Lets just say you are a bit ignorant of such things. A major oil spill in the aquifer area would cause massive issues. I used to do the risk assessments for a plant on the Ogalla aquifer, and oil containment was a huge concern.
If the leak is underground, it can leach into the soil and the water supply before it is detected. There are a lot of small towns in the area that have contaminated wells because of a leaking underground gas tank (why you can still put those in areas that frost is something I have always wondered about).
Depending on your state (and if you are in Oklahoma I will admit to being ignorant about the regs), next time you fill up walk around the pad and look for little iron or plastic well covers. Those are monitoring wells to find any leaks from the tank. They are checked to see if there is any gas getting into the soil, which can get into the soil water supply. Now, as you say, oil doesn’t have that much affinity to water. But there are things in gas that do.
And the Keystone-XL is a crude oil pipeline, not a products pipeline.
According to the national pipeline map, there is already a web of pipelines across the Sand Hills.