Water can flow through sand. (Just look at all the beaches of the world) It  just needs to get so wet that it can't hold anymore water and then  water will flow through it.

While it allows water to fall between the tiny grains of sand, at the same time, the sand grains are so … Read more

HI Amber.  Great question.  Sincerity explained how the sand is able to trap impurities.  Rooster chose another good resource.

This visual might help you further understand.

HI Amber.  Great question.  Sincerity explained how the sand is able to trap impurities.  Rooster chose another good resource.

This visual might help you further understand.

This article tells some about the system to add onto Sincerity's answer. Check it out.

Sophisticated Mayan water management system revealed ...