The copper mine, one of the world's biggest, is on track to begin production in the first quarter of 2016, despite weak prices and relentless protests against the project.
Oil sands companies used 1.5 barrels of water from the Athabasca River to produce one barrel of bitumen in 2014, compared to 2.2. barrels of water in 2012.
The accident would have been avoided had the EPA team checked on water levels inside the abandoned mine before digging into its collapsed and leaking entrance, says document.
The world's largest equipment maker not only reported lower quarterly profits, but also cut its 2015 earnings forecast and warned its revenue would fall even further next year.
The environmental group is after Vattenfall's lignite business in eastern Germany, in a bid to scale back the operations put up for sale by the Swedish state-owned energy giant.