Comprehensive hydrogeochemical characterization and seasonal water quality index analysis for sustainable groundwater management in Valliyur region, Southern Tamil Nadu, India

Abstract

Groundwater contamination and seasonal quality variations pose significant challenges in crystalline terrains where rock-water interactions control hydrogeochemical evolution. Despite increasing demographic and agricultural pressures, the Valliyur region in Southern Tamil Nadu lacks comprehensive seasonal hydrogeochemical characterization. This study aims to characterize seasonal hydrogeochemical evolution, assess drinking and irrigation water suitability using comprehensive indices, and identify controlling processes for sustainable groundwater management. The groundwater showed alkaline nature (pH 7.14–8.35) with electrical conductivity (206-11283 µS/cm) and total dissolved solids (96-7334 mg/L) exceeding WHO guidelines in 32% of samples. Total hardness exceeded standards in 48.33% (pre-monsoon) and 43.33% (post-monsoon) samples. ANOVA statistical analysis confirmed that only potassium showed statistically significant seasonal variations (p < 0.001), while geological processes dominated other parameters. The rock-water interactions and weathering process controlled the groundwater geochemistry. The Piper trilinear diagram reveals water types dominated by Ca²⁺-Mg²⁺-HCO₃⁻-Cl⁻ and Ca²⁺-SO₄²⁻-HCO₃⁻-Cl⁻ during pre-monsoon, shifting to Ca²⁺-Cl⁻-SO₄²⁻-HCO₃⁻ dominance post-monsoon. Saturation indices revealed calcite equilibrium (52%), normal ion exchange dominance (95%), and bivariate analysis confirmed silicate weathering as the primary control. The Water Quality Index showed seasonal improvement from 48.33% “Excellent” (pre-monsoon) to 70% “Excellent” (post-monsoon). Health risk assessment identified 25% of the study area posing moderate to high health risks during pre-monsoon periods requiring water treatment or alternative sources. Irrigation quality assessment confirmed suitability for agricultural use with enhanced quality during post-monsoon. These findings demonstrate groundwater suitability for irrigation but seasonal variability for drinking purposes, highlighting the need for comprehensive monitoring and management for long-term sustainability.

Introduction

Groundwater is an important potable source that has increased extremely in demand globally in the last two decades¹. Demographic expansion, urban development, industrial proliferation, and high-intensity farming contributed to a surge in groundwater extraction, accounting for over 45% of irrigation and 80% of domestic consumption^2,3. This trend is noted in developing countries where surface water resources are limited, as groundwater is the primary irrigation resource that supports 39 million hectares and potable consumption^4,5. Over 80% of the region’s water requirements for domestic needs, farming, and industries are fulfilled by groundwater in Tamil Nadu, representing heavy dependence on the resource⁶. The overexploitation of groundwater resources highlighted serious issues over persistent availability and water quality standards⁷. Recent studies have highlighted the degradation of its quality in many parts of Tamil Nadu, addressing high salinity, fluoride, nitrate, and heavy metal contamination⁸. The complex interplay of geological as well as human-induced such as the composition of bedrock, weathering, mineral dissolution, natural contaminants, erosion, salinization, irrigation, industrial effluents, improper waste disposal, urban sprawl, mining, overexploitation of groundwater, land-use changes and pollution represents groundwater quality issues⁹.

The implications are widespread as access to clean and safe drinking water is a fundamental human requirement and a prerequisite for public health, agricultural productivity, and overall socioeconomic development¹⁰. Addressing water quality challenges aligns with SDG 6 (Sustainable Developmental Goals) Clean Water and Sanitation, which focuses on enhancing groundwater quality by decreasing contamination, preventing waste disposal, and reducing the release of hazardous substances by 2030¹¹. The impacts are far-reaching, as the consumption of polluted groundwater can lead to severe health problems, crop yield reductions, and ecosystem degradation¹². Hydrogeochemical evolution in crystalline terrains is governed by complex water-rock interactions involving mineral dissolution, precipitation of secondary phases, and ion exchange processes that create distinct chemical signatures in groundwater systems^13,14. In hard rock aquifers, weathering profiles develop within the first 100 m below ground surface comprising a capacitive saprolite layer overlying a permeable stratiform fractured layer, where groundwater geochemistry is primarily controlled by the dissolution of primary silicate minerals such as feldspar, biotite, and amphibole¹³. Recent investigations in crystalline terrains have demonstrated that silicate weathering, carbonate dissolution and cation exchange processes create characteristic hydrogeochemical signatures with Ca²⁺-Mg²⁺-HCO₃⁻ waters typically evolving to mixed Ca²⁺-Mg²⁺-Cl⁻-SO₄²⁻ types along groundwater flow paths^15,16. Seasonal variations significantly modulate these baseline geochemical signatures through dilution effects during monsoon recharge, enhanced mineral-water equilibria shifts, and differential mobilization of weathering products from the unsaturated zone¹⁷. Contemporary studies emphasize the importance of ion exchange processes, particularly reverse cation exchange where Ca²⁺ and Mg²⁺ from weathering reactions replace Na⁺ on clay exchange sites, fundamentally altering the ionic composition of groundwater in crystalline aquifers^14,18. Understanding these interconnected hydrogeochemical processes has become increasingly critical for distinguishing natural geogenic variations from anthropogenic contamination in rapidly developing regions where agricultural intensification and urbanization progressively influence groundwater chemistry^15,16.

Sustained groundwater hydrochemistry monitoring is essential to identify problem areas and inform evidence-based management strategies by limiting extraction and proper waste management to maintain quality^19,20. Rural localities depend on groundwater for domestic and irrigation purposes, requiring budget-friendly community approaches and effective management practices to preserve water resources for future generations^3,21. Natural treatment systems, such as riverbank filtration and sand bed filtration, provide cost-effective alternatives for water purification in developing regions where conventional treatment infrastructure is limited²². Advanced water processing techniques efficiently remove contaminants and improve groundwater potability as water quality indices (WQI) facilitate quantitative spatiotemporal comparisons, enabling managing resource and public health interventions^23,24. Emerging water treatment technologies, including photocatalytic purification systems using semiconductor materials, offer promising solutions for degrading organic contaminants in polluted water sources²⁵. The computational fluid dynamics modeling has revolutionized the design and optimization of hydraulic treatment structures, enabling cost-effective and efficient water management infrastructure development²⁶. The water quality indices are a valuable tool that provides an extensive and consistent way of evaluating the overall condition of a water source²⁷. The Weighted Arithmetic method is commonly used for its accuracy and efficient assessment of groundwater quality among numerous methods available^27–29. The method adapts site-specific parameters and adjusts weight factors reflecting local groundwater quality, showing its flexibility in accurately characterizing the quality and detecting contaminant hotspots^27,29–32.

The Latest studies have shown the importance of geochemical processes, water quality index (WQI), and geospatial techniques in assessing groundwater suitability for domestic and irrigation purposes across different locations^33,34. Bakshe et al.¹⁵ found 7% of groundwater samples unfit for drinking in Kerala’s Western Ghats due to Fe/Mn contamination with CaHCO₃ type waters (46%) indicating silicate weathering dominance; Lachassagne et al.¹³ established that crystalline aquifers develop within 100-meter weathering profiles comprising saprolite and fractured layers; Rajmohan et al.¹⁷ demonstrated significant monsoon-induced improvements in Tamil Nadu hard rock aquifers, reducing NO₃⁻ contaminated areas by 95% during high rainfall years; while Arroyo-Figueroa et al.¹⁴ validated a seven-stage framework for hydrogeochemical assessment in Colombian coastal aquifers with distinct seasonal ion variations. Shube et al.³⁵ identified significant fluoride contamination and health risks in Adama town’s rift valley through PIG and GIS analysis, while Tesema et al.³³ found 94.11% excellent to good water quality ratings with five distinct hydrochemical facies in the upper Omo River basin, demonstrating regional variability in groundwater suitability for drinking and irrigation purposes despite increasing anthropogenic pressures. Kumar et al.²⁷ revealed significant concerns in the Anantapur district, where 69% of samples showed very poor drinking water quality, while Gharbi et al.³⁶ found more favorable conditions in the Sidi Bouzid aquifer with 70% of samples were suitable for irrigation under moderate restrictions. Udeshani et al.²⁸ identified concerning trends in the Monaragala region, where over 50% of samples exhibited poor quality, establishing a correlation between high WQI values and chronic kidney disease prevalence. Ghosh & Bera³⁷ utilized multiple indices in the Purulia district, identifying areas of poor water quality and medium to high salinity risks. Hagan et al.³⁸ found more promising results in peri-urban Accra, with 92% of samples suitable for domestic use. In South India, Kamaraj et al.³⁹ investigated coastal aquifers in Tiruchendur, finding 60% of samples suitable for drinking, though irrigation suitability showed mixed results. Kumari & Rai⁴⁰ assessed southern Haryana revealing 45.31% poor drinking water quality and high salinity concerns for irrigation. Urseler et al.⁴¹ analyzed dairy farms in the Pampa Pla’s highly contaminated water unsuitable for domestic needs. Saqib et al.³¹ demonstrated optimistic results in Lucknow, with 86% of the area showing very good water quality. These diverse findings across different regions explain the importance of seasonal assessment of groundwater using multiple indices for effective groundwater quality evaluation and management.

These diverse findings across different regions highlight the need for region-specific groundwater quality assessment. The objectives of the present study are: (1) to evaluate the groundwater quality in the Valliyur region for drinking purposes using the water quality index (WQI), and (2) to assess the suitability of groundwater for agriculture using various irrigation quality indices.

Materials and methods

Research area

The research zone spans around 740 km² and is located between 8°34’07.8” to 8°16’53.2” north latitude and 77°29’45.9” to 77°49’59.0” east longitude with elevations ranging from 23 m to 461 m.amsl. The western part consists of the Western Ghats Mountain range, which has diverse topography, including ridges and valleys. The central and⁴²eastern regions are predominantly flat, sloping gently towards the southeast with occasional hilly terrain in the west⁴³. The region has several major rivers, including the Thamirabarani, Nambiar, Chittar, and Karamaniar rivers. The region experiences a subtropical climate, receives mean annual precipitation of 879 mm (IMD data) mostly from the northeast monsoon, and has relative humidity levels averaging between 79% and 84%. The temperature ranges from a mean minimum of 22.9 °C to a mean maximum of 33.5 °C⁴². The study area consists of various rock types, with garnetiferous biotite gneiss being the most abundant, followed by garnetiferous biotite sillimanite gneiss with graphite and kyanite. Rock types present in relatively lower proportions include Acid to intermediate Charnockite, Calc-granulite, Limestone, Pegmatite, and Coarse sand with rock fragments and Quartzite shown in Fig. 1. These rock types have specific capacities, transmissivity and well yields depending on their weathering and fracture characteristics.

Fig. 1.

Study area location with geology and sample points. This figure was prepared using ArcGIS Desktop 10.8 (https://www.esri.com/en-us/arcgis/products/arcgis-desktop/overview). The geological data and shapefiles were obtained from the Geological Survey of India (https://www.bhukosh.gsi.gov.in) and administrative boundary data from the Survey of India (https://surveyofindia.gov.in/).

Hydrogeological framework

The study area is characterized by distinct hydrogeological units with varying aquifer characteristics and groundwater potential (Fig. 2a). The Migmatitic Gneiss formation constitutes the dominant aquifer system characterized by a single unconfined to semi-confined aquifer with depth ranges of 15–25 m, transmissivity values between 40 and 160 m²/day and specific yield up to 2%, with well yields ranging from 1000 to 2500 L/day. The Alluvium (Younger Alluvium comprising clay/silt/sand with calcareous concretions) forms multiple confined aquifer systems at depths of 40–400 m with higher transmissivity (70–350 m²/day) and significantly higher yields (1000–3000 L/day) with specific yield ranging from 6 to 8%. The Khondalite and Granulites represent single unconfined to semi-confined aquifer systems with depths of 15–20 m, transmissivity of 15–291 m²/day and yields ranging from 1000 to 4000 L/day with specific yields up to 2%. The weathered zone and fracture network in these hard rock formations control groundwater occurrence and movement, with the weathered zone typically extending 10–30 m below ground surface as the primary water-bearing formation.

Fig. 2.

(a) Hydrogeology and (b) land use land cover map of the study area. This figure was prepared using ArcGIS Desktop 10.8 (https://www.esri.com/en-us/arcgis/products/arcgis-desktop/overview). The hydrogeological data were obtained from India-WRIS (https://indiawris.gov.in/wris/#/geoSpatialData), and land use land cover data from ArcGIS Living Atlas Land Cover Explorer (https://livingatlas.arcgis.com/landcoverexplorer/).

Land use and land cover pattern

The land use and land cover analysis reveal that the study area is predominantly agricultural, with crops occupying 479.17 km² (65.03%) of the total area, indicating intensive agricultural activities that potentially influence groundwater quality through fertilizer application and irrigation practices (Fig. 2b). Range land covers 184.67 km² (25.06%). In comparison, built-up areas account for 51.73 km² (7.02%), reflecting moderate urbanization. Forest cover is limited to 10.67 km² (1.45%), concentrated mainly in the western hilly regions, while water bodies occupy 9.56 km² (1.30%) distributed across the region. Flooded vegetation and bare ground constitute minimal areas of 0.06 km² (0.008%) and 0.97 km² (0.13%). This land use pattern dominated by agriculture and moderate urban development creates potential anthropogenic influences on groundwater chemistry through agricultural runoff, fertilizer leaching and domestic wastewater infiltration particularly in areas with high crop density and built-up regions.

Sample collection and analysis

Considering the Northeast monsoon season, the random sampling strategy was implemented comprehensively to investigate the groundwater quality with 60 samples during September 2021 (pre-monsoon) and March 2022 (post-monsoon). The samples collected are stored in plastic bottles initially rinsed with the collected water to ensure sample integrity⁴⁴. The geographic coordinates of the sampling locations were recorded in the field using a Garmin digital GPS device. The on-site measurements of pH, electrical conductivity (EC), and total dissolved solids (TDS) collected by portable Hanna field kit allow for the immediate documentation of the physicochemical parameters of the groundwater at the time of sample collection. The collected samples were analysed for major cations (Ca²⁺, Mg²⁺, Na⁺ and K⁺) and anions (Cl⁻, SO₄²⁻ and HCO₃⁻). The analysis followed IS 3025 standard methods for water quality testing. This study did not analyze nitrate (NO₃⁻) and fluoride (F⁻) concentrations due to analytical constraints, which represents a limitation of the current research and will be addressed in future comprehensive studies.

Spatial distribution of physicochemical parameters

The interpolation approach was utilized specifically the inverse distance weighted (IDW) technique. The IDW assigns greater influence to nearby measurement locations while decreasing the impact of distant points by producing values that reflect weighted neighbourhood averages across the surface⁴⁵. This tool is invaluable for all researchers because it clearly visualizes and identifies the concentration zones and other spatial patterns within the data, providing easily understandable characters and their distribution throughout the study area.

Statistical analysis

One-way Analysis of Variance (ANOVA) was performed to test statistically significant seasonal differences in groundwater quality parameters between pre-monsoon and post-monsoon periods. The null hypothesis (H₀) stated that mean values between seasons are equal, while the alternative hypothesis (H₁) proposed significant seasonal differences. Statistical significance was set at α = 0.05. Effect sizes were calculated using eta-squared (η²) to determine practical significance, where values of 0.01, 0.06, and 0.14 represent small, medium, and large effects⁴⁶. All statistical analyses were conducted using XL Miner Analysis ToolPak in Microsoft Excel.

Hydrogeochemical process analysis

The hydrogeochemical evolution and controlling processes in the crystalline aquifer system were investigated through comprehensive analysis combining traditional graphical methods with advanced process identification techniques. The integrated approach included hydrogeochemical facies classification, mineral saturation analysis, ion exchange mechanisms, and source identification using established geochemical approaches^47–49.

Hydrogeochemical facies classification

Hydrogeochemical facies were determined using the Piper trilinear diagram⁴⁹which systematically classified groundwater types based on major ion composition. The diagram consists of two triangular fields representing cation and anion compositions, with a central diamond-shaped field displaying the overall chemical character. Groundwater samples were plotted according to their relative percentages of major cations (Ca²⁺, Mg²⁺, Na⁺+K⁺) and anions (HCO₃⁻, SO₄²⁻, Cl⁻) to identify distinct hydrogeochemical facies and assess seasonal variations in water types.

Hydrogeochemical process dominance

The Gibbs diagram (Gibbs, 1970) was employed to identify the dominant natural processes controlling groundwater chemistry. Two plots were constructed: (a) Cation Gibbs Plot: Total Dissolved Solids (TDS) versus Na⁺/(Na⁺+Ca²⁺) ratio (b) Anion Gibbs Plot: Total Dissolved Solids (TDS) versus Cl⁻/(Cl⁻+HCO₃⁻) ratio. The diagrams distinguish three primary mechanisms: Rock weathering dominance: Intermediate TDS with moderate ionic ratios, Evaporation dominance: High TDS with elevated Na⁺ and Cl⁻ proportions, and Precipitation dominance: Low TDS with variable ionic compositions.

Mineral saturation indices analysis

Mineral saturation states were evaluated to assess the groundwater system’s equilibrium conditions and precipitation/dissolution potential for major carbonate minerals^50,51. The saturation index (SI) for each mineral phase was calculated using the fundamental relationship:

where IAP represents the ion activity product of the dissociated chemical species in solution, and Kt denotes the equilibrium solubility constant for the specific mineral phase at standard temperature and pressure conditions. Mineral saturation states were systematically classified into three categories based on thermodynamic equilibrium principles^52,53: Undersaturated conditions (SI < -0.5): Indicating mineral dissolution potential; Equilibrium conditions (-0.5 ≤ SI ≤ 0.5): Representing stable mineral-water equilibrium and Supersaturated conditions (SI > 0.5): Suggesting mineral precipitation tendency. The percentage distribution of groundwater samples within each saturation category was quantified for both seasonal periods to assess temporal variations in mineral stability and geochemical evolution patterns.

Ion exchange process identification

Ion exchange processes between groundwater and clay minerals formed were characterized using Chloro-Alkaline Indices (CAI) as diagnostic indicators of cation exchange reactions^47,54. Two complementary indices were calculated:

Primary Chloro-Alkaline Index:

Secondary Chloro-Alkaline Index:

The interpretation of CAI values follows established geochemical principles. Positive CAI₁ values indicate normal ion exchange processes where Na⁺ and K⁺ from groundwater replace Ca²⁺ and Mg²⁺ on clay exchange sites. Negative CAI₁ values suggest reverse ion exchange where Ca²⁺ and Mg²⁺ displace Na⁺ and K⁺ from exchange positions. Cross-plot analysis of CAI₁ versus CAI₂ was employed to identify dominant exchange mechanisms and quantify seasonal variations in ion exchange intensity within the crystalline aquifer system.

Water quality index for drinking (WQI)

The groundwater quality index is a comprehensive metric derived from integrating multifaceted hydrochemical parameters that offer a consolidated quantitative assessment of aquifer suitability for diverse applications, including potable consumption, agricultural irrigation, and industrial utilization^36,55. The Weighted Arithmetic Index method by⁵⁶ was used for identifying the groundwater quality for drinking purposes⁵⁷.

The parameters weights were calculated based on WHO (2017) guidelines using the following steps:

Where, Wi is the relative weight, Sn is the WHO standard value for each parameter, and K is a proportionality constant calculated as:

The Water Quality Index is calculated using formula^27,56,

Where, WQI - Water Quality Index (values between 0 and 100), Wi - weight of the ith parameter and Qi - the quality rating or sub-index of the ith parameter.

Where Vn is the measured value of the parameter and Sn is the Who 2017 standard value of the parameter. Table 1 shows the weightage calculation of the water quality index parameters.

Table 1.

Relative weights (Wi) calculation of water quality parameters as per WHO (2017) guidelines.

Parameters	WHO (2017)	1/Sn	∑1/Sn	K = 1/∑1/Sn	Wi = K/Sn
pH	8	0.1250	0.1913	5.2265	0.6533
EC (µS/cm)	1500	0.0007	0.1913	5.2265	0.0035
TDS (mg/L)	1000	0.0010	0.1913	5.2265	0.0052
TH (mg/L)	300	0.0033	0.1913	5.2265	0.0174
Ca2+ (mg/L)	200	0.0050	0.1913	5.2265	0.0261
Mg2+ (mg/L)	150	0.0067	0.1913	5.2265	0.0348
Na+ (mg/L)	200	0.0050	0.1913	5.2265	0.0261
K+ (mg/L)	30	0.0333	0.1913	5.2265	0.1742
Cl− (mg/L)	250	0.0040	0.1913	5.2265	0.0209
SO42− (mg/L)	250	0.0040	0.1913	5.2265	0.0209
HCO3− (mg/L)	300	0.0033	0.1913	5.2265	0.0174
∑Wi					1.0000

Irrigation water quality (IWQ)

Irrigation quality is an important factor that directly impacts crop growth, soil health, and overall yield⁵⁸. Irrigation water quality is a comprehensive tool for assessing the water suitability for agricultural activities⁵⁹. This index takes into account various parameters that influence water quality, such as salinity, sodicity, toxicity, and other specific factors relevant to the region and crop type⁶⁰. Multiple sub-indices are used to identify IWQ and evaluate different aspects of irrigation quality⁶¹. The sub-indices utilized for IWQ are Sodium Percentage (Na%), Sodium adsorption ratio (SAR), Residual Sodium Carbonate (RSC), Permeability Index (PI), Kelley’s ratio (KR) and Magnesium Adsorption ratio (MAR). The Wilcox plot was used to classify the groundwater samples based on their suitability for irrigation, considering the water’s sodium content and electrical conductivity^62,63. The United States Salinity Laboratory (USSL) diagram is used to assess the groundwater salinity and sodicity hazards, which are crucial factors in determining the water’s suitability for irrigation⁴⁵. The calculation of sub-indices is displayed in Table 2 as follows.

Table 2.

Irrigation water quality indices and their significance in assessing groundwater suitability for agricultural use.

Indices	Formula	Description
Na%		The % of Na was calculated to evaluate the sodium hazard associated with the groundwater, which can impact plant growth and soil62
SAR		The SAR determined to estimate the degree of sodium absorption by the soil45
RSC		The RSC assessed to determine the potential for the groundwater to cause soil alkalinisation and reduce soil permeability64
PI		The PI calculated to assess the groundwater’s ability to maintain soil permeability65
KR		The KR considers the balance between Na+, Mg2+, and Ca2+ ion66
MAR		The MAR determined to assess the potential for the groundwater to cause soil degradation and affect plant growth67

A comprehensive Methodological Flowchart for Groundwater Quality Assessment and Sustainable Management Using Seasonal and Hydrochemical Data is shown in Fig. 3.

Fig. 3.

Methodology framework of the hydrogeochemical study for sustainable groundwater management.

Results and discussion

The groundwater quality is crucial as contaminants and dissolved ions can pose significant health risks if present at elevated levels. The chemical analysis results of pre-monsoon and post-monsoon season compared with WHO 2017 standards are summarized in Table 3(a & b).

Table 3.

Statistical summary of Physico-chemical parameters and their comparison with WHO (2017) guidelines (a) Pre-monsoon and (b) Post-monsoon.

a) Pre-Monsoon
Parameters	Min	Max	Mean	WHO 2017 standard	No. of samples exceeding the limit	Percent of samples exceeding the limit
pH	7.28	8.35	7.7	6.5-8	9	15.00
EC (µS/cm)	206	11,283	1706	1500	19	31.67
TDS (mg/L)	134	7334	1107	1000	24	40.00
TH (mg/L)	59	1424	300	300	29	48.33
Ca2+ (mg/L)	10	222	82	200	06	10.00
Mg2+ (mg/L)	04	275	51	150	01	1.67
Na+ (mg/L)	03	180	42	200	Nil	Nil
K+ (mg/L)	01	373	36	30	21	35.00
Cl− (mg/L)	12	2656	259	250	24	40.00
SO42− (mg/L)	19	1771	290	250	22	36.67
HCO3−(mg/L)	30	958	251	300	03	5.00

b) Post-monsoon
Parameters	Min	Max	Mean	WHO 2017 standard	No. of samples exceeding the limit	Percent of samples exceeding the limit
pH	7.14	8.19	7.63	6.5-8	2	3.33
EC (µS/cm)	150	8219	1244	1500	20	33.33
TDS (mg/L)	96	5260	800	1000	20	33.33
TH (mg/L)	67	5260	371	300	26	43.33
Ca2+ (mg/L)	14	192	69	200	02	3.33
Mg2+ (mg/L)	10	373	65	150	02	3.33
Na+ (mg/L)	02	110	28	200	Nil	Nil
K+ (mg/L)	01	342	22	30	01	1.67
Cl− (mg/L)	12	1771	182	250	14	23.33
SO42− (mg/L)	14	1295	207	250	23	38.33
HCO3− (mg/L)	16	915	237	300	02	3.33

Physical parameters

The pH value is primarily controlled by mineral alteration in the dominant metamorphic terrain⁶⁸. The hydrolysis of minerals such as feldspars and micas releases cations (Na⁺, K⁺, Ca²⁺ and Mg²⁺) and consumes H⁺ ions, leading to generally alkaline conditions⁶⁹. This geological influence is evident in both seasons, with pH ranging from 7.28 to 8.35 (mean 7.7) during pre-monsoon (Fig. 4a) and 7.14 to 8.19 (mean 7.63) during post-monsoon (Fig. 4b). The slightly higher pre-monsoon pH reflects intensified water-rock interactions due to longer residence times and concentration effects⁷⁰. Most samples are within the WHO standard range of 6.5-8.0, and 15% of pre-monsoon samples exceed this limit compared to only 3.33% post-monsoon. This seasonal difference suggests that the geological influence on pH is higher during the drier pre-monsoon period when rock-water interactions are intensified⁷¹. The EC levels were determined by the dissolution of ions from the weathering of metamorphic rocks⁷². The breakdown of silicate minerals in garnetiferous biotite gneiss and garnetiferous biotite sillimanite gneiss releases various ions into the groundwater resulting in elevated EC values across both seasons: 206 to 11,283 µS/cm (mean 1706 µS/cm) during pre-monsoon (Fig. 4c) and 150 to 8219 µS/cm (mean 1244 µS/cm) during post-monsoon (Fig. 4d). The higher pre-monsoon mean EC reflects intensified mineral dissolution due to longer water-rock contact times and evaporative concentration⁷³. During the post-monsoon period, the decrease in mean EC values attributed to dilution from monsoon recharge with a significant proportion of samples in both seasons (31.67% in pre-monsoon and 33.33% in post-monsoon) exceeded the WHO guideline of 1500 µS/cm⁷⁴. Despite different mean values, similar exceedance percentages across seasons suggest that while seasonal factors influence the overall mineralization, the underlying geological control remains strong throughout the year. This higher concentration of EC levels explains the dominant influence of the local geology on groundwater mineralization, with the weathering of silicate minerals continuously contributing dissolved ions regardless of seasonal variations⁷⁵.

Fig. 4.

Spatial distribution maps showing comparison between Pre-monsoon and Post-monsoon seasons for (a,b) pH, (c,d) EC, (e,f) TDS, and (g,h) TH. The figure was prepared using ArcGIS Desktop 10.8 (https://www.esri.com/en-us/arcgis/products/arcgis-desktop/overview). The groundwater quality data were obtained through field sampling and laboratory analysis following standard analytical procedures.

The total dissolved solids (TDS) reflect the mineralogical composition of the aquifer present⁷⁶. The silicate weathering in garnetiferous biotite gneiss and garnetiferous biotite sillimanite gneiss is the main reason for the TDS value. The breakdown of feldspars contributes Na⁺, K⁺, and Ca²⁺ while the ferromagnesian minerals like biotite and garnet on weathering release Mg²⁺ and Fe²⁺⁷⁷. This geological control is observed by high TDS values ranging 134 to 7334 mg/L during pre-monsoon (Fig. 4e) and 96 to 5260 mg/L during post-monsoon (Fig. 4f). Numerous samples in both seasons exceed the WHO guideline (1000 mg/L) highlighting the influence of rock-water interactions on groundwater chemistry with seasonal variations playing a secondary role in modulating these geologically driven TDS levels⁷⁸. The total hardness (TH) of groundwater in the study area is primarily controlled by the presence and weathering of calcium and magnesium-bearing silicate minerals in the metamorphic bedrock⁷⁹. The garnetiferous biotite gneiss and garnetiferous biotite sillimanite gneiss contain minerals such as calcium-rich plagioclase feldspars, hornblende and pyroxenes which contribute to calcium hardness, while magnesium sourced from the weathering of biotite and garnet⁸⁰. This geological influence is reflected in the consistently high hardness values observed 59 to 1424 mg/L during pre-monsoon (Fig. 4g) and 67 to 5260 mg/L during post-monsoon (Fig. 4h). The post-monsoon period shows higher and more variable hardness contrary to the dilution trend seen in EC and TDS⁸¹. Enhanced weathering rates explain this to the influx of slightly acidic rainwater and the flushing of accumulated weathering products into the aquifer⁸². The higher post-monsoon values suggest the possible influence of differential weathering patterns in the region’s varied topography, from Western Ghats to gently sloping terrain. Numerous samples in both seasons exceed the WHO standard of 300 mg/L, highlighting bedrock geology’s dominant role in determining water hardness. Seasonal hydrological changes, such as the influx of rainwater, modulate this fundamental geological control, leading to variations in hardness values between pre-monsoon and post-monsoon periods⁸³.

Major ions distribution

The major ions distributed in the samples analysed followed the order of Ca²⁺> Na⁺> Mg²⁺> K⁺ for cations and SO₄²⁻> Cl⁻ > HCO₃⁻ for anions during pre-monsoon season. Whereas, Mg²⁺> Ca²⁺> Na⁺> K⁺ for cations and Cl⁻> SO₄²⁻> HCO₃⁻ for anions during post-monsoon season. The gneissic rocks in the study area likely contain clay minerals formed from weathering. These clays might preferentially exchange calcium for magnesium during the wetter post-monsoon period, contributing to higher magnesium concentrations⁸⁴. The shift to chloride dominance in post-monsoon is likely due to a flushing effect. Monsoon rains wash accumulated chloride from the unsaturated zone into the aquifer. Chloride, being highly mobile and conservative, quickly became the dominant anion during this period³⁹.

Cations

Calcium (Ca²⁺) levels vary from 10 to 222 mg/L in the pre-monsoon season (Fig. 5a) and 14 to 192 mg/L in the post-monsoon season (Fig. 5b) with average values of 82 and 69 mg/L respectively. The average values were within the WHO 2017 guideline of 200 mg/L, but a few samples exceeded this limit, particularly during the pre-monsoon season. The higher calcium (Ca²⁺) concentrations observed in 6 pre-monsoon and 2 post-monsoon samples due to the dissolution of calcium-bearing minerals like calcite, anorthite (Ca²⁺-rich plagioclase feldspar), and calcium-rich garnet varieties as well as a potential contribution from processes like ion exchange involving Ca-rich clay minerals, reverse weathering reactions⁸⁵. Magnesium (Mg²⁺) concentrations ranged from 4 to 275 mg/L during the pre-monsoon season (Fig. 5c) and 10 to 373 mg/L during the post-monsoon season (Fig. 5d) with average values of 51 and 65 mg/L. The higher magnesium (Mg²⁺) concentrations observed in 1 pre-monsoon and 2 post-monsoon samples due to the weathering and dissolution of Mg-bearing minerals like dolomite, magnesium-rich biotite, and garnet, as well as potential ion exchange processes involving Mg-rich clay minerals in the metamorphic rock present in the study area⁸⁶.

Fig. 5.

Spatial distribution maps comparing major cations between Pre-monsoon and Post-monsoon season: (a,b) Ca²⁺, (c,d) Mg²⁺ (e,f) Na⁺ (and (g,h) K⁺. The figure was prepared using ArcGIS Desktop 10.8 (https://www.esri.com/en-us/arcgis/products/arcgis-desktop/overview). The groundwater quality data were obtained through field sampling and laboratory analysis following standard analytical procedures.

The sodium (Na⁺) levels range 3 to 180 mg/L during the pre-monsoon season (Fig. 5e) and 2 to 110 mg/L during the post-monsoon season (Fig. 5f) with average values of 42 and 28 mg/L. These values were well within the WHO (2017) guideline of 200 mg/L for drinking water, indicating that sodium levels in the groundwater were generally safe for consumption⁸⁷. The sodium present in groundwater is likely due to the plagioclase feldspars weathering, which is common in the gneissic rocks that dominate the region^80,88. The variation in sodium concentrations is attributed to differences in rock-water interaction time and the heterogeneity of the aquifer materials⁸⁹. Potassium (K⁺) levels exhibited a wider range varying from 1 to 373 mg/L in the pre-monsoon (Fig. 5g) season and 1 to 342 mg/L in the post-monsoon season (Fig. 5h) with average values of 36 and 22 mg/L. The higher potassium (K⁺) levels were observed in 21 pre-monsoon and 1 post-monsoon sample results from K-bearing mineral weathering and ion exchange processes^27,90.

Anions

Chloride (Cl⁻) concentrations varied from 12 to 2656 mg/L during the pre-monsoon season (Fig. 6a) and 12 to 1771 mg/L during the post-monsoon season (Fig. 6b), with average values of 259 and 182 mg/L respectively. The average values were nearly equal to the WHO standard value of 250 mg/L with maximum concentrations indicating localized contamination. Elevated Cl⁻ in 24 pre-monsoon and 14 post-monsoon samples results from saline fluid inclusions and chloride-bearing accessory minerals in the bedrock ³⁴⁸⁷. Sulphate (SO₄²⁻) levels vary from 19 to 1771 mg/L in the pre-monsoon season (Fig. 6c) and 14 to 1295 mg/L in the post-monsoon season (Fig. 6d) with average values of 290 and 207 mg/L. While the average values were within the WHO guideline of 250 mg/L, several samples exceeded this limit, with the maximum values being alarmingly high. The oxidation of sulphide minerals, like pyrite or pyrrhotite, is commonly found as accessory minerals in garnet-bearing gneisses leading to the release of SO₄²⁻ ions into the groundwater is the reason for high SO₄²⁻ levels in 22 pre-monsoon and 23 post-monsoon^36,55.

Fig. 6.

Spatial distribution maps comparing major anions between Pre-monsoon and Post-monsoon season: (a,b) Cl⁻, (c,d) SO₄²⁻, and (e,f) HCO₃⁻ concentrations in the study area. The figure was prepared using ArcGIS Desktop 10.8 (https://www.esri.com/en-us/arcgis/products/arcgis-desktop/overview). The groundwater quality data were obtained through field sampling and laboratory analysis following standard analytical procedures.

Bicarbonate (HCO₃⁻) concentrations varied from 30 to 958 mg/L during the pre-monsoon season (Fig. 6e) and 16 to 915 mg/L during the post-monsoon season (Fig. 6f) with average values of 251 and 237 mg/L respectively. While the average values were within the WHO guideline of 300 mg/L, some samples exceeded this limit. The higher calcium (HCO₃⁻) concentrations observed in 3 pre-monsoon and 2 post-monsoon samples collected attributed to the aqueous alteration of carbonate minerals like calcite (Ca²⁺HCO₃⁻) and dolomite (Ca²⁺Mg²⁺(HCO₃⁻)²) present in the gneissic rocks results in liberation of HCO₃⁻ ions into the groundwater^28,29.

Statistical analysis of seasonal variations

Statistical validation of seasonal differences was conducted using ANOVA to determine which observed variations represent significant departures from geological baseline conditions (Table 4). This analysis provides the statistical framework for interpreting the hydrogeochemical processes and water quality implications, which are discussed in subsequent sections.

Table 4.

One-way ANOVA results for seasonal comparison of groundwater quality parameters in the Valliyur region.

Parameter	Pre-monsoon Mean ± SD	Post-monsoon Mean ± SD	F-value	p-value	Significance	Effect size (η2)	Interpretation
pH	7.68 ± 0.24	7.61 ± 0.24	2.83	0.095	ns	0.023	No significant change
EC (µS/cm)	1494.6 ± 1752.1	1353.4 ± 1234.5	0.26	0.611	ns	0.002	No significant change
TDS (mg/L)	1106.5 ± 1090.5	866.2 ± 789.6	1.91	0.17	ns	0.016	No significant change
TH (mg/L)	370.8 ± 282.7	376.3 ± 317.6	0.01	0.919	ns	0	No significant change
Na (mg/L)	38.3 ± 42.5	27.9 ± 26.5	2.59	0.11	ns	0.022	No significant change
K (mg/L)	38.3 ± 42.5	8.53 ± 7.0	28.69	4.25E-07	******	0.196	Highly significant decrease
Mg (mg/L)	33.0 ± 40.4	39.8 ± 53.4	0.61	0.438	ns	0.005	No significant change
Ca (mg/L)	94.1 ± 68.3	85.3 ± 60.6	0.56	0.456	ns	0.005	No significant change
Cl (mg/L)	286.3 ± 372.2	190.8 ± 250.2	2.72	0.102	ns	0.023	No significant change
SO4 (mg/L)	294.4 ± 291.5	247.5 ± 219.4	0.99	0.321	ns	0.012	No significant change
HCO3 (mg/L)	176.3 ± 101.9	194.4 ± 71.6	1.27	0.262	ns	0.011	No significant change

Significance levels: *** p < 0.001; ** p < 0.01; * p < 0.05; ns = not significant SD = Standard Deviation; η² = Eta-squared effect size.

Dominant geological control

The ANOVA results demonstrate a distinct hierarchical structure of controlling factors influencing the groundwater chemistry in the study area. Ten out of eleven parameters showed no statistically significant seasonal differences (p > 0.05), confirming that rock-water interactions dominate groundwater chemistry across both seasons. The high spatial heterogeneity in weathering processes across the metamorphic terrain creates sufficient geochemical noise to mask seasonal signals for most parameters. This finding validates that the seasonal variations described, such as EC: 1495 to 1353 µS/cm; TDS: 1107 to 866 mg/L; Cl: 286 to 191 mg/L, represent secondary processes operating within a framework of dominant geological control. These mean changes indicate real seasonal processes such as dilution and enhanced flow, which remain subordinate to the fundamental geological signatures set by rock-water interactions.

Exceptional potassium dynamics

Potassium concentrations represent a remarkable exception to the geological dominance pattern demonstrating highly significant seasonal variation (F = 28.69, p < 0.001) with a large effect size (η² = 0.196). The 77% reduction from pre-monsoon (38.3 ± 42.5 mg/L) to post-monsoon (8.53 ± 7.0 mg/L) indicates that K⁺ mobility during monsoon recharge overwhelms the buffering capacity of K-bearing mineral weathering. This exceptional behaviour suggests unique mechanisms controlling potassium dynamics in the crystalline terrain. Enhanced leaching of accumulated K⁺ from the unsaturated zone during monsoon infiltration likely contributes to the dramatic seasonal reduction as increased water flux rapidly mobilizes K⁺ ions accumulated during the dry period⁹¹. The rapid mobilization and transport of K⁺ from biotite and K-feldspar weathering products reflects potassium’s high solubility and mobility compared to other major cations in crystalline rock systems. The preferential flushing of exchangeable K⁺ from clay mineral surfaces in expanded flow systems during monsoon recharge demonstrates the particular susceptibility of potassium to hydrological changes⁹². The limited re-equilibration of K⁺ with mineral phases during rapid groundwater flow further explains the persistent depletion during the post-monsoon period, as insufficient residence time prevents the re-establishment of equilibrium conditions⁹³.

Hydrogeochemical processes and evolution

The interaction between rock and water, which encompasses processes such as the solubilization of minerals, secondary mineral formation, cation-anion exchange, and redox transformations along flow paths, determines groundwater chemistry. The spatiotemporal evolution of hydrogeochemical facies reflects the combined influence of lithological composition, residence time, and climatic conditions, which can be effectively visualized and interpreted through specialized graphical methods⁹⁴.

Gibbs diagram

The Gibbs plots provide a preliminary assessment of hydrogeochemical processes controlling groundwater chemistry in the study area. During the pre-monsoon season (Fig. 7a & b) most samples fall within the rock weathering dominance field, reflecting extended residence times and intensified mineral-water contact^40,95. Several samples exhibit characteristics suggesting evaporation dominance in anion is likely due to enhanced evapotranspiration and concentration effects in the semi-arid climate.

Fig. 7.

Hydrogeochemical plots depicting hydrogeochemical processes: Gibbs diagram (a,b) Pre-monsoon and (c,d) Post-monsoon seasons, Piper trilinear diagram (e) Pre-monsoon and (f) Post-monsoon seasons. The Piper diagrams were prepared using Diagrammes Software (http://www.lha.univ-avignon.fr/LHA-Logiciels.htm).

The post-monsoon season (Fig. 7c & d) shows a shift in hydrogeochemical patterns with an increased proportion of samples falling within the rock weathering dominance field. This seasonal transition reflects enhanced rock-water interactions due to increased groundwater flow rates and expanded rock-water contact zones following monsoon recharge rather than simple dilution effects^29,96. The reduced representation in the evaporation dominance field during post-monsoon indicates diminished evaporative concentration processes.

Piper trilinear diagram

The groundwater chemistry exhibits distinct seasonal variations with the pre-monsoon period (Fig. 7e) characterized by diverse water types including Ca²⁺-Mg²⁺-HCO₃⁻-Cl⁻, Ca²⁺-Mg²⁺-HCO₃⁻-SO₄²⁻, Na⁺-Ca²⁺-SO₄²⁻-HCO₃⁻-Cl⁻ and Ca²⁺-Na⁺-SO₄²⁻-HCO₃⁻-Cl⁻ reflecting complex geochemical processes in the gneissic terrain⁹⁷. The dominance of calcium and magnesium indicates active weathering of plagioclase, biotite, and garnet, while sodium enrichment suggests weathering of sodium-rich plagioclase or ion exchange processes in clay-rich zones⁵⁵. Bicarbonate points to CO₂-charged water interacting with silicate minerals, a typical process in crystalline terrains, while some water types indicate more evolved waters influenced by longer residence times or mixing with deeper circulating groundwater. Post-monsoon (Fig. 7f) conditions show an increase in Ca²⁺-Cl⁻-SO₄²⁻-HCO₃⁻ type waters potentially due to enhanced rock-water interaction from increased recharge and flushing of accumulated salts, particularly in fracture zones^85,98. The persistence of sulphate-rich waters (Ca²⁺-SO₄²⁻-HCO₃⁻-Cl⁻) across both seasons suggests the presence of disseminated sulphide minerals within the metamorphic rocks possibly introduced during metamorphic events or anthropogenic inputs from irrigation practices in the region^27,29.

Mineral saturation indices analysis

The mineral saturation indices analysis reveals distinct thermodynamic equilibrium patterns for carbonate minerals in the crystalline aquifer system with significant seasonal variations (Fig. 8a). The saturation distributions provide insights into geochemical evolution pathways and mineral stability. Calcite saturation states demonstrate remarkably consistent patterns across both seasonal periods, with the majority of groundwater samples (52%) maintaining near-equilibrium conditions (SI = -0.5 to + 0.5) during both pre-monsoon and post-monsoon periods. This equilibrium dominance indicates active buffering mechanisms where calcite dissolution and precipitation processes maintain dynamic balance within the crystalline aquifer system⁹⁹. The 38% of pre-monsoon samples exhibit supersaturation conditions (SI > 0.5), slightly decreasing to 36% during post-monsoon, suggesting reduced precipitation potential following monsoon recharge dilution effects. The minimal seasonal variation in calcite saturation (8–10% undersaturated samples) reflects the robust buffering capacity of carbonate minerals present as accessory phases within the metamorphic rock matrix, consistent with findings in similar crystalline terrains¹⁵.

Fig. 8.

(a) Seasonal mineral saturation state classification for calcite, aragonite, and dolomite in pre-monsoon and post-monsoon periods. (b) Ion exchange process identification using Chloro-Alkaline Indices, where negative values indicate reverse ion exchange and positive values indicate normal ion exchange.

Aragonite saturation patterns exhibit greater seasonal sensitivity compared to calcite with undersaturated conditions increasing from 18% (pre-monsoon) to 20% (post-monsoon), while supersaturated samples decrease from 30 to 26%. This increased dissolution tendency during post-monsoon reflects the enhanced reactivity of aragonite compared to calcite under diluted groundwater conditions, supporting enhanced carbonate weathering during high recharge periods¹⁷. The equilibrium state dominance (50–52%) indicates that aragonite maintains significant thermodynamic stability despite its higher solubility within the pH range (7.14–8.35) and ionic strength conditions prevalent in the study area. The higher proportion of undersaturated conditions suggests ongoing aragonite dissolution contributing to Ca²⁺ and HCO₃⁻ enrichment during monsoon-influenced periods when aggressive water conditions promote enhanced mineral dissolution¹⁴. Dolomite exhibits the most pronounced seasonal variations and complex saturation behaviour, with supersaturated conditions increasing from 44% (pre-monsoon) to 50% (post-monsoon), while undersaturated samples remain relatively stable (25–24%). The high supersaturation tendency reflects kinetic limitations in dolomite precipitation under ambient temperature conditions despite thermodynamic favourability consistent with well-documented sluggish dolomite precipitation kinetics in groundwater systems¹⁵. The increased post-monsoon supersaturation paradoxically occurs despite dilution effects suggesting preferential enrichment of Mg²⁺ and HCO₃⁻ through enhanced weathering of magnesium-bearing silicate minerals (biotite, garnet) during active recharge periods. Equilibrium conditions decrease from 28 to 24% indicating seasonal destabilization of dolomite equilibrium relationships under variable hydrological conditions.

Ion exchange process

The chloro-alkaline indices (CAI) analysis reveals predominant normal ion exchange with distinct seasonal variations in exchange intensity and mechanisms (Fig. 8b). The CAI₁ versus CAI₂ cross-plot provides insights into cation exchange dynamics between groundwater and clay minerals. Normal ion exchange processes dominate the hydrogeochemical evolution, with approximately 95% of groundwater samples exhibiting positive CAI₁ values across both seasonal periods. This dominance indicates the systematic replacement of Ca²⁺ and Mg²⁺ from clay mineral exchange sites by Na⁺ and K⁺ from the groundwater solution¹⁰⁰. The consistent normal exchange pattern reflects the abundant clay mineral assemblages (kaolinite, illite, montmorillonite) developed through the weathering of feldspar and biotite within the metamorphic rock, providing extensive cation exchange capacity throughout the aquifer system¹⁵. Pre-monsoon samples demonstrate significantly higher exchange intensities with CAI₂ values ranging from 0.2 to 2.5 compared to post-monsoon clustering, primarily between 0.1 and 1.0. This seasonal variation reflects concentrated groundwater conditions during dry periods that enhance ionic strength and promote more vigorous cation exchange reactions on clay mineral surfaces¹⁷. The broader pre-monsoon distribution pattern indicates heterogeneous exchange conditions likely controlled by varying clay mineral concentrations, residence times, and local groundwater chemistry evolution along different flow paths within the fractured crystalline aquifer system¹⁰¹.

Post-monsoon samples exhibit remarkable clustering in the lower CAI₂ range (0.1-1.0), with one sample reaching CAI₂ ~3.9, indicating a dilution-mediated reduction in exchange intensity following monsoon recharge. The concentrated post-monsoon distribution suggests equilibration of exchange processes under diluted groundwater conditions where reduced ionic concentrations limit the driving force for intensive cation exchange reactions¹⁰². The exceptional high-CAI₂ outlier likely represents a localized zone with enhanced clay-water interaction associated with deep weathering profiles or concentrated clay horizons developed along structural discontinuities within the gneissic terrain¹⁰⁰. Reverse ion exchange processes remain low in both seasons, with less than 5% of samples showing negative CAI₁ values primarily clustered near the zero thresholds. This limited reverse exchange suggests that carbonate mineral dissolution and silicate weathering provide sufficient Ca²⁺ and Mg²⁺ supply to maintain forward exchange reactions, preventing significant reversal to Ca²⁺-Mg²⁺ dominated exchange sites¹⁰³. The absence of extensive reverse exchange indicates mature weathering profiles where clay mineral exchange sites have reached quasi-equilibrium with the prevailing groundwater chemistry, characteristic of long-term water-rock interaction in deep crystalline aquifer systems¹⁵.

Source identification using Ca²⁺/Mg²⁺ vs. SO₄²⁻/Cl⁻ plot

The Ca²⁺/Mg²⁺ vs. SO₄²⁻/Cl⁻ plot demonstrates overwhelming dominance of silicate weathering processes with 85% of groundwater samples in both seasonal periods (Fig. 9a). This pattern indicates primary control by plagioclase feldspar and biotite weathering reactions consistent with the dominant garnetiferous biotite gneiss lithology throughout the study area¹⁰⁴. The impact of halite is negligible, as none of the samples showed SO₄²⁻/Cl⁻ ratios below 0.5, indicating no notable intrusion of saline water or dissolution of evaporites within the crystalline aquifer system. Gypsum dissolution impacts fewer than 10% of samples, mainly during pre-monsoon periods, pointing to localized sulphate minerals likely from pyrite oxidation in specific geological areas rather than extensive evaporite dissolution. Seasonal source variations reveal enhanced silicate weathering intensity during post-monsoon periods, with samples showing slightly elevated Ca²⁺/Mg²⁺ ratios (1.5-3.0) compared to pre-monsoon clustering (1.0–2.0), indicating preferential calcium release from plagioclase weathering under enhanced recharge conditions¹⁴. Extreme outliers (Ca²⁺/Mg²⁺ > 25, SO₄²⁻/Cl⁻ > 2) represent localized zones with distinct hydrogeochemical signatures possibly associated with carbonate mineral dissolution or concentrated sulphide oxidation along fracture networks within the metamorphic bedrock.

Fig. 9.

Geochemical ratio plots for groundwater characterization: (a) Ca²⁺/Mg²⁺ vs. SO₄²⁻/Cl⁻ plot, (b) SO₄²⁻/Cl⁻ vs. HCO₃⁻/Cl⁻ plot and (c) Ca²⁺+Mg²⁺ vs. HCO₃⁻+SO₄²⁻ plot.

Process identification using SO₄²⁻/Cl⁻ vs. HCO₃⁻/Cl⁻ plot

The SO₄²⁻/Cl⁻ vs. HCO₃⁻/Cl⁻ plot reveals limited carbonate weathering dominance, with approximately 85% of samples with both HCO₃⁻/Cl⁻ and SO₄²⁻/Cl⁻ ratios below 2.0 indicating predominantly silicate weathering processes rather than extensive carbonate mineral dissolution (Fig. 9b). The majority of samples exhibit HCO₃⁻/Cl⁻ ratios between 0.5 and 2.0, suggesting moderate alkalinity generation primarily from CO₂-charged groundwater interactions with silicate minerals rather than intensive carbonate weathering. Sulphide oxidation influence remains less, affecting 5% of samples with SO₄²⁻/Cl⁻ ratios exceeding 2.0, indicating the limited occurrence of disseminated sulphide minerals or restricted oxidative weathering conditions within the metamorphic assemblages¹⁰¹. Mixed geochemical processes characterize where both HCO₃⁻/Cl⁻ and SO₄²⁻/Cl⁻ ratios exceed 2.0, indicating concurrent carbonate weathering and sulphide oxidation pathways operating simultaneously within specific hydrogeological zones¹⁵. Enhanced carbonate weathering is evident in approximately 10–15% of samples, showing HCO₃⁻/Cl⁻ ratios between 2.0 and 5.0 primarily concentrated in post-monsoon samples, indicating localized zones of active carbonate mineral dissolution under aggressive recharge water conditions¹⁴.

Carbonate system analysis using Ca²⁺+Mg²⁺ vs. HCO₃⁻+SO₄²⁻ plot

The Ca²⁺+Mg²⁺ vs. HCO₃⁻+SO₄²⁻ Plot reveals systematic deviation above the 1:1 stoichiometric line with 75–80% of samples plotting above theoretical pure carbonate dissolution equilibrium (Fig. 9c). This consistent positive deviation indicates significant additional Ca²⁺+Mg²⁺ contribution from silicate mineral weathering particularly from calcium-rich plagioclase (anorthite component) and magnesium-bearing biotite and garnet assemblages within the metamorphic rock¹⁰⁵. The excess alkaline earth content (ranging 2–15 meq/L above the stoichiometric line) demonstrates the dual-source nature of groundwater chemistry evolution where carbonate buffering provides baseline alkalinity. Seasonal carbonate evolution shows relatively similar distributions between pre-monsoon and post-monsoon periods, with both seasons exhibiting the characteristic above-line clustering pattern. However, post-monsoon samples show slightly more scattered distribution extending to higher total concentrations (up to 25–30 meq/L), reflecting enhanced overall mineral dissolution under diluted recharge conditions¹⁰³. The strong linear correlation above the 1:1 line indicates a systematic proportional contribution from carbonate and silicate sources, demonstrating mature hydrogeochemical evolution where multiple mineral phases achieve coordinated dissolution equilibria¹⁰¹.

Summary of hydrogeochemical controls

The comprehensive hydrogeochemical analysis identifies six primary processes controlling groundwater chemistry in the Valliyur region, with distinct seasonal variations affecting water quality. Table 5 consolidates findings from graphical and statistical analyses, establishing a hierarchical control framework where geological provide the fundamental framework, climatic processes (evaporative concentration, monsoon dilution) create seasonal modulations, and anthropogenic inputs represent localized perturbations that undergo seasonal flushing during monsoon periods.

Table 5.

Summary of hydrogeochemical processes and controls in the Valliyur Region.

Process	Primary controls	Seasonal variation	Evidence	Impact on water quality
Silicate weathering	Garnetiferous biotite gneiss dissolution	Enhanced post-monsoon	Ca2+/Mg2+ vs. SO42−/Cl− plot (85% samples)	Major ion source, TDS increase
Ion exchange	Clay mineral-groundwater interaction	Intensity decreases post-monsoon	CAI analysis (95% normal exchange)	Na+/K+ vs. Ca2+/Mg2+ balance
Carbonate buffering	Calcite equilibrium	Stable across seasons	Saturation indices (52% equilibrium)	pH control, alkalinity
Evaporative concentration	Climate-driven water loss	Dominant pre-monsoon	Gibbs diagram classification	Salinity increase
Monsoon dilution	Recharge water mixing	Significant post-monsoon	Statistical analysis (K+ only significant)	Quality improvement
Anthropogenic input	Agricultural and domestic sources	Variable seasonal flushing	Elevated Cl−, K+ concentrations	Localized contamination

Anthropogenic influences on groundwater quality

The statistical analysis and hydrogeochemical evaluation confirm that geological processes dominate groundwater chemistry in the study area, with anthropogenic activities contributing as a secondary factor that significantly impacts water quality parameters locally. Understanding human influences provide an important context for sustainable groundwater management and interpretation of quality variations within the dominant geological framework¹⁰⁶.

Agricultural contamination sources

Agriculture generates various contaminant pathways impacting baseline geological conditions. Fertilizers applications likely contribute to elevated pre-monsoon K⁺ concentrations with subsequent monsoon mobilization. Agricultural activities enhance the natural weathering process, resulting in potassium having a distinct statistical importance compared to parameters primarily influenced by geological factors¹⁰⁷. Irrigation return flow represents another significant agricultural impact, contributing to elevated chloride concentrations observed in approximately 40% of samples that exceed WHO limits (250 mg/L). The dissolution and concentration of accumulated salts from irrigated soils and evapotranspiration effects during dry periods create localized zones of enhanced salinity that overlay the fundamental metamorphic rock signatures¹⁰⁸. The spatial distribution of high-chloride areas (ranging up to 2656 mg/L pre-monsoon and 1771 mg/L post-monsoon) suggests agricultural influence in intensively farmed areas with inadequate drainage systems.

Domestic and urban contamination

Population growth and urban development in the Valliyur region introduce domestic contamination sources that create point and diffuse pollution impacts on groundwater quality. Septic tank systems widely used in rural and semi-urban areas represent a primary source of chloride contamination through domestic wastewater disposal. Domestic wastewater disposal practices contribute to elevated total dissolved solids (up to 7334 mg/L pre-monsoon and 5260 mg/L post-monsoon) and chloride concentrations in specific localities. Inadequate waste management systems in some areas result in direct or indirect contamination of shallow groundwater, creating localized degradation that modifies the regional geological water quality patterns¹⁰⁹. The elevated TDS values exceeding WHO guidelines in 40% of samples during the pre-monsoon period may partially reflect anthropogenic inputs in addition to natural mineral dissolution.

Land use change impacts

Conversion of agricultural land to urban and semi-urban uses alters groundwater recharge patterns and introduces new contamination pathways that modify natural hydrogeochemical evolution. Increased impervious surfaces reduce natural recharge capacity, potentially concentrating existing contamination sources and reducing dilution effects that normally attenuate anthropogenic inputs¹¹⁰. These changes may explain some spatial variability observed in water quality parameters where land use modifications create local departures from regional geological patterns. The limited industrial presence that is concentrated mainly in western areas in the study area contributes minimal but locally significant contamination sources. Small-scale industrial activities, workshops, and chemical storage facilities create potential point sources that require monitoring to prevent future degradation of groundwater quality.

Temporal patterns of anthropogenic impact

The seasonal framework established through statistical analysis provides insights into temporal patterns of anthropogenic contamination. Pre-monsoon accumulation of agricultural and domestic contaminants is followed by monsoon-induced mobilization, dilution, and transport, creating the observed seasonal improvements in overall water quality. This pattern is most clearly demonstrated by potassium dynamics, where anthropogenic fertilizer inputs likely contribute to elevated pre-monsoon concentrations (38.3 mg/L) followed by dramatic monsoon-induced reduction (8.53 mg/L). On average, chloride concentrations show seasonal reduction from 286.3 mg/L (pre-monsoon) to 190.8 mg/L (post-monsoon), indicating dilution of accumulated anthropogenic salts. Total dissolved solids mean values (1107 mg/L to 866 mg/L) suggest that anthropogenic sources operate within the dominant geological-climatic control system rather than overriding natural processes. The statistical non-significance of most parameters confirms that natural attenuation processes maintain overall system resilience against anthropogenic inputs, with potassium representing the only parameter where anthropogenic practices create detectable seasonal signals.

Water quality index (WQI)

The seasonal WQI variations revealed distinct patterns influenced by geochemical processes, mineralogical compositions and hydrological factors, which affected both cation and anion concentrations in the groundwater¹¹¹. The water quality is classified for pre-monsoon and post-monsoon seasons based on the concentrations of key parameters in Table 6.

Table 6.

WQI classification and grouping of groundwater samples during pre-monsoon and post-monsoon season.

Class	Range	Pre-monsoon		Post-monsoon
		No. of samples	%	No. of samples	%
Excellent	0–25	29	48.33	42	70.00
Good	26–50	16	26.67	16	26.67
Poor	51–75	06	10.00	01	1.67
Very poor	75–100	05	8.33	01	1.67
Unsuitable	> 100	04	6.67	Nil	0.00

During the pre-monsoon period (Fig. 10a), the higher percentages observed in the Poor (10%), Very Poor (8.33%), and Unsuitable (6.67%) categories were attributed to the following reasons. The reduced precipitation and lower groundwater levels concentrated dissolved ions, elevating EC, TDS and TH values due to prolonged rock-water interactions, increased mineral solubilization, evaporative concentration and reduced dilution in the water-bearing stratum^63,112. This concentration effect notably deteriorated the water quality¹¹³. Elevated Na⁺ and K⁺ concentrations in pre-monsoon samples stemmed from the increased dissolution of albite, K⁺-feldspar, muscovite and biotite driven by prolonged rock-water interactions, higher temperatures and concentrated groundwater conditions during the drier season^87,89. The high calcium (Ca²⁺) concentrations in 6 samples were due to the solubilization of calcium-rich minerals like calcite, anorthite, and calcium-rich garnet varieties^36,85. The high chloride (Cl⁻) concentrations observed in 24 samples were potentially due to the presence of fluid inclusions containing saline solutions or the solution effect of chloride-bearing accessory minerals like apatite or biotite during weathering processes^82,94. The high sulphate (SO₄²⁻) concentrations in 22 samples are attributed to the oxidation of sulphide minerals, especially pyrite, which is commonly found as an accessory mineral in these garnet-bearing gneisses⁵⁵. The high bicarbonate (HCO₃⁻) concentrations in 3 samples were likely due to the disintegration of carbonate minerals like calcite or dolomite in gneissic rocks^28,29.

Fig. 10.

Water Quality Index (WQI) map of (a) Pre-monsoon and (b) Post-monsoon season in the study area. This figure was prepared using ArcGIS Desktop 10.8 (https://www.esri.com/en-us/arcgis/products/arcgis-desktop/overview).

In the post-monsoon period (Fig. 10b), the WQI classification showed higher percentages in the Excellent (70%) and Good (26.67%) categories. This improvement in water quality is attributed to^40,114. Despite the overall improvement, some samples still exhibited high concentrations of cations like Na⁺ (2 samples), K⁺ (22 samples) and Ca²⁺ (2 samples), as well as high concentrations of anions like Cl⁻ (14 samples), SO₄²⁻ (23 samples) and HCO₃⁻ (2 samples). These elevated levels persisted due to ongoing geochemical processes in gneissic rocks, including mineral dissolution and oxidation volume^27,68. The infiltrated water from rainfall flushed accumulated ions from the unsaturated zone into the aquifer, maintaining high concentrations despite increased water^29,97. For local water users, the results indicate that pre-monsoon periods require boiling, filtration or alternative sources for wells in poor-unsuitable categories, while post-monsoon enhanced water quality allows direct consumption from most sources. Areas with consistently poor WQI require immediate health risk intervention regardless of season.

Irrigation water quality (IWQ)

The analysis of irrigation water quality involves various geochemical indices such as sodium percentage (Na%), sodium adsorption ratio (SAR), residual sodium carbonate (RSC), permeability index (PI), Kelley’s ratio (KR), and magnesium adsorption ratio (MAR). The irrigation water quality was evaluated using standard indices for the pre-monsoon and post-monsoon periods presented in Table 7a&b.

Table 7.

Evaluation of irrigation water quality through different indices (Na%, SAR, RSC, PI, KR, and MAR) during (a) Pre-monsoon and (b) Post-monsoon season.

a) Pre-monsoon
Parameter	Sample range			Class	Range	No. of samples
	Min	Max	Average
Sodium percentage (%Na)62	7.69	57.54	37.85	Excellent	< 20%	26
				Good	20–40%	31
				Permissible	40–60%	3
				Doubtful	60–80%	Nil
				Unsuitable	> 80%	Nil
Sodium adsorption ratio (SAR)45	0.17	2.81	1.53	Excellent	< 10	60
				Good	10–18	Nil
				Doubtful	18–26	Nil
				Unsuitable	> 26	Nil
Residual sodium carbonate (RSC)64	− 20.61	0.85	0.08	Good	< 1.25	60
				Doubtful	1.25–2.5	Nil
				Unsuitable	> 2.5	Nil
Permeability Index (PI)65	12.54	94.04	74.22	Excellent	> 75%	35
				Good	25–75%	25
				Unsuitable	< 25%	Nil
Kelly’s ratio (KR)66	0.05	0.85	0.5	Suitable	< 1	60
				Unsuitable	> 1	Nil
Magnesium adsorption ratio (MAR)67	3.36	79.39	21.53	Suitable	< 50%	51
				Unsuitable	> 50%	9

b) Post-monsoon
Parameter	Sample range			Class	Range	No. of samples
	Min	Max	Average
Sodium percentage (%Na)62	3.66	39.37	24.72	Excellent	< 20%	51
				Good	20–40%	9
				Permissible	40–60%	Nil
				Doubtful	60–80%	Nil
				Unsuitable	> 80%	Nil
Sodium adsorption ratio (SAR)45	0.09	1.83	0.99	Excellent	< 10	60
				Good	10–18	Nil
				Doubtful	18–26	Nil
				Unsuitable	> 26	Nil
Residual sodium carbonate (RSC)64	− 34.38	− 1.08	− 2.98	Good	< 1.25	60
				Doubtful	1.25–2.5	Nil
				Unsuitable	> 2.5	Nil
Permeability Index (PI)65	23.02	96.18	73.84	Excellent	> 75%	33
				Good	25–75%	26
				Unsuitable	< 25%	1
Kelly’s ratio (KR)66	0.03	0.59	0.34	Suitable	< 1	60
				Unsuitable	> 1	Nil
Magnesium adsorption ratio (MAR)67	5.15	85.77	27.95	Suitable	< 50%	49
				Unsuitable	> 50%	11

Sodium percentage (Na%)

During the pre-monsoon period, Na% ranges from 7.69 to 57.54, an average of 37.85, with the Wilcox plot showing more than half (55%) of the samples come under “Good to Permissible” category while 28.33% were in the “Excellent to Good” category (Figs. 11a and 12a). The geochemical patterns reflect the underlying metamorphic minerals contributing to moderate ionic concentration levels suitable for agriculture through weathering processes^36,37. The presence of 13.33% samples in the “Doubtful to Unsuitable” category and 3.33% in the “Unsuitable” category during the pre-monsoon period can be explained by the presence of other rock types like Acid to intermediate Charnockite, Calc-granulite, Limestone, and Pegmatite. These rocks may contain minerals that contribute to higher concentrations of specific ions, such as sodium from feldspars, calcium, and bicarbonate from carbonates, and sulphates from sulphide minerals, which potentially affects the water quality^38,61. During the post-monsoon period, Na% ranged from 3.66 to 39.37% with an average of 24.72%, and the percentage of samples in the “Excellent to Good” category increased to 43.33%, while 48.33% fell into the “Good to Permissible” category (Figs. 11b and 12b). This improvement in water quality is due to the dilution process of increased precipitation during the post-monsoon period, which reduces dissolved mineral constituents in the phreatic water^40,55. However, the presence of 8.33% of samples in the “Doubtful to Unsuitable” category during the post-monsoon period suggests that some areas are driven by the geological formations containing minerals that can contribute to higher concentrations of specific ions^37,115. Carbonate minerals in aquifers can increase bicarbonate concentrations in groundwater, while the oxidation of sulphide minerals may lead to elevated sulphate levels. These geochemical processes can significantly influence water quality parameters, potentially affecting the agronomic viability of aquifer waters^27,116.

Fig. 11.

Wilcox plots showing sodium percentage versus electrical conductivity during (a) Pre-monsoon and (b) Post-monsoon; USSL diagrams depicting sodium adsorption ratio versus electrical conductivity during (c) Pre-monsoon and (d) Post-monsoon season. Both diagrams were prepared using Diagrammes Software (http://www.lha.univ-avignon.fr/LHA-Logiciels.htm).

Fig. 12.

Spatial distribution maps showing: Sodium percentage (a) Pre-monsoon and (b) Post-monsoon; Permeability Index (c) Pre-monsoon and (d) Post-monsoon; Magnesium Adsorption Ratio (e) Pre-monsoon and (f) Post-monsoon seasons classified according to their irrigation suitability. This figure was prepared using ArcGIS Desktop 10.8 (https://www.esri.com/en-us/arcgis/products/arcgis-desktop/overview).

Sodium adsorption ratio (SAR)

The Sodium Adsorption Ratio exhibited values between 0.17 and 2.81 (average 1.53) and 0.09 and 1.83 (average 0.99) during pre-monsoon and post-monsoon season. The USSL diagram revealed that during both seasons (Fig. 11c & d), most of the samples fell into the C3S1 category, indicating high salinity but low sodium hazard. Metamorphic mineral weathering releases Ca²⁺, Mg²⁺, K⁺ and Al³⁺ contributing to high salinity levels^112,117.

During the pre-monsoon period, some samples fell into the C2S1 category, indicating medium salinity and low sodium hazard, while others were in the C4S1 category, indicating very high salinity but low sodium hazard. These variations are due to the presence of other rock types like Charnockite varieties, Calc-granulite, Limestone, and Pegmatite⁶⁸. These rocks may contain minerals that can contribute to varying salinity levels and specific ion concentrations. During the post-monsoon period, most samples fell into the C2S1 category, and fewer samples were in the C4S1 category, suggesting a slight improvement in salinity levels compared to the pre-monsoon period. This improvement is attributed to the dilution effect caused by increased precipitation during the post-monsoon period, which can reduce the concentrations of dissolved constituents in the underground water bodies^118,119.

Residual sodium carbonate

Residual Sodium Carbonate (RSC) values shifted from a pre-monsoon range of -20.61 to 0.85 (average 0.08) to consistently negative post-monsoon values ranging from − 34.38 to -1.08 (average − 2.98), demonstrating enhanced calcium and magnesium dominance after monsoon recharge. The consistently low Residual Sodium carbonate values across both seasons suggest a low risk of sodium-related soil degradation, which is attributed to the relatively low abundance of carbonate and bicarbonate-bearing minerals in the predominant metamorphic rock types of the study area^40,68,120. The slight decrease in average RSC values during the post-monsoon period due to the dilution effect of increased rainfall on the limited carbonate and bicarbonate ions present in the groundwater system enhancing irrigation suitability^55,121.

Permeability index (PI)

Permeability Index (PI) remained relatively stable, with pre-monsoon values ranging from 12.54 to 94.04% (average 74.22%) and post-monsoon values from 23.02 to 96.18% (average 73.84%), suggesting less impact of seasonal variations on soil infiltration characteristics. The research area’s groundwater resource is suitable for irrigation, evidenced by “Excellent” and “Good” PI values in both seasons displayed in Fig. 12c,d^60,117. This suitability is attributed to the low solubility of minerals in the local metamorphic rocks, resulting in lower dissolved salt concentrations^38,59,73.

Kelley’s ratio

Kelly’s Ratio (KR) exhibited a favorable decrease from pre-monsoon values of 0.05 to 0.85 (average 0.5) to post-monsoon values of 0.03 to 0.59 (average 0.34), with all samples maintaining ‘Suitable’ classification throughout both seasons. KR values below 1 for all samples indicate favorable sodium-to-calcium-and-magnesium ratios for irrigation^29,122with lower average KR values in the post-rainfall period by dilution and reduced sodium from weathered plagioclase feldspar^36,55,117.

Magnesium adsorption ratio (MAR)

Magnesium Adsorption Ratio (MAR) showed a slight increase from pre-monsoon values of 3.36–79.39% (average 21.53%) to post-monsoon values of 5.15–85.77% (average 27.95%) (Fig. 12e,f). The values consistently show irrigation suitability reflecting controlled magnesium release likely influenced by the seasonal recharge patterns and complex hydrochemical evolution along the groundwater flow path^37,73. A slight increase in “Unsuitable” samples during post-monsoon may be due to magnesium flushing from soil or weathering of minor magnesium-bearing minerals^61,115.

Agricultural management implications indicate that post-monsoon planting benefits from improved water quality while pre-monsoon irrigation requires enhanced soil amendments due to higher salinity, though consistent irrigation suitability supports long-term agricultural intensification. The economic advantage allows farmers to rely on groundwater irrigation without expensive treatment costs, enabling sustainable agricultural productivity.

Health risk assessment based on WHO guidelines

The health risk assessment based on WHO (2017) drinking water quality guidelines reveals varying degrees of concern with distinct seasonal patterns (Table 8). The pre-monsoon period show multiple parameters exceedances creating cumulative health risks. Total hardness posed the most widespread concern, with 48.33% of samples exceeding the 300 mg/L guideline, potentially leading to kidney stone formation and cardiovascular complications with long-term consumption^123,124. Chloride concentrations exceeded the 250 mg/L limit in 40% of samples (maximum: 2656 mg/L), which can cause taste and odour problems and may contribute to hypertension in sensitive individuals¹²⁵. Total Dissolved Solids exceeded the 1000 mg/L limit in 40% of samples, indicating overall poor water quality that can cause gastrointestinal irritation and laxative effects affecting children and elderly populations¹²⁶. Potassium levels exceeded the 30 mg/L desirable limit in 35% of pre-monsoon samples (maximum: 373 mg/L), which poses significant risks for individuals with kidney disorders and can cause hyperkalaemia related cardiac complications^127,128. Sulphate concentrations surpassed the 250 mg/L limit in 36.67% of samples (maximum: 1771 mg/L), potentially causing diarrhoea and dehydration in infants¹²⁹. Electrical conductivity exceeded 1500 µS/cm in 31.67% of samples, indicating high mineralization that can affect the taste and contribute to various health issues through cumulative ion exposure. pH levels were above the acceptable range (6.5-8.0) in 15% of samples, which can affect the dissolution of metals and alter the taste of water.

Table 8.

Health risk classification based on parameter exceedances.

Season	Parameter	% Exceeding WHO limit	Health risk level	Primary health concerns
Pre-monsoon	Total hardness	48.33%	High	Kidney stones, cardiovascular stress
	Chloride	40.00%	High	Hypertension, taste/odour issues
	TDS	40.00%	High	Gastrointestinal irritation
	Sulphate	36.67%	Moderate-high	Diarrhoea, dehydration
	Potassium	35.00%	Moderate-high	Cardiac complications, hyperkalaemia
	EC	31.67%	Moderate	Overall water quality degradation
	pH	15.00%	Low	Metal dissolution, taste alteration
Post-monsoon	Total hardness	43.33%	Moderate-high	Kidney stones, cardiovascular stress
	Sulphate	38.33%	Moderate	Diarrhoea, dehydration
	EC	33.33%	Moderate	Overall water quality degradation
	TDS	33.33%	Moderate	Gastrointestinal irritation
	Chloride	23.33%	Low-moderate	Hypertension, taste/odour issues
	pH	3.33%	Low	Metal dissolution, taste alteration

Risk Classification: Low (< 10% exceedance), Low-Moderate (10–25%), Moderate (25–35%), Moderate-High (35–45%) and High (> 45%).

The post-monsoon period significantly improved health risk parameters, with most exceedances substantially reduced due to dilution effects. The total hardness remained a concern in 43.33% of samples, and sulphate levels continued to exceed guidelines in 38.33% of samples, indicating persistent geological influences on water quality. Spatial analysis reveals that the northern and central parts of the study area dominated by garnetiferous biotite gneiss show higher concentrations of multiple parameters, creating compound health risks where residents may be exposed to several contaminants simultaneously. Vulnerable populations, including pregnant women, infants, elderly individuals and those with pre-existing kidney or cardiovascular conditions, face heightened risks in areas where multiple parameters exceed WHO guidelines¹¹¹. The seasonal variation in health risks suggests that pre-monsoon periods require enhanced water treatment or alternative water sources, while post-monsoon water quality improvements offer opportunities for groundwater use with minimal treatment. Long-term water consumption with multiple parameter exceedances, such as high hardness, chloride, and sulphate may lead to cumulative health effects, including the increased burden on kidney function, digestive system stress, and potential cardiovascular complications¹³⁰.

Sustainable groundwater management strategies for Valliyur region

Based on the comprehensive hydrogeochemical characterization and water quality assessment an integrated framework of sustainable groundwater management strategies is proposed for the study area. The significant seasonal variation in water quality with excellent-category samples increasing from 48.33% during pre-monsoon to 70% during post-monsoon periods, demonstrates the critical influence of monsoon recharge on groundwater suitability. This temporal variability necessitates seasonal water use optimization protocols, where communities should preferentially utilize groundwater for potable purposes during post-monsoon periods while implementing alternative water sources during pre-monsoon periods, particularly in areas with consistently poor Water Quality Index values. The 25% of samples requiring treatment during pre-monsoon conditions, predominantly in areas underlain by garnetiferous biotite gneiss formations with elevated hardness, chloride and total dissolved solids concentrations, warrant targeted interventions through community-level reverse osmosis treatment systems and supplementary rainwater harvesting infrastructure. The universal irrigation suitability classification (C3S1) provides opportunities for sustainable agricultural water management through implementation of efficient irrigation technologies and appropriate crop selection strategies, coupled with drainage systems to mitigate potential soil salinization risks.

The implementation framework should incorporate continuous monitoring and early warning systems, leveraging the observed geological control over 95% of hydrogeochemical parameters while addressing the exceptional potassium dynamics that indicate potential anthropogenic influences. A strategically designed monitoring network representing different geological formations with standardized seasonal sampling protocols will enable predictive water quality management. Recharge enhancement measures including construction of check dams, artificial recharge structures and watershed management practices should be prioritized to extend the beneficial effects of natural monsoon recharge throughout the annual cycle. The establishment of wellhead protection zones around high-quality water sources combined with community-based management approaches involving local capacity building in water quality assessment and village-level water management committees will ensure sustainable long-term groundwater resource management. These evidence-based strategies implemented through coordinated multi-stakeholder collaboration will ensure groundwater sustainability while safeguarding public health and maintaining agricultural productivity in the study area.

Conclusion

The comprehensive hydrogeochemical assessment of Valliyur region groundwater reveals the following key findings:

1. Rock-water interactions in garnetiferous biotite gneiss terrain fundamentally control groundwater chemistry with silicate weathering dominating 85% of samples. Statistical analysis confirms geological control supremacy with only potassium showing significant seasonal variation (p < 0.001) establishing a clear hierarchical framework where seasonal processes operate within geological constraints.

2. Normal ion exchange processes dominate (95% of samples), calcite maintains equilibrium conditions (52%) and mineral saturation analysis reveals dynamic carbonate buffering systems. The evolution from Ca²⁺-Mg²⁺-HCO₃⁻-Cl⁻ to Ca²⁺-Cl⁻-SO₄²⁻-HCO₃⁻ water types demonstrates mature aquifer systems with coordinated dissolution equilibria among multiple mineral phases.

3. Monsoon recharge creates improvements in drinking water suitability with excellent quality samples increasing from 48.33% (pre-monsoon) to 70% (post-monsoon). This seasonal transformation reduces health risks from 25% of area requiring intervention to minimal concerns, while irrigation quality remains suitable throughout both seasons.

4. Pre-monsoon health risks concentrate in areas exceeding WHO guidelines for total hardness (48.33%), chloride (40%) and TDS (40%) requiring targeted interventions for vulnerable populations including pregnant women, infants and elderly individuals. The identification of these high-risk zones enables evidence-based resource allocation for treatment systems and alternative source development.

5. The study establishes a scientifically-based management framework combining seasonal use optimization, targeted treatment interventions, enhanced recharge strategies and community-based monitoring systems. This integrated approach leverages the natural monsoon-induced quality improvements while addressing pre-monsoon challenges through technological and policy interventions, ensuring long-term groundwater sustainability for both domestic and agricultural applications.

The future research direction should concentrate on investigation of trace elements including NO₃⁻ and F⁻ concentrations, heavy metal contamination assessment, isotopic studies for source identification and long-term monitoring to validate climate change impacts on seasonal hydrogeochemistry patterns.

Author contributions

A Antony Alosanai Promilton – Conceptualization, Software, Methodology, Writing - Original Draft and Data Curation. A Antony Ravindran - Supervision and Project administration. Stephen Pitchaimani V - Supervision and Writing - Review and Editing. J Vinoth Kinston – Data Collection, Review and Editing. Shankar Karuppannan – Formal analysis, Writing - Review and Editing.

Data availability

The data that support the findings of this study are available on request from the corresponding author.

Declarations

Competing interests

The authors declare no competing interests.