Introduction

A seasonal study on water quality (Rice and Bricker 1995) is very much significant in identifying temporal geochemical processes taking place in the river system. Ca2+, SO4 2−, Cl−, Na+ and K+ ion concentrations seem to fluctuate significantly during different seasons which were well correlated with the environmental changes (Rajmohan and Elango 2004) occurring in the aquatic systems. Dissolved inorganic carbon (DIC) changes could be taken as a good indicator for the fluctuations in productivity status (Chapin et al. 2006) of aquatic systems while variation of dissolved organic carbon (DOC) concentrations is considered as an important parameter for understanding the ecological changes occurring in the aquatic system (Wilson et al. 2013; Dawson et al. 2008). Researchers correlate DOC to increased temperature (Freeman et al. 2001), drought-triggered enzymatic activity (Worrall et al. 2004), increasing atmospheric CO2 concentrations (Freeman et al. 2004) and interactions between temperature and CO2 (Fenner et al. 2007). Hence increasing trend of DOC concentrations in aquatic system (Dai et al. 2009) is also of great concern. Complex interactions exist amongst landscape and stream biogeochemical mechanisms and, therefore, long-term data are valuable to identify major drivers of trends in water quality and are a complement to seasonal trend or other short-term process based studies (Oni et al. 2012).

Study area

The study steered in Meenachil River (L = 78 km, A = 1272 km2), one of the prominent river basins in Central Kerala, formed by several streams originating from Western Ghats (elevation = 1097 m, a.m.s.l). It flows in the E–W direction and finally debouches into Vembanad Lake, a Ramsar site in Kerala. The river basin geographically lies between 9°25′–9°55′N latitude and 76°30′–77°00′E longitude (Fig. 1). The elevation profile of the entire river basin ranges from 77 to 1156 m in the highlands, 8–68 m in the midlands and less than 2 m in the low lands. The highlands constitute about 33% (A = 346 km2), midlands 66% (703 km2) and in lowlands spread only 01% (11 km2).

Fig. 1
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Location map of Meenachil River basin

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The river basin falls within the realm of tropical humid climate (annual rainfall 2700–3000 mm, temperature 25–32 °C). The proximity to the sea and high variations in relief from the west coast to the hilly region of the Western Ghats in the east influences the climatic parameters. The S–W monsoon sets during June and lasts till August. The N–E monsoon onsets in October and continues up to the end of November. It was known that the river had adjusted its morphology according to the coastal changes (Ajaykumar et al. 2012) while geomorphological modifications like channel shifts might have caused the unique and extensive distribution of palaeo-deposit of sand in some areas of basin (Mohan et al. 2005). Urbanization-induced changes on the streamflow records of Meenachil River basin were also well studied (George et al. 2012). Potentially pathogenic bacteria, Vibrio cholerae, Vibrio parahaemolyticus and Salmonella enterica, in the river indicate that the bacteriological quality of the river is poor (Vincy et al. 2017).

The catchment is predominantly covered by rubber plantation (Hevea brasiliensis), but mixed vegetation of plantain (Musa sapientum), coconut (Cocos nucifera) and tapioca (Manihot esculenta) is also found. It was reported that rubber area expansion in Kerala state for the 1955–2000 period was 627% (Kumar 2005), of which Kottayam district ranks top position. Lowlands mainly comprises of paddy (Oryza sativa) cultivation. Compared to other paddy growing areas in Kerala, paddy cultivation in Kuttanad region has certain unique characteristics. Paddy lands in this region are divided into contiguous blocks called padasekharams bound by waterways, rivers and other natural partitions. Many of such padasekharams are manmade in the sense that they are reclaimed lands from the bed of backwaters (Thomas 2002). Thanneermukkom regulator separates the Vembanad lake system into two halves, namely northern part and southern part, in which latter comes in our study region were the Meenachil River debouches with lake.

The basin mainly comprises of pre-cambrian metamorphic rocks which forms a hilly background. The major soil type prevalent in the area is well-drained laterite soils, while main rock types of the area belong to Charnockite group which includes charnockite, cordierite gneiss, magnetite quartzite and pyroxene granulite. Recent sediments of coastal sands and alluvium occupy the areas close to the river mouth zones (Vembanad Lake).

Sample collection and analysis

A total of 21 surface water samples were collected from seven (S1–S7) sampling stations for pre monsoon (PRM), monsoon (MON) and post monsoon (POM) seasons spanning from June 2013 to March 2014. Sampling locations include S1 (Adivaram), S2 (Tekkoi), S3 (Erattupetta), S4 (Pala), S5 (Pattermadam), S6 (Railway bridge) and S7 (Kavanattinkkara) stations.

The portrait of sampling sites is given in Table 1. The station S1 is predominantly covered by forest in its catchment. S2 station seems to have stagnant waters in PRM and POM seasons; S3 has high discharge owing to the confluence of two tributaries just upstream of the sampling point. Station S4 has a small township adjacent to it and water flow was moderate during PRM and POM seasons. Samples collected from S5 and S6 stations generally have foul smell during PRM and POM mainly due to domestic and municipal sewage discharge. The lowland station S7 is the river—Vembanad Lake, interface location having highly dynamic environment with a mangrove belt nearby.

Table 1 Portrait of sampling stations
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Water samples were filtered through GF/C precombusted filters (0.45 µm). Filtered sample was collected and covered in scintillation vials and was analysed for dissolved carbon (DC) and non purgeable organic carbon (NPOC) using TOC-analyzer (TOC-VCSH, Shimatzu). NPOC is treated as DOC, while the DIC was calculated as difference between DC and DOC. Ca2+, SO4 2−, and Cl- were analysed titrimetrically using APHA (1998) standards. Ionic concentrations of Na+ and K+ were determined using Flame Photometer after standard calibrations. Entire statistical analysis was done using SPSS v.22.0 Software.

Results and discussion

A wide seasonal fluctuation in ionic concentrations (Table 2) is observed in the Meenachil River during the study period. PRM shows high concentrations for most ionic forms. Ca2+ was found to be maximum in PRM (avg. 10.98) compared to MON (8.01) and POM (avg. 4.46), whereas SO4 2− shows a similar trend (avg. 4) in all seasons. Wide variations were observed in the case of Na+, K+ and Cl− ions in different seasons with increasing trend from upstream to downstream stations. PRM shows maximum values for Na+ (31.28), K+ (4.0) and Cl− (136.06) followed by Na+ (7.45), K+ (2.05), Cl− (30.1) during POM. Wide variations of these ions (Na+, K+ and Cl−) that were notable in S7 station might be due to the debouching lake water dominance in that region. DIC did not show increasing trend from upstream to downstream although some escalation at S3 and S4 stations were noted with maximum (3.53) during PRM followed by POM (2.54) and MON (1.49). Meanwhile, DOC was maximum (1.58) during PRM when compared to POM (0.97) and MON (0.36).

Table 2 Max, min and average values of ions for different seasons
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Log transformed data of various ionic species are depicted in Fig. 2. This evidently illustrates the upstream–downstream fluctuations occurring in the riverine system. Moreover, wide disparities in the stations are evident during MON. Box plot (Fig. 3) visibly demonstrates the influencing effect of Cl− ion in the study sites. A wide seasonal fluctuation in concentrations of Ca2+, Na+ and K+ is well noted.

Fig. 2
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Log-Y transformed data of ions at various stations during a PRM, b MON and c POM

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Fig. 3
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Box-plot of various ions for a PRM, b MON and c POM

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In order to evaluate the correlation among ions, bivariate correlation analysis was conducted. Correlation diagram (Figs. 4, 5, 6) during PRM advocates strong relationship of Ca2+ with Na+, K+, Cl− and DOC. Ca2+ correlation is evident only during PRM and hence contributes to DOC apart from other seasons. Na+ showed association with K+ and DOC, while affinity of K+ for Cl− is also notable during PRM. Strong correlations of DIC and DOC for Na+, K+ and Cl− ions were remarkable throughout these seasons. Meanwhile, MON table showed significant correlation of Na+ for K+, Cl− and DIC. Correlation of K+ ion for Cl− and DIC also shows similar relations as that of Na+. The scenario is entirely different for POM in which Na+ and K+ show high correlation for Cl−, DIC and DOC. Hence Na+, K+ and Cl− ions were responsible for high DIC and DOC during POM. It is also noted that SO4 2− did not have correlation towards any other parameters during three seasons and hence it was concluded that a different source/process is responsible for this. Relationship of Na+ and K+ is evident in all seasons. Also association of K+ and Cl− is noticeable during different seasons. Positive relationship of DIC for Na+, Cl− (during MON) and Na+, K+, Cl−, DOC (during POM) is evident. Further, correlation of DOC for Ca2+, Na+, K+, Cl− (during PRM) is almost comparable with that of POM, while no significant correlation were noted during MON.

Fig. 4
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Diagram showing correlations during PRM

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Fig. 5
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Diagram showing correlations during MON

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Fig. 6
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Diagram showing correlations during POM

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Factor analysis (Table 3) identifies mainly two factor loads viz. Factor 1 (F1) and Factor 2 (F2) for explaining the percentage of variance considering only Eigen values greater than one and varimax rotation is employed here, which is generally considered worthwhile for factor loading (Manly 1998). The results demonstrate high F1 variance of 73, 68 and 72% followed by F2 comprising of 17, 19 and 21% for PRM, MON and POM, respectively. This F1 and F2 factor loads (Fig. 7) are explained in detail below.

Table 3 Factor loading for PRM, MON and POM, respectively
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Fig. 7
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Factor loading after varimax rotation

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Factor 1 (F1)

During PRM high loading for Ca2+, Na+, K+, Cl− and DOC was noted (73%), while in MON (68%) loading observed was mainly due to Na+, K+, Cl− and DIC. POM is greatly loaded with Na+, K+, Cl−, DIC and DOC constituting total variance of 72%. F1 loading reveals that some common processes were determining the Na+, K+ and Cl− concentrations. Agricultural activity (fertiliser application) which is predominant in the study region could be a possible reason for this loading. Significant loading of Ca2+ ions in PRM could be attributed to the dilution of lime shell deposits prevailing in the downstream locations of the river basin. Substantial contribution of DIC in MON could be owing to the high leaching of inorganic carbon compounds from the catchment area to the river system. Low carbon processing associated with predominant plantation especially rubber was well explicated from this study results. A noteworthy loading of DOC during PRM and POM at S7 station depicts the existence of organic carbon species owing to the proliferation of macrophytes in the lake system, as mentioned in some similar previous studies (Verma et al. 2002).

Factor 2 (F2)

During PRM high loading for SO4 2− was prominent explaining total variance of 17%, while MON quantifies loading for DOC with a total variance of 19%. POM was significantly loaded with Ca2+ constituting total variance of 21%. Appreciable loading for DOC in MON could be due to monsoon effect, which generally plays a vital role in DOC release. Ca2+ loading during POM explains the weathering effect prevailing in the system.

Cluster classification using hierarchical cluster analysis (HCA) is done by employing Euclidean distance and the Ward’s linkage method. HCA is usually considered a powerful tool for exploring water quality data, and articulating geochemical models (Meng and Maynard 2001). HCA dendrogram (Fig. 8) reveals that PRM strictly controls the ion chemistry in S7 (river mouth). Lake process during PRM seems to have significant role in making S7 a different cluster, while intrusion of lake water also plays a major role in making S6 too different. All other stations (S1, S2, S3, S4 and S5) were grouped together making them a separate cluster. During MON it was noted that upstream stations (S1, S2 and S3) were grouped in single cluster; other cluster comprises (S4, S5 and S6) while S7 is purely controlled by lake water chemistry. POM incorporated S6 and S7 forming a cluster while all other stations form separate cluster. High F2 loadings of SO4 2− during PRM have pronounced effect on S6 making it vulnerable to anthropogenic pollution. Anthropogenic pollution impact is well noted in the study, especially at S6 owing to its cluster formation during PRM and POM while dilution effect explains its difference during MON.

Fig. 8
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HCA dendrogram for a PRM, b MON and c POM

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Summary and conclusion

The study clearly catches drastic seasonal variations in ionic constituents in Meenachil River. Though Ca2+ values are low during PRM, a contrary scenario in downstream stations (S6 and S7) was noted. F1 loading also highlights the influence of Ca2+ along with Na+, K+, Cl− and DOC. Saline water ingress will also come into effect contributing to elevated DOC levels at these two stations during PRM. SO4 2− ions did not contribute to much loading during most seasons. Cl− values were generally high in PRM while dramatic increase at S6 and S7 was due to the saline water ingress which was also evident from Na+ values at these stations. Meanwhile, MON was having lower values for most ions mainly owing to dilution effect, and F1 loading highlights prominence of DIC, K+, Na+ and Cl− ions. An extremely high value of DOC value at S7 station during POM, points to the ecological changes occurring in the downstream station adjacent to the river lake interface region owing to the traditional system of applying lime and letting water for remaining period along with the proliferation of different macrophytes. Heterotrophic in-stream processing of DOC may contribute free CO2 to increase DIC and would shift carbonate equilibrium towards HCO3 − (Stumm and Morgan 1981). Hence it was concluded that the shift in ecological processes occurring in Vembanad Lake had pronounced effect on the dissolved nutrient status at river station Kavanattinkkara (S7), especially during PRM and POM.