Colonial Pipeline's Line 1 is flowing again -- which should eventually ease the regional gasoline shortages and price increases caused by the 12-day shutdown -- but significant work remains to cleanup the estimated 336,000 gallons of gasoline that leaked from the pipeline in central Alabama and to dig out the damaged section and determine what caused the leak.

The U.S. Environmental Protection Agency says most of the gasoline flowed overland from the leak site to a mining retention pond about 500 feet away.

Since the leak was discovered, Colonial employees and outside contractors have been pumping water out of the pond when conditions allow. Those efforts have been limited by high concentrations of benzene and gasoline vapors. At one point, the Pelham Fire Department was forced to close the site for 83 hours due to unsafe concentrations of gas fumes.

There are three retention ponds on the property near the spill site. Pond 2 is thought to have collected most of the gasoline from the spill. Pond 1 is uphill from the presumed leak site and has shown no traces of gasoline. Pond 3 is connected to Pond 2 by an underwater culvert which has now been blocked, and has shown trace amounts of broken down gasoline components.

Crews are also using skimmers to collect gasoline on the surface of Pond 2, and have used more than 6,000 feet of boom to corral gasoline and constructed multiple underflow dams prevent it from reaching Peel Creek, a tributary of the Cahaba River.

According to EPA representatives and environmental group Cahaba Riverkeeper, which has been involved in the response effort, water samples in Peel Creek and the Cahaba have shown no evidence of gasoline or gasoline components in either waterway.

Colonial said about 60 percent of the surface of the pond had been cleaned as of Wednesday morning. According to EPA situation reports, 85,732 gallons of gasoline have been recovered from Pond 2, along with more than 292,000 gallons of gasoline-contacted water.

EPA on-scene coordinator Kevin Eichinger said remediation of Pond 2 will be a significant undertaking.

"Ultimately we're fairly certain that Pond 2 is going to have to be pumped completely dry, all that water removed," Eichinger said. "Excavation of the sediment is going to have to occur, excavation of the impacted vegetative debris all around it."

Pond 3, which has shown only dissolved traces of gasoline components, will likely need less remediation.

"Pond 3 still has fish living in it, it's still an active ecosystem," Eichinger said. "Most likely, with that pond, we'll either do aeration to drive those organic chemicals out or do some sort of other treatment, but currently we don't plan to pump Pond 3 down or do any extensive remediation there because these are very low levels of the [gasoline] constituents."

So far, EPA said seven dead mammals have been recovered at the site: a rabbit, two raccoons, one fox, one coyote, one muskrat and one armadillo. Four turtles and two birds have also been found.

The leaking section of the pipeline is in Shelby County, Alabama, approximately 20 miles south of Birmingham, and about four miles from suburbs like Alabaster and Helena.

According to the Alabama Surface Mining Commission, the mining permit for the site where the leak was discovered is held by Twin Pines II, LLC mining company.

ASMC director Johnathan Hall said the mining operations at the site have been completed and the site is going through the reclamation process.

What caused the leak?

The section of pipeline containing the leak is still underground as of Thursday, and company officials said it would be premature to speculate on the cause of the leak before it can be excavated and inspected.

Efforts to dig out the leaking section of pipeline were put on hold early in the response period due to the potentially unsafe vapor levels.

Once Colonial Pipeline made the decision Saturday to build a bypass line around the leak site, priority was given to building the bypass rather than digging out the old line. Now that Line 1 is flowing again, crews can begin digging out leaking line when safe conditions allow.

The U.S. Department of Transportation's Pipeline and Hazardous Materials Safety Administration is "investigating the cause of the failure, any factors that contributed to the severity of the incident, and the operator's adherence to Federal pipeline safety standards," according to its website.

Randy Aldridge, an inspector from the Alabama Surface Mining Commission first reported the potential leak on Sept. 9, during a routine inspection of the mine site. EPA documents state the landowner last visited the site about two weeks prior to that inspection. Aldridge declined a request for an interview with AL.com.

Colonial Pipeline said it had no indication of a leak prior to the inspector's report, either from pressure readings in the pipeline or from routine aerial inspections it performs on the line.