Geophysical Viewpoints for Groundwater Resource Development and Management in Coastal Tracts

Abstract

Ever since the beginning of human civilization, people have settled along riverbanks and coasts. Throughout the world, the extent of coastal regions that have sustainable groundwater bodies is shrinking by the day. The problems that dominate in the use of such groundwater are depletion due to overdraft, and salinisation arising from pollution and/or sea water (saline) intrusion. Around the world, especially in regions with high population density, dynamic tube-well-irrigated agriculture and insufficient surface water, many consequences of the overdevelopment of groundwater are increasingly evident. The most common symptom is decline in water tables. In coastal areas, the most serious consequence of intensified pumping of groundwater for irrigation is saline ingress into coastal aquifers. All these problems will impair the region’s water supply capacity and its ability to meet the demand from its growing population. One of the most serious side-effects caused by groundwater depletion is saline intrusion in coastal aquifers like those in Egypt, Turkey, China and India. Thus the need for sustainable groundwater development warrants detailed mapping of the saline-fresh groundwater interface and monitoring of salt water ingress. In this respect, geophysical investigations can help in the assessment of sub-surface hydrogeological conditions and optimization of the number and location of boreholes to be drilled for sustainable water resource development in any particular coastal area. Use of such techniques is quite economical. The purpose of applying geophysics is to enable development of a picture of the variations in physical properties of the sub-surface horizons and translate them subjectively into a profile of the hydrogeological situation. While surface geophysical techniques help to define the negative and positive areas before taking up the drilling programme, post-drilling down-hole geophysical logging enables identification of the depth zones minutely, confirmation of the hydrogeological characterization and specific emplacement of the cement seals in the boreholes to avoid mixing of groundwaters of different qualities by vertical flow. Geophysical logging techniques also help in deciphering the regional as well as local geometry of the aquifers and the direction of groundwater flow in them, as well as in monitoring variations in water quality. No single geophysical discipline or technique seems to be able to provide the wide range of data required to unravel enigmatic sub-surface hydrogeological conditions. When such unravelling is needed, subjective integration of the results from different techniques becomes essential to minimize ambiguity and make the exploration relatively foolproof. It should also be noted that geophysical exploration in coastal, areas demands a greater level of accuracy in data acquisition and interpretation than similar exploratory work inland. In such areas, resistivity methods face limitations such as the development of very low potential, transition in resistivity with depth, suppression of thin layers with intermediate resistivity values and severe ambiguity in layer parameters, because of the equivalence. In spite of this, the resistivity method has found wide application in coastal areas, mainly in the assessment groundwater quality. Reliability in estimating layer parameters is enhanced if the resistivity method is supplemented with seismic, induced polarization, electromagnetic and/or other geophysical techniques. A detailed geophysical case study from West Bengal in India is presented below. The geophysical inferences concerning coastal hydrogeological conditions, their scope in defining the zones prone to sea water encroachment and the potential areas of groundwater development are highlighted in the text.