I would be interested in what you decided to do. Your e-mail address suggests your job is at the Department of Natural Resources in Michigan. I would expect that the state DPW has some Civil Engineers with lots of experience in drainage, types of soil, and septic systems. It souds like there may be a clay layer beneath the sand. Nothing stops water flow better than clay and silt. The fines fill up the interstices and close off the passages through which water flows.