I think you want 'visible' as an adjective to modify either contaminants or hazards.

 > Although there are no visible contaminants, the stream has a high level of fecal bacteria.
 >
 > The visible hazards - rocks, debris, and large marine mammals - are actually less of a risk than the PCB spill.

[Visible][1] (MW)

 > capable of being seen
 >
 > exposed to view
 >
 > capable of being discovered or perceived


  [1]: https://www.merriam-webster.com/dictionary/visible