The violation of Indigenous rights has been an inevitable consequence of the Line 3 Replacement Project. While the new pipeline does not cross any Indigenous reservations, it does jeopardise usufructuary rights agreements between bands of the Ojibwe people and the US government. These agreements concern the rights to hunt, fish, gather medicinal plants, harvest and cultivate wild rice, as well as the right to preserve sacred or culturally significant sites. The L3RP footprint crosses 180 such sites, including burial mounds, cemeteries and sacred sites. The Ojibwe people use Lake Minnewawa and Big Sandy Lake for fishing and consider both sacred, but both lakes are now under threat of oil spills. Similarly, 20 wild rice lakes are now at risk from L3RP spills. Minnesotan wild rice has traditionally held both economic and cultural importance for its localities.

Projects such as L3RP pose particular risks for Indigenous women. Environmental features that are intertwined with Indigenous women’s identity and culture are often impacted by fossil fuel construction work. Furthermore, temporary accommodation for male pipeline builders ("man camps") correlates with rises in abuse of Indigenous women, girls and two-spirit people.

Enbridge’s abandonment of the old Line 3 Pipeline guarantees an ongoing threat to American Indian reservations at Leech Lake and Fond du Lac. The abandoned pipeline crosses 300 miles of northern Minnesota in total, which leaves landowners at financial and ecological risk as the pipeline disintegrates.

More than 150 North American First Nations and Tribes 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.

Local drinking water supplies are jeopardised by L3RP, which crosses areas that are highly susceptible to groundwater contamination. It also crosses the Mississippi River, which provides drinking water to Minneapolis and several other cities, as well as the Great Lakes, which are home to approximately one fifth of the world’s fresh water. A 2017 Greenpeace report noted that oil spilled into bodies of water is difficult to fully clean up, which poses serious risks to human health and the environment. The potential leakage of diluted bitumen represents a particularly grave threat to water resources, especially because small leaks might go unnoticed by detection systems for months.

Increased militarisation accompanied the project’s construction. In January 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 militarised response at Standing Rock in 2016.” Construction resistance was subjected to heavy surveillance by both police and private security forces. The use of rubber bullets, tear gas, razor wire and water cannons in North Dakota is deeply troubling. The Trump Administration threatened pipeline protestors with 20-year jail sentences.

The exacerbation of global heating is a major consequence of burning tar sands oil. Tar sands oil is 15% more carbon-intensive than conventional oil, due to the extra energy expended during the extraction, upgrading and refining processes. In 2012, climate scientist James Hansen explained that Canada’s tar sands contain twice the amount of carbon dioxide that humanity had so far emitted via our collective oil use. Enbridge’s new pipeline is wider than its predecessor and can transport approximately twice as much oil, indicating that the pipeline will accelerate the extraction of tar sands.

The potential for disastrous oil spills from this pipeline should not be underestimated. Enbridge’s chequered history of oil spills includes the largest inland oil spill in US history: In 2010, 20,082 barrels of diluted bitumen entered the Kalamazoo River under Enbridge’s watch. Furthermore, Enbridge and its joint ventures and subsidiaries caused an average of one “hazardous liquids incident” every 20 days between 2002 and 2018. Expanding the capacity of Line 3 has only increased the risk of such disasters in previously undisturbed ecosystems, from forests and wetlands, to delicate soils, aquifers, freshwater reserves and wild rice beds. According to Enbridge’s Environmental Impact Statement, the new pipeline route impacts upon 389 acres of wild rice in 17 different wild rice water bodies. This rice is a culturally significant staple of the Anishinaabe diet, which would make an oil spill in these areas particularly devastating.

Soil and groundwater contamination is likely to become a legacy of the Line 3 project. For example, toxic waste water is a product of the tar sands oil extraction process. This water is placed in storage ponds or underground wells, which pose a contamination risk to groundwater. Meanwhile, critics of Enbridge’s plan to abandon the old Line 3 pipeline also highlight soil and groundwater contamination. As the pipeline corrodes in the ground, residual oil, lubricants, treatment chemicals and pipeline coatings will escape.

The drainage of important wetlands could be a consequence of leaving the old Line 3 pipeline to disintegrate in the ground. This is because the pipeline may become a water conduit, which could drain and damage wetlands, as well as potentially flood farmland with contaminated water.

Increasing strains upon local water supplies is an inevitable consequence of the pipeline. In January 2021, for example, the breach of an aquifer during construction caused 24 million gallons of groundwater to escape. Minnesota’s Department of Natural Resources (DNR) fined Enbridge USD 3.32 million for the breach, citing the loss of resources and resulting threat to nearby wetlands and lakes. The DNR also said Enbridge failed to notify them at the time of the incident. More generally, tar sands oil extraction and refinement requires triple the amount of fresh water in comparison to conventional oil.