Four Water Samples...

You only have one guess.

Two of the samples were taken from the Mara River and two of the samples were taken from the Talek River. One of the Talek River samples was taken upstream of all of the tourist lodges and camps on the Talek River. The other Talek River sample was taken downstream of all the tourist lodges and camps on the Talek River.

Guess which one was taken downstream of the tourist lodges and camps on the Talek River within the Masai Mara National Reserve...