Dikeledi,  I would further ...

Dikeledi, 

I would further suggest you review the historical records of the water quality at the sampling point, say last 6 months or so.

Also check upstream for water quality sampling results, particularly where there are wastewater treatment facilities depositing the final treatment effluent onto the same river.

In essence, I suggest you conduct a Root Cause Analyses on possible causation factors of the hazard observed.

Do not forget to activate your Incident Management Protocol given the seriousness of the water quality indicator observed.

Best of luck.

Twala.