Imaging saline water ingress in coastal aquifers of Maharashtra using electrical resistivity technique constrained by geochemical data

Abstract

Due to rapid development in and around the coastal zones, there is a growing demand on the water resources. These regions are vulnerable to erosion, flooding due to river floods or tidal waves or rising sea level, saline water intrusion and contamination of the aquifers. The coastal zones are of attraction to tourists, besides activates like agriculture practices, aqua-culture, industries and effluents, which are having an immense bearing on environment apart from economic, social and cultural significance. Hence, appropriate monitoring and protection measures are vital for their preservation and for future use. Proper actions are necessary for planning sustainable resource management of both the marine and terrestrial components for the promotion of economic and social welfare of coastal zones. The rapid pace in developmental activities in coastal Maharashtra has severely affected the water resources, thereby depleting the available groundwater. No systematic geophysical and geochemical studies have been carried out in these coastal regions. In such areas, exploration and delineation of fresh water aquifers from saline water aquifers becomes the principal objective. Need was felt to integrate geophysical data along with geochemical data to evaluate the subsurface geologic stratas, aquifer geometry and seawater intrusion in several coastal aquifers of the country. It is also envisaged that the lowering of resistivity was due to the encroachment of seawater in to the freshwater zones. Water chemistry plays a very important role for the study of its quality in the costal aquifers and thus assessing seawater incursion through an aquifer in coastal belts should be undertaken periodically. Hydrochemical parameters have been used by several workers to delineate seawater incursion process, which can be helpful to control the water quality in coastal areas, evaluate the relationship between total dissolved solids with chloride, sodium, magnesium and sulphate concentrations of groundwater for pre- and post-monsoon seasons. viii Amongst all surface geophysical techniques, the electrical resistivity profiling and vertical electrical sounding (VES) methods have been extensively used to demarcate saline water/fresh water boundaries in different coastal areas with success. Where in electrical resistivity method found useful to map the contaminants. Conventional electrical methods utilize point measurements and thus limited information is obtained. On other hand electrical resistivity imaging (ERI) offers a better understanding of salinity changes both depth and spatial wise. In this thesis, it is proposed to determine the extent and severity of saltwater intrusion in the aquifers underlying northern part of Sindhudurg district, western Maharashtra using electrical resistivity method in addition to locating fresh groundwater pockets to the needs of the society. A total of 86 vertical electrical soundings (VES) and 66 water samples were acquired. Two-dimensional electrical resistivity tomography (ERT) integrated with geo-electrical soundings were applied to delineate diverse water-bearing formations and the arrangement of the interface between them. Besides, collection of hydrochemical data and its analysis is also taken up so as to verify the salt water intrusion. Groundwater chemistry and electrical resistivity results were integrated to investigate the status of coastal groundwater salinity and saline water ingress in northern Sindhudurg district, Maharashtra. The integration showed good correlation between the water resistivity and the chloride concentration of groundwater shows that salinity is the most important factor controlling resistivity. Also, other important geophysical indicators viz. Dar-Zarrouk parameters (longitudinal conductance and transverse resistance) and electrical anisotropy are used to delineate the subsurface fractures which might be favourable zones for groundwater exploration and in resolving clearly the fresh and saline water aquifers occurring in the coastal regions. These results were also compared with the available litho-log data near the observation stations. Thus the present study attempt finds its usefulness in estimating aquifer parameters in the absence of data from pumping tests. ix Thus the present thesis work mainly consists  Utilization of geophysical and geochemical data for delineating the regions contaminated by saline water, as well as to demarcate areas for fresh water in the northern part of the Sindhudurg district, west coast of Maharashtra.  Water quality and hydrogeochemical processes that control groundwater composition of the coastal aquifers to assess its suitability for domestic and irrigation purposes.  Deciphering the complex geology of the area so as to address societal issues like groundwater exploration/ contamination.