Our water has a black sediment after setting for a while. harmful?

Question

it is well water. what can it be?

Answer 1

Could be some black clay minerals settling out of suspension. Probably not harmful but not good either. You may need to filter and boil the water  before drinking. Take a sample to a water lab for testing to be safe. The filter on the end of your bore pump may also be faulty.

Answer 2

It's an algae. It's just annoying.