The ground is composed of several layers of soil and rocks. The top soil layers are porous. Water from rain and melted snow are absorbed into these layers. During heavy rains, the amount of water that falls onto the ground becomes too much for the ground to completely absorb. The result is a layer of water that runs on top of the soil. This water is known as surface water which then forms rivers, streams, lakes, and other bodies of water.
The water that gets absorb slowly permeates through the porous layers of soil and rocks. These layers of soil and rocks that are able to absorb water are called aquifers. Underneath the aquifers is the hard and very compact layer of bedrock that prevents water from moving further downward. The water contained by the aquifers are called groundwater.
The flowing surface water can dissolve and absorb various substances. As a result, surface waters contain all kinds of substances. The water that permeates into the ground gets filtered by the porous soil layers, making groundwater generally less contaminated than surface water.