S. D. Dhiman
A. K. Keshari
Assistant Professor, Department of Civil Engineering
Indian Institute of Technology Delhi, Hauz Khas, New Delhi
The increasing amount of multiple data sets being made available from various sources has created a need for efficient capture, storage, management, retrieval and analysis of geoenvironmental data to address various groundwater pollution problems of varying nature, dimension and complexity, cropping at local, regional and basin scale worldwide. Geographic Information System (GIS) has emerged as an effective tool for relating and integrating vast volumes of different data types, obtained from different sources and compiled on different scales. In this paper, a methodology is presented that utilises GIS to quantify the spatial geologic data and statistical analysis to determine the relation between groundwater quality parameters and geological units. The methodology is illustrated through an application to a study area located in the Western Indian Aquifer (WIA) system. The areal extents of geologic units in the study area are identified and their spatial distributions are quantified using GIS. The spatial extents of identified geological units are then correlated with the groundwater quality parameters using statistical analysis. Results reveal that there is a high negative correlation between calcium and ultrabasic rock, a high positive correlation between fluoride and ultrabasic rock, a moderately high positive correlation between bicarbonate and ultrabasic rock, and a very high positive correlation between pH, gneiss and schists. Such studies within GIS environment help in better understanding of the water-rock interactions, and thus will prove useful in formulating numerical models to study transport mechanisms of various chemical species in different hydrogeologic and chemical environments.
Rajasthan is situated in the western part of Indian subcontinent. The Dungarpur district is located in the southern part of Rajasthan between 23020′ and 24001′ north latitude and 73021′ and 74023′ east longitude. In the west, it has common border with Gujarat. The total area of the district is 3770 square kilometers. In the north and east, the landscape is rugged and wild, but towards southwest border, the features seem to merge in the topography of Gujarat region. The eastern portion slopes down towards the basin of Mahi. The rugged and wild aspect of the region is attributed to the offshoots of the Arravalli’s. Two perennial rivers viz., Mahi and Som flow through the district. The Mahi separates the district from Banaswara, and Som forms a natural boundary with Udaipur district. The non- perennial streams are Jakham, Majhan, Vatrak, Bhader, Gangli, Sapan and VeriGanga. The water table in the district varies from 5 to 15 meters below ground level. The normal annual rainfall in the district is about 761.7 mm. The district has a dry climate with hot season milder than in the desert region of Rajasthan. The maximum temperature in summer is about 380 C and minimum temperature in winter is about 90 C.
The geological system of the district belongs to the Pre-Cambrian Aravalli system. Slates are in abundance in the central region and are largely identified with veins of quartz particularly in the area north and east of Dungarpur town. Some patches of pegmatite and granite intrusions may also be found in the slates. The ultrabasic rocks have been observed as potential sources of asbestos, chromite, magnesite and talc. Among minerals, fluorite, beryl, asbestos and soapstone are the main ones. Besides, copper, mica, kyanite and lead ores have also been found.
The study area for GIS based correlation between groundwater quality parameters and geological units falls between 230 41′ and 240 north latitude and 730 45′ and 740-east longitude. The geological information for the study area has been taken from the work done by Gupta and Mukherjee (1938) and the hydrogeological conditions are based on the investigations carried out by Jethra (1998) for the Dungarpur district of Rajasthan.
|Groundwater chemical species||Min||Max||Avg||S D||SE|
|Parameter/ Bedrock||Gneiss||Quartzite & Quartz-schist||Schists rock||Ultrabasic|
Database Creation and Methodology
The study area boundary, villages, rivers and streams along with ponds, were digitized using the SOI Toposheet 46 E. The drainage pattern and digitized geological map of the study area were obtained. The contour maps of groundwater quality parameter fluoride, calcium, bicarbonates and pH were imported into the GIS and Digital Image Processing software (ILWIS) for georeferencing. The contour interpolation for each groundwater quality parameter was carried out and was rasterized to prepare the spatial variation map. A segment geological map of the study area was also polygonised and then rasterized. Each rasterized groundwater quality parameter map was crossed with the rasterized geological map using the CROSS operation in ILWIS. The CROSS-operation will essentially help in getting the concentration and areal geological unit information. Thus GIS based correlation can be found by knowing the concentration of the parameter of interest and the areal extent of geological unit under it.
The drainage pattern in the study area is shown in Fig. 1.
The digitised geological map of the study area is shown in Fig.2.
The bedrock as delineated in the study area chiefly comprises of ultrabasic rocks, gneiss, schists, quartize and quartz-schists. The groundwater quality parameters considered are namely fluoride, calcium, bicarbonate and pH. The spatial distribution maps for the groundwater quality parameters namely calcium, fluoride, bicarbonate and pH are shown in Fig 3-6.
The degree of fluoride contamination and identification of hydrogeochemical process in mobilising the fluoride contamination for this study area has been published by Keshari and Dhiman (2001).
By using the GIS and a statistical package a significant correlation is found between the groundwater quality parameters and the geological units in the Western Indian Aquifer system. The statistical parameters for groundwater chemical species (in meq/L) are shown in Table 1.
Table 1 reveals that the range for calcium lies between 1 to 3.5 meq/L, fluoride lies between 0.05 to 0.17 meq/L, bicarbonates between 3.69 to 6.2 meq/L and the pH range in the study area is between 7.34 to 7.77. The low standard error exists for all the parameters considered for the analysis.
The correlation coefficients between groundwater quality parameters and the geological units as obtained are shown in Table 2.
A high negative correlation (-0.84) exists between calcium and ultrabasic rock, whereas a moderate negative correlation (-0.55) is observed with the schist and a low positive correlation 0.19 with gneiss for the calcium. A high positive correlation (0.8) exists between fluoride with ultrabasic rock, and a moderate positive correlation with gneiss and quartzite and quaztzite schist. However, fluoride is showing a very poor correlation with schists. A moderately high positive correlation of 0.6 exists between bicarbonate and ultrabasic rocks and a moderate positive correlation of 0.3 with schist and moderately low 0.18 with quartzite and quartz-schists, and a very low correlation of 0.06 with gneiss. A very high positive correlation of pH with gneiss (0.94) and schists (0.93), whereas a moderately high with quartzite and quartz-schist (0.79) and ultrabasic rock (0.62).
Geographical information system based correlation analysis has been carried out to establish the relation between the concentration of the chemical species and the geological units in a spatial domain. Results for the study area reveal that there exists a high negative correlation between calcium and ultrabasic rock, a high positive correlation between fluoride and ultrabasic rock, and a moderately high correlation between bicarbonate and ultrabasic rock and a very high correlation between pH and gneiss and schists. Such studies within GIS environment help in understanding the water-rock interactions and thus will prove useful in formulating numerical models to study transport mechanisms of various chemical species in different hydrogeologic and chemical environments.
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