The quality of river water reflects several influences such as climatic condition, anthropogenic input and the lithology of the basin (Kunwar P. Sing and et.al., 2005; Bricker and Jones, 1995). Climatic conditions, i.e., rainfall, cause rivers to fluctuate in the magnitude of their flow and over time, rivers virtually dilute the concentration of pollutants. Seasonal variations in rainfall, surface runoff, interception and abstraction thus have a significant impact on the discharge of river water and consequently the concentration of pollutants in river water. As a result, during the wet season, the concentration of the contaminants in river water becomes low or moderate, even if anthropogenic activities, such as the disposal of urban and industrial waste water and manure from agricultural land, constitute a constant polluting source. Conversely, during the dry season, the concentration of contaminants becomes high (Vega et al. 1998; Singh et al. 2004).
However, rainfall is a phenomenon that directly influences surface runoff, which washes land into streams, leading to an increase in the concentration of geogenic pollutants like eroded soil and rock in river water (Pejman et al., 2009; Mtethiwa et al., 2008; Khadka & Khanal, 2008; Vega et al., 1998). This phenomenon occurs throughout the entire basin, regardless of rural and urban environments. The amount of rainfall varies from month to month and season to season, creating seasonality within stream water. Various sources of contamination make the hydrogeochemical system a more complex entity that must be explored to develop a strategic plan to avoid further contamination.
The aim of the present research is to assess the seasonality of stream water quality of the Kaljani river at Alipurduar Municipality, West Bengal, India, with two specific objectives: first, to detect whether there is any specific seasonal pattern in water quality status of the concerned river stretch, and second, to estimate the association between water quality and the prevailing local climatic season.
The stretch of the Kaljani river from Bitala to Alipurduar is one of the three most polluted river stretches in North Bengal identified by West Bengal Pollution Control Board (WBPCB, 2022). The other two river stretches are Karola at Jalpaiguri and Mahananda at Siliguri. WBPCB initially declared river Teesta at Jalpaiguri as another polluted river stretch in North Bengal but later on it was later removed from the list.
Previously, no investigation was done on seasonality of water quality and pollution source identification in any of these three rivers stretches and this issue almost remains unexplored even in southern part of the state where thirteen river stretches have been identified by WBPCB as most polluted ones (WBPCB, 2022). In other provinces around the country very few investigations have been conducted (Shiddamallayya & Pratima, 2008; K.P. Singh et. al., 2004, Srinivas et. al., 2018; Sundaray et al., 2006). Most of the work done previously across India focused mainly on assessment of river pollution, its causes and consequence, assessment of suitability of river water for its sectoral use etc. but this issue of seasonal response of stream water quality has yet to be addressed (Bhat et. al., 2011; Ravichandran, 1987; P.K. Singh & Shrivastava, 2015; R.K. Singh et.al.,2019; Tyagi et al., 2020).
However, a number of works on such topics have been reported from various parts of the world. (Lei, 2013; Ogwueleka, 2015; Pejman et al., 2009;Shrestha & Kazama, 2007;Varol & Sen, 2009;Vega et al., 1998; Wang et al., 2013; Yang et al., 2020). Hence, this aspects of hydrogeochemical system and pollution source identification of flowing water demands river specific investigation in regional or local scale in order to find a solution based on the characteristics of the concerned river stretch. Keeping this issue in mind, the objective of the present research has been specified.
The study area
Alipurduar municipality is located on the east bank of Kaljani River (Fig. 1), at the eastern end of the Dooars, one of the physiographic divisions in the state, lying at the foot of the Himalayas. Alipurduar, which had previously been a block within Jalpaiguri district, became a district on June 25, 2018 (Alipurduar Municipality, 2017). Geographically, Alipurduar is a land locked district bordered by Jalpaiguri in the west, Assam in the east and Kooch behar in the south. To the north, the district shares an international border with Bhutan. Total areal coverage is 41.90 km2 (89° E to 89°9´E and 26°4´N to 26°83´N) (Census of India, 2011).
The climate of Alipurduar district is subtropical monsoon, characterized by hot summers from March to May and well distributed rainfall during the southwest monsoon from June to October. The winter season in the area is marked by dry and cold weather from November to February. The mean daily minimum and maximum temperatures are about 6.8 to 9.5˚C and 31 to 35˚C respectively. The normal average rainfall in this area is 1500–1800 mm, with an average of 72 rainy days (IMD, 2008).
The polluted stretch of the Kaljani river surrounds Alipurduar municipality as it passes to the south west. The river receives fresh water from the Himalayas and municipal wastewater from Alipurduar municipality round the year. During field investigations, eight noticeable outfall points were identified.