Normally this happens with septic systems that have a mechanical discharge pump, but it can also be identified on sites with no pumps in the septic system at all. In the cast majority of cases the percolation area has blocked with sludge and scum from the septic tank. This could be due to lack of frequent emptying or the system could be inadequately designed to cater for the loading, causing the effluent to pond on the garden.
Discharge from a septic tank is very dangerous and contains fecal coliforms containing E. coli among billions of other harmful bacteria. Until the issue is rectified you should fence off any area children and pets could make contact with the waste water.
Firstly a site suitability assessment will be completed at design a new system taking into account all the unique features on the site, followed by installation.