Geochemical variations due to salinization in groundwater along the southeast coast of India

Research Article Geochemical variations due to salinization in groundwater along the southeast coast of India Thilagavathi Rajendiran1 · Chidambaram Sabarathinam1,2 · Thivya Chandrasekar3 · Banajarani Panda1 · Mahalakshmi Mathivanan4 · Ganesh Nagappan1 · Devaraj Natesan1 · Meenu Ghai5 · Dhiraj Kumar Singh6 · Ramanathan Alagappan7 Received: 1 October 2020 / Accepted: 1 April 2021 © The Author(s) 2021  OPEN Abstract In recent years, the extraction of groundwater (GW) in coastal aquifers has rendered the fragile aquifers more saline due to the sea water intrusion. Groundwater from the coastal aquifers of the Pudhucherry region were sampled to study the process of salinization. An integrated approach was adopted to identify the salinization process, by coordinating the results of borehole geophysics, rainfall pattern, water level variation, hydrochemical characters and multivariate statistical analysis. A total of 136 groundwater samples were collected during two different seasons, southwest monsoon (SWM) and northeast monsoon (NEM). The major cations and anions were analyzed adopting standard procedures. Resistivity and litholog indicate that the southeastern (SE) part of the study region has lower resistivity than in north. Based on electrical conductivity (EC) and total dissolved solids (TDS) values, most of the samples are potable, except for few samples from southeastern region. The study results indicated that higher values of Na, EC, K, S O4, Mg and Cl were observed during NEM, indicating leaching of salt into the aquifer and ion exchange process. The predominant hydrochemical facies of groundwater was Na-Cl and Ca–Mg–Cl type reflecting the saline water and the mixing process of saltwater and fresh groundwater, respectively. Though more number of samples with higher EC was noted in NEM, the results of PCA and correlation analysis indicate the predominance of leaching of salts and intense agricultural activities. The process of sea water intrusion was observed to be dominant during SWM. Keywords Groundwater · Saline water intrusion · Fresh water · Factor score 1 Introduction Aquifers along the coastal area are important sources of water, and it is usually affected by the diverse landuse practices and geochemical contaminants. Eight percentage of global population live along the coastal region and utilize the local aquifers for their needs [1–5]. The groundwater extraction from the coastal aquifers [6–8], leads to reduction in available fresh water (FW) and thus causes the lowering of water table resulting in the decrease in groundwater quality. Various factors such as seawater intrusion, rock–water interaction, excess usage of groundwater, sea level rise, temperature variance, hydrogeological history of the area, * Chidambaram Sabarathinam, ; * Mahalakshmi Mathivanan, | 1 Department of Earth Sciences, Annamalai University, Annamalai Nagar, Chidambaram, Tamil Nadu, India. 2Water Research Center, Kuwait Institute for Scientific Research, Kuwait City, Kuwait. 3Department of Geology, Madras University, Chennai, India. 4School of Civil Engineering, Sastra Deemed University, Tamil Nadu, Thanjavur, India. 5Department of Chemistry, Kishan Lal Public College Rewari, Haryana, India. 6Grass Roots Research and Creation India (P) Ltd, Noida, India. 7School of Environmental Sciences, JNU, New Delhi, India. SN Applied Sciences (2021) 3:581 | https://doi.org/10.1007/s42452-021-04551-2 Vol.:(0123456789) Research Article SN Applied Sciences (2021) 3:581 salt panning, inadequate removal of manure and use of biochemical foods govern the salinization of groundwater [9–11]. Globally, a complex interplay of the abovementioned facts along with seawater intrusion into these coastal aquifers leads to geochemical hazard in groundwater along the coastal aquifers. The amount of groundwater extracted from the coastal aquifers, hydrogeological conditions, the well design, fresh water sources (ponds, rivers or reservoirs) play a major role in the management of the sustainable utilization. Subsequently, the landuse pattern along the shore line determined the quantum of groundwater extracted from the aquifers which results in seawater intrusion, and in fact, this is also facilitated by sea level rise. [4]. The reduction of fresh water flow to the sea has increased the inland surface backwater flow enhancing the infiltration of brackish water into the adjacent flood plains. One of the most important techniques to determine the saline water intrusion is to enhance the monitoring of the groundwater geochemistry [12–18]. Groundwater with higher TDS are generally more saline in nature [19], and saline groundwater along the coast also have reported higher amount of trace metals apart from major cation and anions [14, 20]. The seawater intrusion (SWI) in the coastal region not only renders the quality of groundwater non-potable but also affects the vegetation, soil conditions and the sustainability of groundwater [21]. The mixing of seawater and freshwater in coastal aquifers leads to the lateral migration of sea water into the aquifer affecting the groundwater chemistry. The chief geochemical process observed in the coastal sedimentary aquifers with clay is ion exchange [22]. The mixing of seawater-freshwater and the ion exchange process are inferred from the ion ratios [23–25]. Further, the geochemical nature of the groundwater and the status of the sea water intrusion have been studied in the sedimentary coastal aquifers by using hydrochemical facies evolution diagram [22, 24, 26, 27]. In numerous coastal aquifers [11, 28–32], the hydrochemical facies evolution diagram had proved to be promising in identifying the spatiotemporal variation in seawater intrusion [33–35]. The lithology also plays a vital role in seawater intrusion as the flow of water in the aquifer is governed by the aquifer properties such as porosity and permeability [36]. The spatial variation of major ions in groundwater of the coastal aquifers and the changes in the geochemical processes can be established by integration the geochemical parameters in the GIS platform [37]. Apart from GIS techniques, other softwares like WATCLAST and CHIDAM are also used to plot, assess and categorize the groundwater [38, 39] and to identify the process of mineral weathering, evaporation and its suitability. Modeling softwares like PHREEQC determine the saturation states of minerals and predict their variation with an increase in evaporation Vol:.(1234567890) | https://doi.org/10.1007/s42452-021-04551-2 and salinity [40]. AquaChem software determines that groundwater type and facilitates the plotting of standard diagrams like Wilcox plot and Piper facies. The statistical softwares plays a key role in determining the hidden process and identify the relationship between ions, thereby the source and process governing the geochemistry of groundwater [41]. Groundwater is unsuitable for irrigation in the coastal area due to the higher concen (...truncated)