Seasonal dynamicity of environmental variables and water quality index in the lower stretch of the River Ganga

Abstract

Rapid human pressure in semi-urban and urban areas along with increasing industrial activities has resulted in release of untreated sewage and other forms of pollutants into major rivers globally including in the Ganga. In this study, nine sites represented by 59 stations along the lower stretch of the River Ganga were monitored seasonally to understand the effect of monsoonal precipitation on environmental variables and Water Quality Index (WQI). Sampling was undertaken in pre-monsoon, monsoon and post-monsoon seasons (2019). In situ surface water temperature, dissolved oxygen, pH, total dissolved solids (TDS) and electrical conductivity (EC) were measured along with dissolved nutrients and Chlorophyll-a. Both pH and DO were strongly influenced by monsoon and affected WQI. TDS was higher in point source (PS) stations during pre-monsoon (113–538 ppm) compared to surface water (SW) stations (113–248 ppm) with strong influence of monsoon (PS-27.4-310.3 ppm; SW-27-68.9 ppm). Dissolved nutrients including nitrate and o-phosphate concentration showed significant seasonal variation and influenced monsoonal precipitation. In PS stations across studied seasons dissolved nitrate concentration varied from 26.33–646 μM while in SW the range was from 21.22–148.06 μM. In the studied sites, higher concentration of dissolved nutrients in PS stations reflected the release of untreated municipal and industrial sewage directly into the river. The effect of precipitation and resulting environmental variables was clearly evident on biological variable (concentration of Chl-a) with observed values in PS stations (0–21 mg l−1) which were lesser compared to SW stations (0–89.3 mg l−1) during monsoon. Non-metric multidimensional scaling revealed three distinct clusters with greater overlap between PS and SW stations in monsoon. The WQI values (14–52) determined for lower stretch of Ganga revealed very poor water quality in majority stations and monsoonal precipitation did not have any influence on the observed trends.

Unusual abundance of bloom forming Aulacoseira spp. diatom populations in an anthropogenically impacted stretch of lower part of the River Ganga

“…Consequently, water quality has significantly deteriorated in several stretches of the river owing to high load of pollutants that has further amplified downstream of the river. The lower stretch of the River Ganga, also known as Bhagirathi-Hooghly as it enters West Bengal, exhibit poor to very poor water quality based on the water quality index (WQI) (Kumar et al 2021). A previous study focusing on estimating the spatial and temporal trends of WQI along the lower stretch of River Ganga revealed a distinct declining trend in the Dakshineswar site irrespective of the season of sampling (Kumar et al 2021).…”

“…In addition to human-induced direct activities, there are several small to medium-sized industries which release effluent into the River. Six sampling points in the Dakshineswar site were identified after initial rigorous monitoring (see details in Kumar et al 2021). The points immediately in the vicinity of municipal discharge drains and industrial effluent release zone were earmarked as point sources or PS stations.…”

“…Three such PS and SW stations were identified and monitored in monsoon and post-monsoon over a span of two years. Site selection criteria and sampling strategies have been discussed previously (Kumar et al 2021). The station nomenclature as laid out previously (Kumar et al 2021) has been modified in this manuscript and detailed in table S1.…”

“…Unabated release of waste, both solid waste and wastewater, has led to high concentration of dissolved forms of nutrients causing eutrophication, decrease in dissolved oxygen concentrations and harmful algal bloom formation, in turn lowering the water quality of the surface water of Bhagirathi-Hooghly (Sarkar et al 2019, Muduli et al 2021. In Bhagirathi-Hooghly, water quality has been found to marginally improve during post-monsoon owing to high seasonal precipitation that spans over July to September and often stretches to October (Kumar et al 2021). Local rainfall results in excess disposal of wastewater into the surface water of many rivers in India (Singh et al 2004).…”

Macrobenthic community of an anthropogenically influenced mangrove associated estuary on the East coast of India: An approach for ecological assessment

“…In India, the estuaries have been reported with altered or degraded environmental quality owing to the inadvertent growth of industries and metropolises on the banks of the major rivers (Sigamani et al, 2015;Feebarani et al, 2016;Mulik et al, 2017;Mitra et al, 2018;Mulik et al, 2020a;Mulik et al, 2020b). The increased loads of municipal and industrial effluents, in amounts higher than the assimilatory capacity of the system, are accumulating pollutants and causing hypoxia-like conditions (Mitra et al, 2018;Mulik et al, 2020a;Kumar et al, 2021). Despite the large deltas with some luxuriant estuarine mangrove cover along the east coast of India (in contrast to the west coast), studies that have looked at the effects of changed environmental conditions on macrobenthic fauna are largely limited (Raut et al, 2005;Ansari et al, 2017;Bhowmik and Mandal, 2021;Dash et al, 2021;Pandey et al, 2021).…”