Ta corresponding to mean d2H rain, Dry Season, Bangui
d18O%o = (0.7* Ta) - 13.75
d2H%o = (5.6* Ta) - 100
SynopsisSynopsis11
• Africa, south of the Great Sahara, has abundant water resources. However its huge aquifers are seldom studied and/or inadequately managed.
• This work presents the first geochemistry-isotope hydrology study on the aquifer of Bangui city (the capital of the Central African Republic, RCA, at the northern border of Congo, RDC with RCA.)
SynopsisSynopsis22
• The obtained hydrochemical data demonstrated the role of biogenic CO2 gas, weathering, solid-phase dissolution, and cation exchange in defining the composition of the studied groundwater.
• The conjunctive use of the major dissolved constituents and the isotope contents (d18O and d2H) showed that the alteration of primary-silicates and the dissolution of solid-phase carbonates are the predominant processes that locally define the zones of dilute and relatively-charged groundwater, respectively.
SynopsisSynopsis33
• The isotope data illustrated that evaporation does not significantly contribute to water-loss from the aquifer, while transpiration (that goes on without isotope fractionation) is prevailing , with a part of the transpired vapor recycled.
• Annual aquifer-recharge is mediocre due to poor infiltration through the Oxisols that induces precipitation to runoff. Recharge during the dry and the humid seasons may be equal.
• The isotopic “inverse” continental-effect (from Cameron to RCA) is explained by differences in air-temperatures, amount and altitude of precipitation, rather by inverse movement of humid air-masses westward in Central Africa.
SynopsisSynopsis44
• The results helped to distinguish between the dynamic GW-flow sections (deep fractured aquifer to the north of the urban-zone) from the less-active parts (the upper porous aquifer to the south of the city.)
• The data showed that the aquifer is void of evaporites, while its carbonate rocks are mostly made-up of Calcite or Mg-Calcite, not dolomite.
• Groundwater pollution, by NO3-, is a serious growing-up
problem in city center, and would spread allover the urban agglomeration.
ملخصملخص
تتمتع أفريقيا جنوب الصحراء بوفرة أحواضها المائية، لكن العديد من خزاناتها الجوفية لم –يخضع للدراسة وقد يستغل بطريقة غير مناسبة. يقدم هذا العمل أول تطبيق للجيوكيمياء
وهيدرولوجيا النظائر البيئية على خزان مدينة بانجى )عاصمة ج. أفريقيا الوسطى( الواقعة )الرافد الشمالى لنهر الكونجو، وهذا الرافد يشكل جزءاI من الحدود "وبانجى"أعلى نهر
الجنوبية ألفريقيا الوسطى مع الكونجو( حبث يعتمد معظم سكان المدينة على نزح المياه )من الخزان المسامى الضحل حر-السطح( وضخها )من الخزان المتشقق العميق شبه-المحصور(،
بالرغم من سريان النهر ووفرة األمطار .أظهرت المكونات الذائبة دور األطوار الصلبة والتبادل الكاتيونى وغاز ثاني أكسيد الكربون –
حيوى-النشأة )بالنطاقين غيرالمشبع ثم المشبع( في تعيين المالمح الهيدروكيميائية لتلك المياه . أوضح االستخدام المتضافر للبيانات الكيميائية والنظائرية أن تجوية السيليكات فى
البداية، ثم ذوبان صخور الكربونات )المتشققة و/أوالمسامية تحت نظام مفتوح أو مغلق( Iهما العمليتان السائدتان في ترسيم تواجد المياه المخففة وتلك غنية التركيز نسبيا ،Iالحقا
بمختلف أجزاء المدينة .أشارت البيانات النظائرية أن البخر ال يdسهم بأى قدر ملموس في فقد مياه من الخزان )وإن –
كان الفاقد بالبخر ملموساI بالموازنة المائية لحوض الصرف السطحى(، على حين أن النتح )بدون تجزئ نظائرى( سائد، ويعاد تدوير جزء من بخاره، فيساهم مجدداI فى نوبات الهطول
بوسط أفريقيا على "التأثير القارى المعكوس للتركيب النظائرى بالهطول"المتتابعة. فسرنا أساس فوارق درجات الحرارة، وتفاوت شدة األمطار، ومدى االرتفاع عن سطح البحر )بدالI من
تفسير سابق، على خزان بالكاميرون، كان أساسه حركة معكوسة – غيرمؤكدة – لكتل هوائية حن الخزان mنحو المحيط األطلنطى(. أوضحت البيانات النظائرية أن ش Iرطبة تسرى غربا
حنهd خالل موسم القحط )بسبب ضعف مسامية mالجوفى خالل موسم األمطار قد ال يتجاوز شتربة األوكسيزول الذى يdفضى إلى تفوق الجريان السطحى خالل موسم األمطار(، مما يعنى
أن الشحن السنوى عموماI ضعيف .ان – Iالصم( ساعدتنا البيانات التحليلية في تمييز جهات تواجد الطبقات العميقة المتشققةKarst )
ذات الدينامية المائية الملحوظة بخزان شمال المدينة عن تكوينات النظام المسامى )ذو السريان المائى الضعيف( بخزان جنوب المدينة، كما بينت النتائج أن األطوار الصلبة تخلو من
األمالح التبخرية، وأن صخورالكربونات العميقة عبارة عن كالسيت وليست دولوميت، كما ظهر تلوث ملحوظ بالنترات بالمياه الجوفية، بوسط العاصمة لكنه سوف ينتشر بكل المنطقة
العمرانية.
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