First Presentation Bangui GW

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


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


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


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


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


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


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


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


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


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


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


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


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


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.

� شكرًا

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Documents

First Presentation Bangui GW