1 Introduction

The impacts and risks of climate change have become more multifaceted, and concurrent climate hazards, prominently an upsurge in flood events worldwide, have intensified (Houghton et al., 1992). Flood events in many parts of the world pose significant risks to people, infrastructure, and the environment. Floodwater can carry a variety of nutrients, including pathogens and chemical pollutants, that may have an impact on the environment and human health. In some cases, floodwaters can carry bacteria that cause diseases, such as typhoid and cholera, posing even greater risks to communities affected by these disasters (Chaturongkasumrit et al., 2013; Lillibridge, 1996; Thilagavathi et al., 2011). Flood events often result in increased water levels and flow rates in rivers, leading to the dilution of pollutants. Contaminants present in floodwaters can mix with existing water bodies, including rivers and groundwater sources. This mixing process can alter the water quality parameters (Lillibridge, 1996). The water quality parameters in flooded agricultural fields were generally more variable than in impounded wetlands, because of the variation in the size of the watershed, the use of wildlife, and the development of vegetation. The water quality parameters recorded at the study sites exhibited temporal variations of 53% (Kabir et al., 2020). Flood events can also damage the groundwater, and the extent to which groundwater levels increase during a flood occurrence will be determined by various factors that are specific to the flood event, such as the volume of rainfall within the city and upstream catchment, the speed at which river levels increase, the duration of fluvial flooding, and the preexisting soil and groundwater conditions (Macdonald et al., 2012; Thilagavathi et al., 2011). According to major ion data, local waters contribute significantly but vary across areas (Amoros & Bornette, 2002). Although groundwater plays a global role, there are inherent complexities (and a lack of understanding) associated with groundwater contamination mechanisms. Flood events are becoming more frequent and more severe, and there has been no comprehensive synthesis of the nexus between surface flooding, groundwater pollution, and enteric disease to date (Andrade et al., 2018).

In south India, the Krishna, Godavari, and Cauvery are major river basins. The Godavari basin covers a larger area and has a greater water volume than the Krishna and Cauvery basins. The Krishna River is the second largest east-flowing river of the peninsula and is significant, however, the Godavari holds the distinction of being the largest in terms of basin size in peninsular India (Ahmad et al., 2024; Chidambaram et al., 2018). Bhutekar et al. (2018) investigated and reviewed the pollution of the Godavari River and concluded that surface runoff contributes most of the pollutants; thus, watershed management will be crucial to the prevention of pollution. Nalawade and Bagul (2020) investigated the relationship between the physicochemical parameter of the water and the phytoplankton diversity in the Godavari River and reported the alarming dimensions of its effect on the algal community. The Cauvery River water is a major source for drinking, agriculture, and industries in Karnataka and Tamil Nadu states. As a result of urbanization and industrialization, water demand has increased, which is threatening its quality. One in nine people worldwide uses drinking water from unimproved and unsafe sources, according to the latest estimation of the World Health Organization (WHO) and United Nations International Children's Emergency Fund. This means that approximately 785 million people lack basic water services, such as piped water into their homes, public taps, protected wells, and boreholes. Instead, they rely on surface water, untreated or poorly treated water, and water from unsafe sources, such as contaminated rivers, ponds, and open wells. The Cauvery River is the most exploited in the country with 95% of its water being extracted (Chidambaram et al., 2018). According to data from the Central Water Commission, the average annual flow of the Cauvery River at its entry point into Tamil Nadu is approximately 22 billion cubic meters (BCM). However, the flow varies significantly from year to year, depending on the monsoon rainfall. In years with good monsoons, the flow can be as high as 40 BCM, whereas in drought years, the flow can be as low as 10 BCM (Department of Water Resources, River Development and Ganga Rejuvenation, 2024). Based on the findings of Kumar et al. (2014), the Cauvery River water quality was assessed to be pure. Physicochemical tests in line with the WHO guidelines for drinking water can be utilized to determine the quality of river water. The physicochemical and microbial diversity studies of the water sample from the Vellar River indicated that it is heavily contaminated with pathogens, and its quality is classified in the D (poor) category (Yoganandan et al., 2020). The evaluation of the water quality aspect of environmental science provides insights into the suitability of water for various uses. To assess real surface water quality scenarios, specific guidelines for constructing an ideal water quality index (WQI) model are crucial (Jain et al., 2022; Uddin et al., 2021). Water quality is a classification system used to assess water based on its level of cleanliness or contamination (Dojlido et al., 1994). Recent advancements in this field have seen the incorporation of statistical methods, machine learning algorithms, and remote sensing techniques to enhance the predictive power and accuracy of water quality assessments (Chidiac et al., 2023; Dey & Vijay, 2021; Sarang et al., 2023). When compared with other regions, such as European countries with the highest water quality scores according to the Environmental Performance Index, the Cauvery River faces more significant challenges. These countries typically have lower population densities, stricter environmental regulations, and more advanced water treatment facilities, contributing to their higher water quality scores (Dell’Uomo & Torrisi, 2011). Many researchers have analyzed the physicochemical parameters in water by calculating the electrical conductivity (EC) of water because it is the cheapest and best way to determine its pollution level, and its value is dependent on the concentration, mobility, and temperature of the impurities (Senthilmanickam et al., 2016; Shrestha et al., 2017; Udhayakumar et al., 2016). Shrestha and Basnet (2018) determine the factors that contribute to EC and estimate their percentage contributions. Hema and Subramani (2013) reported that the water quality in the Cauvery River is influenced by industrial and anthropogenic activities. Industrial activities can release various pollutants, such as heavy metals, chemicals, and organic matter, which can contaminate water bodies and affect the health of aquatic life and humans who rely on the water. During floods, rivers undergo changes in water quality because of increased runoff, sedimentation, and the introduction of pollutants. Several researchers collected water samples during flood events and analyzed them for some parameters, such as turbidity, pH, dissolved oxygen, nutrients, heavy metals, and organic contaminants. These analyses help us understand how floodwaters impact water quality and identify potential risks to human and ecological health.

From a detailed literature survey, we inferred that a comprehensive understanding of the chemical concentration and the level of contamination with time series records of the Cauvery River during flooding time is lacking. Here, we attempted to conduct a time series-based analysis during flooding time to understand the real-time changes. This study may help us understand the impact of a flood on water quality and identify potential sources of pollution.

2 Study area

The Cauvery River is one of the most important rivers in peninsular India (Hema & Subramani, 2013). The Cauvery River flows from its origin point, Talacauvery, to Arkavathi Sangama near Kanakpura in Karnataka. From its origin point, the Cauvery River flows through the states of Karnataka and Tamil Nadu, traversing a total distance of approximately 800 km (497 mi) before reaching its endpoint at the Bay of Bengal. Our study area covers 10°52′21.82″N latitude and 78°56′7.5″E longitude in Kollidam River Dam, Thanjavur (Fig. 1). Tamil Nadu, located in the southern part of India, experiences two main monsoon seasons, i.e., the southwest monsoon, which occurs from June to September, and the northeast monsoon, which occurs from October to December. Compared with other regions, the Cauvery River basin is heavily influenced by the monsoon seasons. The southwest monsoon brings rainfall to most parts of Tamil Nadu, whereas the northeast monsoon primarily affects the coastal regions of the state. During monsoon seasons, heavy rainfall in the catchment areas of the river can lead to flash floods and riverine floods, causing damage to infrastructure, crops, and livelihoods of people living in the flood-prone areas. The annual rainfall in Tamil Nadu is approximately 613 mm, with variations across different regions. Tamil Nadu also experiences other weather patterns, such as cyclones, which can cause heavy rainfall and flooding in the state. In the monsoon season, lasting from June to September, the flow of the river is strong and can reach a discharge rate of 30,000 m3/s. During the dry season, lasting from January to May, the river flow is considerably lower, averaging at approximately 1000 m3/s. From June 2022 to September 2022 an excess of 239 tmcft of Cauvery River water was released to Tamil Nadu.

Fig. 1
figure 1

Study area map shows the Cauvery River basin in Thanjavur district, Tamil Nadu

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3 Materials and method

The surface water samples were collected at 6-h intervals for 6 days (from August 9 to 14, 2022) during and after flooding time from the Kollidam River basin of the Cauvery River (Fig. 1). The collected samples were kept in a 1 L polyethylene bottle. Then, 0.02% of HgCl2 was added to the samples to arrest microbial activity. The water samples were transported to the laboratory for physicochemical and nutritional parameter analyses.

The physiochemical and nutritional parameter analyses of the pH, electrical conductivity (EC), total dissolved solids (TDS), dissolved oxygen (DO), temperature, carbonate (CO3), bicarbonates (HCO3), chlorides (Cl), magnesium (Mg), iron (Fe), calcium (Ca), sulfate (SO4), nitrates (NO3), nitrite (NO2), sodium chloride (NaCl), phosphates (PO4), total hardness (TH), fluoride (F), dissolved ammonia (DA), orthophosphate (OP), alkalinity (A) of the water samples were conducted using standard methods (APHA, 2005). The tested physicochemical and nutritional parameters of the river water samples were compared with the WHO Water Quality Index (WQI).

The water quality formula (Brown et al., 1972) calculated the unit weight (Wn) factors for each parameter using the following formula.

$$W_n=K/S_{n,}$$

where the value of K is K = 1/ 1/S1 + 1/S2 + 1/S3 + ……… 1/Sn = 1/Σ1/Sn (Kumar & Singh, 2021), in which Sn is the standard desirable value of the nth parameters, upon the summation of the unit weight factor of all selected parameters, i.e., ΣWn = 1. 

The sub-index (Qn) value is calculated using the following formula:

$$Q_n=\left[\left(V_n-V_0\right)\right]/\left[\left(S_n-V_0\right)\right]\ast100,$$

where, Vn is the mean concentration of the nth parameters, Sn is the standard desirable value of the nth parameters and V0 is the actual value of the parameters in pure water (generally, V0 = 0 for most parameters, except for pH) (Jain et al., 2022).

$$Q_{pH}=\left[\left(V_{pH}-7\right)\right]/\left[\left(8.5-7\right)\right]\ast100$$

Combining Steps 1 and 2, WQI can be calculated as follows:

$$Overall\;WQI=\frac{\Sigma W_nQ_n}{\Sigma W_n}$$

4 Results and discussion

This study assessed the water quality parameters during flooding time in the Cauvery River basin. Through statistical-trend–based and time–series–based analyses, the researchers evaluated the spatial and temporal variations in water quality. Our study found that urbanization along the river banks, excessive groundwater use, and human inputs were the main contributors to water pollution in the Cauvery River basin. The findings of Chakraborty et al. (2023) showed that urbanization along the river banks is a major contributor to water pollution in the Cauvery River, along with excessive groundwater use and human inputs. Moreover, their study identified chemical inputs from leaks and cross-connections in the water supply system as another significant factor leading to deteriorating water quality. Furthermore, the study conducted by Keerthan et al. (2023) found that anthropogenic activities such as industrial waste disposal and agricultural runoff were the main sources of heavy metal contamination in groundwater. Yoganandan et al. (2020) evaluated the water quality and microbial diversity of Vellar River and concluded that Vellar River falls into the D (poor) category. The current study investigates various water quality parameters by conducting tests of the physicochemical, chemical, and nutritional parameters.

4.1 Physiochemical test

We analyzed various physicochemical parameters in the collected water samples. The most important physicochemical parameter is pH. The average pH noted in flooding time is 8.24, with a range of 7.9 to 8.9 (Fig. 2a). After flooding time, the average pH is 8.09. According to the WHO, a pH range of 6.5–8.4 is considered safe for human consumption (Yehia & Said, 2021). According to Hall et al. (1977), a close examination of the pH values revealed that metals, such as cadmium and zinc, may have a greater negative impact on the environment when the pH level is lower, specifically at a pH of 6.5. pH and temperature will vary in every season (Vijayan et al., 2018). The temperature of the entire study period ranges between 28°C and 32°C (Fig. 2b). The interval between flooding time and temperature increases after the temperature becomes stable. The main factors that vary in river water temperature are the season, location, timing of the sampling, and temperature of the effluents entering the stream, as highlighted in a study conducted by Hall et al. (1977).

Fig. 2
figure 2

Physiochemical parameters during flooding time and after flood. (a) pH; (b) temperature; (c) dissolved oxygen; (d) EC; (e) TDS. The cyan colour shade part shows the after-flood value. Four lines represent different times of the day. The pink line represents 6 am, the blue line represents 12 am, the green line represents 6 pm, and the orange line represents 12 pm, respectively

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Dissolved oxygen (DO) refers to the amount of gaseous oxygen (O2) that is dissolved in water. DO is an important indicator of the capability of water to support aquatic life (Chandra et al., 2011; Udhayakumar et al., 2016; Vijayan et al., 2018). In our study, we observed that, at 12 pm and on Tuesdays, the DO value suddenly increases (Fig. 2c). However, after a flood event, the DO value suddenly decreases. This information highlights the impact of time, day, and environmental events on water quality. During flooding time, the DO value ranges from 5 mg/L to 7 mg/L. After flooding time, the DO value is similar to that of a recent study of the water quality of the Cauvery River, i.e., 5.59 mg/L (Appavu et al., 2016). Electrical conductivity (EC) is a measure of the capability of water to conduct electricity. The average EC value in the study area is 220 mg/L, with the maximum and minimum EC values of 243.0 mg/L and 114.5 mg/L respectively (Fig. 2d). This finding indicates that the study area is affected by the presence of dissolved ions in the water. In a previously published paper, the minimum EC value recorded in the Cauvery River is 98.6 mg/L (Appavu et al., 2016; Chandra et al., 2011), whereas, in the current study of the Cauvery, Kollidam River basin, the maximum EC value is 243 mg/L, which shows that the water is more polluted and is unsuitable for irrigation. As the EC value of the soil solution increases, the availability of usable water for plants decreases significantly because plants can only transpire pure water (Joshi et al., 2009). Solaraj et al. (2010) analyzed the monthly variation of physicochemical parameters in the Cauvery delta region, and their results indicated p < 0.001. Measuring the total dissolved solids (TDS) level in drinking water is essential to identify organic and inorganic compounds in our study area. On average, the TDS level is 157.1 mg/L, with the highest and lowest levels being 172.0 and 105.3 mg/L, respectively (Fig. 2e). Patil et al. (2012) proposed that the increased TDS value can cause undesirable taste, gastrointestinal irritation, corrosion, or incrustation. After flooding time, the average TDS level is 296 mg/L. Studies of the Cauvery River water around the KRS Dam indicate that the maximum TDS value is 340 mg/L because of wastewater in downstream (Gholami & Srikantaswamy, 2009). As evident from the major ion data, flood events exhibit a notable but fluctuating contribution from local water sources, such as groundwater (Amoros & Bornette, 2002).

4.2 Chemical parameters

Chloride in river water can come from various sources, such as natural mineral deposits, seawater intrusion, or airborne sea spray. The average chloride concentration during flooding time is 1.91 mg/L, with a maximum value of 2.9 mg/L, and a minimum value of 1.3 mg/L. The WHO recommends a human permissible limit of 200 mg/L for chloride. After flooding time, the chloride value concentration will increase, with an average value of 4.03 mg/L (Fig. 3a). Hema and Subramani (2013) conducted a study of the Cauvery River basin and determined the average chloride levels during two monsoon periods, i.e., 75.4 mg/L in the post-monsoon period and 34.1 mg/L in the pre-monsoon period.

Fig. 3
figure 3

Chemical parameters during flooding time and after flood. (a) chloride; (b) calcium; (c) fluoride; (d) iron; (e) magnesium; (f) NaCl. The cyan colour shade part shows the after-flood value. Four lines represent different times of the day. The pink line represents 6 am, the blue line represents 12 am, the green line represents 6 pm, and the orange line represents 12 pm, respectively

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Our results show calcium levels ranging from 1.5 mg/L to 4.3 mg/L (Fig. 3b), with the human permissible level of calcium being 75 mg/L (Table 1). Compared with the WHO recommended flooding time calcium levels, our results are low because of the heavy load of nutrition. Therefore, the levels of these minerals in river water need to be monitored to ensure that they remain within safe limits. Agricultural activities are often concentrated near rivers as floodplains are known for their high fertility resulting from the deposition of abundant nutrients in the soil during river overflow (Kumar et al., 2014).

Table 1 Average value of physiochemical, chemical, and nutritional parameters in comparison to the recommended value by the WHO and BIS
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Fluoride is a naturally occurring mineral that can be found in various sources, including river water. The average fluoride content during flooding time is 11.5 mg/L, with the maximum fluoride content 11.6 mg/L obtained on Thursday. After flooding time, the fluoride ranged between 0.1 mg/L and 0.4 mg/L (Fig. 3c). In the study area, the fluoride content was higher than the WHO and Bureau of Indian Standards (BIS) human consumable levels (Table 1). Excessive levels of fluoride in drinking water can be harmful to human health. Srinivasamoorthy et al. (2012) reported that high levels of fluoride can cause dental fluorosis, a condition that affects the appearance and strength of teeth. Shankar et al. (2022) proposed the use of groundwater quality to generate models for assessing water quality. In their study, the groundwater fluoride level in Thanjavur District was determined to be 0.35 mg/L. By contrast, in our study area, the fluoride level was 12 mg/L because of flooding. Floods can increase the fluoride levels, which, in turn, decreases the groundwater quality. Iron and calcium are important minerals that can be found in river water. During flooding time, iron ranges between 11.3 mg/L to 11.8 mg/L (Fig. 3d), and the WHO recommended human permissible level of iron is 0.1 mg/L. After flooding time, high levels of contamination with iron from various industrial sources may affect the groundwater.

Magnesium is an important mineral that can be found in river water. For drinking water, the BIS recommended that the magnesium content range between 0 mg/L and 50 mg/L. The average magnesium content during flooding time is 2.26 mg/L, with a maximum value of 2.9 mg/L and a minimum value of 1.2 mg/L (Fig. 3e). After flooding time, the magnesium content was low in the 6-h interval. Gholami and Srikantaswamy (2009) observed increased hardness at downstream stations, which indicates significant solubilization of magnesium in the downstream waters of the Cauvery River. During flooding time, the concentration of sodium chloride (NaCl) in the water of the Kollidam River basin is expected to decrease, with NaCl levels ranging from 2.92 mg/L to 8.76 mg/L (Fig. 3f).

Alkalinity is an important parameter in river water that needs to be monitored because it can affect the pH of the water and impact aquatic life. The average alkalinity in the study period is 0.3 mg/L with a maximum value of 0.5 mg/L (Fig. 4a). The average value of bicarbonate level during flooding time is 5.9 mg/L, and that after flooding time is 0.45 mg/L (Fig. 4b). The Cauvery River water bicarbonate value is lower than the WHO standard range for drinking water. In general, bicarbonate in drinking water first increases and then decreases, and is not considered to have any health and environmental effects. According to study of the Uyyakondon channel of the Cauvery River conducted by Jameel and Hussain (2009), the levels of carbonate and bicarbonate were determined to be 500 and 606 ppm, respectively, which exceed the safe levels for human consumption.

Fig. 4
figure 4

Chemical parameters during flooding time and after flood. (a) alkalinity; (b) bicarbonate; (c) carbonate; (d) sulfate; (e) total hardness. The cyan colour shade part shows the after-flood value. Four lines represent different times of the day. The pink line represents 6 am, the blue line represents 12 am, the green line represents 6 pm, and the orange line represents 12 pm, respectively

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Carbonate content in water is an important aspect of water quality that affects both human health and the environment. The average carbonate content in the study area is 0.23 mg/L (Fig. 4c). The maximum carbonate content is high at 6 pm (i.e., 0.4 mg/L). Carbonate is important because it affects the pH of water. Carbonate acts as a buffer, which means that it can help stabilize the pH of water. This is important because changes in pH can have negative effects on aquatic life and can affect the taste and appearance of drinking water (Patil et al. 2012).

During flooding time, alkalinity was low because of the continuous flow of water. Many rivers contain significant amounts of sulfate, which is important for several reasons, including its role in supporting cellular function and aiding in the absorption of nutrients. The average level of sulfate in the study area is 0.145 mg/L, with a maximum value of 0.743 mg/L and a minimum value of 0.04 mg/L (Fig. 4d). High levels of sulfate have been linked to an increased risk of certain health conditions, such as kidney stones.

The total hardness (TH) of the water of the Cauvery River averaged 101 mg/L, with a maximum value of 112 mg/L and a minimum value of 84 mg/L (Fig. 4e). Matić et al. (2016) measured the concentration of calcium and magnesium ions in the water of the Drava River in Croatia, calculated the TH using a standard method and determined that the TH of the water ranged from 135 mg/L to 310 mg/L. During flooding time, the water sample from the Cauvery River is classified as moderately hard according to the WHO guidelines for drinking water quality. After flooding time, the TH of the water decreased, with an average value of 18.6 mg/L (Fig. 4f; Table 1).

4.3 Nutritional parameters

Regular monitoring of phosphate levels in river water is important to ensure that the water quality is maintained and any necessary actions can be taken to prevent eutrophication and protect aquatic life. The phosphate range in river water is between 0.026 mg/L and 0.2 mg/L (Fig. 5a). Compared with the WHO and Bureau of Indian Standards (BIS) human consumable limit, our study revealed that 25% of phosphate is present in floodplains (Table 1). Orthophosphate is a common phosphorus found in river water. The maximum orthophosphate value in Cauvery River water is 0.33 mg/L; this high level of orthophosphate in river water can indicate pollution from human activities such as sewage discharge or industrial runoff, and can cause an overgrowth of algae and aquatic plants, which can deplete the oxygen levels in the water and harm aquatic plants. An average of 0.13 mg/L of orthophosphate is present in floodplains (Fig. 5b).

Fig. 5
figure 5

Nutritional parameters during flooding time and after flood. (a) phosphate; (b) orthophosphate; (c) ammonia; (d) nitrate; (e) nitrite. The cyan colour shade part shows the after-flood value. Four lines represent different times of the day. The pink line represents 6 am, the blue line represents 12 am, the green line represents 6 pm, and the orange line represents 12 pm, respectively

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Our results showed that the average value of ammonia is 4.47 mg/L, with a maximum value of 10.2 mg/L and a minimum value of 0.8 mg/L (Fig. 5c). Floodwaters can contain high levels of nitrogen in the form of nitrate and ammonia. Ammonia can be found in river water from natural sources, such as the decomposition of organic matter, and anthropogenic sources, such as agricultural runoff and discharge from wastewater treatment plants. These nutrients can contribute to the growth of harmful algal blooms, which can be toxic to aquatic life and humans. Nitrate concentrations in the water range from 0.73 mg/L to 0.83 mg/L, with an average of 0.79 mg/L (Fig. 5d). In the current study, the nitrate value is 1.5% compared with the WHO value, indicating a decrease that can be attributed to high freshwater input or dilution from rainfall (Table 1). Nitrites can be rapidly converted into nitrates through the nitrification process, which is carried out by bacteria. In well-aerated river systems, nitrites may be rapidly converted into nitrates, resulting in low nitrite concentrations (Hill, 1996). Andrade et al. (2018) reported that riverine floodplain water bodies have the potential to impact groundwater quality and lead to contamination. During flood events, water from the river can infiltrate the surrounding soil and interact with the underlying groundwater.

In the current study, the average value of nitrite in the Cauvery River is 2.11 mg/L with a maximum value of 2.9 mg/L and a minimum value of 1.2 mg/L (Fig. 5e). The results of these analyses can be used to assess the overall health of the river ecosystem and identify potential sources of pollution that need to be addressed. Finlay (2020) discussed potential solutions to reduce nitrate pollution in rivers, including best management practices in agriculture, wetland restoration, and targeted policy interventions. The nitrite content in river water can vary depending on several factors, including the sources of pollution in the river and the overall health and biodiversity of the ecosystem.

4.4 Relationship between EC and physicochemical parameters

To assess the relative impact of physicochemical parameters on EC, correlation and regression analyses of the parameters of river water were conducted. The correlation matrix was generated from 12 parameters: TDS, fluoride, total hardness, magnesium, phosphate, NaCl, alkalinity, pH, calcium, iron, temperature, and chloride. The results showed some positive, moderately positive, and negative correlations among various physicochemical parameters. EC is positive correlated with TDS (R2 = 0.2480), chloride (R2 = 0.0983), iron (R2 = 0.0082), NaCl (R2 = 0.0149), TH (R2 = 0.5119), and phosphate (R2 = 0.0741); the greater impact of these individual parameters on EC could be attributed to their strong correlation and higher concentration in water (Shrestha & Basnet, 2018). EC is negatively correlated with pH (R2 = 0.0723), temperature (R2 = 0.1591), fluoride (R2 = 0.1064), calcium (R2 = 0.0186), alkalinity (R2 = 0.0180), magnesium (R2 = 0.0071). Among them, the correlations of iron, NaCl, calcium, alkalinity and magnesium with EC are relatively weak (Figs. 6 & 7). The correlation between EC and physicochemical parameters can be used as an indicator of water quality because it is influenced by the concentration of ions in the water and the impacts of pollution in aquatic environments. Our findings are consistent with the results of the research conducted by Shrestha and Basnet (2018), which indicated that 39.65% of TDS and 19.9% of TH are responsible for EC. In the water body, the main natural sources of heavy metals are chemical leaching of bedrock, water drainage basins, and runoff from river banks (Klavinš et al., 2000). In summary, studies of the flooding time of river water are essential for environmental monitoring as they provide critical information about water quality, hydrological dynamics, ecological impacts, and climate change effects.

Fig. 6
figure 6

Correlation between EC and pH, temperature, TDS, fluoride, chloride, and iron in the Cauvery River

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Fig. 7
figure 7

Correlations between EC and calcium, sodium chloride, alkalinity, total hardness, phosphate, and magnesium in the Cauvery River

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4.5 Water quality index

According to Shankar et al. (2022), the WQI of Thanjavur District shows that the Sengipati, Budalur, and Pudupatti regions have poor water quality. Studies of the irrigation indices showed that groundwater was also unsuitable for irrigation. Our WQI indicated that, during flooding time, the maximum value of WQI is 568.1100, and the minimum value of WQI is 103.4869 (Table 2). We observed a consistently high level of contamination in the Cauvery River at 12 pm in the time series record. Such time series records are valuable for monitoring contamination levels. During floods, the dilution of pollutants can not only temporarily improve water quality, but can also introduce new contaminants from runoff (RamyaPriya & Elango, 2022). Long term trends can reveal improvements or deteriorations in water quality over time and help understand the periodic characteristics of water quality variation (Jiang et al., 2020). Our findings emphasized the importance of treating groundwater before consumption and implementing measures to protect it from potential contamination hazards.

Table 2 Water quality analysis of flood water in Cauvery River basin
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5 Conclusions

This study emphasizes the importance of physicochemical parameters in assessing the quality of river water. Nutrient loads in floodwater can have harmful effects on the environment and human health, thus, these impacts need to be managed and mitigated through proper land use practices and infrastructure planning. We understand that the carbonate content in water is crucial to maintaining good water quality and protecting both human health and the environment. During flooding time, the levels of iron, fluoride, pH, and total hardness increase due to the excessive nutrient load. Finally, our finding indicates that flooding groundwater can cause a decrease in water quality and an increase in contamination levels, which can have detrimental effects on both the environment and human health. By monitoring and understanding these aspects, decision-makers can make informed choices regarding flood risk management, water resource planning, and conservation efforts to protect both human health and ecological systems.