Geochemical evaluation of groundwater and suitability of groundwater quality for irrigation purpose in an agricultural region of South India

The objective of the present study was to evaluate the geochemical processes controlling the groundwater chemistry and also to assess the groundwater quality suitability criteria for irrigation purposes. An agricultural region of Telangana, South India, was selected for the present study. A total of 100 groundwater samples were collected and estimated for pH, electrical conductivity (EC), total dissolved solids (TDS), calcium (Ca2+), magnesium (Mg2+), sodium (Na+), potassium (K+), bicarbonate (HCO3-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{HCO}}_{3}^{ - }$$\end{document}), chloride (Cl−), sulfate (SO42-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{SO}}_{4}^{2 - }$$\end{document}), nitrate (NO3-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{NO}}_{3}^{ - }$$\end{document}), and fluoride (F−). The groundwater was characterized by mostly alkaline conditions with a dominance of Na+ and HCO3-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{HCO}}_{3}^{ - }$$\end{document} ions, indicating the prevailing conditions of weathering and dissolution of silicate minerals. The various geochemical signatures such as Na+ vs Cl−, Ca2+  + Mg2+ vs HCO3-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{HCO}}_{3}^{ - }$$\end{document}, Ca2+  + Mg2+ vs HCO3-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{HCO}}_{3}^{ - }$$\end{document} + SO42-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{SO}}_{4}^{2 - }$$\end{document}, HCO3-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{HCO}}_{3}^{ - }$$\end{document} vs Cl− + SO42-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{SO}}_{4}^{2 - }$$\end{document}, Ca2+  + Mg2+ vs total cations, and Ca2+  + Mg2+ vs Na+  + K+ and the saturation indices with respect to calcite, halite, and gypsum suggest obviously the dominant conditions of carbonate weathering associated with the reverse ion exchange and evaporation processes as the geogenic factors. The linear trend of TDS vs NO3-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{NO}}_{3}^{ - }$$\end{document} + Cl−/HCO3-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{HCO}}_{3}^{ - }$$\end{document} clearly specifies the influence of non-geogenic sources on the aquifer system. These are the important contributors to the variation in the groundwater chemistry. However, the impact of the geogenic source is masking the influence of the anthropogenic source in some areas of the present study region. According to the salinity vs sodium adsorption ratio, residual sodium carbonate, magnesium ratio, and Kelly ratio, 99.9%, 7.06%, 63.07%, and 51.27% of the total study region come under the unsuitable categories for irrigation purposes, respectively. Therefore, the findings of this study recommended some site-specific appropriate management strategies for the safe supply of groundwater for proper crop growth and consequently for sustainable development of the rural environment.


Introduction
Globally, groundwater is a natural resource for domestic (65%), agricultural (20%), and industrial (15%) purposes (Saeid et al. 2018). Studies on groundwater resources have become vital in semiarid regions for various purposes (Sundraiah et al. 2014;Faten et al. 2016;Chetan et al. 2017;Laxman et al. 2021;Subba Rao et al. 2021a, b). However, once the groundwater quality becomes an inferior type due to the influences of geogenic and nongeogenic sources, it will not be suitable for any purposes in general (Subba Rao et al. 2021c). The quality of groundwater is controlled by geochemical processes that depend upon the topographical features, flow path, recharge, nature of lithology, and rock weathering associated with mineral dissolution, ion exchange, and evaporation (Li et al. 2013;Saravanan et al. 2015;, 2021a. Besides this, increasing exploitation of groundwater, rapid urbanization and industrialization, over-application of chemical fertilizers and pesticides for higher plant growth, animal waste, and improper drainage systems are important humaninduced activities, which damage the natural occurrence of 1 3 142 Page 2 of 13 the chemical quality of groundwater and consequently not only affect human health but also reduce crop production (Deepali et al. 2015;Rahmati et al. 2015;Li et al. 2016a, b;Kouakou et al. 2017;Wagh et al. 2019).
Some recent studies have been exampled here: Contamination was caused by human and agricultural activities that directly and/or indirectly damage the groundwater quality due to the influence of dissolution and transport of excess quantities of fertilizers, and also through the changes in the water-rock reactions in the soils and aquifers in the deltaic region of the Cauvery River in Tamil Nadu, India (Vetrimurugan and Elango 2014). Because of the dissolution of minerals, evaporation, and non-geogenic impact, the quality of groundwater has considerably deteriorated in the Jazan arid area of Saudi Arabia (Alfy et al. 2015). Groundwater quality was mainly controlled by the lithology, dissolution, precipitation of minerals, and ion exchange in irrigated land on the Southeastern edge of the Tengger Desert, China (Li et al. 2016a). The lithological variation was caused by the changes in the groundwater quality due to the weathering of the rock minerals and evaporation processes in the East Wasit Province, Central Iraq (Ghalib 2017). The chemical weathering of the country rocks associated with the dissolution of minerals and carbonates, and ion exchange, and also man-made pollution mainly were influenced the groundwater quality in Northwestern Tunisia (Ayadi et al. 2018). In Western India, not only the geogenic processes (silicate weathering, ion exchange, and evaporation) as the main governing factors of the chemistry of groundwater, but also the nongeogenic sources (domestic wastes, irrigation-return-flows, and animal wastes) as the secondary sources were damaged the chemical groundwater quality (Wagh et al. 2019) The water-rock interactions with ion exchanges were caused by the inferior groundwater quality for drinking and irrigation purposes in the Lower Ketar Watershed, Ethiopia (Tolera et al. 2020). The water-rock interactions associated with the ion exchange and evaporation as the prime geochemical regulating factors of the geochemical characteristics and also the anthropogenic activities as the secondary factors were caused by the deterioration of the chemical quality of groundwater used for irrigation and drinking purposes in a rural part of Telangana, South India (Subba Rao et al. 2021c).
From the review literature, it can be said that knowledge of the geochemical processes controlling groundwater chemistry is a very important aspect of dealing with groundwaterrelated issues. The identification of geochemical processes helps in understanding the changes occurring in groundwater quality due to the interaction with aquifer material. On the other hand, understanding the changes in the chemical quality of groundwater caused by human-induced activities like agriculture is also an essential issue, especially in arid and semiarid regions. This type of study in turn assists in management planning for taking the appropriate remedial measures to protect the aquifers from contamination caused by natural (geogenic) and anthropogenic (non-geogenic) origins. Therefore, the geogenic processes and agricultural activities on the aquifer conditions need to be understood everywhere for taking the appropriate suitable measures.
The residents in the State of Telangana, South India, mainly rely on groundwater resources for their drinking and irrigation purposes due to the insufficient supply of surface water sources. The present study region faces the deterioration of the groundwater quality in some places due to the lack of proper awareness on the planning of disposal of household wastes, septic tank leakages, animal wastes, and uncontrolled usage of chemical composts, application of irrigation water flows, etc. As a result, the groundwater quality may not be suitable for drinking purposes as well as for irrigation purposes. However, nobody has so far been carried out any research work from the study region so that the present study would be useful as baseline information for further work in the future. Therefore, the main objective of the present study was to evaluate (a) the weathering processes associated with the ion exchange and evaporation controlling the groundwater chemistry by using the various bivariate diagrams and saturation indices, and (b) the suitability of groundwater quality for agricultural purposes by adopting various irrigation chemical factors like sodium adsorption ratio, residual sodium carbonate, magnesium ratio, and Kelly ratio. This assists in implementing the proper site-specific remedial measures for the sustainable development of society.

Location
The study region lies between latitude 17°23′-17°25′ N and longitudes 77°45′-78°50′ E in the State of Telangana, South India, covering an area of 632.45 km 2 (Fig. 1a), which falls in the toposheets of 56 G/15 and 56 G/16 with a scale of 1:50,000 of the Survey of India. The region experiences a dry climate with an average annual temperature of 14 °C (winter) to 41 °C (summer) and an average annual rainfall of about 940 mm. The stream pattern comes under the subdendritic type.

Hydrogeological conditions
The region has a gentle slope. The calcium carbonate concretions are intermixed with soil. The important geological formations are basalts and granite (Fig. 1b). Laterite occurs in patches. The former types are fine-grained with dark-colored rocks, which contain the minerals calcium plagioclase feldspars and clinopyroxene with olivine, quartz, hornblende, nepheline, orthopyroxene, etc. The latter types are medium-to coarse-grained rocks, which are composed of quartz, plagioclase and potassium feldspars, biotite, apatite, hornblende, etc. These rocks are hard. However, the basalts become water-bearing formations due to the occurrence of vesicular structures, cracks, and joints in them, while the granites become aquifer formations due to the presence of weathered and fractured rock portions. Though the laterites are porous, they are poorly permeable. Groundwater exists under the unconfined to semi-confined conditions. The depth of groundwater level varies from about 18-28 m below ground level.
In some areas, the taste of the groundwater quality appears to be slightly brackish due to the influence of the anthropogenic sources (domestic waste, septic tank spillages, irrigation-return-flows, chemical fertilizers, animal wastes, etc.) on the groundwater system, which was observed during the field work.

Sampling and analytical procedure
Sampling was carried out in the present study region during the summer month of May 2015. A hundred groundwater samples were collected (Fig. 1a) in one-liter polythene bottles. They were cleaned with 1:1 dilute HCl and washed away with distilled water three times before the sampling, following the standard field procedures suggested by APHA (2012).

Computation of chloro-alkaline indices
To assess the role of cation exchange and reverse ion exchange in the aquifer system, the chloro-alkaline indices (CAI-1) and CAI-2) were adopted here (Eqs. 1 and 2) as proposed by Schoeller (1977). A negative index of CA (base ion exchange process) indicates an exchange of Ca 2+ and Mg 2+ ions from the groundwater with Na + and K + ions from the aquifer material (Deeapli et al. 2015), whereas a positive index of CA (reverse ion exchange) reflects the adsorption of Na + and K + ions on the aquifer material with the release of Ca 2+ and Mg 2+ from the groundwater.

Computation of saturation index
The saturation index (SI) is widely used to envisage the chemical activities of particular minerals in the chemistry of groundwater (Subba . The SI was computed to know the role of evaporation through the evaluation of equilibrium between water and respective minerals (Eq. 3) by comparing the ion activity product (IAP) with the solubility product (Ksp), using the geochemical software PHREEQC (Parkhurst et al. 1999).
If SI exceeds zero, it represents the oversaturation (precipitation) concerning a particular mineral (Subba ). If SI is below zero, it indicates the unsaturation (dissolution) in respect of a concerned mineral. If SI is equal to zero, it signifies the saturation (equilibrium) associated with a particular mineral.

Application of GIS analysis
In the analysis of GIS, the toposheets were digitized, using ArcGIS 10.7 software to generate the thematic spatial maps for observing the variation in the chemical quality of groundwater. Then, the spatial variation maps were extracted, using inverse distance weighted interpolation tools (Subba Rao et al. 2021b).

Generalized groundwater chemistry
The pH measured from the groundwater samples of the present study region was between 6.30 and 8.90, and its average was 7.14 ( Table 1), indicating mostly alkaline nature (Deepali et al. 2015). The EC, which expresses the level of transmitting capacity of current for the property of the water medium, ranged from 88 to 1600 µS/cm with an average of 454.76 µS/cm, indicating multiple processes taking place in the groundwater system (Subba Rao 2017). The TDS varied from 56.32 to 1024 mg/L, which is an average of 291.05 mg/L controlled by both geogenic and non-geogenic sources (Subba ). Among cations, the Ca 2+ content was from 8.02 to 152.30 mg/L, being an average of 49.60 mg/L (Table 1). Calcium feldspars are the main source of higher concentration of Ca 2+ (Subba Rao and Maya Chaudary 2019). The Mg 2+ was between 2.43 and 92.42 mg/L with an average of 23.53 mg/L. This is mainly attributed to the ferromagnesian minerals (olivine, pyroxene, biotite, etc.) present in the basement rocks, apart from the sources of non-geogenic origin (domestic wastes, septic tank leakages, etc.) (Subba Rao 2021). The Na + ranged from 3 to 146 mg/L, and its average was 54.13 mg/L. This could be due to the influence of sodium feldspars, household wastes, irrigation-return-flows, etc., on the groundwater (Deepali et al. 2020). The K + content varied from 1 to 118 mg/L with an average of 6.20. The potassium feldspars are the chief sources, and the potassium fertilizers are the secondary sources of K + in the groundwater (Subba Rao 2002). The concentrations of groundwater were in the decreasing order of Na + > Ca 2+ > Mg 2+ > K + . The dominance of Na + in the groundwater indicates the weathering and dissolution of the sources of silicate minerals and anthropogenic origins (Subba Rao and Surya Rao 2010;Subba Rao et al. 2020). Among anions, the HCO − 3 content ranged from 20.70 to 584.79 mg/L with an average of 147.29 mg/L ( Table 1). This is a result of the release of the CO 2 into the soil zone due to the consequence of the decay of organic matter and weathering of silicate minerals, apart from the influence of atmospheric CO 2 (Subba

Controlling processes of groundwater chemistry
The dissolved ions in the groundwater system are derived from geogenic origins (rock-forming minerals) as a chief source (Manikandan et al. 2020). The non-geogenic origin (domestic waste, septic tank spillages, irrigation-returnflows, chemical fertilizers, animal wastes, etc.) may also contribute as a secondary source (Sakram et al. 2019). However, sometimes secondary sources overtake the influence of geogenic origin (Subba . Generally, the relative dominance of dissolved ions in groundwater depends on the sources and solubilities of ions (Hem 1991;. Since the present study region faces problems relating to the geogenic and anthropogenic activities, it needs an assessment of the sources of dissolved ions in the aquifer system, using the different bivariate diagrams (Na + vs Cl − , Ca 2+ + Mg 2+ vs HCO − 3 , Ca 2+ + Mg 2+ vs HCO − 3 + SO 2− 4 , HCO − 3 vs Cl − + SO 2− 4 , Ca 2+ + Mg 2+ vs total cations, Ca 2+ + Mg 2+ vs Na + + K + and TDS vs NO − 3 + Cl − /HCO − 3 , ion exchange indices (chloro-alkaline indices of CAI-1 and CAI-2), and saturation indices with respect to calcite, halite, and gypsum for taking the site-specific suitable measures for sustainable development of the rural community. These geochemical signatures have been widely used (Subba Wagh et al. 2019;Deepali et al. 2020;Manikandan et al. 2020), which are discussed below:

Influence of rock weathering on groundwater chemistry
Rock weathering (or cation exchange) and halite dissolution play a significant role in dissolving the ions in the aquifer system (Eqs. 8 and 9). If the groundwater system received the Na + and Cl − ions by the dissolution of halite, the ratio of these ions would be equal to unity (Deepali et al. 2015;). Excess Na + over Cl − represents the rock weathering or cation exchange, while the excess Cl − over Na + indicates the reverse ion exchange (Subba Rao 2008; Subba . Nine percent of the groundwater sampling points in the present study region fall on the uniline of Na + and Cl − ions (Fig. 2a). This reflects the halite dissolution process. A few sampling points (13%) are found above the theoretical line of Na + and Cl − ions, specifying the rock weathering or cation exchange process. Most sampling points (78%) are observed below the equiline of Na + and Cl − ions, which measures the contribution of ions from the reverse ion exchange process as the chief source. Since the present study region experiences a semiarid climate and comes under the agricultural area, it leads not only to the formation of soil salts (NaCl, CaSO 4 , etc.) but also adds ions associated with the Na + and Cl − to the groundwater body through the application of irrigation-return-flows (Subba Rao et al. 2012a, b). Because Cl − is a non-geogenic origin (Subba Rao 2014), the excess Cl − ion over the Na + ion can also be considered as a source of non-geogenic origin.
(8) Na + , K + , Ca 2+ , Mg 2+ silicates + H 2 CO 3 Rock weathering → Na + + K + + Ca 2+ + Mg 2+ + HCO − 3 + H 4 SiO 4 + clay product (9) NaCl Halite dissolution → Na + + Cl − The diagram Ca 2+ + Mg 2+ vs HCO − 3 is frequently used to confirm rock weathering or cation exchange as the dominant controlling process of groundwater quality (Sakram and Admilla 2018;Vinnarasi et al. 2021). Above 1:1 equiline of (Ca 2+ + Mg 2+ ) and HCO − 3 indicates the dominance of Ca 2+ and Mg 2+ ions over the HCO − 3 caused by rock weathering or cation exchange, and below it confirms the release of HCO − 3 into the groundwater caused by feldspar minerals with carbonic acid (H 2 CO 3 ; Eq. 8). In the present study region, 8% of the plotted points are observed below the uniline of (Ca 2+ + Mg 2+ ) and HCO − 3 (Fig. 2b). This indicates that the Ca 2+ , Mg 2+ , and HCO − 3 ions are derived from the dissolution of carbonate rocks, which is further supported by the plotting of the groundwater samples that move toward HCO − 3 from the theoretical line of HCO − 3 :(Cl − + SO 2− 4 ; Fig. 3a). The maximum plotting points (92%) of the chemical data of the groundwater samples between (Ca 2+ + Mg 2+ ) and ( HCO − 3 + SO 2− 4 ) also show the deviation from the equiline and run toward HCO − 3 + SO 2− 4 (Fig. 3b), indicating an excess of HCO − 3 . Generally, the dissolution of Ca 2+ and Mg 2+ silicates and HCO − 3 and SO 2− 4 associated with the soils give an equal amount of these ions in the groundwater system (Deepali et al. 2021). High Ca 2+ + Mg 2+ compared to HCO − 3 indicates the reverse ion exchange due to the derivation of Ca 2+ and Mg 2+ ions from the aquifer material. If excess ( HCO − 3 + SO 2− 4 ) over (Ca 2+ + Mg 2+ ) reveals the removal of Ca 2+ and Mg 2+ ions from the water by the cation exchange process, or excess HCO − 3 that is added to the groundwater body by weathering of Na + and K + silicates (Wagh et al. 2019), apart from the soil CO 2 released from the decay and decomposition of organic matter (Eqs. 10 and 11).
Further, the computed partial pressure of carbon dioxide (PCO 2 ) in the groundwater samples of the present study region varied from − 3.05 to − 0.43 with an average of − 1.18 (Table 1), which is more than the atmospheric PCO 2 (− 3.50). The higher PCO 2 of the groundwater indicates the prevailing conditions of the open system weathering with a relatively higher rate of solubility (Vinnarasi et al. 2021). Therefore, the groundwater of the present study regions shows a higher concentration of HCO − 3 (Table 1).
Further, it is also noted that most groundwater sampling points in the plots of (Ca 2+ + Mg 2+ ) vs (Na + + K + ) move toward Ca 2+ + Mg 2+ (Fig. 4a), so that they appear as the major contributors, which exceeds the Na + and K + ions in the groundwater (Subba Rao et al. 2006 (Ca 2+ + Mg 2+ ) vs total cations (Ca 2+ + Mg 2+ + Na + + K + ) also deviate from the equiline and change their trend toward the total cations (Fig. 4b). This is a result of the total contribution of the cations from the source (Subba Rao 2008). Therefore, these diagrams clearly explain the carbonate weathering as the major controlling process of groundwater chemistry due to the occurrence of soil CO 2 .

Influence of ion exchange process on groundwater chemistry
As discussed earlier, the excess Cl − ion over the Na + ion in the majority (78%) of the groundwater samples (Fig. 2a) explains the reverse ion exchange process taking place in the aquifer system. To verify this phenomenon further in the groundwater of the present study area, the chloro-alkaline indices (CAI-1) and CAI-2) were computed (Eqs. 1 and 2) and the results are shown in Table 2. The values of CAI-1 varied − 3.64 to 0.94 with an average of 0.12, while those of CAI-2 were from − 0.78 to 3.36 with an average of 0.34. They demonstrated in Fig. 5 representing the cation ion exchange (Eq. 12) and reverse ion exchange processes (Eq. 13). From Fig. 5, it is noted that 83% of the groundwater sampling points fall toward the positive indices of CAI-1 and CAI-2. It enlightens the reverse ion exchange as the chief controlling process in the groundwater system (Deepali et al. 2015).

Influence of evaporation process on groundwater chemistry
The soils in the present study region contain calcium carbonate concretions, which suggest the prevailing conditions of the semiarid climate. As stated above, evaporate dissolution (halite) occurs in the groundwater system. Thus, it is also essential to confirm the role of the evaporation process in the groundwater body. Saturation indices (SI) were computed concerning calcite (CaCO 3 ), halite (NaCl), and gypsum (CaSO 4 ), using Eq. 3. The results are shown in Fig. 6. They predict the reactive minerals with the help of the chemistry of groundwater (Deepali et al. 2015

Non-geogenic source
The present study region is a part of an agricultural rural area and also shows poor disposal conditions for Ionic relations a (Ca 2+ + Mg 2+ ) vs (Na + + K + ) and b (Ca 2+ + Mg 2+ ) vs total cations household waste and septic tank spillages, animal waste, unlimited use of irrigation-return-flows and agricultural fertilizers, etc. Generally, they modify the groundwater chemistry due to the adding of additional concentrations of ions and thereby inferior groundwater quality occurs.
To know the influence of non-geogenic sources on the chemistry of groundwater, the relationship of TDS and ( NO − 3 + Cl − /HCO − 3 ) is widely used (Li et al. 2019;Subba Rao et al. 2021c). This relation illustrates a linear trend (y = 0.0077x + 1.9265 and R 2 = 0.1066; Fig. 7) obviously supporting the influence of the non-geogenic sources on the groundwater system. As a result, the impact of the anthropogenic source is masking the influence the geogenic source. This is the main reason that the groundwater quality in some areas in the present study region appears to be slightly brackish, depending upon the hydrogeological environmental conditions.

Salinity (EC) vs Sodium adsorption ratio (SAR)
In the present study region, groundwater is the main source of crop growth. However, its poor quality reduces crop growth considerably (Aravinthasamy et al. 2020). Hence, the assessment of the suitability of the groundwater quality is essential for taking management measures to ensure healthy crop growth (Ghalib 2017). Groundwater salinity expressed in terms of EC plays a significant role in crop growth. The salinity of groundwater is classified as low, C1 (EC < 250 μS/cm), medium, C2 (250-750 μS/cm), high, C3 (750-2250 μS/cm), and very high, C4 (> 2250 μS/cm). With the increase in the groundwater salinity, crop production can be reduced (Subba Rao 2017). The sodium adsorption ratio (SAR) evaluates the influence of the Na + content about Ca 2+ and Mg 2+ (Eq. 4), which is expressed in terms of alkalinity. If SAR increases, it reduces soil permeability, which has adverse effects on crop growth Subba Rao et al. 2021c).

Residual sodium carbonate
Residual sodium carbonate (RSC) is also used to evaluate the groundwater suitability for crops, which is a relationship between CO 2− 3 + HCO − 3 and Ca 2+ + Mg 2+ (Eq. 5). The RSC is classified into three types (Alaya et al. 2014). They are suitable (< 1.25 meq/L), marginally suitable (1.25-2.50 meq/L), and unsuitable (> 2.50 meq/L) types for irrigation purposes. The RSC varied from − 4.22 to 8.01 meq/L, and its average was 0.14 meq/L ( Table 2). According to the classification of RSC, the suitable, marginally suitable, and unsuitable types are observed in the areas of 86.24%, 6.70%, and 7.06%, respectively, for irrigation purposes (Fig. 9a). The unsuitable groundwater quality is observed from the eastern part of the study region.

Magnesium ratio
Generally, Mg 2+ damages the soil structure, when the water has higher Na + and salinity, which decreases the crop yields. The magnesium ratio (MR) was computed, using Eq. 6 (Szaboles and Darab, 1964). The MR was between 4.86 and 81.10, and its average was 44.79 (Table 2). If MR is higher than 50% in water, it is harmful to irrigation, and if it is less than 50%, it is suitable for irrigation purposes (Faten et al. 2016;. Accordingly, 36.93% and 63.07% of the spatial areas come under the suitable and non-suitable types for irrigation purposes, respectively (Fig. 9b). Groundwater quality is observed to be unfit for irrigation purpose from the areas of the central part spreading from western to eastern side of the study region.

Kelly's ratio
Kelly's ratio (KR) is used to assess the irrigation water quality (Kelley 1963), which measures the levels of Ca 2+ and Mg 2+ ions (Eq. 7). If KR is below one, it is suitable for irrigation, and if it is above one, it is not suitable for irrigation purposes (Aravinthasamy et al. 2020). The KR was from 0.06 to 8.09 with an average of 1.21 (Table 2). As per the classification of KR, 48.73% and 51.27% of the study region fall into the suitable and unsuitable water quality types for irrigation purposes, respectively (Fig. 9c). The unsuitable groundwater quality zone for irrigation purpose is mainly observed from the southern part of the study region.

Recommendations for groundwater quality management
According to the EC vs SAR, RSC, MR and KR, 99.9%, 7.06%, 63.07%, and 51.27% of the total study region come under the poor groundwater quality type for irrigation purposes. These zones spread mainly in the whole area, eastern, central, and southern parts, respectively. The quality of groundwater in these zones may not support crop yields due to the reduction in the soil permeability (Subba Rao et al. 2012aRao et al. , b, 2021c. Thus, the plant roots are unable to receive water properly and, consequently, nutrients from the soils. Therefore, the following site-specific suggestions are recommended for the sustainable development of groundwater • Inferior groundwater quality should not be used for crop growth without treatment, • Application of gypsum as an amendment is necessary to increase the soil permeability for proper plant growth, • The quality of groundwater should be improved through the implementation of the rainwater recharging strategies in the entire study region, • Drainage systems and septic tanks should be maintained properly to reduce groundwater pollution, especially where the build-up areas occur, • Biological treatment plants and recycling of solid waste are necessary for a clear and clean rural environment, particularly where the population is more, and • Creating public awareness on environmental issues must be needed for building a healthy society.

Conclusions
The following important conclusions were drawn from the present study region of Telangana, India, after observing the geochemical processes controlling the chemistry of groundwater chemistry, using the various geochemical ratios, and groundwater quality suitability, using the irrigation chemical parameters: • The groundwater was mostly alkaline with a characterization of Na + and HCO − 3 ions. • The geochemical ratios such as Na + vs Cl − , Ca 2+ + Mg 2+ vs HCO − 3 , Ca 2+ + Mg 2+ vs HCO − 3 + SO 2− 4 , HCO − 3 vs Cl − + SO 2− 4 , Ca 2+ + Mg 2+ vs total cations, and Ca 2+ + Mg 2+ vs Na + + K + and the saturation indices of calcite, halite, and gypsum suggest that the groundwater chemistry was mainly controlled by carbonate weathering associated with the reverse ion exchange and evaporation processes as the geogenic factor. The ratio TDS vs NO − 3 + Cl − /HCO − 3 indicated the non-geogenic origin as the secondary source in the groundwater system. The impact of the later source is masking the influence the first source in some areas of the present study region. • As per the groundwater salinity vs sodium adsorption ratio, residual sodium carbonate, magnesium ratio, and Kelly ratio, 99.9%, 7.06%, 63.07%, and 51.27% of the spatial study region were unsuitable for irrigation purposes, and • Site-specific appropriate management measures were suggested to improve the groundwater quality for proper crop production and consequently for better living conditions of the rural community.