Is the existing Northern Border Pipeline (which is natural gas as opposed to DAPL's crude oil) a comparable environmental hazard, or is DAPL provably more risky?
Natural gas and crude oil spills really aren't comparable, especially in terms of environmental hazard.
A natural gas spill mainly evaporates into methane gas and dissipates. Wikipedia describes this process as follows:
LNG is normally stored and transported in liquid form at a temperature of approximately −161 °C (−258 °F). If this cooled liquid is released from a storage facility, pipeline, or LNG transport ship, then it begins to warm. As LNG warms above its storage temperature, the liquid begins to vaporize. The resulting gas produced by this warming is typically methane, which is the major component (with some ethane) of natural gas.
— LNG spill
So long as the gas does not ignite, and people don't suffocate from the gas, it just dissipates into the atmosphere. Of course, methane is a greenhouse gas which would still impact global warming to some extent, but let us not get side-tracked.
On the other hand, crude oil spills are messy and do not dissipate so-readily. They tend to cause lasting environmental problems and require expensive cleanups that aren't completely effective.
I think it's worth mentioning, even if the risks of environmental damage by a spill were comparable, doubling the number of pipes would effectively double that risk for that area, which by itself would still be problematic.
This claim is rather vague, as safety could refer to a lot of things. Is DAPL less likely to cause environmental damage than existing oil transport methods in the area?
I suppose this claim depends on how you define "safety". If we define it as "amount of oil spilled", which seems the most-relevant to the environmental concerns, then the statistics may not back this claim.
A December 2014 report by the Congressional Research Service on this issues contains the following bar-chart.
Assuming that the most-recent stats are still roughly accurate, then pipelines spill a smaller percentage than tanker trucks, but more than both railroads and tanker ships combined.
One small caveat that I think should be addressed here is that they specifically say "underground pipelines", but this chart does not differentiate. However if we take one industry website's word, most pipelines are actually underground.
Most crude oil pipelines are underground, except for pump stations and valves.
How the different transportation methods specifically impact this one area is perhaps a little less-clear, aside from the obvious difference of not having a crude oil pipeline crossing the river. A similar earlier plan involved crossing the Missouri River north of Bismarck was rejected at least in-part due to protest and concern for water safety there.