3 6 /an) m Prélèvements (10

Arbia Jendoubi, Rachida Bouhlila ; Mounira Zammouri & Lamia Guellouz

Lab. Modélisation en Hydraulique et Environnement Ecole Nationale d’Ingénieurs de TUNIS

Bouhlila.rachida@enit.rnu.tn

Hydrogeology and Nitrates

Contamination of Mater Aquifer,

North Tunisia

9 Weak Precipitations :

limited aquifer recharge rate 9 high water resident time

9 low flow rates

long contact time :

water – rocks & pollutants

Ground water quality problems in arid area

Ground water quality problems in arid area

9 But also, in many countries :

Legislation & regulation is absent or is being born..

9 Absence of drillings protection perimeters

9 water quality monitoring : not

frequent

Ground water quality problems In arid area

Major groundwater quality threat in arid area :

9 Salinisation in relation to the significant evaporation and the weak recharge :

major ions

9 Seawater and Sebkha brine intrusion or up coning in relation to the overexploitation

9 Trace elements related the geology and to the sedimentary processes : fluorine

Ground water quality problems in arid

But also : Pollution related to the

agricultural and industrial activity - Nitrates and pesticides

- Hydrocarbons and industrial solvents - Other specific pollutants : fluorine and phosphogypsum for the industry of

phosphates, etc..

Mater : 60 km north – west of Tunis

2.00 4.00 6.00 8.00 10.00 12.00 14.00

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00

T u n i s M A T E R

3 6 °

3 4 °

3 2 °

8 ° 1 0°

0 4 8 12 Km

Mater and his aquifer ….

9Agricultural vocation zone : Field crops and intensive breeding

9Upper aquifer: not very deep and in hydraulic communication with the

subjacent layers

9Groundwater : drinking water supply (SONEDE)

But...Absence of drillings protection perimeters

Schematic cross section & water budget

Est Ouest

Lac Ichkeul Drainage par

les oueds

Evaporation

Fuites vers le lac

Drainage par les émergences

Apports des crues des oueds

Recharge latérale en provenance des terrasses de bordure

Pompage Niveau

piézométrique

Est Ouest

Lac Ichkeul Drainage par

les oueds

Evaporation

Fuites vers le lac

Drainage par les émergences

Apports des crues des oueds

Recharge latérale en provenance des terrasses de bordure

Pompage Niveau

piézométrique Ouest

Lac Ichkeul Drainage par

les oueds

Evaporation

Fuites vers le lac

Drainage par les émergences

Apports des crues des oueds

Recharge latérale en provenance des terrasses de bordure

Pompage Niveau

piézométrique

Aquifer water volume abstraction

0 2 4 6 8 1 0 1 2

1 9 7 0 1 9 7 3 1 9 7 6 1 9 7 9 1 9 8 2 1 9 8 5 1 9 8 8 1 9 9 1 Prélèvements (106 m3 /an)

What is the extent of the nitrates contamination ??

{

the only available water quality data are those of the SONEDE drillings : Around the Mater town

{

Sampling and analyzes on 30 points

distributed on the entire plain

Nitrates contamination….

9 Nitrates average concentrations in SONEDE drillings

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0

A n n é e

M a te u r 1 1 M a te u r 5 M a te u r 2 M a te u r 4 M a te u r 1 0 M a te u r 9 M a te u r 8 M a te u r v ille N T :0 9 1 4

Nitrates Iso concentrations, measured in April 2000 from 0 to 210 mg/l

Km

1 3 2

4

5 6

7

8 9

10 12 11

13 15 14

16

17

18 19

20

21

23 22

24 25

26 28

29 30 32 33

34

35 36

2.00 4.00 6.00 8.00 10.00 12.00 14.00

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

0 4 8 12

Ferme Puits

O bjectives..

9quantify the aquifer resources

9quantify space and time nitrates fluxes to the aquifer

9specify the aquifer water contamination mechanisms

9propose and study remediation solutions

Groundwater and solute transport model for the aquifer

Hydro geological model

{Finite elements, single layer model

{the dense hydrographic network: feed the aquifer upstream and drains it downstream

{the lake Ichkeul : general discharge system of the aquifer

{Steady state regime up to 1966 : transmissivities calibration

{Unsteady state regime of 1966 to 2000 : storage coefficients calibration

Piezometric level map measured for the year 1965

0 2 4 6 8 10 12 14 16

X (m )

0 2 4 6 8 10 12 14

Y( m)

0 4 8 12

E che lle :

Km

Measured and calculated water head level in 4 wells

Point d'e au n°367

0 2 4 6 8 10

A nnée

Point d'eau n° 215

2 2,5 3 3,5 4 4,5

A nné e

Point d'e au n° 486

6,5 7 7,5 8 8,5 9 9,5

A nné e

Point d'e au n° 308

10 10 ,5 11 11,5 12 12 ,5 13 13 ,5

A nné e Point d'e au n°367

0 2 4 6 8 10

A nnée

Point d'eau n° 215

2 2,5 3 3,5 4 4,5

A nné e Point d'e au n°367

0 2 4 6 8 10

A nnée

Point d'eau n° 215

2 2,5 3 3,5 4 4,5

A nné e

Point d'e au n° 486

6,5 7 7,5 8 8,5 9 9,5

A nné e

Point d'e au n° 308

10 10 ,5 11 11,5 12 12 ,5 13 13 ,5

A nné e Point d'e au n° 486

6,5 7 7,5 8 8,5 9 9,5

A nné e

Point d'e au n° 308

10 10 ,5 11 11,5 12 12 ,5 13 13 ,5

A nné e

The aquifer Water budget, from 1965 to 2000

1965 1968 1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 Lac

Drainage

Alimentation DrainaExploitge ation Evapo

ration Lac

0 200 400 600 800 1000 1200 1400 1600 1800

Année

L/s

The plain Nitrogen assessment : Actualization of the ground occupation map

z Nitrogen budget:

+350 – (260 -30) = 120 kg/ha/an

* 54 kg/ha/an in Valence, Spain (Geng; 1991)

* 300 kg/ha/an, in Kerbenez, France (Beaudoin et al.;

1996)

- 260

TOTAL

+ 350

TOTAL

30

Lixiviation

12

Volatilization

200

fertilization

18

denitrification Biological 30

Fixing

140

Crops consumption

20

Rain contribution

60

Immobilization

100

Mineralization

Nitrogen losses Kg/ha/an

Nitrogen Contributions

Kg/ha/an

Nitrates transfer model

z NO₃^- : a tracer aqueous solute

z Solute conservation equation :

a dispersion – convective equation

z Sequential Resolution of the flow & the transport at each time step using a

finite elements method

Nitrates transfer model

z the calibration is related to:

•

•

•

•

z The calibration data are :

•

•

Nitrates concentrations measured and simulated for the year 2000

Km

1 32

4

5 6 7

8 9

11 10 12 13 15 14

16

17

18 19

20

21

23 22

24 25

26 28

29 30 32 33

34

35 36

2.00 4.00 6.00 8.00 10.00 12.00 14.00

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

0 4 8 12

Ferme Puits

Km

1 32

4

5 6 7

8 9

11 10 12 13 15 14

16

17

18 19

20

21

23 22

24 25

26 28

29 30 32 33

34

35 36

2.00 4.00 6.00 8.00 10.00 12.00 14.00

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

0 4 8 12

Ferme Puits

Measured and calculated

nitrates concentrations evolution in 4 wells

Forage M ville

20 25 30 35 40 45 50 55 60 65 70 75

A n né e calmes

For age M 5

5 10 15 20 25 30

A nn é e

Forage M8

0 2 4 6 8 10 12 14 16 18

A nné e

Fo rag e M 4

0 5 10 15 20 25 30 35

A n n é e Forage M ville

20 25 30 35 40 45 50 55 60 65 70 75

A n né e calmes

For age M 5

5 10 15 20 25 30

A nn é e

Forage M8

0 2 4 6 8 10 12 14 16 18

A nné e

Fo rag e M 4

0 5 10 15 20 25 30 35

A n n é e

Quelques résultats….Concentrations calculées et mesurées au droit des forage SONEDE pour 1998

5.50 6.00 6.50 7.00 7.50 8.00 8.50 9.00

4.00 4.50 5.00 5.50 6.00 6.50 7.00 7.50 8.00 8.50

Calculated & measured drillings concentrations

Mean Cal.

Mean Meas.

St.. Deviat.

Cal.

St.. Deviat.

Meas.

Correl. Coef.

R²

Mateur ville 47,38 47,00 11,36 9,83 0,83

Mateur 9 31,51 31,82 4,83 5,14 0,93

Mateur 5 18,54 17,97 4,74 5,12 0,80

Mateur 11 23,76 22,10 6,69 7,43 0,77

Mateur 4 20,95 19,19 5,56 5,22 0,75

Mateur 8 8,08 9,17 3,94 4,70 0,92

Mateur 2 19,84 19,53 5,64 5,33 0,73

Mateur 10 19,46 19,05 6,73 6,68 0,91

Predictive simulations & remediation scenarios

z

Calibrated flow and transport models : predictive tools

z

Simulations of scenarios from

2000 and until reduction of the

concentrations to the drinking

standards

Remediation scenarios

•

•

Very localized and slow impact to bring back the concentrations to the

standards ( 45 mg/l)

Remediation scenarios

z

Sc. 3 : Storage of the bovines breeding farms effluents

Impact generalized but perceptible at the end of 80 years

z

Sc. 4 : Rationalization, or reduction of the manure contributions :

Impact generalized and perceptible at the end of 70 years

Sc. 5 : combination of all these protection scenarios

Reduction of the contamination of all the tablecloth at the end of 30 years (2030)

2.00 4.00 6.00 8.00 10.00 12.00 14.00

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

0 4 8 12 Km

Conclusions et ..recommendations…

z

agricultural diffuse pollution :

Complex problem and slow Phenomena

z

Data : A systematic, quantitative and qualitative follow-ups of the aquifers

z

modeling contributes to the systems and phenomena knowledge

z

Prevent... or cure ?