runoff

noun

• That portion of precipitation or irrigation on an area which does not infiltrate or evaporate, but instead is discharged from the area.
• Dissolved chemicals, etc, included in such water.

  "The runoff of nitrates is poisoning the lake."

• A second or further round of an indecisive election, after other candidates (often all but the last two) have been eliminated.

  "There will now be a runoff as neither front runner received more than 50% of the vote."