These "mounds" may be due to water from the drainfield which has frozen during the winter and then when it thaws, it leaves "soft" spots in the soil. This happens in many yards which do not have good drainage also. To determine if your drainfield is failing, you would need to determine the water level in the field. This can be done by opening the distribution box, if you have one, and observing the water levels in the laterals. If a satuated field is your probelm, I would expect to find water well up into the lateral pipes. Another way is to dig a posthole in the drainfield down to the stone soil interface. This should be about 24-36 inched deep and observe the water levels. In an ideal system, the water should not accumulate in the stone layer for any significant period of time. If there is any water in the stone, the level should not change from day to day. This indicates that the system is absorbing as much water each day as is being added. If the water levels are above the stone layer into the soil above, then the drainfield has failed and remediation work needs to be done. Please call if you have any quesitons.