With the pipeline dozens of feet below layers of sand, clay, and rock, the earth would act to slow the movement of oil should a leak occur, giving responders time to detect and address the problem. “If you’re 92 feet below a river and we’re talking about maybe a barrel of oil a day, that’s not going to transport anywhere quickly,” Horn said. “It’s not as though you’ll have a leak and see a giant oil slick on the river right away.”

Other possibilities of increased risk buried deeper, and the length of the pipe:

The above statements mostly say the pipeline planned to go under Lake Oahe has increased safety due to the depth below the lake that the pipeline is planned for. The statement by Horn suggests that the monitoring system would definitely detect a leak before enough oil spilled to contaminate the lake. This assumes the monitoring system works correctly and works at all. The system is made by people, therefore it could fail. If it does fail, there is a lot more oil that will be contained in the increased depth of soil, than would be in a shallow tunnel, also the increased depth increases pressure on the pipe, as well as the pressure from the body of water, the lake resting on the soil above the pipe. If the monitor does not work, by the time the oil percolates to the top of the soil, which is the bottom of the lake, and then the oil rises to the top of the lake where it can be seen, after a possibly significant amount of time that a person happens to be there to observe the oil, the quantity of oil will be so large, the clean-up will be extremely difficult, in addition to the logistics of removing oil from soil underneath a lake. In addition the logistics of assembling and installing a pipeline that deep, for a length of 7,800 feet without any mistakes is extremely difficult; adding to the risk there will be mistakes which could cause a leak. Also the wider the body of water that is being crossed also increases the risk that there will be an oil leak into that body of water, so a 20 foot wide tributary would be safer to cross than a mile wide lake. If there is a one in a million chance of any one inch of pipe developing a leak, than two sequential inches has a 2 in a million chance, 2000 inches has a 2000/1000000 or 0.2 percent chance, and any one of the inches in 7,800 feet has a 9.36 percent of a leak, almost 10 percent, that is way too high of a risk for contaminating a major water source. Any one of those 93,600 inches that develops a leak will contaminate the lake equally.