Groundwater is water that comes from rain, snow, sleet and hail that soak into the ground and is accumulated in lower layers of soil that act as a sponge that hold the water. Groundwater is used as a source of drinking water when pumped into a well. However, this valuable water source can be polluted by leaky landfills, leaky tanks that contain gas or chemicals, and when too much fertilizer or pesticides are used on fields or lawns.