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The geology where the spill happened is problematic. The surface consists of deep unconsolidated glacial sediments that serve as a local aquifer. Glacial sediments can be highly variable in grain size and thus in permeability and porosity. Yet there is no doubt in my mind that the spill will contaminate the surficial aquifer in the area. However, the seriousness of the problem will depend upon the particular characteristics of the site where the spill happened.
Of course, the spill brings into question the wisdom of the permitting of the Keystone XL Pipeline near the Standing Rock Indian Reservation's source of drinking water (note that the spill happened on the Keystone Pipeline which is different from the Keystone XL Pipeline). For more background on this issue, you can read my series about the Keystone Pipeline and the Standing Rock Sioux here.