Surface waters are constantly moving through the water cycle as
they move downstream rivers or evaporate, condense, and precipitate.
This means that a once polluted body of surface water can often
be restored, reducing the amount of future treatment it needs. The
same is not true of groundwater. Groundwater is water that fills
the small spaces between loose soil and rock particles or in cracks
in the bedrock and can be hundreds of thousands or even millions
of years old. If this water gets contaminated, it is impossible
to clean it at the source, which means that it will always have
to be treated before it is used, or it may become unusable forever.