Geospatial and multi-criteria analysis to rank suitable sites of

Geospatial and multi-criteria analysis to rank suitable sites of shallow
aquifer recharge with reclaimed water: Application to Nabeul - Hammamet
groundwater
Khadija Gdoura, Makram Anane, Salah Jellali
Laboratoire de traitement des eaux usées, Centre de Recherches et Technologies des Eaux, Technopole de Borj
Cédria.
Abstract
The reuse of treated wastewater in arid and semi-arid areas to recharge aquifers contributes to decrease the
pressure on conventional water. This paper aims is to develop a methodology to locate and rank suitable sites for
groundwater recharge with treated wastewater using fuzzy-AHP based on GIS. The Nabeul-Hammamet
(Tunisia) shallow aquifer is selected as the target area. Several important parameters are identified considering
simultaneously : technical, economical and environmental aspects. These parameters are weighted using pairwise matrix, layered using geo-spatial techniques and then standardized using fuzzy functions. The final map is
obtained by a weighted linear combination of the standardized parameters. The results show that the total
suitable area covers 5400 ha which represents about 19% of the total aquifer area. This constitutes quite a large
zone that can absorb the entire volume of available treated wastewater. The best selected sites to receive the
surplus amount of TWW are located near of the treatment plants and inside agricultural lands. A detailed
hydrodynamic study of the non-saturated and saturated zone is necessary to know quantitatively and
qualitatively the impact of this charge on groundwater.
Key words: Wastewater, Groundwater recharge, AHP, GIS, Multi-criteria analysis, Nabeul-Hammamet.
Résumé
La réutilisation des eaux usées traitées dans un but de recharger des aquifères dans les zones arides et semiarides contribue de façon significative à réduire la pression croissante sur les eaux conventionnelles. L’objectif
de ce travail de recherche est d’élaborer une méthodologie pour la localisation et le classement des sites
favorables à la recharge des nappes phréatiques par les eaux usées traitées. Celle-ci se base sur la combinaison
des techniques géo-spatiales avec celles d’analyses multicritères. Plusieurs critères d’aspects techniques,
environnementaux, sociaux et économiques ont été identifiés, spatialisés, pondérés par comparaison par pair et
normalisés selon des fonctions de logique floue. La carte finale qui classe les sites favorables dans la région de
Nabeul-Hammamet a été obtenue par une combinaison linéaire pondérée des critères normalisés. La superficie
totale favorable à la recharge de cette nappe par les eaux usées traitées dépasse de loin celle prévue par la théorie
pour faire infiltrer la totalité des effluents des stations d’épuration existantes dans la région. Cette superficie se
situe principalement au bord de la mer au Sud- Est de Hammamet, autour du Grand-Nabeul, dans certains
domaines agricoles et au niveau des dunes côtières. Les sites les plus adaptés sont localisés près de Nabeul dans
des zones agricoles. Une étude hydrodynamique détaillée de la zone non saturée et saturée sur ces sites est
nécessaire pour évaluer quantitativement et qualitativement l’impact de cette recharge sur la nappe.
Mots clés : Eaux usées, Recharge des nappes, AHP, SIG, Analyse multicritère, Nabeul-Hammamet.
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1. Introduction
Tunisia is facing scarcity and spatial disparity of water resources. Infact, the estimated
available freshwater is only about 480 m3/capita/year (Ennabli and al.2005). The reuse of
treated wastewater (TWW) is considered as a solution to satisfy part of the increasing water
demand. However, only 28% of the total annual amount of TWW produced from the 109
wastewater treatment plants is reused (from l’ONAS in 2011), mainly for irrigation. Artificial
aquifer recharge using this water is currently applied in pilot sites for experimental reasons.
The selection of suitable recharge sites is a key issue for the project success.
An appropriate choice of suitable sites for aquifer recharge with TWW requires a combination
of the Multi-Criteria Decision Analysis techniques (MCDA) and the Geographic Information
Systems (GIS). MCDA considers economic, environmental and technical criteria and weights
them according to their importance for a specific use. GIS manage and treat spatially these
criteria and get results in illustrative maps (Engel and al. 2003). Several studies have been
conducted combining GIS-MCDA such as selection of suitable sites for landfill (Nouri and al.
2011) and for airports (Ding and al. 2011). On the other hand, several MCDA methods have
been developed as ELECTRE, PROMETHEE, AHP, TOPSIS, AIM (Gilliam and al. 2005;
Gomez and Barredo and al. 2005; Zhong -Wu and al. 2006). However, few of them can be
integrated into a GIS (Gomez and Barredo and al. 2005; Marinoni and al. 2004). The Analytic
Hierarchy Process (AHP) is one of the most used.
The main goal of this work is to develop an innovative methodology to select best site for
shallow groundwater recharge with TWW, coupling GIS and AHP.
2. Methodology
2.1. Characterization of the study area
The case study area corresponds to Hammamet–Nabeul aquifer limits. It is located at ‘Cap
Bon’ peninsula at the North Eastern part of Tunisia and belongs to Nabeul District (Figure 1).
It covers a total area of 286 km2 with a length of 34 km and a width varying from 8 to 14 km.
The climate is semi-arid with an average temperature equal to 19°C and an average annual
rainfall equal to 400 mm. The altitude varies between 0 and 500 m and the depth to
groundwater varies between 4 and 31 m. The main activities of the region are tourism and
agriculture.
TUNISIA
Limit of the study area
Wastewater treatment plant
Road
Figure 1 - Location map of Nabeul-Hammamet shallow aquifer limit showing wastewater
treatment plants and main roads
Two wastewater treatment plants are currently operating in the study area, namely SE3 and
SE4. Currently SE4 is under extension and rehabilitation to increase the capacity and improve
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the quality of wastewater treatement. The Commisariat Regional de Développement Agricole
of Nabeul (CRDA-Nabeul) predicted that the SE3 and SE4 outflow will reach 4.8 Mm3/year
in 2025 (CRDA, 2008).
2.2. Methodology
To locate the best sites to recharge the shallow aquifer with the 4.8 Mm3 produced
annually by SE3 and SE4, a methodology was developed coupling AHP with GIS. It involves
the following steps:
2.2.1. Constraints identification
The constraints and their discarding conditions considered to delineate the suitable areas for
recharge with reclaimed water of the aquifer are: depth of the unsaturated zone (> 5m), slope
(<12%), soil texture (<10% clay), geology (excluding clay, sandstone and limestone) ,
distance from lakes and hill dams (> 500m), distance from residential areas (> 200 m) and
distance from roads (<50m). These criteria and subcriteria are selected after consulting
national experts and reviewing international guidelines and technical documents published
especially in (Kallali and al. 2007) ; (Anane and al. 2008), (Pedrero and al. 2011), (Rahman
and al. 2012) and (Rahman and al. 2013).
2.2.2. Hierarchical structure and weighting of criteria and sub-criteria
The hierarchical structure organizes the decisional problem in a set of levels. In this case,
three levels were established Figure 2. The first one defined the objective of the study which
is ranking suitable sites for shallow aquifer recharge with TWW. The second level presents
the main criteria and the last level displays the sub-criteria.
Choice of
suitable
to install basinsdes
of infiltration
Choix
des
sitessites
d’installation
bassins
d’infiltration
Technical
Soil texture
Environmental
Economic
Geology
Elevation
soil
occupation
Depth of the
unsaturated
zone
Water salinity
Distance
from urban
area
Distance from
tanks
Slope
Distance from SE
Concentration of
nitrates
Soil salinity
Figure 1 - Graphical representation of the hierarchical structuring
Three main criteria were selected: the technical, economic and environmental criteria. From
each criterion derives three to six sub-criteria Figure 2. To classify the suitability of sites for
groundwater recharge, a local weight was assigned to each criterion and sub-criterion using
pair-wise comparison matrix (Saaty et al. 1980). Then the global weight of each sub-criterion
was obtained by multiplying its local weight by the local weight of the corresponding main
criteria Table 1. The criteria and sub-criteria identification and weight assignment were based
upon experts consulting and reviewing of international published guidelines and technical
documents.
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Sub –
Criterion
local weight
criterion k
0.33
Technical
0.33
Economic
Environnemental
0.33
overall
weight p
Soil texture
0.269
0.089
Geology
0.269
0.089
Depth of
unsaturated zone
Soil salinity
0.269
0.089
0.077
0.025
Water salinity
0.077
0.025
Slope
0.036
0.012
Elevation
0.571
0.188
Distance from SE
0.285
0.094
Soil occupation
0.142
0.047
Distance from
urban zone
Distance from
tanks
Concentration of
nitrates
0.623
0.206
0.238
0.079
0.137
0.045
k
Table 1 - Local weight and overall weight of each sub-criterion.
2.2.3. Geospatialization of sub-criteria and constraints
Geospatial and semantic data were collected from several competent institutions (CRDA,
Ministry of Agriculture, National Office of Metrology ...) and from satellite images (Landsat
SAT; SRTM ...). Then, these data were treated and sub-criteria were obtained in geospatial
layers using spatial analysis techniques.
2.2.4. Standardization of sub-criteria
Ranking potential sites for aquifer recharge deals with sub-criteria with different types
(qualitative and quantitative), forms (continuous and discrete) and units of measurement (m
dS/cm,... %). In order to combine these heterogeneous data, it is mandatory to standardize all
sub-criteria by bringing them into a common domain of measurement. Thus, pixel values of
each sub-criterion raster layer were transformed into a scale of suitability ranging from 0 to 1
using fuzzy functions. Sub-criteria values were processed differently depending on their
continuous or discrete form. For continuous criteria, such as slope, depth to groundwater and
distances from WWTP, values were stretched over 0–1 interval using increasing, decreasing
or symmetric sigmoid function where 0 is assigned to the least suitable for aquifer recharge
and 1 to the most suitable. For discrete values, such as soil texture and land use, the classes
were grouped according to their capacity to recharge aquifer with TWW. Then, their weights
were assigned to each class by means of Saaty matrix (Gomez and Barredo, 2005). The
obtained weights were stretched linearly from 0 to 1.
2.2.5. Calculating the decision index
A final composite map of the study area was obtained by calculating the composite decision
value for each pixel obtained using weighted linear combination of standardized sub-criteria.
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The values vary between 0 and 1 where 0 corresponds to the least suitable area and 1 to the
most.
3. Results and Discussion
3.1. Suitable sites identification
The total suitable area for aquifer recharge with TWW occupies 5400 ha which represents
about 19% of the entire surface of Nabeul-Hammamet shallow aquifer (Figure 3). This area is
quite large and can by far absorb the entire amount of SE3 and SE4 outflow. The total suitable
area is made by enclaves varying from less than one hectare to more than 100 ha. The most
restrictive constraints are distance from urban zone and distance from roads. They limit the
suitable area to 64.34% and 69.23%, respectively. The least restrictive constraints are
distances from water bodies and depth to groundwater (96.2% and 94.9% respectively).
a
c
b
d
e
f
g
h
Figure 2 - Favorable site for recharging groundwater by TWW according to the constraints a)
Depth to groundwater, b) Slope, c) Soil texture, d) Geology, e) Distance from water bodies, f)
Distance from urban areas, g) Distance from roads, h) Superposition of all constraints.
3.2. Ranking suitable areas
The index values used to rank the suitable 5400 ha for aquifer recharge by TWW range from
0 to 0.60 for SE3 and from 0 to 0.57 for SE4 (Figure 4).
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b
a
Figure 4 - Ranking suitable sites for aquifer recharge with the outflow of a) SE3 and b) SE4.
3.3. Location of the best required land
According to the methodology established by EPA (EPA in 1984), 7.19 ha and 19.36 ha of
land are required to use of the outflow of SE3 and SE4 (1.3 Mm3/yr and 3.5 Mm3/yr
respectively) for groundwater recharge. The best 7.19 ha and 19.36 ha in area of the shallow
aquifer are presented in Figure 5. The most suitable sites are located in the north west and the
south east of the city of Nabeul for SE4 and in the north west of the city of Nabeul for SE3.
SE4
SE3
a
b
Figure 5. Localization of the most suitable sites to install basins of recharging groundwater by
TWW, a) for the station SE3, b) for the station SE4.
They are located in agricultural areas occupied by olive trees and cereals with easy
accessibility. In these areas the soil is non saline and the salinity of water is about 2 g/l.
4. Conclusions
In the present work, a single-objective AHP integrated with a GIS was carried out to identify
and rank potential sites to recharge the Nabeul-Hammamet’s aquifer with treated wastewater.
Many criteria were selected taking into account technical, environmental and economical
aspects. Constraints analysis shows that 19% of the Nabeul-Hammamet aquifer’s watershed is
suitable to the recharge with reclaimed water, which exceeds the required land for the reuse of
the total SE3 and SE4 outflow. Ranking these suitable sites, using AHP integrated in a GIS,
reveals that the best sites are located in the north west and the south east of the town of
Nabeul for SE4 station and the north west of the city of Nabeul for station SE3.
This methodology uses available and easy-to-get data from Tunisian official institutions and
satellite images. Hence, it is easily extendable to other Tunisian regions and other countries.
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This work constitutes a helpful technical support for decision makers for a better integrated
water management in the Nabeul-Hammamet aquifer watershed.
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