To build the new Line 3 pipeline across northern Minnesota, the Enbridge Energy company needs to dig a trench — and temporarily pump groundwater out of the construction area, in a process called dewatering.
Enbridge originally asked for permission to pump about 510 million gallons of water from the pipeline corridor, as it builds the replacement to the current Line 3 pipeline along a new, 340-mile route across northern Minnesota.
But as construction moves forward, the company encountered more groundwater than it anticipated and requested to significantly increase the amount it’s authorized to pump, according to the Minnesota Department of Natural Resources.
On June 4, the DNR issued an amended permit that allows Enbridge to pump up to nearly 5 billion gallons — almost 10 times more than the original amount the company had requested — for the remaining 145 miles of pipeline it has left to build.
Opponents of the project worry that the pumping could reduce the overall quantity of groundwater and potentially affect sensitive wetlands, lakes and streams along the route, which are already under stress due to current drought conditions statewide.
But the DNR says it has determined that the increase in dewatering would not threaten groundwater sustainability or have other harmful impacts on natural resources.
The agency’s permit only allows Enbridge to pump shallow groundwater from the construction area, not from lakes or wetlands, said Randall Doneen, a senior DNR administrator who oversees ecological and water resources. The water is temporarily stored and treated, then discharged nearby, where it infiltrates back into the ground, he said.
“Our assessment is that the actual pumping of it will have limited impact to the wetlands, streams and lakes and the shallow aquifer,” Doneen said.
Dewatering to dig a trench
Dewatering is required for construction projects like new roads, buildings or sewer lines that require digging a hole or trench, which tend to fill with water if they’re below a certain depth.
“Everywhere that you go down into the ground, you eventually run into the water table,” explains Kelton Barr, a consulting hydrogeologist who’s worked on other dewatering projects in Minnesota, but is not involved in Line 3.
“Below that point, the ground and all the pores in it are saturated with water,” Barr said. “And so, if your construction project needs to be doing things below that water table, then you have to do dewatering.”
Large dewatering projects — those that involve removing more than 10,000 gallons a day, or 1 million gallons a year — require a permit from the Minnesota DNR.
In an email, Enbridge spokesperson Juli Kellner attributed the need to pump more water in part to the company’s decision to use more wellpoint systems — a series of wells installed along the excavated trench that lower the shallow groundwater.
Dewatering with wellpoints produces cleaner water with less dirt and sediment than…