Two ideas on Bill's problem. Excessive water usage beyond system design and questionable soil conditions. In a retention system, if water usage exceeds the drainfields capacity, bleeding will occur at the shallowest point on the lowest line of the system. If that is the case, expanding the drainfield could remedy the problem. Although with the mention of clay soil, I would consult with my local health department or regulatory agency to make sure the soil conditions are proper. Hope my comments may be of some assistance.