Water criteria evaluation for drinking and irrigation purposes: a case study in one of the largest rivers of Sundarbans World Heritage region

Water is a vital natural resource for the survival of human beings and other living organisms. Human needs such as drinking, domestic and agriculture activities depend greatly on water (Ehya & Saeedi 2019). Global water use is presently categorized as 10% for domestic, 70% for agriculture (mostly irrigation), and 20% for industrial uses (Boretti & Rosa 2019). The global demand is expected to grow at an alarming rate in all the sectors i.e. domestic, agriculture, and industry. Water demand for various purposes is projected to increase by 20–30% by 2050 (Boretti & Rosa 2019). With the increasing growth of population throughout the world, this extra demand for quality water needs to be fulfilled to accomplish human activities. While water demand is on the rise, the availability of freshwater is shrinking due to pollution and reduced sources. As a result, 12% of the global population use drinking water from unsafe sources (WHO/UNICEF Joint Water Supply and Sanitation Monitoring Programme 2015; Boretti & Rosa 2019). The scarcity of water is already faced by many countries including Bangladesh; many more are expecting to face optimum water availability by 2050 (Veldkamp et al. 2017). In this situation, least developed countries, such as Bangladesh, need to search for alternative sources of surface water, especially in the Sundarbans World Heritage region, which will fulfill the requirements for fresh water to meet the low living standards of the people of that area.

Confirming water quality for domestic, drinking, and agriculture purposes is paramount and desirable, since polluted water resources have detrimental health impacts and significantly affect overall human wellbeing and the environment (Egbueri et al. 2019). Furthermore, the quantity and quality of agriculture products can also be greatly affected if poor quality water is utilized on farms. Thus, examining water quality and its factors is most expedient for the sustainability of water resources. Many scientific methods such as stoichiometry, hydrochemistry, multivariate statistics, and numerical models, have been effectively exploited in water quality assessments (Subba Rao et al. 2019; Egbueri & Mgbenu 2020; Rao et al. 2021). Hydrogeochemistry shows the main chemical ions in water and multivariate statistical techniques are widely used tools for pollution source identification. Water quality indices such as water quality index (WQI), sodium percentage (Na%), sodium adsorption ratio (SAR), magnesium hazard (MH) and residual sodium carbonate (RSC) are widely accepted mathematical ratings which delineate the overall water quality (Egbueri et al. 2019, 2021) Comprehensive and sophisticated studies on the assessment of water quality across the world have been conducted for industrial uses, irrigation purpose and human consumption (Haghiabi et al. 2018; Ezugwu et al. 2019; Son et al. 2020; Agbasi & Egbueri 2022; García-Ávila et al. 2022).

Surface water usually occurs on the earth's surface as precipitation, snow, ice, streams, ponds, lakes, rivers, and wetlands (Chigor et al. 2012). Surface water from rivers, ponds, and reservoirs are considered as the most reliable sources to meet human needs in Bangladesh, which are accelerating significantly with social, economic, cultural, and various industrial development. The rivers are the main sources of water supply for household, irrigation, and industrial consumption in some parts of Bangladesh, as the underground water level in that area has been continuously downgrading over the past few years due to excess use for various purposes (Bhuiyan et al. 2011; Islam et al. 2020). Unfortunately, water pollution has been continuously worsening for the last three decades, affecting almost all the rivers in Bangladesh (Bhuiyan et al. 2011). Over 30% of the country's population lives without sanitation facilities, and approximately 80% of sewage as well as industrial effluents from the state also discharge as untreated into landfill (Nelson et al. 2014; Islam et al. 2018; Alam & Mondal 2019). Now is the right time to take the necessary steps to utilize hundreds of rivers throughout the country for fulfilling the future need for fresh and clean water throughout the year. Therefore, proper monitoring, evaluation, and management of the water quality are necessary to save the river water from pollution, which will eventually ensure a cost-free source of water to the rural people for use in various purposes. In order to establish well-managed water sources for industry, agriculture, drinking purposes, and for the protection of aquatic life, river water quality monitoring through physicochemical, hydrogeochemical, and graphical representations has been exploited effectively (Etteieb et al. 2017). The key focus of this study is to examine the chemical contamination and the suitability of ground water for irrigation and drinking purposes in the countryside district using multiple indices approaches such as WQI, Pearson's correlation, SAR, Na%, RSC, and MH.

This study examined the water quality of Pasur river, one of the largest rivers, and important source of surface water in the Sundarbans region, situated in the southwestern part of Bangladesh. Most of the inhabitants of the Pasur river basin area are dependent on the river for drinking water as well as agriculture works. Classical scientific techniques and indicators such as WQI, Na%, SAR, MH, RSC were investigated to reach the research objectives. The physicochemical properties, drinking water chemistry, and agriculture criteria of the river water were examined and compared with the standards of WHO (2004) as well as other water quality indices for drinking and irrigation applications.

The Pasur river (21 ° 45′ N, 89 ° 30′ E), one of the biggest rivers in the Sundarbans region, is located on the second largest seaport in Khulna city in the southwestern part of Bangladesh. It departs the Madhumati river northeast of Khulna city and moves 177 kilometers southward of Mongla port and through the Sundarbans areas into the Bay of Bengal (Ali et al. 2018). The average flow of the river fluctuates between 213 cubic meter/second (m/s) in pre-monsoon (January–March) and 1,680 cubic m/s in post-monsoon season (August to October). The region experienced humid equatorial tropical climate with temperature ranging from 22 °C to 35 °C and an annual rainfall of 1,600 to 1,800 mm along with occasional severe cyclones (Rahman et al. 2013). The humidity normally varied from 70 to 90% in the region. Approximately 1 million people are living in villages in the Pasur river basin area. More than 80% of the inhabitants are living in abject poverty, and they mostly depend on the river water throughout the year for drinking and irrigation (Islam et al. 2018). A great number of inhabitants depend on agriculture for their livelihood and most of the local residents have been using ground water in agriculture and household applications, resulting in lowering of the water table. In this countryside, unmonitored use of agrochemicals and poor treatment of agricultural byproducts were suspected to be a probable key factors for water pollution in the region.

Six sampling locations were selected on the basis of activities of people on the banks of Pasur river as well as agricultural activities. The sampling stations of the Pasur river area are shown in Figure 1. The water samples were collected from three locations in Dacope Upazila (Bajua S1, Laudove S2, Banishanta S3) and three locations in Mongla Upazila (Mongla ferry ghat S4, Kanainagar S5, Karamjol S6) during pre- and post-monsoon seasons. Water samples were collected in the month of February during pre-monsoon, and September during post-monsoon in December 2020. Water samples were collected in polypropylene bottles (1,000 ml) previously washed with distilled water and acidic solution, and dried. Samples were manually collected at a depth of 25 cm, preferably from locations with high water flow, to ensure homogeneous samples. The samples were then transferred to the laboratory of Dexterous Engineering Bangladesh following the guidelines of American Public Health Association (APHA) at favorable temperature (<4 °C). Electrical conductivity (EC), total dissolved solids (TDS), and pH of the water samples were measured at the sampling site with conductivity/TDS meter (Hach-HQ14D, USA), and pH meter (HANNA, USA), respectively (Brahman et al. 2013). The turbidity was measured with a turbidimeter (HACH 2100P, USA). Total hardness (TH) as CaCO₃ was measured by EDTA titration method using Eriochrome Black T as indicator. Titrimetric method was also followed to measure the concentration of bi-carbonate. The major anions concentration (Cl⁻, ⁠, ⁠, F⁻ and ⁠) were measured through ion chromatography (IC) (Shimadzu LC10ADvp, Japan). The concentration of major cations (Na⁺, K⁺, Ca²⁺, Mg²⁺, and Fe²⁺) was carried out by flame atomic absorption spectrophotometer (FAAS). Biochemical oxygen demand (BOD), and chemical oxygen demand (COD) measurements were also carried out according to the standard procedure for water and wastewater of described by Basheer et al. (2017).

The hydrochemical characteristics of Pasur river water were analyzed through construction of a Piper diagram. The criteria of Pasur river water chemistry across Sundarbans World Heritage region was established using Table 1. The diagram exhibits the ionic composition of river water samples to distinguish the major trends of data during pre and post monsoon seasons (Logeshkumaran et al. 2015; Ehya & Saeedi 2019). Grapher 15.0 scientific software was used for the plotting of the Piper trilinear diagram. The major cations and anions composition of Pasur river water was analyzed after assessing geochemical evaluation by using this diagram. Then the diagram was used to classify the river water quality by the distribution of important cations like Na⁺, K⁺, Ca²⁺, Mg²⁺ and main anions like Cl⁻, ⁠, and ⁠.

The Gibb's diagram can be used to determine not only the dominant factors and hydrochemical processes of water, but also the source of dissolved ions in the river water. By using the Gibb's plot, effects of processes that determine water composition such as evaporation and precipitation, rock dominated weathering, and sea water can be determined (Logeshkumaran et al. 2015; El-Aziz & Hassanien 2017).

Pearson's correlation coefficient is a statistical technique used to compare the level of associated variables (El-Aziz & Hassanien 2017; Shil et al. 2019). The matrix measures the strength of a linear association between two continuous variables. The correlation coefficient ranges from −1 to +1, where +1 represents a perfect positive relation, and −1 indicates a perfect negative association between the variables. A coefficient of 0 (zero) indicates the absence of dependency between the variables, and values greater than 0.7 (r > 0.7) are considered an indicator of strong dependency of one variable on the other.

Inappropriate salinity in the water is considered as one of the main criteria of the river water, which may create many adverse effects on agricultural fields. Various types of salt may enter into the agricultural fields with irrigation water, causing high salinity as well as reduction of crop yield in any area (Zhang et al. 2019). The critical physicochemical properties such as hardness, pH, calcium, magnesium, sodium, potassium, chloride, sulfate, bicarbonate, nitrate, carbonate, EC, and TDS determine the feasibility of the river water for irrigation (Haritash et al. 2016). Water assessment indices such as SAR, Na%, RSC, and MH ratio were also calculated to assess the suitability of the Pasur river water for irrigation uses (Adimalla et al. 2018).

The Wilcox diagram which was proposed by Wilcox in 1948 was used to evaluate the probability effects of the river water on the soil for irrigation activities in the area. At present, it is widely utilized for the classification of waters for agriculture purposes. The major factors in this diagram are SAR and EC. The vertical axis of the Wilcox diagrams shows the SAR and the horizontal axis shows salinity of water. Salinity is calculated by EC and SAR. Besides, the US Salinity Laboratory diagram assists in identifying the salinity impacts of groundwater in the agriculture fields. A groundwater data plot on the US Salinity Laboratory diagram is used, in which the SAR is considered an alkalinity hazard and EC a salinity hazard.

The concentrations are measured in meq/l.

The concentrations are measured in meq/l.

The hydrochemical parameters of water samples collected from Pasur river during pre- and post-monsoon seasons were characterized, and the statistics are demonstrated in Tables 2 and 3. The results showed that calcium (Ca²⁺) was dominating among the cations in both pre- and post-monsoon seasons. The concentration of Ca²⁺ exceeded the permissible limits of WHO (2004) of sampling station S4 during pre- and post-monsoon seasons. Another major cation, Mg²⁺ was observed which exceeded the limit (150 mg/L) of WHO (2004) at S2 and S4 locations during both the seasons. The concentration of K⁺ of all the water samples was found higher than the WHO standards (2004) in both the seasons, while the concentration of Na⁺ was seen above the permissible limits on locations of S2 and S4 only in pre-monsoon. Iron concentrations fall with in the permissible limits of WHO (2004) during both the seasons, and contributed less than 1% of the total ion concentration. On the other hand, Cl⁻ was the major dominating ion during both the seasons in the Pasur river water among anion measurements. The concentrations were within the acceptable limits in both the seasons. Other major anions, ⁠, nitrate ⁠, and ⁠, were found within the acceptable limits in all the sampling locations during both seasons. Fluoride and phosphate have the lowest contribution among all the anions, and accounted for less than 1% of the total ion concentration.

The results also showed that pH values of Pasur river water ranged from 7.1 to 8.1 in both the seasons, which fall with in the WHO (2004) permissible standard. The value of EC indicated that the concentration exceeded the maximum permissible limit of 1,500 μS/cm of WHO (2004) in locations S2 and S4 only in pre-monsoon season. The TDS values of river water samples were found within the WHO (2004) permissible limits in both the seasons. The values of water hardness varied from 243 to 352 mg/l, and from 155 to 302 mg/l in pre-monsoon and post-monsoon, respectively, which fall with in the WHO (2004) acceptable range. The water turbidity values fall in the acceptable range at all sampling locations during post-monsoon, while the values crossed the permissible limits (10 NTU) according to WHO (2004) in S2 and S4 locations during pre-monsoon season. The results of BOD and COD of Pasur river water showed that the values exceeded the standard limits in both the seasons (Tables 2 and 3).

Pearson's correlation matrix was carried out to investigate the relation among various parameters in Pasur river water. The actual values of the important parameters of the water samples were taken for statistical analysis to evaluate the relation among the variables as represented in Tables 4 and 5. The correlation coefficients of the pH of the Pasur river water indicated that the alkalinity of water is greatly dependent on bicarbonate ion (r = 0.64) in pre-monsoon, and it displayed a strong correlation (r > 0.70) with Ca²⁺, ⁠, Cl⁻, and ion in post-monsoon season. The correlation matrix analysis showed very strong positive correlations (r ≥ 0.70) exist between EC and other parameters, TDS, TH, Ca²⁺, Na⁺, Mg²⁺, and Cl⁻ during pre-monsoon, while TDS, TH, Fe²⁺, and influenced positively to increase the value of EC during post-monsoon. The higher correlations values (r ≥ 0.70) between the parameters and EC indicated that the EC value of Pasur river water was affected by the ions. Ca²⁺, Mg²⁺, Na⁺, Cl⁻, ⁠, and were the major ions that cause the higher salinity in river water. The TDS showed high positive correlations (r ≥ 0.70) with TH, Na⁺, Ca²⁺, Mg²⁺, and, Cl⁻ in pre-monsoon, while in post-monsoon season TDS values were highly dependent on Na⁺, K⁺, and ⁠. The positive correlations of TH were found higher with Na⁺, Ca²⁺, Mg²⁺, and in pre-monsoon, and with Na⁺, Ca²⁺, ⁠, ⁠, and in post-monsoon seasons. High positive correlations (r ≥ 0.70) were observed between Ca and Na in both the seasons. It was also observed that Mg²⁺ has good correlations (r ≥ 0.5) with Cl⁻, ⁠, and in pre-monsoon and only with in post-monsoon season. On the other hand, K⁺ has good positive correlations (r ≥ 0.70) with and in pre-monsoon, and with in post-monsoon season. Na⁺ also has good positive correlations (r ≥ 0.50) with Cl⁻ and in both seasons. The results indicated that various types of salts in Pasur river water dominated depending on seasons, as the activities of the local people were not same throughout the year.

The results indicated that Ca²⁺ was the major cation in the Pasur river water during both the seasons as shown in Tables 2 and 3. Water samples from S2 and S4 locations in pre-monsoon and S4 location in post-monsoon exceeded the maximum permissible limit of Ca²⁺ (200 mg/L) in drinking water according to WHO (2004). High concentration of Ca²⁺ in those two locations occurred due to pollution from untreated municipal effluents as well as unplanned development of a few industries and local housing in those locations. Some cement industries and oil refinery industries were established very close to the locations of S2 and S4, which mainly contributed to the increased concentration of Ca²⁺ in river water (Wahid et al. 2007). It was reported that incomplete treatment of wastewater could increase the level of Ca²⁺ by more than 30% in the surface water (Potasznik & Szymczyk 2015). Usually, Ca²⁺ contributes to the water hardness significantly and a high concentration of Ca²⁺ causes stomach problems and makes the water unsuitable for domestic use due to scale formation and encrustation (El-Aziz & Hassanien 2017).

The results also indicated that Na⁺ was another dominating cation in Pasur river water during both seasons. The maximum permissible limit of Na⁺ in drinking water was 200 mg/L according to WHO (2004). The Pasur river water could be used for drinking purposes based on Na⁺. The low concentration of Na⁺ in Pasur river water may be due to the physical structure of soil and rock as well as the humidity and temperature of the semi-arid area. Thus, the water of Pasur river is safe for human consumption based on sodium ion concentration. K⁺ concentrations in all sampling locations were higher due to anthropogenic impact (Skowron et al. 2018). High concentration of K⁺ in drinking water causes various diseases such as high blood pressure, arteriosclerosis, hyperosmolarity, and oedema (El-Aziz & Hassanien 2017). To utilize the Pasur river water for drinking purposes, the concentration of K⁺ in water should be within the permissible limit of WHO (2004). For this purpose, chemical fertilizers should be applied in a more scientific way during cultivation of crops, and the use of organic fertilizers should be increased in the river basin area.

Anions in Pasur river water were dominated by Cl⁻, ⁠, and in all the sampling locations during both the seasons (Tables 2 and 3). Pasur river is situated very close to the Bay of Bengal and Cl⁻ concentration in Pasur river water may be increased by mixing of sea (salt) water at the time of cycle storm or any other calamities in that area, industrial salt precipitation, weathering, leaching of sedimentary rocks and soils, as well as domestic, industrial, and municipal effluents. The concentrations in Pasur river water might come from the sulfate content in fertilizer dissolution after irrigation as well as human and animal wastes. It may also enter river water by direct dissolution of evaporation deposits. Sulfate concentration in drinking water is considered to have significant effects on human health. Diarrhoea may be induced in the people of Pasur river basin area with concentrations of 500 mg/L or more (Mallick 2017). Another important anion in Pasur river water that determines its alkalinity is bicarbonate. Dissolved salts, dissolved carbon dioxide, pH, and temperature might lead to the formation of bicarbonates in Pasur river water. Other sources of bicarbonate salts may be the carbonate rocks, as in the Pasur river water dominated by Ca²⁺ and Mg²⁺, which were usually found in calcite (CaCO₃), and dolomite (CaMg(CO₃)₂). Chemical fertilizers are the source of nitrate in the Pasur river basin region, as the majority of inhabitant depend on agriculture activities for surviving, using various nitrogen and nitrate containing fertilizers for improved crop yields. Besides the fertilizers, food preservatives as well as human and animal wastes also increase the concentrations of in river water. Excessive amount of concentration in drinking water may cause illness such as methemoglobinemia of children (Mallick 2017). The concentrations of Cl⁻, ⁠, ⁠, and in Pasur river water during both the seasons were within the maximum permissible limits of WHO (2004) for drinking water of 600 mg/l, 400 mg/l, 240 mg/l, and 100 mg/l respectively. Therefore, Pasur river water is safe in all the sampling stations during both the seasons and it can be categorized as risk free for drinking purposes based on major anions.

Many researchers reported that the rural population including children suffers from endemic fluorosis disease due to elevated fluoride content in drinking water (Adimalla et al. 2018). Fluoride concentration in Pasur river water ranged from 0.37 to 0.57 mg/l in pre-monsoon, and from 0.41 to 0.74 mg/l in post-monsoon season, and was found within the maximum acceptable limit of 1.5 mg/l of WHO (2004).

Phosphates enter water systems from many sources, including sewage, manure, and different artificial fertilizers used in agricultural fields. Excess phosphates lead to decreased oxygen in river water, which then affects the consistency of the material and nutrient cycling processes. The presence of phosphorus is often meager in the well-oxygenated river water and significantly, the poor level of phosphorus limits the production of freshwater systems. Usually, phosphates are not noxious to human beings unless the concentration is very high in drinking water. Digestive difficulties could arise due to an extremely high concentration of phosphate. The results highlighted that water samples in all the locations of Pasur river were found within the recommended limits of 1.5 mg/l according to WHO (2004), and can be used for drinking water without any further treatment.

Water hardness, which usually depends on the quantity of calcium, magnesium along with other cations, is another indicator of drinking water quality. Hard water is a potential problem, as it may cause some diseases like urolithiasis and cardiovascular disorders in humans (Gizaw et al. 2014). The Pasur river water based on TH demonstrated that all the sample water in both seasons did not exceed the maximum limit of WHO (2004) (Tables 2 and 3).

TDS usually reflects the contents of dissolved mineral salts in water that measure the water suitability for use. TDS mainly consists of inorganic salts in water, and measures the amount of dissolved minerals, which occurs due to interaction between water and geological materials. An inappropriate amount of TDS in water may cause adverse taste effects as well as gastrointestinal irritation (Ibrahim & Nofal 2020). High TDS value may also cause chronic, acute, and carcinogenic health issues to the consumers (Sajil Kumar & Kuriachan 2020). According to WHO (2004), the water TDS value of 1,500 mg/L is considered the maximum permissible limit for drinking water and the results indicated that all investigated water samples in Pasur river water were well within the maximum permissible limit in both seasons (Tables 2 and 3). The variation of TDS value in pre- and post-monsoon seasons was clear and it may be due to variation of river water flow rate and steady nature from season to season. The variations of TDS values in river water also may be connected to the anthropogenic sources including agricultural activities in the region, domestic sewage, and septic tanks.

EC of water is generally related to the concentration of ions dissolved in water. According to the maximum permissible limits of EC recommended by WHO (2004), the river water samples in post-monsoon season in all the locations were suitable for drinking (Table 3). But water in the locations S2 and S4 in pre-monsoon were not suitable for drinking (Table 2). The relatively higher values of EC indicated a salt enrichment in those two locations of the river water, which may be due to the improper disposal of adjacent domestic and industrial wastewater.

pH is one of the most important quality indicators for assessing the quality of river water. The pH values of the Pasur river water samples in all the locations ranged from 7.1 to 8.1 (Tables 2 and 3) in both the seasons, indicating neutral to slightly alkaline river water. According to WHO (2004), the pH value for drinking water should be between 6.5 and 8.5. Hence, pH values of Pasur river water were fit for drinking purposes.

The water turbidity measures the water clarity and results from the dispersion of extremely fine colloidal particles in water. Excessive turbidity of water increases the risk of bacterial growth during storage, and can even be harmful to human health by affecting the life cycle of aquatic organisms (Latha et al. 2013). The values of Pasur river water turbidity in S2 and S4 sampling locations were higher in pre-monsoon season, but within the permissible limit of 10 NTU of WHO (2004) in all sampling locations during post-monsoon season (Tables 2 and 3). Unplanned and untreated industrial effluents discharge were observed in the locations of S2 and S4, which contributed to higher turbidity in those locations of the river. In post-monsoon season, the Pasur river was full of water and the flow rate was much higher than pre-monsoon season, which made the river water cleaner, and more suitable for drinking purposes according to the WHO standard (2004).

Typically, both BOD and COD are the two main parameters to be analyzed to indicate the degree of pollution of the river water. Monitoring or control of biochemical parameters is a routine-based water quality assessment for river quality where pollution is of major concern due to uncontrolled industrialization and urbanization that can significantly affect the sustainability of river conservation. Thus, BOD and COD are two extensively used parameters for organic pollution measurements. The river water contained higher biodegradable organic pollutants and non-biodegradable organic pollutants in both the seasons, which are not appropriate according to WHO (2004). The anthropogenic organic pollutants discharged into river water, human substances in soil near aquatic systems, and phytoplankton activity in the river may be the reason for the increased BOD and COD in Pasur river water. The industries in the Mongla export processing zone (EPZ) area, located 2–3 kilometers away from the river, released huge amounts of biochemical oxygen demanding wastes such as many colorants containing organic compounds that also increased the BOD and COD levels in river water.

Use of WQI is a recognized method for evaluating river water quality and its suitability for drinking purposes, as it is quite impossible to provide the overall water quality based on a single parameter (Boretti & Rosa 2019). WQI provides the rating of water quality in terms of an index number by considering all the important individual water parameters for the overall river water quality. WQI expresses the grading of water quality for drinking purposes and it is computed from the point of view of human consumption. The concept is widely used and represents the overall suitability of water appropriately (Amiri et al. 2014; Varol & Davraz 2015). The grade of water quality for determining suitability for drinking purposes according to the WQI scale can be categorized into five types as shown in Table 6 (Shil et al. 2019). The WQI values of Pasur river water were calculated by considering the mean of respective pre-monsoon and post-monsoon seasons, and were provided in Tables 7 and 8. In the present study, the quality of Pasur river water samples in pre-monsoon and post-monsoon was good to very good. According to the classification of WQI, 100% of the river water samples were found safe with the average grade of B in both seasons and can be used for drinking purposes by rural people in Sundarbans region of Bangladesh.

The Piper diagram as shown in Figure 2 indicates the suitability of Pasur river water for drinking purposes based on the type of the water sample in accordance with the classification standards. It clearly explains the variation or domination of cation and anion concentrations. According to the diagram, metal salts of Ca and Mg in Pasur river water exceeded the salt of Na and K, whereas concentrations of strong acids (Cl⁻, and ⁠) were lower than the weak acids (⁠⁠, and ⁠) in both seasons, which indicated that the hardness and the chemical properties of Pasur river water were dominated by alkaline earths and weak acids. The majority of the Pasur river water samples were plotted in the Ca–Mg–HCO₃ field during both seasons. So, it is obvious that the Pasur river water samples in all the locations fall under the category of bicarbonate, which increased the temporary hardness of river water and could be easily removed by heat treatment process. So, according to the Piper diagram, Pasur river water can be used for the purpose of drinking and household activities just after a mild heating process.

Although the Gibbs diagram did not perfectly reflect the influences of the activities of humans on hydrochemical mechanisms, it was widely applied for the analysis of change of hydrochemistry in river water. Precipitation, rock weathering, and evaporation regions were categorized by the diagram to identify the source of dissolved ions and hydrochemistry of the river water. The results indicated that the Pasur river water falls in the rock dominant zone during pre- and post-monsoon seasons (Figure 3). The diagram suggested that the chemistry of Pasur river water was influenced by rock weathering. TDS, Ca²⁺, Mg²⁺, Na⁺, Cl⁻, and ions in Pasur river water were derived through the rock-water interaction, which caused weathering of minerals present in river water of the semi-arid region of Sundarbans area. The fluctuations in river water chemistry reacted not only due to the household, industrial, and agricultural activities but also the surrounding rocks, and the chemical compositions were altered by the hydrosphere, atmosphere, and the biosphere. As the river water samples showed a little tendency towards the precipitation dominant zone during both the seasons, the assessment of TDS and other contaminants should be regularly monitored as the water of Pasur river is used for drinking purposes.

The arid and semi-arid region of Sundarbans in Bangladesh requires appropriate water with some soluble ions for irrigation. The water should contain some essential ions within the permissible limits for proper plant growth. The physical and chemical properties of the agricultural soil as well as the growth of plants are affected by the quantity of soluble ions in the irrigation water (Shil et al. 2019). The quality of Pasur river water for irrigation is evaluated by some important indices including SAR, Na%, RSC, and MH. All the parameters are taken into consideration for evaluating the river water suitability for agricultural uses. The results of all the indicators of Pasur river water samples are presented in Tables 7 and 8 with their mean value.

SAR investigates the activity of Na⁺ in agricultural soil exchange, and excess SAR is responsible for decreasing soil permeability, which inhibits the required amount of water for agricultural crops (Singh et al. 2014). Water samples with SAR<10 are considered as excellent and above 26 as unsuitable for irrigation. SAR values of 10–18 and 18–26 was classified as good and doubtful, respectively (Table 9). In the present study, the amount of SAR value varied from 1.6 to 2.6 during pre-monsoon, and from 0.59 to 1.63 during post-monsoon (Tables 7 and 8), putting Pasur river water samples in the excellent category for irrigation uses.

The permeability of soil is affected by the sodium concentration; therefore, water grading based on sodium concentration could be effectively used for irrigation in the basin area of Pasur river. During pre-monsoon and post-monsoon, the Na% varied from 18.94 to 26.26 and from 9.62 to 17.82, respectively (Tables 7 and 8) and according to Wilcox, 100% water samples of Pasur river can be effectively used for irrigation purposes during both seasons (Table 9). The Wilcox diagram is also very popular with many researchers for determination of the water quality for irrigation (Adimalla et al. 2018). The Wilcox diagram (Figure 4) of Pasur river water showed that 100% of samples could be considered as falling in the good water category for irrigation during both the seasons. Thus, Pasur river water can be effectively used for agricultural purposes during both the seasons.

RSC is another indicator for measuring hazardous effects of irrigation water. High amount of bicarbonate in irrigation water increases soil erosion, and is responsible for disruption of plant growth, low soil permeability, and reduced water penetration. The amount of RSC in water samples varied from −28.4 to −9.13 meq/l (Tables 7 and 8), indicating that the Pasur river water can be used for irrigation during both seasons (Table 9).

Excess amount of magnesium in irrigation water is responsible for soil alkalinity, which reduces crop yield. MH values of Pasur river water varied from 50.8 to 62.5%, and from 28.2 to 74.2% during pre-monsoon and post-monsoon, respectively (Tables 7 and 8). Therefore, all the samples during pre-monsoon, and 66.7% of water samples during post-monsoon were of an inferior quality for irrigation uses based on MH (Table 9).

The range of pH of Pasur river water is seen within the permissible limit (7.1–8.1) of WHO (2004) with slightly alkaline nature in both seasons (Tables 2 and 3). Therefore, Pasur river water is suitable for irrigation purposes in both the seasons with respect to pH, i.e. no alkalinity hazard was observed in the water.

On the basis of EC, water samples of the Pasur river fall under the slight to moderate category for irrigation purposes (Table 9). The water salinity depends on the amount of dissolved salts and the EC. By plotting SAR value against the salinity hazard (EC) in the US salinity diagram, the diagram can be analyzed to measure the usability of Pasur river water for irrigation by dividing the water into categories as shown in Figure 5 and Table 10. High salinity and low sodium water was observed in Pasur river water samples during both the seasons, which requires a proper drainage system. Irrigation water with higher salinity may affect crop growth and might cause osmotic effects and nutritional disorders (Adimalla et al. 2018). In the study area, almost all the samples of river water fall into the category C3S1 during both the seasons, which means high salinity and low sodium hazard and can be considered as suitable for irrigation after mild treatment, i.e. controlling the salinity of the river water by heat treatment.

Water samples from the Pasur river, one of the largest rivers of the Sundarbans World Heritage region in Bangladesh, collected during pre-monsoon and post-monsoon seasons were analyzed to assess the feasibility for drinking and agriculture uses. All the investigated anions in the Pasur river water are found within the maximum permissible limits of drinking water of WHO (2004) in both the seasons. However, the concentrations of Ca²⁺ and Mg²⁺ among cations exceeded the maximum limit of drinking water at locations of S2 and S4 in both the seasons. The pollutants (Ca²⁺ and Mg²⁺) in the locations of S2 and S4 should be controlled in order to make the Pasur river water more suitable for drinking. The WQI rating of Pasur river water showed that all the samples were categorized as good quality of B class for drinking water in both the seasons. The Piper diagram of the Pasur river water samples showed that water samples were dominated by bicarbonate of calcium and magnesium, which could be easily removed by heat treatment process. The Gibbs diagram indicated that the chemistry of Pasur river water is influenced by rock weathering, and TDS, Ca²⁺, Mg²⁺, Na⁺, Cl⁻, and ions in Pasur river water were derived through the rock-water interaction. SAR, Na%, and RSC indices signify that the quality of water is favorable for irrigation uses in both the seasons. However, MH of Pasur river water quality should be controlled as it may create hazardous effects during irrigation. Wilcox diagram suggested that all the water samples in both the seasons can be used for irrigation purposes. The USSL diagram classified Pasur river water samples as low sodium and high salinity water (C3S1 category) during both the seasons, which indicated that the water could be used after controlling the salinity.

The authors declare no conflict of interest.

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