The Search For Unpolluted Water

Exodus 7:22-25
 
22  And the magicians of Egypt did so with their enchantments: and Pharaoh's heart was hardened, neither did he hearken unto them; as the LORD had said. 
23  And Pharaoh turned and went into his house, neither did he set his heart to this also. 
24  And all the Egyptians digged round about the river for water to drink; for they could not drink of the water of the river. 
25  And seven days were fulfilled, after that the LORD had smitten the river.