Older and poorly designed septic tanks can frequently overfill during times of wet weather especially in the winter months. The main indication of this is the manholes around the house backing up or blocking during wet weather. The backing up is caused by the ground water table rising to a level that matches or exceeds the effluent level in the septic tank.
As the Septic System is discharging directly into the water table, the level of pre-treatment is likely to be extremely poor. Discharge from a Septic Tank is very dangerous and contains fecal coliforms containing E. coli and Kryptos Sporidium among billions of other harmful bacteria. If you have a well on site you should have your drinking water tested without delay.
You will need a Site Suitability Assessment to design a new Waste Water System taking into account all the unique features on the site, followed by Installation and Certification.