Kolkata City's water supply is dependent on both surface water sources from the river Hooghly and groundwater sources. A large part of the Kolkata Metropolitan Area (KMA) is currently served by groundwater pumped up from an aquifer deep below the city. The city draws water from the aquifers more rapidly than they can recharge themselves. The adjacent town of Salt Lake City covering an area of 12.35 km(2) was a conglomerate of several salt lakes 50 years back. Though planned as a satellite town, the water level is also decreasing fast due to overuse of groundwater at the rate of 20 cm/year. Therefore the objectives of this study are to find the reason for depletion of the water level, depict the present piezometric level in both Kolkata and Salt Lake City, identify the threat of land subsidence and arsenic contamination already begun in the area and classify the areawise subsidence rate to assume the future threat. The methodology followed in this chapter is mainly the analysis of the secondary data related to the temporal change of groundwater level, counting the land subsidence rate, and so on. The result shows that the city's water level has receded by 7-11 m between 1958 and 2003, especially in some areas of both north and south Kolkata. In around 30 % of the city area the level comes close to or falls below the clay layer which is ideal for subsidence and in some areas subsidence ranges from 10 to 14 mm a year creating high-risk zones. The remedial measures are given in the form of groundwater recharge by rainwater harvesting and mandatory installation of rooftop harvesting in Kolkata and Salt Lake along with improvement of the chemical quality of the water.