A key piece of data related to the biggest tar sands oil spill in U.S. history has disappeared from the Environmental Protection Agency's website, adding to confusion about the size of the spill and possibly reducing the fine that the company responsible for the accident would be required to pay.

The July 2010 accident on an Enbridge Inc. pipeline dumped thousands of barrels of Canadian dilbit into the Kalamazoo River and surrounding wetlands. But almost three years and two federal investigations later, one of the most important questions about the spill remains unanswered: Exactly how much oil spilled from the pipeline?

The same question is being asked about a more recent dilbit spill—a March 29 accident on ExxonMobil's Pegasus pipeline in Mayflower, Ark. Estimates for that spill, which is still being cleaned up, have risen from 80,000 gallons to more than 200,000 gallons.

Determining the size of an oil spill is important, because every barrel of oil that reaches a navigable waterway triggers a statutory fine of $1,100 per barrel under the Clean Water Act. The fine rises to $4,300 per barrel if a company is proven to have acted with gross negligence.

In the case of the Michigan spill, the EPA has posted numerous updates on its website about the amount of oil recovered from the site. It was 766,000 gallons in March 2011, then grew to 1,148,229 gallons in June 2012. (There are 42 gallons in a barrel.)

But speculation about the magnitude of the spill took a new twist sometime in March, when the EPA's website for the accident stopped showing how much oil has been collected at the site—1,149,460 gallons at last count. Web archives show that number was deleted between March 9 and March 27.

The only spill estimate remaining on the website is another, much lower number that has also been tracked since soon after the spill: The amount of oil—843,000 gallons—that Enbridge says was discharged from the line.

In other words, 36 percent more oil has been recovered than was spilled.

In a series of emails with InsideClimate News, an EPA spokesman did not explain why the larger number had been removed or address the discrepancy between the two numbers.

"The estimated oil recovered remains at 1,149,000" gallons, said the spokesman, who requested anonymity.

When questioned about the discrepancy between the numbers, he said the agency "does not comment on ongoing investigations."

Former EPA official Al Armendariz offered two explanations of why the agency might have dropped the larger spill estimate from its website. "Either the number was changed by a very deliberate decision and it's exactly what they want for a legalistic reason. Or it could be lack of communication between the staff [at EPA]. That wouldn't surprise me either."

Armendariz was regional administrator of the EPA's Region 6 Office from 2009 to 2012. He resigned last year and now works for the Sierra Club's Beyond Coal campaign.

Two federal investigations of the Marshall spill have already been conducted, but neither examined the volume of oil spilled. In July 2012, the National Transportation Safety Board concluded that the accident was caused by a "complete breakdown of safety" at Enbridge. A separate investigation by the U.S. Department of Transportation fined Enbridge $3.7 million for violating 22 pipeline safety regulations.

The EPA is conducting its own investigation of the spill, and its findings will determine whether Enbridge will be fined under the Clean Water Act. On March 15, the agency ordered Enbridge to dredge several sections of the riverbed where oil continues to accumulate.

If Enbridge is fined $1,100 per barrel, the company would pay $22 million for an 843,000-gallon spill. The fine would increase to $29 million for a 1.1 million-gallon spill.

If the company is fined $4,300 a barrel, it would pay $86 million for an 843,000-gallon spill and $113 million for 1.1 million gallons.

Armendariz said the difference between paying fines for an 843,000-gallon spill and a 1.1 million-gallon spill probably wouldn't be significant for a major company like Enbridge, Canada's largest transporter of crude oil.

But Armendariz also said that "oil companies are in the business of making money and they won't write a check for oil they don't think they spilled...The government should expect that Enbridge will try to use an estimate that is most favorable to them."

Enbridge spokesman Larry Springer said the 1.1 million figure exaggerates the size of the spill, because it includes substances other than oil.

It "is a culmination of everything collected during cleanup of the Kalamazoo River and [Talmadge] Creek – the product released from Line 6B, non-petroleum organic materials, and other petroleum-based products in the river (hydrocarbons)," Springer said in an email. "There are a number of conservative factors involved in calculating this number that we believe contribute to an overestimation of the total amount collected."

The EPA spokesman also said the larger number includes "oil, oily water, soil and debris containing oil."

But spreadsheets and other documents that InsideClimate News obtained through the Freedom of Information Act show that the 1.1 million-gallon estimate refers to the amount of oil after it has been separated from debris.

The spreadsheets, which Enbridge provided to the EPA in regular updates on the recovery, contains separate estimates for the volume of oily water, soil and debris. Another column is reserved for the oil alone, expressed in gallons. That column shows the 1.1 million gallon estimate.

According to other documents, these numbers were derived by analyzing sediment cores and other samples collected during the cleanup. The documents also indicate that the methods used to determine these volumes were at least partially developed by the EPA.

For instance, an EPA directive from Nov. 20, 2012 ordered Enbridge "to complete the quantification of submerged oil in the sections of the Kalamazoo River affected by the Enbridge Line 6B oil spill." The directive notes that the EPA and Enbridge "have been developing methodologies for quantification of the submerged oil" since summer of 2012.

Other documents describe quality assurance plans that are supposed to ensure that the data are gathered and analyzed using reliable scientific methods.

Neither the EPA and Enbridge responded to follow-up questions about the techniques used to obtain the 1.1 million gallon figure.

In response to InsideClimate News' questions, however, the EPA did address another discrepancy on its website.

Since at least Sept. 2010, the site had said that the spill discharged 819,000 gallons of oil, even though Enbridge's own engineers had increased that estimate to 843,000 gallons in Nov. 2010. The EPA spokesman acknowledged that the agency should have changed the estimate. The website was updated last week.