The regional groundwater dynamics caused by the “low fluid-potential drain” (Hitchon, 1969) is associated with flow through highly permeable karst. The amounts of winter discharge from the Paleozoic karst into the Athabasca River allows the general estimation of the rate of groundwater with oxygen annually penetrating the oil sands layer. The associated oxygen and biodegradation are assumed to be important agents for the transformation of oil into heavy oil and tar. The paper puts emphasis on the often-neglected impact of groundwater dynamics on geochemical processes.