in rivers or lakes. The rest is underground water. This groundwater is plentiful and dependable, because it doesn't depend on seasonal rain or snow. It is the major source of water for many cities. But as the population increase and the need for water also increases, the groundwater in some area is getting dangerously low. Added to this problem is an increase amount of pollution that seeps into the groundwater. In the future, with a growing population and more toxic waste. The hydrologic cycle we depend on could become dangerously imbalanced