Despite being billed as a replacement, the old Line 3 will not be removed when the new Line 3 is put into service. The old pipeline that will be left abandoned crosses 300 miles of northern Minnesota, but the state authorities have no plan to protect landowners from the financial and ecological risks associated with the decomposition of the pipeline.

The new Line 3 will endanger many valuable sources of drinking water since the proposed route will cross areas of high susceptibility for groundwater contamination. Moreover the proposed route crosses the Mississippi River, which provides drinking water to several cities (including Minneapolis and St Paul).

The creation of new pipelines facilitates the expansion of tar sands projects, since these projects require pipelines to be financially viable. Tar sands are 21% more carbon-intensive than and require three times as much fresh water to extract and refine as conventional oil. The toxic wastewater is stored in storage ponds or underground wells depending on the extraction methods, risking contaminated groundwater.  

The environmental issues associated with Line 3 concern both the new and the old pipeline. Critics of the plan to abandon the old pipeline, including environmentalists and Indigenous groups, as well as landowners along the pipeline's route, believe that the pipeline's removal is preferable to its abandonment. This is due to the fact that US laws on pipeline abandonment are inadequate to ensure that Enbridge will take proper responsibility for the pipeline once it is decommissioned. Abandoning the pipeline risks contamination of soil and groundwater with residual oil, lubricants, treatment chemicals, and pipeline coatings as the pipeline corrodes. There is also a risk of the pipeline becoming a water conduit, causing the unnatural drainage of wetlands and damaging their ecosystems.

The new Line 3 will cross several important ecosystems including undisturbed forest, important wetlands, some of the most delicate soils, aquifers and pristine lakes in northern Minnesota and the Great Lakes, home to one fifth of the world’s fresh water.  A July 2017 Greenpeace report noted that oil spills anywhere pose serious risks to human health and the environment, and oil spilled into bodies of water is difficult to fully clean up. Diluted bitumen transported from Canada’s tar sands fields represents a particular threat to water resources along the routes of proposed pipelines. The oil spill might go unnoticed for months since detection systems are only able to detect leaks above 150.000 litters per hour and the preferred route crosses many remote areas.

Enbridge has been responsible for many oil spills in the past, including 20,082 barrels of diluted bitumen into the Kalamazoo River in 2010 - the largest inland oil spill in US history. Expanding the capacity of Line 3 only increases the risk of spills in the future.

The old Line 3 – which will be left abandoned - runs through American Indian reservations Leech Lake and Fond du Lac.  Although the proposed new Line 3 will not cross any reservations it will still jeopardize so-called usufructuary or use-rights between Ojibwe bands and the United States government. These use-rights include the rights to hunt, fish, gather medicinal plants, harvest and cultivate wild rice, and preserve sacred or culturally significant sites.

 Among the newly proposed route over 180 archaeological sites have been identified, including burial mounds, cemeteries and sacred sites. Lake Minnewawa and Big Sandy Lake which are sacred to the Ojibwe people and used for fishing will be threatened by the new Line 3. Both the construction as well as the spills of the new Line 3 will threaten traditional foods and medicines. This also applies to the wild rice which is of economic as well as cultural importance in Minnesota. Twenty wild rice lakes are within the reach of the oil spills from the new Line 3.

Over 150 First Nations and Tribes across Canada and the U.S. have signed the Treaty Alliance Against Tar Sands Expansion, opposing the use of the signatories’ Indigenous territories and coasts for new or expanded pipeline infrastructure projects that would facilitate the expansion of the tar sands.