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Vulnerability assessment in fractured aquifer using improved vulnerability index: Applied to Gabes aquifer, Southeastern Tunisia
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Abstract: The Gabes aquifer system, located in southeastern Tunisia, is a crucial resource for supporting local socio-economic activities. Due to its dual porosity structure, is particularly vulnerable to pollution. This study aims to develop a hybrid model that combines the Fracture Aquifer Index (FAI) with the conventional GOD (Groundwater occurrence, Overall lithology, Depth to water table) method, to assess groundwater vulnerability in fractured aquifer. To develop the hybrid model, the classical GOD method was integrated with FAI to produce a single composite index. Each parameter within both GOD and FAI was scored, and a final index was calculated to delineate vulnerable areas. The results show that the study area can be classified into four vulnerability levels: Very low, low, moderate, and high, indicating that approximately 8% of the area exhibits very low vulnerability, 29% has low vulnerability, 25% falls into the moderate category, and 38% is considered highly vulnerable. The FAI-GOD model further incorporates fracture network characteristics. This refinement reduces the classification to three vulnerability classes: Low, medium, and high. The outcomes demonstrate that 46% of the area is highly vulnerable due to a dense concentration of fractures, while 17% represents an intermediate zone characterized by either shallow or deeper fractures. In contrast, 37% corresponds to areas with lightly fractured rock, where the impact on vulnerability is minimal. Multivariate statistical analysis was employed using Principal Components Analysis (PCA) and Hierarchical Cluster Analysis (HCA) on 24 samples across six variables. The first three components account for over 76% of the total variance, reinforcing the significance of fracture dynamics in classifying vulnerability levels. The FAI-GOD model removes the very-low-vulnerability class and expands the spatial extent of low- and high-vulnerability zones, reflecting the dominant influence of fracture networks on aquifer sensitivity. While both indices use a five-class system, FAI-GOD redistributes vulnerability by eliminating very-low-vulnerability areas and amplifying low/high categories, highlighting the critical role of fractures. A strong correlation (R2 = 0.94) between the GOD and FAI-GOD indices, demonstrated through second-order polynomial regression, confirms the robustness of the FAI-GOD model in accurately predicting vulnerability to pollution. This model provides a useful framework for assessing the vulnerability of complex aquifers and serves as a decision-making tool for groundwater managers in similar areas.
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Key words:
- Groundwater /
- Aquifer vulnerability /
- Fractured media /
- FAI-GOD index /
- GOD index /
- GIS
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Figure 1. Location and geological map of the study area (Based on Tunisia geological map 1/500,000)
Figure 4. Lineament density illustration using ArcGIS functions (after Tam et al. 2004)
Figure 5. Parameters and results of the GOD vulnerability assessment in the study area: a) Groundwater occurrence (G), b) Overlying lithology (O), c) Depth to water table (D), (d) Final GOD vulnerability map.
Figure 6. Derived geospatial maps for the FAI analysis in the study area: (a) Digital Elevation Model (DEM) of the study area, (b) fracture network, (c) Lineament density map, and (d) resulting FAI index map.
Figure 7. Spatial distribution maps of the FAI-GOD index and associated vulnerability classes
Figure 8. Projection sampled point on the C1×C2 factorial plane according to the pollution vulnerability indices
Notes: C1 and C2 represent the first and second principal components derived from the PCA analysis, which together explain the largest proportion of variance in the dataset
Figure 9. Hierarchical cluster analysis (HCA) of sampled points based on Euclidean distance derived from FAI-GOD vulnerability scores, revealing three distinct vulnerability groups in the study area
Figure 10. Comparison between GOD and FAI-GOD methods of aquifer vulnerability assessment in fractured aquifer. The shift toward higher vulnerability classes reflects the impact of fracture-controlled flow.
Figure 11. Polynomial regression of vulnerability assessment indicators AFI-GOD and GOD indices
Table 1. GOD vulnerability parameters, scores and index classification (Foster, 1987)
G Groundwater occurrence Score Confined and artesian 0.1 Confined 0.2 Semi-confined 0.3 Semi-unconfined covered 0.5 Unconfined 1 O Overlying lithology Score Unconsolidated sediments 0.4 Consolidated porous rocks 0.5 Aeolian sand 0.6 Alluvial sands, fluvio-glacial, sand gravels 0.7 Gravel alluvial 0.8 Unconsolidated dense rocks 0.9 Fractured or karstic Consolidate dense rocks 1 D Depth to water table (m) Score >100 0.4 50–100 0.5 20–50 0.6 10–20 0.7 5–10 0.8 2–5 0.9 0–2 1 GOD index value Vulnerability classes GOD index 0–0.1 Very low 0.1–0.3 Low 0.3–0.5 Moderate 0.5–0.7 High 0.7–1 Extremely high 
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Table 2. Lineament density values and scores
Factor Value Risk to pollution class Assigned weight to Fracture index Lineament density (km/km2) < 0.5 Low 0.1 0.5–1 Moderate 0.4 1–1.5 High 0.8 > 1.5 Very High 1.0
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Table 3. Scores adapted to the GOD and FAI index according to aquifer type
Aquifer type GOD score (Sg) Fracture score (Sf) Porous media 1.0 0.0 Fractured rock 0.4 0.6 Karst 0.2 0.8
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Table 4. FAI-GOD vulnerability index classification (five classes)
FAI-GOD index Vulnerability classess < 0.1 Very low 0.1–0.4 Low 0.4–0.7 Moderate 0.7–1.0 High >1 Very high
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Table 5. Results of the GOD vulnerability assessment for the study area
Classes Percentage (%) Very low 8 Low 29 Moderate 25 High 38 Extremely high 0
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