Thank you for your inquiry. Water from a well functioning drainfield does not evaporate up to the surface. The reason not to pave over the field is to help keep the soil aerated down to the level of the laterals. If the field is paved over, then air cannot penetrate the surface. It is important to help keep the drainfield aerobic so that the build up of anaerobic sludge is minimized and the field keeps functioning within design limits.

Whether 30 feet is significant is your situation depends on the size of the field and how much of the remainder of the field is left uncovered. If this is an insigificant portion of the total area of the drainfield, then the risk is less. 

If you do pave it over, you may want to consider Septic-Scrub additions several times a year to help maintain the aerobic conditions and drainage.