Hydrogeochemical delineation of groundwater fitness for drinking and agricultural utilities in Thiruvallur district, South India

Appraisal of groundwater fitness to drinking, agricultural, and domestic purposes was attempted within the Thiruvallur district of South India since groundwater is the significant sources for the above utilities. Groundwater samples collected from a sum of 110 locations have to be analyzed for major concentrations of which higher values of total dissolved solids (TDS), chloride (Cl−), and sodium (Na+) ions were noted during pre-monsoon (PRM) specifically besides the east along with the southeastern part of the study region and higher calcium (Ca2+) and nitrate (NO3−) values disseminated besides the central and eastern parts of the study region during the post-monsoon (POM) season. Suitability of water quality depends upon total dissolved solids; Water Quality Index (WQI) suggests 70% and 62% of the study area representing hard and sedimentary formations are appropriate for consumption utilities. Irrigation aptness of water for utility suggests the majority of study area is suitable in view of electrical conductance, sodium adsorption ratio, USSL plot, Na%, Wilcox’s plot, Kelly’s ratio, and Doneen’s plot. Dominant hydrochemical facies were observed to be Na-Cl, mixed Ca-Mg-Cl, and Ca-HCO3, and parameters like Na+, Ca2+, HCO3−, Cl−, and NO3− ions have been observed to be higher in the central region next to the east and may be appropriate to influence geogenic, anthropogenic, and seawater encroachment.


Introduction
Groundwater forms the main source for drinking, household, industrial, and agricultural convenience in coastal regions of numerous countries. India is covered by 2.20% of global land and 4.0% of global water reserves, with 16% of the world's population. Due to increasing human utilities for household, farming, and industrialized utilities, the pressure on good water resources has drastically increased (Ramakrishnaiah et al. 2009).
The groundwater eminence is subjective in litho-units, land use pattern, and anthropogenic activities (Srinivasamoorthy et al. 2011 andSaravanan et al. 2016;Senthilkumar et al. 2017, andGopinath et al. 2018). Overexploitation of water and intense urbanization process with lack of water management activities has led to deteriorating water quality. As a result, water management and their quality are required in rising countries like India. In this country, severe irrigation behaviors have improved the need for groundwater wealth. Hydrogeochemical investigations provide a better understanding of hydrologic conditions and their usefulness in any region.
The process of interacting with aquifer material next to the flow paths within the subsurface is the contribution to the groundwater substance concentration (Srinivasamoorthy et al. 2008 andSenthilkumar et al. 2014a;Gopinath et al. 2017 andPrakash et al. 2018). Now, awareness is needed to understand the natural concentration of many ions and metals in water to regulate and discriminate geogenic and anthropogenic sources influencing the quality of groundwater within the aquifer. Many ions naturally occurring in drinking water have a significant impact on human health through deficiency or excessive intake (Frengstad et al. 2001).
Concurrently, the factors to influence the chemical value of groundwater are lithology, soil type, parent rock character, composition of recharge water into the aquifer, and the duration of water being held in the aquifer material (Giridharan et al. 2008;Atikul Islam et al. 2016). Therefore, it is important to understand groundwater quality for its suitability for drinking and agricultural uses (Subramani et al. 2005;Chidambaram et al. 2011;Ramesh and Elango 2011;Krishnakumar et al. 2013;Senthilkumar et al. 2014a;Narsimha 2018;Jain and Vaid 2018). Appraising of water quality by water quality indices (WQIs) which integrates water quality data to mark has also been tried by many worldwide investigators (Nazeer et al. 2014;He et al. 2018;Khadse et al. 2016;Li et al. 2018aLi et al. , 2018bWu et al. 2017;Akakuru and Akudinobi 2018;Gopinath et al. 2019).
The aim of the present study is to delimit the quality of ground water in Thiruvallur district and areas where groundwater is suitable or unsuitable for drinking and irrigation purposes. Thiruvallur district faces acute water shortage due to large usage of groundwater for agricultural, industrialized, and urbanization applications. A total utility of groundwater in this district for all purposes is about 1058.46 million cubic meters (CGWB 2007). Groundwater is being pumped to Chennai city for the last few decades for drinking water supplies, which have created a large decrease in water level (Senthilkumar et al. 2019). The factors influencing groundwater quality in this area are not restricted to rock water interaction, groundwater flow congestion of aquifer and excess groundwater withdrawal, use of chemical fertilizers, and municipal, manufacturing, and farming activities. The study area is enclose by surface water bodies like Poondy reservoir on the central portion of the study region and Puzhal, Cholavaram tanks in the eastern component of this area for agricultural activities (CGWB 2007), but due to their tendency to receive water only during monsoon, people have been urged to rely on groundwater potential for their everyday activities. At the same time, this area is very important, and potential aquifers provide water to all industries and domestic needs of the emerging Chennai city. The local level evaluation of groundwater suitability for drinking and irrigation utilities has been studied by Parimala renganayaki and Elango (2013), Brindha et al. (2013), and Krishna Kumar et al. (2017) in this study area. Senthilkumar et al. (2017b) have identified available groundwater potential zones in the sedimentary terrain of this study area using electrical sounding technique. The present study would be helpful to modify pumping schemes with more reasonable pumping hours and more suitable recharge methods to improve and sustain the groundwater quality for long-term planning and management to ensure saline-free water supply to future residents for sustainable utility of groundwater in this region. Therefore, the present study attempts to isolate groundwater suitability for drinking and agricultural purposes in Thiruvallur district of South India.

Study area
Thiruvallur district is placed north of Tamil Nadu state in  India between northern latitude 12°57′ 32″ and 13°33′ 0″  and eastern longitude 79°17′ 32″ and 80°20′ 15″ according to  the survey of Indian Toposheet No 57O /7, 8, 11, 12, 15 and 16, 66C/2, 3, 4, 7 and 8, with an aerial extent of 3524.77 sq. km (Fig. 1). This area is bordered on the north by Andhra Pradesh, on the west by Vellore district, in the east by the Bay of Bengal, and in the south by Chennai and Kancheepuram districts. This area experiences tropical climatic condition with an annual mean least and most temperature of about 24.3°C and 32.9°C correspondingly. The mean yearly rainfall of the study area is 1104 mm, out of which 52% of the precipitation is received during the northeast monsoon (October to December) and 41% contributed from southwest monsoon experienced during July to September (Krishna Kumar et al. 2017). For hard rock aquifer, the water level is 5.5 m (bgl) during pre-monsoon and 1.5 m in post-monsoon time, and at the same time, sedimentary aquifer is 4 m (bgl) and 0.5 m (bgl) for pre-and post-monsoon seasons. These water level variations indicated that the rainfall well is influenced to rise about 3-4m. The district traces rivers such as Araniyar, Koratalayar, Koum, Nagri, and Nandi, which flow in the eastern parts of the study area as well as west to east flowing directions. Among the rivers, Araniyar and Korattalayar are strongly influenced by backwater and tidal activities from the Bay of Bengal (Senthilkumar et al. 2019). The drainage pattern is dendritic, and all rivers are ephemeral in nature and have significant flows during the rainy season. The groundwater flow pattern is generally running from north to southward in the western part, and it turns from west to eastward direction in the eastern region.
The important geological units ( Fig. 1) noted in the study area are confined to upper Gondwana formations with essential litho-units encompassing sand, coastal alluvium, silt, clay, sandstone, and conglomerate noted along the eastern and central parts of the study region (GSI 2005). In the western part, such litho-units representing Archean formations like granite gneiss, epidotehornblende gneiss, and charnockites are exposed. Geomorphologic features (Fig. 2) like alluvial plain is noted beside the central parts, coastal plain observed by the side of the eastern parts, and denudational hills and pediments distributed in the western part; flood plain and sedimentary high land represent a central segment of this study region. Groundwater into Archaean formations mainly occurs in weathered, fissured, and fractured portions of hard rock, and in recent alluviums, groundwater occurs in unconfined to semi-confined conditions (CGWB 2007). The major land use pattern of this area is followed by irrigation area, forest cover, wasteland, water bodies, and built-up land (Senthilkumar et al. 2017). Dolerite dyke, lineaments, fracture, and fissures are major geological structures are present in the western part of the study area (GSI 2005). The width of the unconfined aquifer in the rock region varies from 2 to 12 m, and the average thickness of alluvium formation is 15 m in the study area. The yield of dug wells in the hard rock aquifer was 100 to 500 lpm and that in sedimentary bore wells ranges between 20 and 400 lpm. Transmissivity of the hard rock formation ranges from 14 to 750m 2 /day and in alluvium formation between 40 and 625m 2 /day. Specific yield of the study area in the hard rock formation varies from 0.092 to 0.000057 and sedimentary formations between  (CGWB 2007). The tanks are major water catchment areas that are mostly rain fed tanks withholding water during the monsoon seasons, and in non-monsoonal periods, the tanks are dry.

Material and methods
Fifty-five groundwater samples were collected from representatives dug wells and bore wells for the period of June 2014 (pre-monsoon) and January 2015 (post-monsoon). From 55, 13 samples were collected from gneiss and charnockite formations along the western region, and the remaining 42 samples were collected from sedimentary formations limited to the central and eastern portions of the study area. Field measurements like electrical conductivity (EC) and hydrogen ion concentration (pH) were measured using Systronics Water Quality Analyzer-371 at the time of sample collection. Each sample was collected in 1-l capacity of clean polyethylene bottles, after pumping the sampled wells for 5 min, and all the samples are filtered by 0.45 μ Millipore paper to analyze major ion chemistry using standard procedures (APHA 2012). Volumetric titration methods were used to analyze calcium (Ca 2+ ), magnesium (Mg 2+ ) with 0.05 N EDTA solution, and carbonate (CO 3 2− ); bicarbonate (HCO 3− ) was estimated by titration with 0.01 N H 2 SO 4 standard solution. Chloride (Cl − ) was determined by titration with 0.02 N AgNO 3 solution. The flame photometer (ELICO CL 354) was used for measuring sodium (Na + ) and potassium (K + ). Spectrophotometer ELICO SL 164 was used for measuring the sulfate (SO 4 2− ) and nitrate (NO 3 − ) concentrations. The analytical error percentage of chemical analysis of ±5 was confirmed by ionic error balance. Parameters like Na%, sodium adsorption ratio (SAR), residual sodium carbonate (RSC), Permeability Index (PI), Kelly's ratio (KR) have been calculated using the following equations:

Result and discussion
The statistics of physicochemical parameters like electrical conductivity, pH, and total dissolved solids and major elements (Na + , K + , Ca 2+ , Mg 2+ , Cl − , HCO 3 − , CO 3 2− , SO 4 2− , NO 3 − ) in pre-monsoon (PRM) and post-monsoon (POM) are accessible in Table 1 in view of varying litho-units. Meanwhile, pH is the most significant marker of its quality and prohibited by the sum of dissolved CO 2 , bicarbonate, and carbonate (Ghandour et al. 1985). The pH ranges from 6.8 to 8.1 with mean of 7.5 irrespective of seasons and litho units. pH, irrespective of seasons and terrain, represents alkaline in nature because of bicarbonate from dissolved carbonate (Nagarajan et al. 2010). The value of pH in the study samples of drinking water is below the permissible limit as prescribed by BIS (2003) and WHO (2011). The electrical conductivity for groundwater samples during PRM varies from 520.0 to 2346.0μS/cm with a mean value of 1406.0μS/cm in hard rock samples and 219.0 to 5870.0μS/cm with an average value of 1387.0 in sedimentary formations. For the duration of POM, values range from 500.0 to 2384.0μS/cm with an average of 1345.0μS/cm in hard rock; in sedimentary terrain, the values vary from 150.0 to 4930.0μS/cm with a mean value of 1330.0μS/cm. Higher electrical conductivity rate occurs in sample no.18 at the eastern part of the study region encompassing sedimentary terrain. Higher electrical conductivity (5870 μS/cm) observed in sedimentary aquifer during PRM might be due to seawater encroachment (Senthilkumar et al. 2019;Gopinath et al. 2019). Total dissolved solids in hard rock aquifers ranges between 333.0 and 1501.0 mg/L with a mean value of 900.0 mg/l during pre-monsoon and between 320.0 and 1526.0 mg/l with an average of 861.0 mg/l during post-monsoon season. Higher total dissolved solids (3757.0 mg/L) observed in sample no. 18 (Kalpakkam) irrespective of seasons indicate groundwater deterioration due to a seawater intrusion phenomenon Venkatramanan et al. 2017;Senthilkumar et al. 2019;Gopinath et al. 2019). This sample location is situated 9.5 km away from the coast in the centraleastern part of this area.
Total dissolved solids are classified into four different categories based on Davis and DeWiest (1966) for drinking and agricultural fitness (Fig. 3 a and b) such as that desirable for drinking (500.0 mg/L) and permissible for drinking (500.0-1000.0 mg/L). Water covered by the west and entire central regions irrespective of seasons is useful for irrigation (1000.0 -3000.0 mg/L), and water covered by the entire eastern region is unfit for irrigation and drinking (>3000.0 mg/L) purposes as noted in only sample location 18 during the PRM season. In Table 2, samples representing hard rock aquifers (n=13) are desirable and permissible for drinking and useful for irrigation purposes irrespective of the seasons. In sedimentary aquifers (n=42), about 29% of the samples are desirable for drinking irrespective of seasons, and about 33% of the samples are within the permissible state during the post-monsoon but found to be increased by 38% during the post-monsoon time. About 36% of the samples for the period of pre-monsoon are suggested to be useful for agricultural utilities and that trend decreased up to 31% during the post-monsoon season, which might be due to the influence of recently recharged precipitated water to the aquifers. Sample no. 18 alone is found to be unsuitable for irrigation and drinking utilities during both the seasons which might be due to heavier groundwater extraction resulting in seawater intrusion into the aquifers (CGWB 2007).
The ascendancy of the chief elements follow the array Na + > Ca 2+ > Mg 2+ > K + and HCO 3 . Na + within hard rock samples ranges between 27.0 and 164.0 mg/l with a mean value of 113.0 mg/l during pre-monsoon and between 57.0 and 173.0 mg/l with an average of 117.0 mg/l, which is higher than that observed during the post-monsoon season indicating the influence of silicate weathering process from Na-rich minerals like plagioclase and pyroxene (Vasanthavigar et al.      , 9, 11, 20, 21, 34, 35, 40, 44 -46, 50, 51, 54 33 9, 11, 14, 20, 21, 33-36, 39, 40, 44, 45, 50, 51, 54 38 1000-3000 Useful for irrigation 2, 12, 14, 16, 19, 24, 25, 28, 36, 39, 41, 48, 49, 52, 55 36 2,12,16,19,24,25,28, 41,46, 48, 49,52  presence of higher SO 4 − in drinking water will lead to respiratory-related problems (Subba Rao 1993). The major reason for higher sulfate in sedimentary environment is the availability of gypsum, anhydrite, and oxidation of sulfate minerals from basement rock (Han et al. 2016).  value of 30.0 mg/l. In sedimentary aquifers, the value ranges from 0 to 111.0 mg/l with average of 23.0 mg/l during PRM, and increase in value was observed during POM from 4.0 to 199.0 mg/l with a mean value of 28.0 mg/l. Higher nitrate (199.0 mg/l) is observed in location 45 irrespective of seasons, and its long-term consumption may lead to methemoglobinemia and gastric cancer (Naidu et al. 1998;Chen et al. 2017). Sources of nitrate might be derived from animal wastages and festering tank leakages, poultry farms, irrigation practices, and decomposing of natural domestic wastes (Narsimha and Sudarshan 2017). According to WHO (2011), the maximum permissible level for nitrate is 45 mg/l, where about 92% of the hard rock samples were under the permissible limit during both the seasons and 90% of the sedimentary samples were found to be below the permissible boundary during PRM and POM seasons.

Piper classification
Important ions like Na + , K + , Ca 2+ , Mg 2+ , Cl − , HCO 3 − , CO 3 2− and SO 4 2− (in meq/l) were charted in a piper trilinear diagram (Piper 1944) to estimate the hydrochemical facies of water through the PRM and POM periods in view of terrain variations, as these ions are the more common components in controlling groundwater geochemistry. Piper plot (Fig. 4)  Most of samples are clustered at the facies of mixed Ca-Mg-Cl and NaCl types during both PRM and POM seasons indicating older groundwater owed to expand rock water contact with superior Na + leached from feldspar-rich reserves and Cl − derived from anthropogenic activity . In sedimentary terrain, the leading water types follow the array of NaCl > Mixed CaMgCl > CaHCO 3 > CaCl > mixed CaNaHCO 3 . Majority of the samples are found to be scattered in Na-Cl and mixed CaMgCl facies during both the seasons, revealing the prominence of seawater intrusion (Senthilkumar et al. 2019) restricted to the coastal traces of the study area. Fewer representations are also noted in Ca-HCO 3 and mixed CaNaHCO 3 -type facies during PRM season indicating the recent recharge of terrestrial water restricted to the western portion of the study area encompassed by sedimentary aquifers.

WQI
The demarcation of groundwater quality for drinking wa-  (2003) have been considered for calculating the water quality index. For the calculation of the WQI of 9 parameters (total dissolved solids, Na + , K + , Ca 2+ , Mg 2+ , Cl − , HCO 3 − , SO 4 2− , and NO 3 − as a whole), a weight (according to its relative importance in the overall quality of water for drinking purposes) is assigned (Table 3). The water quality index provides the overall effect of individual water quality for drinking. It is classified into five, such as excellent (> 50), good (51-100), poor (101-200), very poor (201-300), and unsuitable for drinking (<300). Depending on the importance of the water quality  ,5,7,8,15,26,30,37 19 1,4,5,10,26,33,9,10,11,14,20,21,33,34,35,40,46,50,51,54,55 38 3,6,7,8,9,11,14,15,20,21,34,35,40,50,51,54 38 >300 Very hard water 2, 12, 16, 18, 19,24, 25, 28, 36, 39, 41, 44, 45, 48,49, 52 38 2,12,16,18,19,24,25,28,36,39, 41,44,45,46,48,49,52,55 43 parameters, maximum and minimum weights were assigned. The maximum weight of 5 was assigned to the parameters total dissolved solids, chloride, and nitrate, as they play an important role in groundwater quality (Srinivasamoorthy et al. 2008;Vasanthavigar et al. 2010;Krishnakumar et al. 2013). Other parameters such as bicarbonate, sodium, potassium, calcium, and magnesium were assigned weights 1-4 based on their importance in the evaluation of water quality. After assessing the weights for each parameter, the relative weights were calculated (Table 3). In addition, a quality rating (QI) is assigned to each rating based on the concentration of samples (C i ) divided by the drinking water quality standard (S i ) using the following ratings: At last, WQI is calculated according to below method: where SI i is the sub-index of ith factor. W i is the relative weight Consequently, the quality of water score of the studied samples obtained ranges from 21 to 443 during the PRM period and Fig. 6 a Spatial distribution map of total hardness (TH)-PRM, b Spatial distribution map of total hardness (TH)-POM decreases from 17 to 340 during POM seasons. The WQI assortment, water type, and percentage of the sample have been classified in Table 4, and its spatial distribution maps are shown in Fig. 5a for PRM and Fig. 5b for POM season, revealing that the excellent and good water types occupied the northwestern part and the remaining area is covered by poor to very poor water types irrespective of season. In hard rock samples, about 46% of the samples were established to be excellent to good category and 54 % were poor to very poor category for drinking during PRM, and for POM seasons, 23% of the samples were good for drinking utility and 77% were found to be poor to very poor category. In sedimentary aquifers, about 46% of samples represents excellent to good water type during PRM and POM seasons, but the poor water type during PRM and POM periods were represented by 43% and 45% of water samples respectively. Very poor water type is found represented by 9% and 7% of samples at the time of PRM and POM correspondingly, but sample no. 18 is found to be unsuitable for drinking purposes irrespective of seasons representing sedimentary aquifers. Comparison between spatial distribution maps of total dissolved solids with WQI reveals that good-quality water occurred in the northern and northwestern parts of the study region irrespective of the seasons. The poor and very poor quality of groundwater identified as packets for the entire study area might be due to ion exchange process and unplanned irrigation practice (Vasanthavigar et al. 2010;Bodrud-Doza et al. 2016;Gopinath et al. 2018).

Total hardness
The total hardness in ground water is due to increase in Mg 2 + , Ca 2 + , HCO 3 − , Cl − , and SO 4 − ions in water. Based on Sawyer and McCarty (1967), total hardness (TH) classification of water in hard rock terrain shows hard water type for about 46% and 54% of the samples during PRM and POM respectively and very hard water types represented by 54% and 46% of samples during PRM and POM season in that order (Table 5).
In sedimentary terrain, about 5% of samples are observed to be soft, 19% of the sample as moderately hard water, 38% as hard water type, and 38% as very hard water type during PRM period. During POM season, 2% of the sample represents soft, 17% as moderate, 38% as hard water, and the remaining 43% as very hard water. The spatial delivery of TH in the entire study area with respect to season is presented in Fig. 6 a and b. Mostly hard and very hard water types occupied the entire study area, and except small patches, moderate hard water types occupied the north-central portion during PRM and POM. Very high hard water consumption in the long term will result to many health problems to human beings (Durvey et al. 1991;Agrawal and Jagetai 1997).

NA + %
More sodium issue in irrigated water will distress the soil permeability and is toxic to plants (Durfer and Backer 1964;Todd 1980;Bangar et al. 2008). Na + % can be evaluated based on Raghunath (1987), by the below equation: Based on Na + %, groundwater samples were classified into five major types as represented in Table 6 in view of lithology variations. The sodium percentage for hard rock aquifers in about 7% of the samples is doubtful during PRM, and increase in the sample amount of about 13% has been observed during POM season. The rest of the samples irrespective of seasons represent the good and permissible categories. In sedimentary Fig. 7 a Spatial distribution map of suitability for irrigation purposes based on Na%-PRM, b Spatial distribution map of suitability for irrigation purposes based on Na%-POM terrain, about 32% and 2% of samples are doubtful and not suitable during PRM season. During POM, about 29% and 2% of samples are doubtful to not suitable; the rest of the samples signifies an excellent and permissible category (Fig.  7 a and b). The Na + % spatial distribution map for PRM period is shown in Fig. 7 a and b for POM period revealing that the good and permissible types occupied the north, west, and eastern portions, but the southeastern part is covered by doubtful to unsuitable condition irrespective of season for agricultural purposes. Groundwater classification based on Na + % and electrical conductivity was attempted by Wilcox (1955)'s diagram for different litho-units in the study area and is represented in Fig. 8. The diagram represents the majority of hard rock samples in excellent to good and good to permissible except in few locations that represent doubtful to unsuitable for irrigation purposes irrespective of PRM and POM seasons. On the other hand, for sedimentary terrain, majority of the samples scatter in excellent to good and good to permissible irrespective of seasons. About 7 and 5 samples from the period of PRM and POM, respectively, are found to represent doubtful to unsuitable condition for irrigation utility. Samples no. 18 and 46 are unsuitable water for irrigation purposes during POM season. The agricultural yields are normally low in locations that are doubtful to unsuitable, which is due to the presence of Na salts and indulges osmotic effects in the soil-plant system. Excess sodium in waters produces undesirable effects on soil properties and reduces the soil permeability. The Na + % spatial distribution map shows that most of the location is covered by permissible category for irrigation in PRM and good to permissible category in POM season, except a few samples that are unsuitable for irrigation in the southeastern part of the study area irrespective of season. In the study area, groundwater at Ramapuram shows high Na which may be due to the influence of seawater, and these locations can be encouraged to irrigate salt-tolerant plants like cotton, coconut.

SAR
Sodium adsorption ratio is a significant parameter to determine the groundwater suitability for agriculture due and thus determine alkali/sodium risk to crops. Sodium adsorption ratio can be calculated by adopting the equation below where all ions are expressed in meq/l: Based on Richard (1954)'s classification, sodium adsorption ratio values in hard rock terrain range from 1.2 to 5.24 during PRM and POM season SAR ranges from 1.74 to 5.44. On the other hand, in sedimentary aquifers, the values range from 0.31 to 14.09 in PRM and between 0.5 and 6.92 during POM season. It is noted that all the samples represent excellent category for irrigation purposes in all season with respect to litho-units except location no. 55 during PRM that represents good-quality water for irrigation utilities as represented in Tables 7 and 8. Shrinking and swelling in clayey soils will be due to sodium adsorption ratio values greater than 9, (Saleh et al. 1999). Higher sodium adsorption ratio in groundwater will develop alkaline soil (Todd 1980), with high salt leading to saline soil. The analytical data are plotted in the USSL chart (Richards 1954) for different seasons and litho-units. Majority of the samples from hard rock terrain (>90%) are found to represent in C 3 S 1 and few samples in C 2 S 1 irrespective of seasons, suggesting high salinity and low sodium waters. For sedimentary terrain, majority of the samples represent C 3 S 1 and C 2 S 1 type, and minor representation is also noted in C 1 S 1 and C 4 S 1 during PRM and POM. This water can be used for irrigation in any type of soil with tiny danger of transferable sodium. C 4 S 3 -type water occurs at Ramapuram (sample no. 55) during PRM indicating very high salinity and high sodium, which is suitable for salt tolerance in plants. C 4 S 3 -type water restricts good drainage in soils which in turn is unsuitable for irrigation (Karnath 1989;Mohan et al. 2000). The higher electrical conductivity and sodium concentrations are the result of saline soil, irrigation activities, and leaching of animal waste and other domestic wastes (Senthilkumar et al. 2014b;Gopinath and Srinivasamoorthy 2015).

RSC
The hazardous effect of CO 3 2− and HCO 3 − along with calcium and magnesium ions is used to determine the residual sodium carbonate (Eaton 1950) calculated using the formula mentioned below (Lloyd and Heathcote 1985): When residual sodium carbonate <1.25, the water is found to be safe for irrigation, and values ranging 1.25 to 2.5 mean moderate for irrigation, while greater than 2.5 groundwater seems to be unsuitable for irrigation utilities (USEPA 1999). Groundwater from this area is classified on the root of residual sodium carbonate values as terrain shown in Table 9. The residual sodium carbonate in about 85% of hard rock samples is safe during the PRM and POM seasons; 15% of the samples are moderate during PRM, and during POM, about 7.5% of the samples are found to be unsuitable for irrigation utilities. In sedimentary terrain, 98% and 93% of water samples represent safe category during both PRM and POM seasons, and the remaining 2% and 7% of samples show that continued usage of high residual sodium carbonate water will result in crop yielding and burning of plant leaves.

Kelley's ratio
Based on Kelly's ratio, groundwater can be classified for irrigation purposes through the relationship of Na + ions against Ca + and Mg + ions measured by Kelly (1957) as noted below: Kelley's ratio >2 indicates excess Na + in water which is unsuitable for irrigation (Table 10); ratio <1 is suitable for irrigation, and ranges between 1 and 2 are marginally utilized for agricultural purposes. In hard rock regions, 85% of the samples (<1) are suitable for irrigation during PRM and about 77% during POM season are suitable for irrigation. The remaining 15% of the samples in PRM and 23% during POM represent a marginal category for irrigation utilities. On the other hand, in sedimentary terrain, about 91% of samples represent a suitable category through PRM and POM seasons, the and marginal category is represented by 7% and 9% of the samples during PRM and POM seasons. Only 2% of the samples during PRM season is found to be not suitable for irrigation (>2) activities. The release of Na + or Ca 2+ into groundwater and the adsorption of Ca 2+ or Na + are due to montmorillonite clays which are situated in the waterlogged area, marshy/swampy land, and creeks (Alison et al. 1992;Krishnakumar et al. 2013).  -12, 14-16, 18-21,24-26, 28, 30, 33-37, 39-41, 44-46,48-52, 54 98 1-4, 6-9, 11,12, 14-16, 18-21, 24-26, 28, 30, 34-37, 39-41, 44-46,48-52, 54

Conclusion
The current understanding of groundwater quality in Thiruvallur district of Tamil Nadu, South India, for drinking and agricultural utility indicated that the water quality of the samples exceeded the BIS and WHO limits in particular locations, which have potential adverse effect on human health. Groundwater is generally alkaline, hard to very hard type, and fresh to brackish irrespective of seasons and litho-units. The dominance of major ions followed the order Na + > Ca 2+ >Mg 2+ >K + and HCO 3 − > Cl − > SO 4 − > NO 3 − in hard rock samples and Na + > Ca 2+ >Mg 2+ >K + and Cl − > HCO 3 − > SO 4 − > NO 3 − in sedimentary formations. Groundwater shows sodium is principal in cations and chloride in anions in sedimentary region, and HCO 3 dominates in samples collected from hard rock aquifers. The WQI balanced with total dissolved solids and Cl observed high values representing the poor quality of groundwater for drinking purposes along the east and south parts of the study area dominated by domestic, agricultural, and industrial activities. Mixed CaMgCl > NaCl > CaHCO 3 > CaCl for hard rock samples and NaCl > Mixed CaMgCl > CaHCO 3 > CaCl > mixed CaNaHCO 3 in sedimentary samples dominate. Further, sodium adsorption ratio, Na %, RSC, PI, and Kelly's ratio calculated in determining the irrigation suitability suggests about 85% of groundwater in the hard rock region and 70% of groundwater samples in the sedimentary region are useful for irrigation utilities. In loc no. 55, higher sodium hazard and very high salinity are noted during PRM season indicating higher evaporation. The permeability index values suggest all the groundwater samples irrespective of the terrain and seasons are suitable for irrigation except loc no.15 and 33. The study of water quality parameters and its suitability for drinking and irrigation suggests the majority of the groundwater samples are apt except few samples flabby for irrigation purposes due to the high salinity. These areas with high salinity need adequate drainage to overcome salinity problems for irrigation purposes. The land use practice and the ion exchange course participate in a significant position in determining major geochemical processes of the study area.

Declarations
Conflict of interest The authors declare that they have no competing interests.
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