Analysis of seasonal spatio-temporal variations in river water quality and its influencing factors in the Periyar River Basin, Southern Western Ghats, India

One of the most valuable natural resources that humans have access to is water. The quality of this resource is completely influenced by the surrounding environment, how it is used, treated, and reused as needed, as well as by human activities including mining, domestic, industrial, and agricultural processes. The need for clean water has surged dramatically worldwide driven by a rapid population boom. Access to clean water resources is an indispensable requirement to meet the drinking and irrigational necessities of a region or a nation. However, increasing pollution in surface waterways in both developed and developing nations has a detrimental effect on the availability of this clean water (Bhutiani et al. 2016; Omonona et al. 2019). The management of sustainable water resources is largely hampered in developing nations by water contamination and scarcity. Although the World Health Organization (WHO 2017) has guidelines for water quality measurements, most countries witness a continuing rise in water pollution from various sources. The primary causes of the decline in surface water quality are nutrient pollution linked to agricultural production and discharge from industry, agriculture, and sewerage. Furthermore, natural variables (such as the amount, frequency, and severity of rainfall, quantity, kind, and topography of the vegetation, landscape, soil, and river discharge) also contribute to deterioration of water quality. According to numerous studies (Sundaray et al. 2006; Dessu et al. 2014; Garaba & Zielinski 2015; Aliyu et al. 2020; Achite et al. 2024; Chianeh et al. 2024), many freshwater bodies are being distressed and contaminated, leading to alterations in the water quality, hydrologic regime and biodiversity of aquatic systems. Environmental sustainability and public health are both significantly influenced by the quality of the water.

According to estimates from the Union Ministry of Water Resources, water demand will surge from 813 billion cubic meters (BCMs) in 2010 to 1,093 billion BCMs in 2025. Approximately 76 million individuals in India face a lack of access to safe water, as reported by the WaterAid India Country Strategy 2016–2021. The research also stated that diarrheal illnesses brought on by contaminated water claim the lives of nearly 140,000 children under the age of 5 years. According to World Bank estimates, contaminated water is responsible for 21% of communicable diseases in India. As a result, in most nations, water degradation has emerged as a critical and dangerous environmental concern. Maintaining the integrity of aquatic ecosystems and human health depends on periodic observation and evaluation of the water quality. To enhance the quality of India's air and water, the Central Pollution Control Board (CPCB) and State Pollution Control Board (SPCB) make sure that these environmental laws are upheld and implemented.

Since the quality of river waters has a profound effect on river environment management, ecologists and water resource managers are particularly interested in understanding the different sources, processes, and pathways that regulate it. The interactions among physical, chemical, and biological systems in a catchment are collectively manifested in the dissolved chemical load present in river water. Consequently, an analysis of the hydrogeochemistry of a river basin can unveil a wealth of information regarding the circulation of dissolved elements within the continent–river–ocean system, including insights into the rate and pattern of chemical weathering processes (Hu et al. 2004; Moon & Kim 2021).

In the Indian context, several studies established the geochemical budget as a result of catchment weathering in large river systems and elucidated the dissolved ionic flux in the Himalayan and Peninsular rivers (Thomas et al. 2015a). Several researchers also studied the hydrochemical differences in a number of the southern Western Ghats' small and medium-sized rivers. Rivers in tropical mountainous environments should be examined from such a perspective because observations indicate that tropical climates enhance both rock weathering and landscape depletion (Thomas 2015b). However, there exists a notable gap in the understanding regarding the hydrochemistry of the isolated mountain river basins situated in the southern Western Ghats. These basins are distinguished by their reduced basin size, homogenous lithology and conspicuous but incomplete human signatures. Moreover, climate change has a significant impact on tropical mountain fluvial-hydro systems, so it is imperative to continuously evaluate the hydrochemical composition of mountain rivers.

Kerala, a state in southwest India, is endowed with 44 rivers and approximately 3,000 mm of annual precipitation. Due to the steep slope of the study area toward the sea, most of the water from the rivers rapidly flows into the Arabian Sea. The quality of river water has become a concern for the people of Kerala due to factors such as urbanization, developmental activities, population growth and altered land use patterns. The ecologically fragile Periyar River not only serves as the primary source of drinking water but also provides water for domestic, agricultural and industrial development.

A detailed and systematic hydrogeological study of the whole basin is still lacking as per the literature while region-specific studies have been conducted. Khalid et al. (2018) reported on the emerging pollutants in the Periyar River in the lowland region, Ayyappan Vasantha et al. (2022) studied the remediation of polluted water quality as an impact of flood from ten sampling stations in Periyar with natural fibers; Singh et al. (2022) studied the reconstruction of extreme flood events by integrated real-time and probabilistic flood modeling in Periyar. Sudheer et al. (2019) investigated on the effects of dams on 2018 Kerala floods. A few studies were focused on the saline water intrusion in the Periyar lowland regions (Kumar et al. 2015; Damodaran & Balakrishnan 2018), effect of COVID-19 lockdown on the industrial belt in Eloor-Edayar region (Aditya et al. 2023), effect of 2018 Kerala floods on the Periyar lowland groundwater systems (Krishnakumar et al. 2022), soil geochemistry (Krishnakumar et al. 2021, 2023), environmental impact of quarrying (John et al. 2014; Vandana et al. 2020), environmental impact of sand mining from the in-stream and floodplain areas (Sreebha & Padmalal 2011), textural characteristics of sediments (Paul 2001; Arun et al. 2019), surface water quality (Maya et al. 2007; Jency et al. 2015; Mahesh & Prasanth 2015), sea level changes on the groundwater quality along the coast of Ernakulam district (Sreekesh et al. 2018), changes in rainfall pattern (Sreelash et al. 2018), morphometric studies (Jaganathan et al. 2015), land cover landuse changes (Jacob & Dwarakish 2015; Jency et al. 2015), delineation of paleochannels estimation using remote sensing and resistivity (Priju et al. 2018), simulation studies of flood along Periyar basin (Vijayan et al. 2021), isotopic investigations on moisture recycling and evaporation (Saranya et al. 2021) and the effect of reservoirs and drought on water cycle dynamics (Saranya et al. 2020).

A comprehensive sampling of a river is essential for meaningful insights into the regions causing problems within a river system, as activities in the upper catchments are reflected downstream. To date, no extensive study has examined the entire stretch of the Periyar River, from highlands to lowlands, concerning its suitability for drinking and irrigation. This study aims to address this research gap through a detailed and extensive investigation of the spatiotemporal variations, geochemical processes, factors governing hydrochemical composition, and water suitability in the Periyar River Basin (PRB), which was severely impacted during the 2018 Kerala Floods. Given that the river serves as a principal source for rural and urban water supply schemes, the importance of this study is further underscored. The findings will provide a valuable data repository and a framework for tool selection for future research, aiding urban planners, legislators, and environmental managers in effective river basin management. This is particularly significant for the Periyar River, being one of the largest and most important rivers in Kerala, hosting several dams and reservoirs for drinking, irrigation and electricity generation.

Situated on the southernmost region of India's west coast, the state of Kerala is bordered to the east by the Western Ghats and to the west by the Arabian Sea. Indian Summer Monsoon Rains often begin over the Kerala coast and therefore the state receives the highest monsoon rainfall. The mean annual rainfall of Kerala is nearly 3,000 mm, with notable regional variations within the state. The months of June and July account for the majority of this precipitation, contributing roughly 50% of the total rainfall.

Alluvial, brown hydromorphic, lateritic, and forest loam soils are the main soil types found in the basin; of these, 60% is composed of laterite and forest loam soil (GSI 1995). According to Krishnakumar et al. (2022, 2023), the LULC features include urban and rural settlements, croplands, plantations, fallow lands, rocky outcrops, deciduous woodland, bushes, barren areas, and other man-made settlements. Nevertheless, 59% of rural residents live in midland plateaus, and 65% of urban homes are located near the coast. Rubber, coconut, and pepper are grown in the lowlands, while cardamom, tea, and pepper are the main plantations in the highland regions. The highlands are home to some of India's biggest cardamom-producing facilities, with high-yielding types (Krishnakumar et al. 2023).

In this investigation, a total of 75 samples in each season, i.e. NEM (October 2020), POM (January 2021), PREM (April 2021) and SWM (July 2021) were collected (n = 75 × 4 = 300 samples) to assess the quality of river waters (Figure 1). Sampling was carried in the mainstream and tributaries (n = 55 × 4 = 220 samples), major dams and reservoirs (n = 12 × 4 = 48 samples) from highlands to lowlands and the locations were identified and fixed using GPS. Samples in the lowland region near to the river mouth exhibited characteristics of estuarine behavior (n = 8 × 4 = 32 samples). To ensure the accuracy of data, samples were acquired from the middle region of the river (over bridges wherever possible) to a depth of 20–30 cm beneath the water surface to avoid surface scum and debris including macrophytes. Samples were collected in High-Density Polyethylene (HDPE) bottles that had been pre-washed three times with sample water before collection. The bottles were properly labeled and sealed on-site upon collection. Following the established analytical protocols of APHA (1995) samples were transported to the laboratory and evaluated for various parameters within the residence time. Physical parameters such as temperature, pH, conductivity and Total Dissolved Solids (TDS) were evaluated in situ by Aquaread multiparameter water quality analyzer (AP-2000-D) while the concentrations of major cations (Ca²⁺, Mg²⁺, K⁺ and Na⁺) were analyzed through Microwave Plasma Atomic Emission Spectroscopy (MP-AES, Model: Agilent 4210) while those of the predominant anions like ⁠, and Cl⁻ were determined by Continuous Flow Analyser (CFA, Model: Skalar SAN + +) and UV-VIS-NIR spectrophotometer available at the Central Chemical Laboratory, NCESS, Thiruvananthapuram. Bicarbonate ions (⁠⁠) were determined by acid titration. Samples for cation analysis were acidified using 2M HNO₃ to all the samples instantly after collection (Figure 2).

The Water Quality Index (WQI) is a mathematical calculation to integrate physical, chemical and biological parameters into a numerical score that represents the overall status of the water quality under consideration (Saha et al. 2021). Brown et al. (1972) proposed the WQI method based on the National Sanitation Foundation Water Quality Index (NSFWQI).

The five important steps applied to determine the WQI are:

Step 1: Collection of data regarding the relevant physicochemical water quality parameters.

V_i refers to the ideal value of the nth parameter in pure water. Also, S_n refers to the standard permissible value of the nth parameter.

pH levels typically ranged between 6.50 and 8.50 in places that are not susceptible to pollution (Hem 1985). In Periyar R̥iver, pH varied from 5.45 to 8.11 in NEM, 5.92–10 during POM, 5.9–8.61 in PREM and 5.22–8.37 in SWM suggesting mild acidic to slightly alkaline in all the seasons (Table 2). The acidic nature of Periyar River (PR) is due to the characteristics of acidic tropical soils in the study area (Aditya et al. 2023) and laterites since laterites have the capacity to generate acidity. The acidity of soils due to the reaction of water with exchangeable Al³⁺ on the surface of soil particles generated a strong Al-water reaction which repels H⁺ from the water molecules into the soil solution. Year-round high rain and high temperature leach the soil particles into the river system which ultimately results in the acidic pH of the river (Krishnakumar et al. 2023). Electrical Conductivity values ranged from 66 to 2,453 μS/cm in NEM, 76–4,176 in POM, 77–15,366 in PREM and 96–1,840 in SWM. Similarly, TDS also exhibited analogous variations. Larger values were observed during PREM (50–9,988 mg/l) and POM (50–2,715 mg/l) compared to NEM (42–1,594 mg/l) and SWM (62–1,196 mg/l). The majority of rivers worldwide have TDS levels of less than 500 mg/L, according to Gaillardet et al. (1999); the few exclusions are indicative of either pollution or semi-arid or arid climates. According to Stallard & Edmond (1983, 1987), river waters with moderately high TDS concentrations indicate weathering of evaporites, whereas streams with low TDS concentrations indicate weathering of silicates. The mean TDS were relatively greater than those of other rivers draining the Western Ghats. This discrepancy may be caused by variations in the basin size, lithology, discharge, climate, and level of anthropogenic intervention.TH concentration varied between 2 and 25 mg/l (NEM), 6.3–360 (POM), SWM (3.27–32.6 mg/l) with the maximum concentration during PREM (7.89–405).

The dispersion of different silicate minerals is the primary factor regulating the amount of Ca²⁺ and Mg²⁺ in stream water (Magaritz et al. 1988). Ca²⁺ concentration in the present study was between 0.16 and 8.8 mg/l in NEM, 1.5–511 mg/l in POM, 1.89–65 mg/l in PREM and 1.09–11 mg/l in SWM whereas Mg²⁺ varied from 0.41 to 2.91 mg/l in NEM, 0.49–67 mg/l in POM, 0.65–70 mg/l in PREM and 0.40–2.91 mg/l in SWM. Ca²⁺ is primarily found in igneous and metamorphic rock minerals, particularly in carbonate rocks and chain silicates like amphiboles and pyropes, as well as plagioclase feldspars (Thomas et al. 2015a). While mafic minerals like pyroxenes and amphiboles, as well as dark-colored micas like biotite, act as a potential source of Mg²⁺ in river water. The weathering of minerals, primarily plagioclase feldspars, clay minerals, and atmospheric inputs are the sources of Na⁺ in natural systems (Hem 1985). The higher concentration of Na⁺ can also signify the dominance of silicate weathering or anthropogenic activities. It is observed from the results that Na⁺ concentration was fairly minor during monsoon seasons relative to POM and PREM seasons due to the dilution effect. K⁺ appears to behave conservatively in river systems, as indicated by relatively lower levels of K⁺ in comparison to Na⁺ (Mackenzie & Garrels 1971). K⁺ is less abundant in the crustal rocks than Ca²⁺, Mg ²⁺, and Na⁺, as evidenced by the numerically lower and temporally consistent values of the study area. The leaching or weathering of K-feldspar, specifically orthoclase and microcline, serves as the source of K+ in water.

Sulfate (⁠⁠) is typically found in natural waters as soluble salts of calcium, magnesium, and sodium. During POM and PREM, concentration was found to be high (0.12–574 mg/l and 1.10–599 mg/l, respectively) compared to NEM (0.1–84 mg/l) and SWM (0.01–16 mg/l). The and PO₄ contents were found to be notably negligible across all seasons.

It was observed from the results that the abundance of predominant cations and anions were Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ and > Cl⁻ > > for NEM, POM and PREM while during SWM cations were in the decreasing order of Ca²⁺ > Na⁺ > K⁺ > Mg²⁺. The ionic ratios were generally higher for the PREM season than NEM, SWM and POM. The observed variations in the pattern of major ions across different seasons are attributed to the monsoonal weather conditions prevailing in the basin. During the monsoon season, high precipitation dilutes the solute load in the river, while the high temperatures in the pre-monsoon season concentrate the solutes in the river water. These seasonal differences in river water chemistry are a characteristic feature of major Indian rivers influenced by the monsoonal climate.

Anion facies classification shows dominance suggesting active groundwater flushing in all seasons with the exceptions of a few samples belonging to Cl⁻ type. The Cl⁻ dominant samples suggest the impact of anthropogenic sources of inputs in the study area. However, the interactions between water, the soil matrix, and the litho units are highlighted by Ca²⁺ and Mg²⁺ rich water. In PRB, CaHCO₃ type water prevails in all seasons followed by mixed CaMgCl type. A small proportion of samples during PREM is observed in the NaCl type. CaCl water type also shares a minute percentage. The CaHCO₃ water types represented groundwater dominating discharge while the other facies denote anthropogenic inputs into the system. Generally, the river water samples in the study area were dominated by alkaline earth (Ca²⁺ and Mg²⁺) and weak acids (⁠⁠). Waters exhibiting a dominance of mixed cation-HCO₃, possibly arise from the interaction of ‘fresh recharge waters’ with Mg-rich minerals present in the lithology of the basin. The prevalence of mixed cation-HCO₃ waters serves as an indicator of recharge influenced by rainfall. Additionally, the mixed ion classification of these waters may also contribute to chemical inputs associated with fertilizers (Delpla et al. 2009). The Ca–Mg–HCO₃–Cl– type signifies recharge waters undergoing a more extended reaction time with subsurface materials.

After combining the ideas of Clarke (1924), Gibbs (1970) suggested a basic model of water chemistry in which the main mechanisms governing water chemistry are rock weathering, climate via atmospheric precipitation, and climate via evaporation and fractional crystallization. The weight ratio of Na⁺/(Na⁺ + Ca²⁺) versus TDS in a bivariate plot indicates the primary natural mechanisms governing the chemistry of surface water (after Gibbs 1970) and offers important insights into the relative importance of lithology and climate from three different angles: (a) climate via rock weathering, (b) climate via atmospheric precipitation, and (c) climate via evaporation and fractional crystallization.

According to Gibbs' model, higher TDS (>300 mg/L) and a high Na⁺/(Na⁺ + Ca²⁺) ratio (0.5–1) indicate the dominance of evaporite dissolution and are controlled by evaporation–precipitation mechanisms. In contrast, medium TDS levels (70–300 mg/L) and Na⁺/(Na⁺ + Ca²⁺) ratios less than 0.5 are indicative of water–rock interactions and are governed by chemical weathering mechanisms. Meanwhile, lower TDS levels (<70 mg/L) and higher Na⁺/(Na⁺ + Ca²⁺) ratios (0.5–1) suggest precipitation dominance, controlled by atmospheric dry and wet deposition mechanisms (Ciba & Upendra 2024).

The results of the present study when plotted in Gibb's diagram (Figure 4a) show rock weathering as the primary factor responsible for regulating the hydrochemical composition. In PRB during all the seasons most of the samples were clustered in the zone of rock dominance illustrating the importance of mineral dissolution and rock weathering in determining the composition of the river water. However, it is also observed that some samples were plotted near evaporation during POM and PREM implying the effect of the dry season in the study area since river water in the tributaries experiences a comparatively more pronounced evaporation process than that in the mainstream (Cui & Li 2015). The results indicate that continuous leaching due to abundant precipitation during the wet season enhances the dissolution of minerals through rock-water interactions, thereby contributing to the chemical composition of the water. Surface runoff causes erosion of weathered rock minerals, exposing fresh surfaces and accelerating chemical weathering. The findings of Bricker & Garrels (1967), Hem (1985), and White et al. (2010) further support the importance of chemical weathering in defining the hydrochemical composition of natural waters.

The mean HCO₃⁻/Ca²⁺ + Mg²⁺ ratios during NEM, POM, PREM and SWM are 2.98, 4.52, 3.25 and 2.69 respectively indicating the significance of silicate weathering in controlling the major ion concentration of PRB waters. The relative proportion of Ca²⁺ and Mg²⁺ ions derived from carbonate and silicate weathering are inferred from the ratios of Ca²⁺/Na⁺ and Mg²⁺/Na⁺ (Hem, 1985). In the present study the mean Ca²⁺/Na⁺ ratios obtained were 1.00, 1.38, 1.50 and 1.49 during NEM, POM, PREM and SWM whereas mean Mg²⁺/Na⁺ ratios were 0.96, 0.91, 0.82 and 0.81 respectively. The ratios suggest the dominance of Ca²⁺ and Mg²⁺ over Na⁺ referring the weathering of mafic minerals or congruent dissolution of carbonate minerals.

The productivity and dynamic status of the rivers are often expressed through the aqueous pCO₂. Riverine pCO₂ indicates the comparative source or sink of river carbon pool for atmospheric carbon pool, as well as the dynamics of carbon and terrestrial biogeochemical processes (Liu et al. 2019; Wang et al. 2021). According to various studies (Cao et al. 2020; Liu & Han 2021), the generation and conveyance of soil CO₂, the introduction of chemical weathering byproducts, in-stream bio-degradation, respiration, and photosynthetic activities collectively exert influence on the pCO₂ levels in rivers.

The mean pCO₂ during NEM was around elevenfold higher than atmospheric pCO₂ while during SWM and POM, it was roughly five times. Lower pCO₂ during the monsoon periods might be due to the dilution effect (since the quantum of rainfall is significantly >3,000 mm). But during PREM nearly 26 times the atmospheric pCO₂ was estimated. The higher values during PREM can be a result of groundwater recharge to the river since the stream discharge of rivers draining the Western Ghats primarily originates from aquifers during the non-monsoon season. It is observed from Figure 5, that, as the pCO₂ increases, the pH value decreases. With a rise in pCO₂ there occurs a decrease in pH due to the production of and H₃O⁺ ions (Ciba & Upendra 2024). Elevated pCO₂ levels are linked to urban and agricultural influences, as well as deep and murky waters, which are ideal for in-stream oxidation and respiratory processes (Zhang et al. 2009). Since Periyar river is over saturated with respect to the atmospheric CO₂ indicating a higher value of pCO₂ production, the river can be regarded as net heterotrophic. According to several studies, most rivers are around tenfold supersaturated, whereas the tributaries of the Amazon are about fortyfold supersaturated because of root respiration and organic matter decomposition.

The Periyar basin is one of the most densely populated river basins in Kerala and an important river for agricultural and food production catering to two principal functions, i.e. drinking and irrigation. These two functions are of great significance to the livelihoods of residents and social development.

The classification of the WQI category was assessed on the drinking water suitability recommended by Brown et al. (1972), such as Excellent (WQI < 25), Good (26–50), Poor (51–75), Very Poor (76–100) and water unsuitable for drinking (>100) considering pH, Conductivity, TDS, DO, Ca²⁺, Mg²⁺, K⁺, Na⁺, TH, ⁠, ⁠, and Cl⁻.

The criteria for water suitability vary for drinking and agriculture purposes. In the present study, the results of WQI suggested locations with poor and very poor drinking quality. Water unsuitable for drinking may still be suitable for irrigation because the assessment of irrigation water primarily focuses on the presence of undesirable constituents that are not beneficial for crop growth (Sarkar & Islam 2019). Therefore, irrigational water quality has been assessed in the present study to find the agricultural suitability of the Periyar River since dams are devoted to irrigation in the study area. The hydrochemical composition of river water governs its relevance for agricultural and irrigation requirements, which are evaluated by assessing SAR, Percent Na (Na%), Magnesium Hazard (Mag Haz), PI and RSC and the results are summarized in Table 3.

Excess carbonate and bicarbonate-containing water frequently precipitate calcium and magnesium from the soil as their carbonates. Consequently, the fraction of salt in the soil increases and becomes fixed, leading to a decrease in the permeability of the soil. A negative RSC suggests that there is unlikely to be any sodium build-up because there is more calcium and magnesium present than what may precipitate as carbonates. A positive RSC suggests that there may be a salt buildup in the soil. The results suggest RSC of the study area is excellent during NEM and SWM and as marginal quality during POM and PREM. Increased magnesium concentration in water affects soil quality, turning it alkaline and reducing crop productivity. MH values ranged from 14–93, 0–82, 0–76 and 0–68 during NEM, POM, PREM and SWM. Long-term irrigation water use affects the permeability of the soil influencing the soil's Na⁺, Ca²⁺, Mg²⁺ and composition. PI values varied from 16.5 to 723 in NEM, 15 to 568 in POM, 11 to 490 in PREM and 30 to 554 in SWM.

Corrosion is the partial dissolution of the materials constituting the treatment and supply systems, tanks, pipes, valves and pumps which becomes a concern of public health and economic aspects. According to WHO guidelines, corrosion minimalization is an important aspect of securing safe drinking water. In the present investigation, the corrosion index was calculated since preliminary investigations suggested the riverine samples of PRB as slightly acidic in nature (5.2–10). Low pH waters have been shown to be corrosive (WHO 2004); however, other physical and chemical characteristics of water contribute significantly to the corrosive tendency of water (Agatemor & Okolo, 2008).

In PRB, the mean Cc was found to be 0.91, 0.95, 1.02 and 0.63 during NEM, POM, PREM and SWM, respectively. Except SWM, in all other seasons chlorides and sulfates may interfere with natural film formation on metal pipes which may cause higher than desired corrosion rates, while in SWM chlorides and sulfates are likely to have no significant impact on the formation of the natural film (Table 4). A ratio greater than 1.2 indicates higher corrosion rates (Larson & Skold 1958).

It is evident that the effect of various land use patterns has a substantial influence on seasonal water quality metrics at the sub-basin scale particularly in river basins subjected to intensive human disturbance. In this study the landuse landcover (LULC) variations assessed for 2017 and 2023 years using Sentinel 2 Landcover Explorer indicated a rise in waterbody, cropland/agriculture and settlements by 1%, 0.5% and 1.5%, respectively, from 2017–2023, while a decrease was observed in forest and grasslands by 2% and 0.9%, respectively. According to Sadhwani et al. (2023), the future LULC change predicted for PRB shows cropland and urbanization land uses to increase up to 5.1 and 17.63%, respectively, till 2,100 with a decrease in forest cover (24.58%). Cropland, barren land and forest were found to be decreased alternating by an increase in settlements and plantations. Water bodies exhibit minimal change, experiencing a marginal decrease of 0.4%. Settlements and plantations amplified from 3 to 14% and 0.48 to 2.7%, whereas cropland, barren land and forest decreased from 40 to 34%, 5 to 3.7%, and 46 to 40%, respectively.

Prominent transitions observed include shifts from plantations to settlements and from forests to plantations. Additionally, instances of interconversion between forests and plantations were also noted. Within the PRB, the continuous reduction of barren land is predominantly attributed to its conversion into plantation areas. Since the highland regions of PRB have reserved forests, the buffer zones of these forests are being slowly converted to plantations and as a result, the existing plantations are getting converted to croplands and croplands to settlements. According to the findings, future deforestation and urbanization will result in land degradation and have a substantial impact on the hydrological balance of the watershed. Elevated runoff results in diminished percolation and baseflow, ultimately lowering soil water content. This, in turn, adversely affects soil properties and diminishes fertility. These changes can affect the natural river flow pattern eventually leading to floods and other environmental repercussions.

The present study highlights the hydrochemical characterization and water quality patterns of the Periyar River, draining the Western Ghats to assess the suitability for drinking, irrigation and industrial usage using various quality indices to better understand the diverse natural and human-caused processes in the basin.

• It was observed from the results that the abundance of predominant cations and anions were Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ and > Cl⁻ > > for NEM, POM and PREM while during SWM cations were in the decreasing order of Ca²⁺ > Na⁺ > K⁺ > Mg²⁺. The ionic ratios were generally higher for the PREM season than NEM, SWM and POM. This might be due to the input of monsoon rainfalls in the study area.

• The acidic nature of riverine samples throughout all the seasons can be due to the presence of laterites and lateritic soil since laterites have the capacity to generate acidity. The same has been thoroughly established through previous studies conducted in the soil and groundwater phases in the study area.

• In this study, it is revealed that an increasing ionic concentration trend is observed in the downstream regions in contrast to Probst's theoretical dilution curves. This may be due to the ingression of ocean water into the riverine system through the Vembanad estuary.

• The prevailing hydrochemical facies in the surface water are characterized by CaHCO₃ and mixed Ca–Mg–Cl types, reflecting both natural and anthropogenic influences on water chemistry. Despite variations in hydrochemical facies across seasons, a notable commonality is the presence of transitional waters throughout the riverine system.

• Gibb's plot illustrated that the Periyar River water chemistry is mainly controlled by rock–water interaction rather than evaporation/precipitation.

• In the majority of samples, riverine pCO₂ exceeded atmospheric levels, indicating the outgassing of CO₂ from river water into the atmosphere which may lead to increased concentration of CO₂ in the atmosphere.

• The WQI analysis revealed the majority of samples were in the excellent and good category for drinking needs while on the other hand irrigational suitability assessment indicated more than 95% of samples as best suited for agriculture purposes.

• As per the previous studies conducted, it has been reported that the diminishing intact natural vegetation and the impacts of urbanization have a considerable effect on the factors responsible for the hydrological balance which may lead to floods or droughts and increase or decrease in rainfall along with deterioration in water quality.

The major limitation of the present study was the inaccessibility to collect samples from the small streams and minor tributaries of the river since about half of the portions in the highland region and some portions in the lowland region belong to forested areas. However, samples were acquired from the confluence points where forest streams merge with the mainstream, thereby encompassing the entire river and adequately reflecting the study area. Despite this limitation, the findings of this study by collecting 300 samples during four different seasons will surely contribute to a comprehensive understanding of the transformations within the river basin, facilitating strategic planning and effective management.

Since, the lower reaches of the PRB, namely ‘Kochi’, are experiencing rapid urbanization, while the uplands and midlands encompass ecologically sensitive regions of Western Ghats, periodic monitoring of hydrochemical variations from upstream to downstream is essential, as changes in the upstream can directly and adversely affect downstream regions. Additionally, the highlands are active economic centers, with significant contributions from tourism and plantations (tea and cardamom). Consequently, any land use and land cover (LULC) change and uncontrolled usage of agrochemicals for better yield may deteriorate the water quality in these areas. Therefore, in future, continuous monitoring may be done to decipher the spatiotemporal changes if any, due to natural or anthropogenic activities including the impacts of climate change in water resources.

This study has been conducted as part of MoES-NCESS core programme (W.P.3B.4) “Assessment of Global Environmental Changes in Sahyadri”. The authors are very much thankful to the Director, NCESS for providing research facilities, encouragement and support. The support rendered by the Forest and Wildlife Department, Government of Kerala for the smooth implementation of the program is also gratefully acknowledged. The authors express their gratitude towards the anonymous reviewers and the editor for their constructive comments in improving the quality of the manuscript. One of the authors, Ms. S.K. Aditya is thankful to University of Kerala (UoK) for granting PhD research opportunity and the Council of Scientific and Industrial Research (CSIR) for the research grants.

Funding is provided by the Ministry of Earth Science, Government of India.

All authors have read, understood, and have compiled as applicable.

S.K.A. contributed to conceptualization, sample collection, formal analysis, data curation, investigation, concept, writing original draft, validation, methodology, visualization. A.K.contributed to conceptualization, formal analysis, data curation, supervision, concept, writing original draft, validation, project administration, resources. K.A.K. contributed to conceptualization, data curation, formal analysis, visualization.

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

The authors declare there is no conflict.