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).

New tar sands projects require new pipelines to be financially viable - the impact of these new pipelines should, therefor, include the expansion of tar sands extraction. 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, 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, Indigenous groups, and other landholders along the pipeline's route, believe that removing the pipeline would be better than abandoning it. 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 contaminating the soil and groundwater with residual oil, lubricants, treatment chemicals, and pipeline coatings as the pipeline corrodes. The pipeline may also become a water conduit, draining and damaging wetlands.

The new Line 3 will cross several important ecosystems including undisturbed forest, important wetlands and some of the most delicate soils, aquifers and pristine lakes in northern Minnesota and the Great Lakes, which are home to one fifth of the world’s fresh water. A July 2017 Greenpeace report noted that oil spilled into bodies of water is difficult to fully clean up, posing serious risks to human health and the environment. Diluted bitumen transported from Canada’s tar sands represents a particulary grave threat to water resources along the routes of proposed pipelines. Leaks might go unnoticed for months since detection systems are only able to detect leaks above 150,000 liters 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 such disasters in the future.

In January of 2019, The Intercept reported that “law enforcement has engaged in a coordinated effort to identify potential anti-pipeline camps and monitor individual protesters, repeatedly turning for guidance to the North Dakota officials responsible for the militarized response at Standing Rock in 2016.”  Resistance to Line 3 construction has been the subject of heavy surveillance by both private security and publicly funded police forces, and collaboration with the actors responsible for the rubber bullets, tear gas, razor wire and water cannons in North Dakota is deeply troubling. The Trump Administration is backing legislation to punish pipeline protestors with up to 20 years in jail.

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 of and spills from the new Line 3 will threaten traditional foods and medicines, including wild rice which is economically as well as culturally important 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.