On Sept. 30, BP PLC will attempt to convince a federal judge that its efforts to stop the leak from the Deepwater Horizon disaster were sufficient. The jury-less civil trial will also seek to determine how much oil was leaked in to the Gulf of Mexico and will determine what fines the company will pay for violations of the federal Clean Water Act.

At the heart of the trial is a disagreement as to how much oil was actually leaked in the 2010 disaster. While both BP and the Department of Justice agree that roughly 810,000 barrels were captured by spill containment tactics, BP claims that only 2.45 million barrels were leaked in total. The DOJ estimate is considerably higher at 4.2 million barrels.

BP claims that these estimates were not affected by the falsifications of the spill rates it made to the government at the onset of the crisis.

The British oil company has already paid more than $42 billion in settlements, fines and efforts to clean up the spill. The Wall Street Journal is reporting that the trial could impose additional fines as steep as $18 billion, but BP has said it expects the number to be closer to $3.5 billion.

This is the second leg of three civil trials that will shore up the price tag for these violations. The first, which took place in February, focused on the culpability of each of the companies involved in the spill. BP, Transocean Ltd, and Haliburton each held ownership on Deepwater Horizon, with BP owning the oil well itself, and Transocean and Haliburton responsible for the equipment used to harvest the plot.

While the oil giant has paid considerable damages and fines already, it has been accused of dragging its feet on payments owed to businesses impacted by the spill. It has also actively sought renegotiation of the terms of those settlements.