The cone of depression is a valley, an upside-down cone of water, created by a well when it pulls groundwater to the surface of the earth to use for drinking water or electricity.
A cone results from changing groundwater flow direction. As the well pulls water up from underground, soils falls inward, which creates a cone shape. As the water shifts and rises upward, the water level around the well lowers and adjusts by producing a water discharge.
When the well gets pumped, groundwater near the pump discharges, flowing upward toward the well in all directions. As a result, pollution--chemicals, trash, debris--may flow into the water source depending on the direction of the cone of depression.
Mostly captured in the cone of depression, remaining groundwater contaminants can usually be safely removed by continued pumping and disposing of contaminated water.
If a well owner pumps water out faster than it can be replaced by natural groundwater, the water table (groundwater surface) will drop. The well may go dry temporarily but will soon fill with fresh groundwater.