UNESCO-IHP vulnerability mapping demonstration project at the Ghar El

Melh coastal aquifer in TunisiaDott. Salvatore Carrubba

Consultant Geologist

Ghar El Melh Coastal Aquifer

Studied Area

UNESCO-IHP undertook this project to demonstrate the use of vulnerability mapping at the Ghar El Melh coastal aquifer,

about 50 km North of Tunis

VULNERABILITY MAPPING IN COASTAL AREAIn coastal area we must consider two components of the evaluation of aquifer vulnerability:•vertical vulnerability against vertical pollution from surface•and horizontal vulnerability due to the salt water intrusion phenomena. Sea salt water is heavier than fresh water and in coastal area, it, can intrude itself into the aquifer flowing under the fresh water (of the aquifer) in a delicate equilibrium of weights

• At the moment there are many methods for studying vertical vulnerability, but there are no methods for studying horizontal and vertical vulnerability at the same time.

VULNERABILITY MAPPING IN COASTAL AREA

The goal of the UNESCO-IHP project is to create a single parameter – “comprehensive vulnerability” – able in

describing simultaneously these two types of vulnerabilities

Simplifying the hydro geological model, the main aquifer is constituted by the “Grès de Porto Farina” (Upper Pliocene) that are sands and calcarenite, each other mixed.

This aquifer is a multilayer aquifer and this aquifer is in hydraulic connection with the "younger" permeable deposits and sea water.

On the other hand the “Marnes de Raf Raf” (Pliocene) are impermeable clays rocks and constitute the bed of the aquifer.

m3 - mc2 - mb2 - ma2 - mab2 m'1

P'a

P'b

a

D – qm2 – Aa – qm1 – q"

SCHEMATIC HYDRO GEOLOGICAL MODEL

groundwater level

Ghar El Melh Coastal Aquifer

VULNERABILITY MAPPING IN COASTAL AREA

• Keeping in mind that vulnerability is an intrinsic characteristic of the aquifer in defending itself from an external threat that can be a pollutant or a contaminant.

• we must look for the intrinsic characteristics of the aquifer that can be able in opposing to a pollutant from surface or to a contaminant like salt water intrusion.

VERTICAL VULNERABILITY MAPPING

Ghar El Melh Coastal Aquifer

m3 - mc2 - mb2 - ma2 - mab2 m'1

P'a

P'b

a

D – qm2 – Aa – qm1 – q"

SCHEMATIC HYDRO GEOLOGICAL MODEL

groundwater level

Elevate Vertical Vulnerability Grade

Low Vulnerability

The natural intrinsic physical properties of an aquifer that can protect it from a pollutant coming from surface

(VERTICAL VULNERABILITY) are:

Depth-to-Water, Infiltration range, Attenuation action of unsaturated zone, Soil attenuation capacity, Aquifer types, Hydraulic Conductivity, Slope, Other intrinsic characteristics ...

Ghar El Melh Coastal Aquifer

HORIZONTAL VULNERABILITY MAPPING IN COASTAL AREA

The main “defence” that aquifer has in opposing sea salt water intrusion is the aquifer specific energy that is linked to groundwater level. In fact, more fresh groundwater level is higher above sea level, and more able the aquifer will be in opposing salt water intrusion pushing out salt water

HORIZONTAL VULNERABILITY MAPPING IN COASTAL AREA

Aquifer specific energy is the main “defence” that aquifer has in opposing sea salt water intrusion

Elevate Vulnerability Grade.

In this part of the aquifer groundwater level is the same of sea level. Aquifer is fully exposed to the "threat" and don't have any “means of defence" for pushing out salt water

HORIZONTAL VULNERABILITY MAPPING IN COASTAL AREA

All the areas of the aquifer where groundwater level is above sea level and the bottom of the aquifer is under sea level

Elevate Vulnerability Grade

High Vulnerability Grade

In this areas it is probable that salt water is naturally under fresh water and human activities can increase salt water intrusion and mix salt and fresh water.

HORIZONTAL VULNERABILITY MAPPING IN COASTAL AREA

Naturally it is improbable that salt water can contaminate fresh water but human activities can induce salt water intrusion or soil salinization.

Elevate Vulnerability Gr.

High Vulnerability Grade

Medium Vulnerability

Grade

Low

Very Low

HORIZONTAL VULNERABILITY MAPPING IN COASTAL AREA

GHAR EL MELH COASTAL AQUIFER

VULNERABILITY MAPPING IN COASTAL AREA

1

1

4 0

0

2

34

7

6

1

1

8

COMPREHENSIVE VULNERABILITY MAP

COMPREHENSIVE VULNERABILITY MAP GHAR EL MELH COASTAL AQUIFER

This map is an integrated vulnerability map because human activities data are also plotted: the pollution sources (punctual or diffused); subjects at contamination risk (target of pollution);Drainage network.

COMPREHENSIVE VULNERABILITY MAP GHAR EL MELH COASTAL AQUIFER

– Therefore, this map is a great instrument for territory planning and for safeguarding environment and groundwater resources against pollution and overexploitation.

– The methodology developed for mapping the “comprehensive vulnerability” of this system is a new concept and could be replicated at other Mediterranean coastal aquifers.

Thank you for your attention

Merci pour votre attention

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