4.1. Thematic Layers and Features in the CRB
Mapping and analysis of slope
Slope is an important geomorphological feature that affects the groundwater potential of a region and an important parameter in identifying groundwater recharge prospects (Fasche et al., 2014). Groundwater potential is greater in gentle slopes as more infiltration occurs due to the increased residence time. On the other hand, the increased runoff rate for steep slopes makes them less suitable for groundwater recharge. In this study, slope varies from 0 to 80.44%, the majority of the area having a slope between 0 to 4.73 %. The highest slopes were found mostly in the western region of the study area. Based on this, the slope range between 0-4.73% was given a weightage of 7 (very good) with 4 (moderate), 3(moderate) and 2 (poor)given to subsequent classes (see Fig. 3). Generally, steep slopes are given lower weights and gentle slope with higher weights (Agarwal and Garg 2016).
Figure 3 Slope Map
Mapping and analysis of aspect
Aspect is an important terrain characteristic that affects the groundwater recharge characteristics of a basin. It is the direction of slope usually measured clockwise from 0 to 360°. Zero means the aspect facing north, 90 ,180 is south-facing, and 270 is west-facing. In arid and semi-arid regions, microclimatic changes are dependent on slope exposure direction and drainage basin development. Thus, aspect has a direct influence on the microclimates (Hadley 1961; Al-Saady et al., 2016). An aspect map of the study area is shown in Fig. 4. The aspect of CRB is trending towards all the directions, however higher weightage is given to the flat terrains and the lowest to those areas trending north. terrains and the lowest to the north trending areas.
Figure 4 Aspect Map
Mapping and analysis of groundwater level
In Unsaturated conditions, the upper level of saturated underground surface in which water pressure equals the atmospheric pressure is known as groundwater table (Freeze and Cherry 1979). Depth to the water table is a measure of groundwater recharge or discharge. When the water table is deep, the flow is towards the water table via percolation and infiltration. On the other hand, when the water table meets the land surface, the flow is away from the water table. (Poehls and Smith 2009). So, for potential recharge zones, the higher depth to the water table is an essential factor. The groundwater level in the study area varies from 0 to 21m below ground level. Most of the region in the study area falls between 6 and 11m below ground level(mbgl) (Fig. 5). As the depth to the water table increases, the possibility of recharge increases because of the increased storage in aquifers. Greater weight is given to those regions where the depth to the water table is high and vice versa.
Figure 5 Groundwater Level Map
Mapping and analysis of rainfall
Rainfall data for the past 44 years has been collected by the India Meteorological Department (IMD). A spatial variation map of the rainfall was created with the IDW interpolation method. The minimum and maximum rainfall received in the Chennai Basin were 770 and 1570 mm, respectively. The coastal part of the basin is receiving a high amount of rainfall, compared to the western part. A spatial map of rainfall in the Chennai Basin is given in Fig. 6.
Figure 6 Rainfall map
Mapping and analysis of Lithology
The geology of an area is one of the key factors in groundwater potential zone delimitation. Various geological formations have different water bearing capacities and subsurface flow characteristics. A considerable variation in the water bearing capacities may be found between sedimentary to Igneous and metamorphic rocks of recent to Precambrian periods (see Fig. 7). The other principal factor is the weathering of the rocks, which increase the groundwater potential of the area. The Chennai basin exhibited a wide range (sedimentary-Metamorphic-Igneous) of geological formations. Starting from the eastern coastal region, a long stretch of coastal Alluvium is observed throughout the study area and charockites in the southern edge. From the middle to north alluvial formation begins and extend to greater areas towards the west. Laterites are found in the northern part of the basin and also spread in between the alluvial formations. In the southern part, just near to the charnockite, there are thick shale sandstone formations. The western end of the area is marked by biotite hornblende gneiss, with lengthy patch of hornblende-epidote. Geology of the area suggests that the possible high groundwater bearing formations are alluvium and sandstones Considering the geology of the area, alluviums, sandstone are promising locations for groundwater development. However, the degree of weathering, lineament and fractures determine the same for the hard rock formations.
Figure 7Geological of the study area
Mapping and analysis of Drainage
The drainage network map of the Chennai Basin is shown in Fig. 8. The Chennai Basin has many rivers, tanks and reservoirs. Since the basin has mostly permeable formations as well as built-up areas, the drainage density of the basin is very low. Thus, the main features are classified as rivers, tanks/reservoirs and others. Suitable ranking is given to each feature depending on their groundwater potentiality.
Figure 8 Drainage Map
Mapping and analysis of soils
Soils in the study area can be classified into Clay, clay loam, loamy sand, Sand, Sandy Clay, Sandy-clay- loam, Sandy loam, as shown in Fig. 9. Along the beeches sandy and sandy clay loam types are present, and these formations are permeable and can be a aquifer. These formations are extensively found along the East Coast Road (ECR), and holds good for agricultural activities.
Clayey soils are found in northern region, namely Gummidipoondi, Ponneri, Minjur, Madhavaram and Manali, and in the western portion of the East Coast Road around Thiruporur. These soils have much lower infiltration rates. Weights assigned for the soil layer are mainly based on the infiltration rate. As a result, clayey soils have been given the lowest weights, while sandy soil receives the highest.
Figure 9 Soil map
Mapping and analysis of land use
The rapid increase in population resulted in extensive changes in the land use pattern of the CRB. Groundwater recharge is largely controlled by the landuse. Hence, a proper understanding of land use is necessary for the sustainable groundwater development. Overexploitation of water resources for various purposes has a severe impact on the water system. Increased water exploitation has led to a reduction in water recharge and groundwater storage of the area. The various land use patterns of the study area are presented in Fig. 10. Cropland, mangroves, shrubs, and Casuarina cover a majority of the study area.
Figure.10 Land-use map of Chennai Basin
Mapping and analysis of Lineaments
Lineaments are rectilinear alignments observed on the surface of the earth, which are representations of geological or geomorphological events. They can be observed as straight lines in digital data, which represent a continuous series of pixels having similar terrain values. Large scale lineaments can be identified from remotely sensed images. Lineaments are the primary indicators of secondary porosity and also for potential sources of water supply. The presence of lineaments is observed in all directions in the study area. The lineament density seems to be very high in Takkolam, Cooum, Sriperumbudur, Thiruvallur, Thiruthani, etc (Fig. 11).
Figure 11: Lineaments Map
Mapping and analysis of geomorphology
The Chennai Basin has exceptionally versatile geomorphological features with beaches, Beach Ridges, Beach terraces, Buried Pediments, Wash Plains, Salt Pans, Swamps, Swale, Deltaic Plains, Deep Pediment, Pediment and Shallow Pediment, Buried Course & Channels, Tertiary Uplands, Flood Plains, Piedmont, Inter Fluveo. The presence of rivers, coastal regions, hills and plain land make this area an example of a complex geomorphological set up. It has a long coastal belt on the eastern boundary where the city of Chennai is located, with one of the thickest populated regions in southern India. The NE boundary of the study area has a long portion with Duricrust, a hard mineral layer on top of the sedimentary formations. Tertiary laterites are found as patches all along the basin. In the western part structural hills are visible. Lower Gondwana formations are seen in the southern and central parts. Upper Gondwana formations are Pediments seen in the Tambaam region pat of the city. At the northern part, along the state boundary of Andhra Pradesh, tertiary uplands form a larger area and the same is present in available north of the city. A detailed geomorphological map of the study area is shown in Fig. 12.
Figure12 Geomorphology map of the study area.
Mapping and analysis of Depth to bed Rock
Depth to bed rock is a representation of the thickness of unconsolidated or weathered formations in the area. The depth to bed rock of CRB varied from11 to 829m (Fig. 13). Southern coastal regions and western part of CRB has weathered thickness upto 45m. The deepest depth to bed rock is found in the extreme north region. Based on these values, three major categories such as poor, moderate and very good, with corresponding weights 5 ,6 and 8 were assigned for the layer.
Figure 13: depth to bed rock
4.2. Normalized weights for thematic maps
The pairwise comparison matrix of the groundwater prospecting thematic layers were derived based on the AHP method. The weights were normalized and the weights for individual thematic layers are calculated by Eigen vector method (Table 4).
Table 4
Pairwise comparison matrix of 11 groundwater prospecting parameters for AHP
Thematic Layer | Sp | A | GWL | RF | GEO | D | Sl | LU | Ln | GM | DBR |
Slope (Sp) | 1.00 | 0.33 | 0.33 | 0.50 | 0.50 | 0.33 | 0.33 | 0.20 | 0.25 | 0.25 | 0.25 |
Aspect (A) | 3.00 | 1.00 | 0.50 | 0.50 | 0.33 | 0.33 | 0.50 | 0.50 | 0.50 | 0.33 | 0.50 |
Ground Water level (GWL) | 3.00 | 2.00 | 1.00 | 0.25 | 0.25 | 0.25 | 0.25 | 0.33 | 0.25 | 0.25 | 0.50 |
Rainfall (RF) | 2.00 | 2.00 | 4.00 | 1.00 | 0.33 | 0.25 | 0.33 | 0.25 | 0.50 | 0.25 | 0.25 |
Geology (GEOL) | 2.00 | 3.00 | 4.00 | 3.00 | 1.00 | 0.50 | 0.33 | 0.33 | 0.50 | 0.33 | 0.25 |
Drainage(D) | 3.00 | 3.00 | 4.00 | 4.00 | 2.00 | 1.00 | 0.50 | 0.25 | 0.50 | 0.33 | 0.33 |
Soil (SL) | 3.00 | 2.00 | 4.00 | 3.00 | 3.00 | 2.00 | 1.00 | 0.50 | 0.33 | 0.50 | 0.33 |
Landuse (LU) | 5.00 | 2.00 | 3.00 | 4.00 | 3.00 | 4.00 | 2.00 | 1.00 | 0.33 | 0.50 | 0.33 |
Lineament (Ln) | 4.00 | 2.00 | 4.00 | 2.00 | 2.00 | 2.00 | 3.00 | 3.00 | 1.00 | 0.50 | 0.25 |
Geomorphology (GM) | 4.00 | 3.00 | 4.00 | 4.00 | 3.00 | 3.00 | 2.00 | 2.00 | 2.00 | 1.00 | 0.50 |
Depth to bed rock (DBR) | 4.00 | 2.00 | 2.00 | 4.00 | 4.00 | 3.00 | 3.00 | 3.00 | 4.00 | 2 | 1.00 |
SUM | 34.00 | 22.33 | 30.83 | 26.25 | 19.42 | 16.67 | 13.25 | 11.37 | 10.17 | 6.25 | 4.50 |
Table 5 shows the normalized weights of each layer and their corresponding total weightage. The maximum weightage shows the most influential parameter, and the minimum weightage represents the least influential parameter. In the CRB, depth to bed rock or aquifer thickness play the most important role with 20.33% weightage. With 15%, geomorphology was the second most important parameter. The relative importance of the other parameters are as follows, lineament (12.37%), land use (12%), soil (9%), drainage (8.2%), geology (6.6%), rainfall (4.9%), aspect (4.5%), water level (4.2%), and slope (2.6%).
Table 5
Calculation of Normalized weights for 11 thematic layers of CRB
| Sp | A | GWL | RF | GEO | D | Sl | LU | Ln | GM | DBR | Normalized weight |
Sp | 0.03 | 0.01 | 0.01 | 0.02 | 0.03 | 0.02 | 0.03 | 0.02 | 0.02 | 0.04 | 0.06 | 0.0257 |
A | 0.09 | 0.04 | 0.02 | 0.02 | 0.02 | 0.02 | 0.04 | 0.04 | 0.05 | 0.05 | 0.11 | 0.0455 |
GWL | 0.09 | 0.09 | 0.03 | 0.01 | 0.01 | 0.02 | 0.02 | 0.03 | 0.02 | 0.04 | 0.11 | 0.0429 |
Rf | 0.06 | 0.09 | 0.13 | 0.04 | 0.02 | 0.02 | 0.03 | 0.02 | 0.05 | 0.04 | 0.06 | 0.0491 |
GEOL | 0.06 | 0.13 | 0.13 | 0.11 | 0.05 | 0.03 | 0.03 | 0.03 | 0.05 | 0.05 | 0.06 | 0.0665 |
D | 0.09 | 0.13 | 0.13 | 0.15 | 0.10 | 0.06 | 0.04 | 0.02 | 0.05 | 0.05 | 0.07 | 0.0822 |
Sl | 0.09 | 0.09 | 0.13 | 0.11 | 0.15 | 0.12 | 0.08 | 0.04 | 0.03 | 0.08 | 0.07 | 0.0911 |
LU | 0.15 | 0.09 | 0.10 | 0.15 | 0.15 | 0.24 | 0.15 | 0.09 | 0.03 | 0.08 | 0.07 | 0.1188 |
Ln | 0.12 | 0.09 | 0.13 | 0.08 | 0.10 | 0.12 | 0.23 | 0.26 | 0.10 | 0.08 | 0.06 | 0.1237 |
GM | 0.12 | 0.13 | 0.13 | 0.15 | 0.15 | 0.18 | 0.15 | 0.18 | 0.20 | 0.16 | 0.11 | 0.1512 |
DBR | 0.12 | 0.09 | 0.06 | 0.15 | 0.21 | 0.18 | 0.23 | 0.26 | 0.39 | 0.32 | 0.22 | 0.2033 |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1.000 |
To check the consistency of the assigned weights, the consistency ratio was calculated using the formula mentioned in the methodology. For the 11 layers (n = 11), the consistency ratio was found as 0.98, which is < 0.10. This means that the weight assessment was consistent.
Table 4: Pairwise comparison matrix of 11 groundwater prospecting parameters for AHP
Table 5: Calculation of normalized weights for 11 thematic layers of CRB
Table 6: Weight assessment and normalization of different features of groundwater prospecting thematic layers
Table 6
Weight assessment and normalization of different features of groundwater prospecting thematic layers
Factor | Class | Value | Normalized weight of features | Level of Suitable |
Geomorphology | Chennai City | 2 | 0,0122 | Poor |
Pediment | 2 | 0,0122 | Poor |
Buried Pediment Shallow | 2 | 0,0122 | Poor |
Buried Pediment Moderate | 3 | 0,0183 | Moderate |
Tank | 8 | 0,0488 | Very Good |
Buried Pediment Deep | 6 | 0,0366 | Very Good |
Structural hill | 2 | 0,0122 | Poor |
Valley Fill | 8 | 0,0488 | Very Good |
River | 9 | 0,0549 | Very Good |
Flood Plain | 9 | 0,0549 | Very Good |
Lateritic Gravel | 3 | 0,0183 | Moderate |
Duricrust | 2 | 0,0122 | Poor |
Marshy Land | 7 | 0,0427 | Very Good |
Tertiary Upland | 5 | 0,0305 | Good |
Sand Dune | 6 | 0,0366 | Good |
Pediment Outcrop | 2 | 0,0122 | Poor |
Settlement | 2 | 0,0122 | Poor |
Swales | 2 | 0,0122 | Poor |
Beach | 5 | 0,0305 | Good |
Paleo Deltaic Plain | 7 | 0,0427 | Very Good |
Quartz-Graval Tertiary | 4 | 0,0244 | Moderate |
Upper Gondwana | 8 | 0,0488 | Very Good |
Pulicate Lake | 7 | 0,0427 | Very Good |
Alluvial Plain | 8 | 0,0488 | Very Good |
Laterite Tertiary | 4 | 0,0244 | Moderate |
Creek | 5 | 0,0305 | Good |
B Canal | 7 | 0,0427 | Very Good |
River Island | 7 | 0,0427 | Very Good |
Lower Gondwana | 7 | 0,0427 | Very Good |
Dyke | 2 | 0,0122 | Poor |
Gullies | 2 | 0,0122 | Poor |
Pedi Plain | 2 | 0,0122 | Poor |
Old River Course | 9 | 0,0549 | Very Good |
Geology | Biotite Hornblend Gnies | 4 | 0,0727 | Poor |
Quartz Gravel | 5 | 0,0909 | Moderate |
Sandstone Conglomarate | 5 | 0,0909 | Moderate |
Laterite | 7 | 0,1273 | Good |
Shale Sandstone | 5 | 0,0909 | Moderate |
Waterbodies | 4 | 0,0727 | Poor |
Alluvium | 8 | 0,1455 | Very Good |
Epidote Hornblend | 5 | 0,0909 | Moderate |
Granite | 5 | 0,0909 | Moderate |
Charnockite | 7 | 0,1273 | Good |
Drainage | River | 8 | 0,4000 | Very Good |
Tank/Reservoir | 9 | 0,4500 | Very Good |
Others | 3 | 0,1500 | Poor |
Water Level | 0–6 | 2 | 0,1429 | Poor |
6–11 | 5 | 0,3571 | Moderate |
6–21 | 7 | 0,5000 | Good |
Soil | Sandyloam | 3 | 0,0667 | Moderate |
Loamysand | 3 | 0,0667 | Moderate |
Habitation | 2 | 0,0444 | Poor |
Waterbody | 8 | 0,1778 | Very Good |
Sandyclayloam | 6 | 0,1333 | Good |
Sandyclay | 6 | 0,1333 | Good |
Clay | 3 | 0,0667 | Poor |
Sand | 6 | 0,1333 | Good |
Clayloam | 6 | 0,1333 | Good |
Misce | 2 | 0,0444 | Poor |
Rainfall | 770–930 | 1 | 0,1000 | Poor |
930–1090 | 2 | 0,2000 | Moderate |
1090–1250 | 3 | 0,3000 | Good |
1250–1410 | 4 | 0,4000 | Very Good |
Landuse | Barren Land | 2 | 0,0211 | Poor |
Brickiln_industries | 2 | 0,0211 | Poor |
Beach | 3 | 0,0316 | Moderate |
HF Ind_IT | 4 | 0,0421 | Moderate |
Airport | 2 | 0,0211 | Poor |
Alkalinity Salinity | 2 | 0,0211 | Poor |
Back Water | 2 | 0,0211 | Poor |
casurina | 3 | 0,0316 | Moderate |
City | 2 | 0,0211 | Poor |
Estuary | 2 | 0,0211 | Poor |
Groves | 4 | 0,0421 | Moderate |
Crop Land | 5 | 0,0526 | Good |
Juliflora | 4 | 0,0421 | Moderate |
Marshy Land | 5 | 0,0526 | Good |
Navey | 2 | 0,0211 | Poor |
Plantation | 5 | 0,0526 | Good |
Pulicat Lake | 5 | 0,0526 | Good |
River | 8 | 0,0842 | Very Good |
Salt Pan | 2 | 0,0211 | Poor |
Sand | 8 | 0,0842 | Very Good |
Shrub | 5 | 0,0526 | Good |
Waste Land | 3 | 0,0316 | Moderate |
Landwithscrub | 4 | 0,0421 | Moderate |
Land without Scrub | 2 | 0,0211 | Poor |
Hills with Shrub | 2 | 0,0211 | Poor |
Dry Crop | 7 | 0,0737 | Good |
Lineament | Buffer 500 | 6 | 0,4000 | Good |
Buffer 750 | 8 | 0,5333 | Very Good |
Others | 1 | 0,0667 | Poor |
Depth to Bed Rock | 11–45 | 5 | 0,2632 | Poor |
45–75 | 6 | 0,3158 | Moderate |
75–829 | 8 | 0,4211 | Very Good |
Aspects | Flat | 9 | 0,1957 | Very Good |
North 0-22.5 | 7 | 0,1522 | Very Good |
Northeast 22.5–67.5 | 5 | 0,1087 | Good |
East 67.5-112.5 | 6 | 0,1304 | Good |
Southeast 112.5-157.5 | 8 | 0,1739 | Very Good |
South 157.5-202.5 | 4 | 0,0870 | Moderate |
Southwest 202.5-247.5 | 3 | 0,0652 | Moderate |
West 247.5-292.5 | 2 | 0,0435 | Poor |
Northwest 292.5-337.5 | 1 | 0,0217 | Poor |
North 337.5–360 | 1 | 0,0217 | Poor |
Slope | 0-2.42 | 7 | 0,4375 | Very Good |
2.42–7.58 | 4 | 0,2500 | Moderate |
7.58–15.61 | 3 | 0,1875 | Moderate |
15.61–38.81 | 2 | 0,1250 | Poor |