Your description indicates many potential problems with your system. If I interprete corrently, the drainfield is over four feet deep and fifty years old. Most people would put the life of a drainfield as 20-30 years. Therefore your system is past due to be replaced. The best way to evaluate the field would be to try Septic-Scrub. You would want to dewater the field. You can do this by pumping the septic tank and then digging a posthole in the drainfield through the stone to the soil below, in your case about five feet. Pump water from this hole back into the septic tank until no more water flows into the hole. Your system probably has clay tile for the drainfield lines. There is no easy way to add a riser to this system. You might add the product through the tank outlet as per the directions found on this web page. If you have any questions, please do not hesitate to call.