A tl:dr; Earthquakes and rain.
A: If you bury drums of radioactive material you want to make sure that they stay sealed. However, on a 10,000 year time span, you cannot assume much. A very strong drum right now could be very weak in 5,000 years. Or, with one strong earthquake 2,000 years from now, it could crush the drums you've buried deep in the desert. As rainfall seems from the surface and into the water table, it will pull the radioactive material with it, then making the water supply unsafe.
Yucca Mountain was a popular site for a long time because it's a deep salt mine. Salt mines are interesting because it means that it's been impossible for water to flow thru them for millennia, which is the time scale you're concerned about here. From an engineering stand point, Yucca Mountain was just about the perfect solution, but it got killed politically because being 100 miles away from Las Vegas was "too close" for elected officials to stomach.
http://en.wikipedia.org/wiki/Yucca_Mountain_nuclear_waste_re...
Although the design questions are fascinating, the bigger issue that we need somewhere to store all of the nuclear waste. It's actually sitting in a variety of temporary facilities right now that aren't as safe as WIPP or Yucca mountain simply because it's very hard to get politicians to agree whose backyard gets to be the forever home for our spent fuel rods.
Source: My father has spent the last 20 years of his career with the Department of Transportation and Department of Energy working with transportation of hazardous and nuclear waste and took trips out to Yucca Mountain and WIPP.