Labor tests after a mass die-off of fish in the Oder River have found high levels of salinity but no mercury poisoning its waters.
Analyzes of river samples taken in both Poland and Germany showed elevated salt levels, according to Anna Moskwa, Poland's Climate and Environment Minister.
She said that comprehensive toxicology studies are still underway in Poland as the mystery continues as to what killed tonnes of fish in Central Europe.
She said Poland's state veterinary authority has tested seven species of dead fish and ruled out mercury as the cause of the die-off, but was still waiting for results of other substances.
German laboratories have not shown a high presence of mercury, but German laboratories have detected atypical levels of salts that could be linked to the die-off, but wouldn't fully explain them on their own.
German media had suggested that the river had been poisoned with mercury.
The Oder River - one of the largest rivers in Central Europe - runs 854 kilometres from the Czech Republic to the border between Poland and Germany before flowing into the Baltic Sea.
Polish prime minister Mateusz Morawiecki said on Friday that huge amounts of chemical waste had been dumped intentionally into his country's second-longest river, causing environmental damage so severe that it would take years for the waterway to recover.
On Saturday, Mr Morawiecki pledged to do everything possible to limit the environmental damage.
Poland's Interior Minister, Mariusz Kamiski, said a reward of 1 million zlotys $309,460 would be paid to anyone who helped track down those responsible for polluting the river.
Authorities in the north-eastern German state of Mecklenburg-Western Pomerania have warned people not to fish nor use water from the Szczecin lagoon because the river's contaminated water was expected to reach the estuary area on Saturday evening.
The extent of the fish die-off is shocking, said Alex Vogel, the Environment Minister for Germany's Brandenburg state.
This is a blow to the Oder as a waterway of great ecological value, from which it will probably not recover for a long time. On Thursday, the head of Poland's national water management authority said 10 tons of dead fish had been removed from the Oder River.
Hundreds of volunteers were working to collect dead fish along the German stretch.
Two of the officials have been dismissed after huge numbers of dead fish were seen floating and washing ashore, and he acknowledged that Polish officials were sluggish in response.
The most important thing is to deal with this ecological disaster as soon as possible, because nature is our common heritage, according to Morawiecki.
His comments were echoed by Schwedt Mayor Annekathrin Hoppe, whose German town is next to Lower Oder Valley National Park.
She called the contamination of the river an environmental catastrophe of unprecedented scale for the region.