Hydrochemical fingerprinting and effects of urbanisation on the water quality dynamics of the Quaternary aquifer of south Bengal Basin, India

Hydrochemical fingerprinting of groundwater helps to analyse the physico-chemical dynamics and assesses pumping-induced hydrochemical changes within a complex and stressed deltaic aquifer system of an urban-peri-urban area in south Bengal Basin in India using Kolkata and Howrah cities as case study. Statistical analysis of geochemical parameters of 287 groundwater samples shows that eight hydrochemical facies occur in the study area, which can be broadly categorized into fresh, blended and brackish water covering 53, 186 and 171 km 2 area, respectively, excluding the area covered by River Hugli. In the past two and half decades, due to over-withdrawal of groundwater, the groundwater trough in south-central Kolkata increased by 266 km 2 and resulted in the deterioration of fresh water (mean TDS = 518 mg/L) into blended (mean TDS = 722 mg/L) and brackish water (mean TDS = 1385 mg/L) due to hydraulic mixing, flushing and ion-exchange. A comparison of the common area of the hydrochemical facies of mid-nineties and present study reveals a reduction of area of fresh water by 74% and increase of area of blended and brackish water by 42% and 32%, respectively. Contamination of groundwater by anthropogenic arsenic clustering around a palaeo-channel named Tolly’s Nala, wastewater from sewerage, copper, lead, cadmium and zinc surrounding the industrial areas are detected in wells where the upper aquitard is either absent or < 10 m in thickness. At the present rate of withdrawal, the quality of groundwater may further deteriorate at an accelerated pace and the groundwater of the entire area may become unsuitable for drinking in future.