We also don't get 20,000+ crowds as often as we'd like (or would like others to think we do).
There's 2,000 spaces at the stadium, as far as I remember, plus another 300 odd in the speedway stadium next door, and another 1,000 or so just down the road at HP. There's still a lot use buses...
It is possible to build a stadium on a contaminated site - we did it at the Madejski. When you come next month have a look for the methane vents - tall metal tubes dotted around the site. It does cost a barrowload of cash, though, and is to be avoided if there is an alternative. With us there...