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.