I know that in our case, the leach field IS under the driveway. At least that's what the sketch showed when we bought the house 4 years ago.
To that end, I want to eventually replace the concrete driveway with pavers, to be able to get more moisture down through the leach field. The pavers are a better idea in our area anyway, since they won't deteriorate the way that concrete will.
But, that project is a few years in the future...