Authorities say the spill began Tuesday, when someone opened the cisterns at an oil refinery turned depot near Monza, letting tens of thousands of liters (thousands of gallons)of oil pour unimpeded into the Lambro River, a tributary of the Po.
Prosecutors have launched an investigation into the spill. Authorities say it's certain someone intentionally opened the cisterns.
By Wednesday, despite efforts to contain the slick with absorbent pads and the closure of hydroelectric locks, the oil seeped from the Lambro into the Po, Italy's longest river, which flows west-to-east across the country.
And Thursday, the country's disaster relief chief, Guido Bertolaso, said he expects most of the slick to be cleaned up over the next day.
"I believe this is not an irreparable situation," Bertolaso said after meeting with regional officials amid criticism from environmental groups and opposition lawmakers that the government had been slow to respond.
"I believe that in the next 24 hours most of this oily mass will be recovered and then, following the course of the river, before it reaches Ferrara and obviously before it reaches the delta, we will be able to recover all the rest," said Bertolaso, head of the civil protection agency.
The World Wildlife Fund for Nature says thousands of birds — ducks, herons and others — are nesting and reproducing in the area, which it called one of the most important in Europe. In addition, several fish species — eel, shad and mullet — reproduce in the waters.
"The entire ecological and economic system is at risk," WWF warned in a statement.
Officials have said water in the area is safe to drink, but provinces have issued fishing and boating bans for affected parts of the Po.
Coldiretti said food was safe since farm production is low anyway at this time of the year, and heavy rains have meant that the Po won't be needed for irrigation for some time.
"There are no risks for food on the table or damage to cultivation," Coldiretti said in a statement, adding that the rain forecast in coming days means that the oil will be further diluted and the residue dispersed.
But those same rains are worrying environmental groups, which have warned that high water levels in the Po mean the oil will spread to the Po's other tributaries and streams, causing broader environmental degradation.
And the Confagricultura farm group said the repercussions of the spill will be felt in small tributary farm communities, particularly as water demands increase with the spring planting of rice.
Sludge from an oil spill snaked down the Po River on Thursday to reach the province of Parma, raising fears that the home of Italy's famed prosciutto, parmesan cheese and other agricultural staples might be at risk of water contamination. Italian farm lobby Coldiretti insisted Italy's food chain was safe since the Po is not being used for irrigation these days. But another group of farm owners, Confagricultura, warned that the spring planting season - particularly for water-intensive rice crops - might be at risk unless clean water is ensured. The Po River valley, which extends 71,000 square kilometers (27,400 square miles) across several northern regions, produces a third of Italy's agricultural output and represents 40 percent of the country's GDP. Because of its economic importance, officials are warning that farm output might be affected, in addition to the already extensive damage the slick has caused to the area's wildlife.