Groundwater salinization and suitability assessment in the coastal aquifer of the Thiruvallur district, India

Water, often considered the cornerstone of civilisation and life on Earth, is an indispensable resource that underpins the functioning of ecosystems, supports agricultural production, and is vital for human health. The urgency of water quality monitoring is emphasised as a global priority to mitigate potential health risks and enable effective planning and management of water resources (Mishra et al. 2014). With the ongoing developments in agriculture, industry, urbanisation, and population growth, there is a discernible rise in overall water demand (Singaraja et al. 2014). This escalating demand, coupled with the looming spectre of freshwater scarcity, presents one of the most formidable challenges of the 21st century (Magesh et al. 2018; Rao et al. 2019; Rey et al. 2020). In essence, water quality becomes a pivotal factor influencing not only the sustenance of life and the productivity of agriculture but also the broader socio-economic and geopolitical landscape (Cellone et al. 2019). Hydrochemical processes play a pivotal role in unveiling the suitability of groundwater in coastal regions for various purposes, such as drinking, farming, and industrial use (Kaliraj et al. 2015; Seenipandi et al. 2019). These processes not only help in understanding the natural changes in quality resulting from interactions between rocks and water but also shed light on the impacts of human activities on groundwater chemistry (Srinivasamoorthy et al. 2013; Chandrasekar et al. 2014; Vahab et al. 2016). Groundwater unavailability has occurred in various parts of the coastal zones in India due to uneven temporal and spatial distribution, increased consumption and usage, pollution, contamination, misuse, and waste. Preventive measures, land-use planning, and regulatory frameworks are implemented to safeguard coastal aquifers to control and manage potential contamination sources (Mastrocicco & Colombani 2021). Sustainable groundwater management practices, including aquifer recharge strategies and public awareness initiatives, are crucial for maintaining and improving groundwater quality over time (Selvakumar et al. 2017; Devaraj et al. 2020; Sundriyal et al. 2021). Groundwater quality degradation in coastal zones is a frequent issue, typically caused by factors such as effluent discharge, the composition of the underlying rock, vegetation cover, agricultural practices, biological activities, evaporation rates, rainfall, topography, and unregulated waste disposal, among other environmental influences (Selvakumar et al. 2018; Faizal Khan et al. 2020; Zhao et al. 2021). Moreover, the over-extraction of groundwater for different commercial purposes directly affects the groundwater table by creating a cone of depression; finally, fresh groundwater is converted into saline groundwater, and encroachment of urbanization with industrial developments also affects the groundwater salinity (Krishna Kumar et al. 2017; Panthi et al. 2022; Sellamuthu et al. 2022). Common threats to groundwater quality in the urban region include industrial discharges, leaking underground storage tanks, untreated sewage, and agricultural runoff with fertilizers and pesticides. In recent decades, the changes in land-use patterns have severely stressed the coastal habitats. Indeed, the pressure on coastal zones due to urbanization, industrial activities, and population growth poses a significant threat to coastal ecosystems in India. These pressures lead to habitat loss, pollution, groundwater depletion, and salinization – all of which impact biodiversity, water quality, and the health of ecosystems. Particularly in Tamil Nadu, the coastal aquifers are mostly shallow and moderate depth of groundwater resources and they are being utilized for different activities (Senthilkumar et al. 2014). The over-extraction of groundwater in the coastal regions raises an alarm about the huge vulnerability level and creates various problems due to seawater intrusion. Several studies around the globe and India documented that the over-extraction of groundwater in the coastal zones can lead to seawater intrusion into the freshwater aquifer (Selvakumar et al. 2012; Habtemichael & Fuentes 2016; Klassen & Allen 2017; Senthilkumar et al. 2017; Abd-Elaty et al. 2022). Regular sampling and analysis are integral components of groundwater quality assessment to detect potential contaminants with salinity and ensure the groundwater's suitability for different uses, particularly in the coastal regions. Under these circumstances, the present research aims to understand the hydrochemical processes controlling groundwater salinization and assess the suitability for drinking and irrigation in the coastal regions of the Thiruvallur district, Tamil Nadu, India.

Groundwater from 54 dug and bore wells at locations covering most of the coastal aquifer within 10 km from the coast were sampled. The sample size depends on the availability of bore and dug wells in the study area. The sampling dates were selected to avoid taking samples the week after any rainfall event. The groundwater is mainly used for drinking, domestic, irrigation, or blending with desalinated and recreational purposes. The collection and analysis of groundwater samples were followed by standard methods (APHA 2012). The sample was collected in a suitable HDPE polythene bottle and was labelled clearly by denoting the groundwater location, the sampling date, and the station number. From the time of sample collection to the time of actual analyses, many physical, chemical and biochemical reactions would change the quality of the water sample; therefore, to minimize this change, the samples were preserved soon after the collection. The physical parameters such as temperature, pH, electrical conductivity (EC), total dissolved solids (TDS), and salinity were measured using the Horiba Laqua multi-probe water quality analyzer. Calcium (Ca²⁺) and magnesium (Mg²⁺) were measured by titrimetric method against EDTA, and based on the Ca²⁺ and Mg²⁺ concentrations total hardness (TH) was calculated. Sodium (Na⁺) and potassium (K+) were measured by flame photometry. Sulphate (SO₄²⁻, nitrate (NO₃⁻), and phosphate (PO₄³⁻) concentrations were measured by spectrophotometry. The presence of bicarbonate (HCO₃) and chloride (Cl⁻) were analyzed titrimetrically against H₂SO₄ and AgNO₃, respectively. Fluoride concentration was analyzed using the SPADNS spectrophotometric method. The analytical error of the hydrochemical analyses was verified by calculating the ion-balance errors, taking the relationship between the total cation and the total anion for each set of complete analyses of water samples. The analytical error percentage of all 54 samples was less than the accepted limit of ±10% (Domenico & Schwartz 1990), and different irrigation water quality indices were calculated based on the analytical values. For understanding the hydrochemical facies, groundwater salinization and mechanisms controlling groundwater chemistry in the coastal aquifer using different graphical, geospatial and geostatistical representations with the support of Aqua-Chem, Arc GIS, and SPSS softwares, respectively.

The detailed statistical summary for major physicochemical parameters is shown in Table 1. The analytical results show that the groundwater in the study area is slightly alkaline to alkaline, with a pH varying from 6.74 to 8.57, with a mean value of 7.05. The EC, TDS, and salinity values show variation in different parts of the study region, particularly some patches of coastal region moderately fresh and the many regions moderately saline to highly saline. EC is a very important parameter responsible for changing the fresh to saline, and it is also directly proportional to TDS and salinity. The present study area EC value ranged from 370.5 to 10,235 μS/cm with an average of 1,840 μS/cm, and the TDS values varied from 256 to 6,648 mg/l with a mean value of 947 mg/l. Spatial variation of EC and TDS values in the central parts of the study area shows high values compared to northern and southern portions. TDS values are indicators of fresh or saline, and many classifications were framed from fresh (<1,000 mg/l) to brine (>35,000 mg/l). For suitability of drinking based on these two parameters, almost 73% of the groundwater samples show an elevated concentration and exceed the maximum permissible limits per national and international standards. However, the rest of the groundwater samples are within the permissible limits; perhaps these groundwater samples can also not be consumed directly and need further different purification methods. TDS is also an important parameter to determine all other major ions.

Table 1

Descriptive statistics of groundwater quality data and drinking water specifications of the study area in comparison with WHO (2017) and BIS (2012) 

Parameter .	Minimum .	Maximum .	Average .	WHO (2017)  .	BIS (2012)  .
pH	6.74	8.72	7.05	6.5–8.5	6.5–8.5
Temperature (°C)	22	34	28	–	–
EC (μS/cm)	370.5	10,235	1,340	1,500	1,500
TDS (mg/l)	256	6,648	917	1,000	500
Salinity (mg/l)	0.307	0.857	0.562	0.5	0.5
TH (mg/l)	63	1,852	175.3	300	200
Ca2+ (mg/l)	78	615	148	200	200
Mg2+ (mg/l)	13	274	84	150	100
Na+ (mg/l)	54	1,568	165	200	200
K+ (mg/l)	2.8	76.5	14	12	12
HCO3− (mg/l)	68	1,637	540	400	400
Cl− (mg/l)	89	3,364	328	250	250
(mg/l)	26	835	74	250	250
(mg/l)	4.5	71	26	45	45
(mg/l)	0.3	5.6	1.26	0.5	0.5
F− (mg/l)	0	1.83	0.35	1.5	1.5

Parameter .	Minimum .	Maximum .	Average .	WHO (2017)  .	BIS (2012)  .
pH	6.74	8.72	7.05	6.5–8.5	6.5–8.5
Temperature (°C)	22	34	28	–	–
EC (μS/cm)	370.5	10,235	1,340	1,500	1,500
TDS (mg/l)	256	6,648	917	1,000	500
Salinity (mg/l)	0.307	0.857	0.562	0.5	0.5
TH (mg/l)	63	1,852	175.3	300	200
Ca2+ (mg/l)	78	615	148	200	200
Mg2+ (mg/l)	13	274	84	150	100
Na+ (mg/l)	54	1,568	165	200	200
K+ (mg/l)	2.8	76.5	14	12	12
HCO3− (mg/l)	68	1,637	540	400	400
Cl− (mg/l)	89	3,364	328	250	250
(mg/l)	26	835	74	250	250
(mg/l)	4.5	71	26	45	45
(mg/l)	0.3	5.6	1.26	0.5	0.5
F− (mg/l)	0	1.83	0.35	1.5	1.5

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The dominance of significant ion concentrations was found in the following order: Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ = Cl⁻ > HCO₃⁻ > SO₄²⁻ > NO₃⁻ > PO₄³⁻ > F⁻. The Ca²⁺ and Mg²⁺ values ranged from 78 to 615 mg/l with an average of 148 mg/l and 13 to 274 mg/l with an average of 84 mg/l, respectively. 56% of the groundwater samples exceeded the maximum permissible limit for drinking. The higher values of both concentrations were recorded in the central and southern regions due to the ion exchange process, rock weathering, and diverse anthropogenic inputs. The K⁺ values varied between 2.8 and 76.5 mg/l, with a mean value of 14 mg/l. Out of 54 groundwater samples, 21 exceeded the permissible (12 mg/l) limits for drinking, the higher concentration of potassium mainly due to the trapped saline water from past geological history, weathering from sedimentary rocks, and anthropogenic sources such as municipal wastewater and industrial effluents. The Na⁺ values ranged between 54 and 1,568 mg/l, averaging 346 mg/l. For drinking, 62% of the groundwater samples exceeded the maximum limit of 200 mg/l, particularly in the central portion of the study region, and had higher concentrations (>1,000 mg/l), which mainly due to the aquifer systems may be mixing of fresh-saline groundwater with seawater intrusion.

The HCO₃⁻ varies from 68 to 1,637 mg/l, with a mean value of 540 mg/l. The higher values of bicarbonate in the coastal aquifers are mainly derived from the uptake of CO₂ either from soil zone or direct atmospheric inputs, action on carbonate-rich non-clastic sedimentary rocks such as limestone and dolomites, and dissolution may arise in mixing with both freshwater and seawater end members being saturated with carbonates. Cl⁻ values ranged from 89 to 3,364 mg/l, with an average value of 628 mg/l. In the study, chloride is one of the most important dominant anions. Owing to salinization, in central parts of the study area, groundwater samples near the coastline showed very high concentrations (>3,000 mg/l) and gradually decreased towards the inland regions due to the saline groundwater intruding into the fresh groundwater aquifer due to the saltwater intrusion process. However, some groundwater wells near the coast show lower values (<150 mg/l), for the wells are shallow with wet atmospheric deposition through the rainfall, extraction of groundwater, and much less usage. The sulphate concentration ranges from 26 to 835 mg/l, averaging 114 mg/l. Saltwater mixing results in increased sulphate concentration near the coastal wells; however, most groundwater samples showed low-medium sulphate concentrations due to the lack of sulphate minerals in the host rock. The nitrate values ranged from 4.5 to 71 mg/l, averaging 26 mg/l. The higher concentration of more than 45 mg/l is unfit for drinking. It represents groundwater contamination caused by various manmade inputs such as liquid waste discharged from septic tanks, manure on the land, wastewater, sanitary landfills, industrial wastewater, agricultural runoffs, and other local pollutions. The phosphate values ranged from 0.3 to 4.6 mg/l with mean values of 0.9 mg/l. Higher (>) PO₄³⁻ concentration is noticed in the seven wells; it may be due to geogenic factors with the dissolution of phosphate-rich minerals in aquifer sediments and anthropogenic inputs like urban runoff from urban areas, wastewater infiltration, etc.

The present study region's groundwater is mostly used for agricultural purposes compared to drinking. Hence, the groundwater suitability for irrigation is also needed to understand the nature of quality. For this purpose, five significant irrigation water table quality indices, such as SAR, Na%, PI, RSBC, and MH, were assessed. Table 3 shows that based on SAR, 30% of the samples found no problem, and 54% belonged to increasing problems for irrigation. However, 16% of the groundwater in the study regions shows a severe problem with unsuitable irrigation due to higher alkalinity and salinity levels. However, PI values reveal that 28% of the groundwater was found to be an unsuitable category.

The present research focused on evaluating the hydrochemical characteristics, suitability for drinking and irrigation, and sources of groundwater salinization in the coastal regions of the Thiruvallur district. The results highlight that the degradation of groundwater quality is very high at central parts of the study regions, particularly sampling wells 9–11, 27, 28, and 34–36, which show very high WQI values with very poor conditions due to the enhanced levels of TDS, salinity, significant major ionic concentrations, and these regions are highly vulnerable to seawater intrusion. The salinity problem is very high at all the locations near the central regions; hence, it cannot be used safely for drinking and irrigation. Five hydrochemical facies include Ca–Cl, mixed Ca–Na–HCO₃, Ca–HCO₃, mixed Ca–Mg–Cl, and Na–Cl. The HFE plots reveal that many samples show the overlapping in the mixing line between fresh groundwater and saltwater caused by seawater intrusion. During the encroachment of seawater intrusion to the right and beneath the line, there is an initial increase in salinity and a rapid and marked reverse ion exchange of Na/Ca, which is recognised by the characteristic Ca–Cl facies. The evaporation and rock water interaction are the main natural mechanisms controlling the groundwater quality in the study region.

V.A. contributed to original draft, sampling, methodology, software, formal analysis, visualization, writing. K.G. contributed to writing – review and editing, resource, visualization. S.S. contributed to review, editing, data curation, and visualization.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.