Map of Chicago-Milwaukee area showing water level decline, 1864-1980.
Chicago has been using groundwater since at least 1864 and groundwater has been the sole source of drinking water for about 8.2 million people in the Great Lakes watershed. This long-term pumping has lowered groundwater levels by as much as 900 feet.
Source: Alley, William & Reilly, T.E. & Franke, O.L.. (1999). Sustainability of Ground-Water Resources. U.S. Geological Survey Circular 1186.