Groundwater Potential Zone Mapping Using Analytical Hierarchy Process(AHP) And GIS,
FOR NAGAPATTINAM DISTRICT, TAMILNADU, INDIA

-Shanmuga Priya,
Student of AGSRT

ABSTRACT

Groundwater is the most reliable source of fresh water. Due to several criteria such as increased population, urbanization, and industrialization, the groundwater sources are under severe threat. Climate change plays a vital role in the quality and quantity of groundwater sources. Also, the climate variability severely affects the parameters influencing the groundwater recharge. Unreliable monsoons and poor quality of the surface water resources tend to increase the decline in the groundwater levels. Hence, it is necessary to identify and delineate the groundwater potential zone (GWP) which can be used to augment the groundwater source. The study is carried out for Nagapattinam district where the groundwater serves as the main source for domestic and agricultural purposes rather than the surface water.

The parameters such as topography, geology, drainage density, soil, land use and land cover rainfall, and the lineament density are generated as different layers in the GIS background and are subjected to weighted overlay analysis to obtain the potential zones of groundwater. The weights for the various layers were generated using the multi-criteria decision-making technique and analytical hierarchy process which allows the pairwise comparison of criteria influencing the potential zone. Further, the GWP map has been reclassified into five different classes, namely Excellent, Good, moderate, poor, and very poor. The results of the study revealed that the Excellent potential zone comprises (80.87 ha), Good (178.3 ha), moderate (377.8 ha), poor (588.9 ha), and very poor (178.8 ha), respectively.

1.Introduction:

Groundwater is one of the most valuable natural resources, which supports human health, economic development and ecological diversity. Because of its several inherent qualities it has become an immensely important and dependable source of water supplies in all climatic regions including both urban and rural areas of developed and developing countries. Groundwater is a form of water occupying all the voids within a geological stratum. Water bearing formations of the earth’s crust act as conduits for transmission and as reservoirs for storing water. The groundwater occurrence in a geological formation and the scope for its exploitation primarily depends on the formation of porosity. High relief and steep slopes impart higher runoff, while topographical depressions increase infiltration. An area of high drainage density also increases surface runoff compared to a low drainage density area. Surface water bodies like rivers, ponds, etc., can act as recharge zones. Over the years the growing importance of groundwater based on an increasing need has led to unscientific exploitation of groundwater creating a water stress condition.

This alarming situation calls for a cost and time effective technique for proper evaluation of groundwater resources and management planning. A groundwater developing program requires a large volume of data from various sources. Hence, identification and quantization of these features are important for generating a groundwater potential model of a study area. Currently groundwater is gaining more attention due to drought problem, rural water supply, irrigation project and low cost of development it requires. Despite the extensive research and technological advancement, the study of groundwater has remained more risky, as there is no direct method to facilitate observation of water below the surface. Its presence or absence can only be inferred indirectly by studying the geological and surface parameters.

The different hydrogeological themes can be used to identify the groundwater potential zone of the present area. The remote sensing and Geographic information system (GIS) tool can open new path in water resource studies. Analysis of remote sensing data along the survey of India (SOI) topographical sheets and collateral information with necessary ground truth verifications help in generating the baseline information for groundwater targeting. Identification of groundwater occurrence location using remote sensing data is based on indirect analysis of directly observable terrain features like geological structures, geomorphology, and their hydrologic characteristics. Also lineaments play significant role in groundwater exploration in all type of terrain. Application of GIS and RS can also be considered for multi criteria analysis in resource evaluation and hydro geomorphological mapping for water resource management.

2.Study Area:

2.1 GENERAL

The primary objective of the study is to contribute towards systematic groundwater studies utilizing Remote Sensing and
Geographic Information Systems (GIS) in the delineation of groundwater potential areas. Nagapattinam district is one of the 38 districts of Tamil nadu state in southern India. It is a coastal district and covering a total area of 2,715.83 km2. Out of the total area, around 1,261.49 km2 are classified as wetland, 618.80 km2 as dry land and the remaining 835.48 km2 as government land. It is located in the longitude between 79°35'0" Eto79°50'0"E and latitude between 10° 35’0"N to 11°25’0"N with the MSL 9 m up.

It is situated in the Cuddalore district north, Thanjavur district west, southern and western sides totally covered by Bay of Bengal. The physiographic terrain is a plain topography with gentle gradient towards the coast. Major River is Coleroon, recent formations with alluvium. Type of aquifer is fairly thick discontinuous confined fresh groundwater overlaid by saline water towards the coast with the water level ranged from 1 m to 8 m. The soil is predominantly sandy in texture and clayey in certain pockets, with slight salinity/alkalinity.

The soil in the region belongs to Valudalakudi series; dark brown to brown, deep, sandy and possessing characteristics of mild-to-moderate alkalinity levels. The area lying between Nagapattinam and Vedaranyam is dominated by sand dunes, and cultivated soils are mostly sandy in texture. Regarding the water table, fresh water is overlying saline groundwater. The cultivation depends primarily on rainfall, supplemented by underground water. The district receives rainfall under the influence of both south-west and north-east monsoon.

A good part of the rainfall occurs as very intensive storms resulting mainly from cyclones generated in the Bay of Bengal especially during north-east monsoon. The area receives an average of 1,372 mm of rainfall annually; nearly 76 % occur during the north-east monsoon, followed by 17.3 % during the south-west monsoon. The rainfall pattern in the district shows interesting features. Annual rainfall, which is 1,500 mm at Vedaranyam, the south-east corner of the district, rapidly decreases to about 1,100 mm towards west of the district. The district enjoys humid and tropical climate with hot summers, significant to mild winters and moderate to heavy rainfall. The temperatures vary from 40.6 to 19.3° C with sharp fall in night temperatures during monsoon period. The relative humidity ranges from 70 to 77 %, and it is high during the period of October to November.

Fig.1 Study Area Map

2.2 HYDROGEOLOGY

The study of groundwater Hydrogeology deals with how water gets into the ground, how it flows in the subsurface and how groundwater interacts with the surrounding soil and rock. Ground Water occurs under the phreatic condition and wherever there are deep seated fractures, it occurs under semi-confined to confined conditions. Granites and gneisses yield moderately compared to the yield in Charnockites.
Occurrence and storage of groundwater depend upon three factors viz., Geology, Topography and rainfall in the form of precipitation. Apart from Geology, wide variation in topographic profile and intensity of rainfall constitutes the prime factors of groundwater recharge. Aquifers are part of the more complex hydro geological system and the behaviour of the entire system cannot be interpreted easily. In hard rock terrain the occurrence of Ground Water is limited to top weathered, fissured and fractured zone which extends to maximum 30 m on an average it is about 10-15 m in Nagapattinam District.

In Sedimentary formations, the presence of primary inter granular porosity enhances the transmitting capacity of groundwater where the yield will be appreciable. The sedimentary area which occupies the eastern part of the district along the coastal tract is more favourable for groundwater recharge. Ground Water occurs both in semi confined and confined conditions. A brief description of occurrence of groundwater in each formation is furnished below.

Alluvial Formations: In the river alluvium groundwater occurs under water table condition. The maximum thickness is 37 m and the average thickness of the aquifer is approximately 12 m. These formations are porous and permeable which have good water bearing zones
Tertiary Cuddalore Sandstone: Tertiary formations are represented by Cuddalore Sandstone and characterised as fluvial to brackish marine deposits. Predominantly this formation is divided into Lower and Upper Cuddalore formations. In the Upper Cuddalore formations the groundwater occurs in semi confined conditions, whereas in the Lower Cuddalore the groundwater occurs in confined condition with good groundwater potential.
Cretaceous Formations: Groundwater occurring in the lens shape in the sandy clay lenses and fine sand is underlain by white and black clay beds which constitute phreatic aquifer depth which ranges 10m to 15m below ground level. Phreatic aquifer in Limestone is potential due to the presence of Oolitic Limestone.
Hard Rock Formations: Groundwater occurs under water table conditions but the intensity of weathering, joint, fracture and its development is much less in other type of rocks when compared to gneissic formation. The groundwater potential is low, when compared with the gneissic formations.

3.METHODOLOGY

The methodology of the study involves collecting and preparing all the seven thematic layers using ArcGIS software. First stage includes development of spatial data base map by using survey of India (SOI) toposheet nos. 58N/13, 14, and 15 on 1: 50000 scale and satellite data. GIS and remote sensing technology is applied to prepare various thematic maps with reference to groundwater like drainage density.

The second stage involved preparation of digital elevation model (DEM). DEM is used to prepare slope, aspect, flow accumulation and stream order. Methodology is widely used for preparing runoff potential map for small to medium size engaged drainage basin.

In the third stage, digital image processing of the satellite data has been done. Then followed by creation of land use and land cover map by using supervised method. Then creation of other important data which is used to determine the ground water potential at the later stage like geological map, lineament density map, soil map and rainfall map.

In the fourth stage all above themes are further processed and analysed in overlay and ranking is given to evaluate suitable groundwater potential zone. All the thematic layers will overlay by using GIS to find the final integrated output of groundwater potential zones in the present study, slope, drainage density, Land use and land cover, geology and lineament density are considered for the identification of groundwater potential.

Fig.2 Flowchart of methodology for the study

4.FACTORS INFLUENCING GROUNDWATER

4.1 DRAINAGE DENSITY

The drainage density is inversely related to permeability which influences runoff and quantity of infiltration. By determining the flow direction and flow accumulation of the region using ArcGIS software, the line density or the drainage density was obtained and was grouped into five classes. The drain age density of the study area varies between 0 and 1.49 km/km2. About 75% of the total area constitutes low and very low drainage density as the region has flat terrain. Only 15% of the total area constitutes a very high drainage density between 0.591 and 1.49 km/km2.

An area of high drainage density increases surface runoff compared to a low drainage density area. Surface water bodies like rivers, ponds, etc., can act as recharge zones. Factors such as geology, land use, geomorphology, and topography affect the drainage density of a region. The highest value of drainage density represents the highest chance of runoff which eventually leads to less percolation. Hence the lesser drainage density, the higher is the probability of recharge or potential groundwater zone. The entire drainage map is divided into five categories as in Table 1 and depicted in (Fig.3).

Fig.3 Drainage density map of Nagapattinam

Table1.Drainage Density Category

4.2 LINEAMENT DENSITY

Lineaments are the presence of joints, faults, and fractures which provide a route for percolated water and also an indirect indicator of a potential zone. Lineaments are structurally controlled linear or curvilinear features, which are identified from the satellite imagery by their relatively linear alignments. Lineament density of an area can directly reveal the groundwater potential since the presence of lineaments usually denotes a permeable zone.

Lineament density maps is a measure of quantitative length of linear feature expressed in (Km/Km2). All lineaments are also classified based on their length into the following two types. - For quantification purpose, lineament with length < 3 km is classified as a minor lineament. Lineament with length > 3 km is classified as a major lineament. Lineament density for the study area varies from 0 to 2.55 Km/Km2 and was categorized into five classes for the convenient of assigning weights. The highest value of lineament density indicates the highest potential for groundwater recharge. In present study area with very high lineament density (1.46 - 2.55) having good groundwater potential whereas area with very low lineament density (0-0.27) having poor groundwater potential. The entire map classified in five categories as follow and depicted in (Fig.4).

Fig.4 Lineament Density Map of Nagapattinam

Table2.Lineament Density Category

4.3 SLOPE

Slope is one of the important terrain parameters which are explained by horizontal spacing of the contours. In general, in the vector form closely spaced contours represent steeper slopes and sparse contours exhibit gentle slope whereas in the elevation output raster every cell has a slope value. Here, the lower slope values indicate the flatter terrain (gentle slope) and higher slope values correspond to steeper slope of the terrain. In the elevation raster, slope is measured by the identification of maximum rate of change in value from each cell to neighboring cells.

The slope values are calculated either in percentage or degrees in both vector and raster forms. (Fig.5). In the nearly level slope area (0 - 0.51) degree, the surface runoff is slow allowing more time for rainwater to percolate and consider good groundwater potential zone, where as strong slope area (3.81 - 16.4) degree, facilitate high runoff allowing less residence time for rainwater hence comparatively less infiltration and poor groundwater potential. The entire slope map is divided into five categories as in Table 3.

Fig.5 Slope Map of Nagapattinam

Table3. Slope gradient and category

4.4 LAND USE/ LAND COVER

Land use/land cover mapping is one of the important applications of remote sensing. Land use plays a significant role in the development of groundwater resources. It controls many hydrogeological processes in the water cycle viz., infiltration, evapotranspiration, surface runoff etc. surface cover provides roughness to the surface, reduce discharge thereby increases the infiltration. In the forest areas, infiltration will be more and runoff will be less whereas in urban areas rate of infiltration may decrease. Remote sensing provides excellent information with regard to spatial distribution of vegetation type and land use in less time and low cost in comparison to conventional data. Supervised classification of study area shows that major portion in land use is crop land covering area 74921 ha, barren land covering area 55465.592ha., water body covering area 15595.75ha., vegetation covering area 20888.67ha., settlement covering area 32990.40ha and sandbar covering area 15124.56ha as in Table 4 depicted in (Fig.6).

Fig.6 Land use and land cover Map of Nagapattinam

Table4. LULC areas and classification

4.5 SOIL

The topmost layer above the earth which serves as the medium for water percolation is soil. The rate of infiltration depends on the permeability and water holding capacity of the soil. The amount of water reaching the water table depends on the extent of the unconsolidated zone lying beneath the soil. The soil map was obtained from world soil map and has two classes of soil. (Fig. 7) shows various soil groups in the study area. 46 % of area contained eutric fluvisols and 54 % of the region was found to have dystric regosols of the study area, respectively.

Among these two group Fluvisols occur only on the alluvium of the water flow of the area. These soils have been influenced by flood and significant fluctuation of ground water level. Eutric Fluvisols contain, generally, high content of clay which is not a favouring condition for groundwater recharge as it having very poor to good category of potential zone in the study area. Dystric means deficient, and Regosols present weakly developed soils from the Regolith, which means loose, unconsolidated and broken rock material covering bedrock. Dystric Regosols in this area are characterized as Carbonate deficient weakly developed soils formed from non-carbonate wind-blown sand as it poor to excellent category of potential zone in the study area.

Fig.7 Soil Map of Nagapattinam

4.6 GEOLOGY

The properties of different water-bearing geological formations play an important role in the occurrence and movement of groundwater. Geologic setting plays a vital role in the occurrence and distribution of groundwater in any terrain. The published geological map of the Geological Survey of India was used for delineating geological units of the study area. The study area falls on only one geological parameter as quaternary formations such as aeolian, coastal, alluvial and fluvial sediments.
The Quaternary deposits represented by the river deposits of Ponnaiyar and Varahanadhi spread over as patches in Nagapattinam District. The alluvium consists of unconsolidated sands, gravelly sands, clays and clayey sands. The thickness of the sands ranges between 15 and 25 m in the alluvial formation which also form potential aquifers. In some areas, sand stone of tertiary formation are the potential groundwater reservoirs. (Fig.8) depicts the geology map of the study area.

Fig.8 Geology Map of Nagapattinam

4.7 RAINFALL

Rainfall is the major water source in the hydrological cycle and the most dominant influencing factor in the groundwater of an area. For the present study, the rainfall data of 2022 is used. The annual rainfall ranges from 1250mm to 1640mm. The spatial distribution map of rainfall was prepared using IDW interpolation method. Based on the maximum and minimum values, the rainfall has been reclassified into five categories such as Very Low (1250–1260mm), Low (1370–1430 mm), Moderate (1440– 1500 mm), High (1510–1560 mm) and Very High (1570–1640 mm) rainfall.

Infiltration depends on the intensity and duration of rainfall. High intensity and short duration rain influence less infiltration and more surface runoff; Low intensity and long duration rain influences high infiltration than run-off. High weights are assigned for high rainfall and vice versa. (Fig.9) depicts the rainfall spatial interpolation map of the Nagapattinam district.

Fig.9 Rainfall Map of Nagapattinam

Table5. Classification of Rainfall distribution

5. ASSIGNING RANK AND WEIGHT

The groundwater potential zones are obtained by overlaying all the thematic maps in terms of weighted overlay method using the spatial analysis tool in ArcGIS 10.8. During the weighted overlay analysis, the ranks have been given for each individual parameter of each thematic map and the weight is assigned according to the influence of the different parameters. These weightages were assigned based on the expert’s opinion in the different fields.

All the thematic maps are converted into raster format and superimposed by weighted overlay method (rank and weight wise thematic maps and integrated with one another through GIS (Arc/Info grid environment). For assigning the weight, the geology and rainfall were assigned higher weight, whereas the lineament density and drainage density were assigned lower weight. After assigning weights to different parameters, individual ranks are given for sub variable. In this process, the GIS layer on lineament density, rainfall, slope and drainage density were analyzed carefully and ranks are assigned to their sub variable.

The maximum value is given to the feature with highest groundwater potentiality and the minimum given to the lowest potential feature. As far as slope is concerned, the highest rank value is assigned for gentle slope and low rank value is assigned to higher slope. The higher rank factors are assigned to low drainage density because the low drainage density factor Favors more infiltration than surface runoff. Lower value followed by higher drainage density. Among the various lineament density classes, the very high lineament density category is assigned higher rank value as this category has greater chance for groundwater infiltration. Lower value is assigned for very low lineament density. In LULC high rank is assigned to crop land and low value is assigned to barren land. The overall analysis is tabulated in (Table 6). Multiplying the weightages with corresponding rank gives weighted average, which is called as Cumulative Suitability Index (CSI).

CSI = Σ (Rank x Weightage)

Based on the CSI values, the groundwater potential zones may be categorized as Excellent, Good, Moderate, poor, and very poor.

Table 6. Rank and weight for different parameter of groundwater potential zone

6.RESULTS AND DISCUSSION

The groundwater potential map obtained from the Multi-Criterion Analysis is shown in the (Fig. 10). Based on the CSI values, the area in the map has been prioritized into five categories as Excellent, Good, Moderate, poor and very poor groundwater potential zones. The result shows that the most of the parts of the study area comes under poor groundwater potential zone, because of eutric fluvisols soil content which consists high amount of clay that is not favouring for potential zone and moderate parts comes under excellent category. Even though the groundwater level is very high in many parts of the study area as per the analysis, the over exploitation of the groundwater experienced in the study area causes ground-water depletion i.e., longterm water-level declines caused by sustained ground water pumping.

Since the groundwater is pumped at a faster rate than it can be recharged, it creates negative effects over the environment and the people who make use of the groundwater in this study area. Since the study area is situated near the coastal zone, the over exploitation of the groundwater makes entry of the salt water into the land. The entered salt water mixes with the groundwater and cause the groundwater unfit for the portal use. Due to this groundwater depletion, Land Subsidence occurs in many places and sometimes when water is taken out of the soil, the soil collapses, compacts, and drops. Hence, there is an urgent need for artificial recharge of groundwater by augmenting the natural infiltration of precipitation into subsurface formation by some suitable method of recharge in order to overcome all these problems.

Fig.10 Groundwater potential zone map of Nagapattinam

Table7.Groundwater potential zones of study area

7.CONCLUSION

Since Remote Sensing and GIS techniques were used for the study, the outputs achieved are nearly accurate compared to the other conventional methods. The generated groundwater potential map can be utilized for the optimum utilization of the groundwater and for the identification of the zonation for the artificial recharge. The optimum utilization can be achieved by placing the well in the higher groundwater potential zones.

At higher groundwater potential zone, natural infiltration will be surely good, so these zones can easily recharge up during the rainy seasons. If the groundwater is pumped from these zones, then there will be no need for providing the separate recharge structures in these zones, it will automatically recharge up when it rains and there will be corresponding reduction in the cost of boring and pumping, since the depth of boring will be reduced. The zones that have the low groundwater potential (i.e., where the natural infiltration is not desirable) have to be recharged artificially and these zones can be identified from the groundwater potential map generated for the study.

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Groundwater Potential Zone Mapping Using Analytical Hierarchy Process(AHP) And GIS, FOR NAGAPATTINAM DISTRICT, TAMILNADU, INDIA

-Shanmuga Priya, Student of AGSRT