Analysis and control of saline water intrusion in coastal aquifers of Purba Midnapur district of West Bengal, India

Abstract

Saline water ingression into fresh groundwater aquifers happens in the region of coastal areas of whenever seawater displaces or mixes with fresh groundwater. This situation happens in coastal regions having hydraulic continuity with the sea. Seawater intrusion into freshwater coastal aquifers is likely to cause serious problems if only such aquifer is tapped for domestic water supply, irrigation or any other specific purposes. Salinity in coastal area is a widespread problem of Purba Midnapur district in West Bengal. Amongst significant contributions in the relevant area of investigation, the work of Wang (1965), Bennett et al. (1968), Yeh (1968), Goswami (1968), Pinder and Cooper (1970), Fetter (1972), Mughal and Awan (1977), Dasgupta and Gaikwad (1987), Reilly and Goodman (1987), Fand et al. (1987), Schincariol and Schwartz (1990), Motz (1992), Kececioglu and Jiang (1994), Xue et al. (1995), Mahesha (2001), Barlow (2003), Raghu Babu et al. (2004), Chandra (2005), Chachadi (2005), Papadopoulou et al. (2005), Mohan and Pramada (2005), Gallardo and Marui (2007), Mahesha (2009),Werner and Simmons (2009), Olufemi et al. (2010), Bianchi et al. (2011), Sherif et al. (2012), Ayolabi et al. (2013), Das et al. (2014), Sahu (2014), Adedotun et al. (2015), Grundmann et al. (2016), Toste et al. (2017) and Alfarrah and Walraevens (2018) were found to be worthy of note. This research is aimed towards conducting theoretical investigation, laboratory experimentation, simulation of groundwater level modelling by computing software and field investigation and control of saline water ingression into coastal aquifers with special emphasis on Purba Midnapur. Here the derivation of formula for permissible discharge of wells from shallow aquifers is done in general. The horizontal infiltration galleries called qanat coupled with vertical risers are presented as a viable solution to the problem of upconing below deep vertical wells. The vertical wells may not be feasible in several situations and qanat coupled with vertical risers can be effectively used in such cases. An experimental study has also been conducted to determine the hydraulic conductivity of well-graded fine sand experimentally using a Falling-Head Permeameter and the variation of hydraulic conductivity of the sand sample with saline concentration and period of submergence has been studied. Further, an experimental study regarding variation of kinematic viscosity and density of sodium chloride solutions for the variation of saline concentration and temperature has been conducted. Based on these findings, the governing non-linear flow Forchheimer’s equation has additionally been modified. The flow pattern of seepage due to saline water intrusion and freshwater recharge into granular porous medium has been conducted using standard model test. Three dimensional views of piezometric surface, pre-monsoon and post-monsoon depth to groundwater level and premonsoon piezometric surfaces contours had also been developed. For the year 2014 and 2015, the pre-monsoon piezometric surface contours show depressions near Ramnagar, Bhagbanpur, and Sutahata. In the predominant climatic situation in the zone, the pre- monsoon condition is undeniably critical as compare the post-monsoon condition. Consequently, assessment of the probable for salinisation is conducted considering the pre-monsoon position. It is very clear that the Ramnagar area which includes the tourist resort of Digha and the town of Contai, is highly prone to saline water intrusion as is the Sutahata area which includes the industrial town of Haldia. The Visual MODFLOW 2000 has been utilized to analyze the groundwater level simulation for entire Purba Midnapur. The simulated data for the year 2012 based on the measured data of 2002 and the observed data of 2012 have been compared and a correlation coefficient is found out to signify data validity. Also simulated groundwater level data of 2019 to 2023 based on well discharge data of 2002, 2012 and combined data of 2002 and 2012 have been correlated for its justification. The groundwater flow occurs from south to north direction of Purba Midnapur, as the saline water intrusion from the Bay of Bengal takes place into the aquifers towards inland direction. In the real field situation saline water encroachments have affected the aquifers and it has covered up to 50 km of location from Kalindi (near seashore) to Nandakumar (inland) location. This chapter focuses on the prediction of future groundwater levels as a potential groundwater management scenario in the region concern. During field investigation, the water samples were collected from different depths at selected locations of coastal West Bengal, India. To expedite the process, reasonable assistant is taken from the Public Health Engineering Directorate under Government of West Bengal. Collected samples were analyzed at the laboratory of State Water Investigation Directorate under Government of West Bengal to investigation the chemical and hydro geological characterization. Based on the field data, contours are plotted at the selected coastal zone for Chloride, Specific Conductivity, Iron content, Total Dissolved Solids, etc. These results are used to determine the probable path of saline water ingression. Owing to heterogeneity of the aquifer in the selected areas, the spatial irregularity in salinity is significant. An innovative method of control of saltwater intrusion into the coastal aquifers has been recommended by the author. This new method comprises of withdrawal by qanat-well structures with reasonable compensation by rainwater harvesting by methods for recharge tank and recharge well. The salient features of the methodology are depicted by considering a design example adopted by the author in the Contai Polytechnic Institute Campus of the area of Purba Midnapur. The plan of the study area in Purba Midnapur has been divided into six zones based on aquifers soil characteristics and status of piezometric surface which is recognized as aquifer improvement plan and imposing certain restriction of abstraction of groundwater by deep tube in the area concern, will help in recharging the aquifer, control the danger of saline water intrusion and enhancing the quality of groundwater. Other plan of solution to meet future freshwater demand in the area, using alternative freshwater sources has also been additionally been encouraged, the existing Subarnarekha river water are collected and distribution to the various purpose after proper treatment. Lastly, a series of important conclusions are drawn based on the specific research findings. Keywords: Coastal Environment, Coastal Structures, Water Sampling, Chemical Analysis, Case Studies, Geotechnical Investigation, Hydrogeology, Laboratory Experimentations, Mathematical Analysis, Quantitative Analysis, Simulation of Groundwater level Modelling, Management and Control of Seawater Intrusion, Porous Media Flow.