As I understand it (at the risk of over-simplification) there are a couple of environmental concerns.
1. Nebraska is concerned that a pipeline leak would damage their aquifer. Legitimate concern but way overblown IMO. If the Alaskan pipeline could be designed some 40 years ago and built across terrain infinitely more difficult (including three mountain ranges), more pristine & environmentally sensitive, one would think a reasonably safe pipeline could be placed through Nebraska.
2. This one I have even more difficulty with. "Environmentalists" don't like oil from oil sands because it's "dirtier" than oil from a drilled well. It's harder to get and harder to refine. Here's the thing -- it's gonna be produced (by Canada) and we we don't want it, other nations will happily take it. Meanwhile, our supplies will continue to suffer.
Canada is our next-door neighbor, and a friendly neighbor at that. It makes more sense to trade with them than to depend on unreliable sources. Obama's siding with the "environmentalists" on this issue is, IMHO, incredibly short-sighted and irresponsible. If he gets his way, we'll be passing up a golden opportunity to help ourselves as a nation, creating greater economic strength and a more stable energy supply.
