A burst pipe leaves the water unswimmable for the public along the North Ayrshire coastline.
The Scottish Environmental Protection Agency (Sepa) said routine water testing had found high levels of bacteria last week.
Certas Energy says other possible explanations for the leak at the Gulf fuel station at Lix Toll have emerged.
Volunteers have handed out about 40,000 bottles of water since the spillage, which a local petrol station blamed on an attempted fuel theft.
The Scottish Environment Protection Agency has warned that fuel contamination in the River Dochart may have spread to the River Tay.
Residents in the village of Killin reported a solvent smell and taste to their water on Friday.
The Scottish Environment Protection Agency has issued 19 flood warnings and 12 flood alerts around the country.
ExxonMobil said there was no actual danger but it identified failings and has made improvements since.
The Lower Tweed has been escalated, after the Upper Don and Lower Don also reached significant scarcity.
