Ross Craig, it appears that the leach field is failing. The septic tank effluent cannot percolate into the soil at a rate equal to, or graater than, the application rate so it is surfacing. If the effluent cannot go down, it comes up. Placing more soil on top of the leach field will not improve, nor fix the failing system.
There are many factors that could be the cause of this failure. These factors include hydraulic overloading, organic overloading, improper leach field design, improper leach field construction, and several other factors in combination. I recommend that you contact the local on-site system permiting authority to investigate the failure, and get their recommendation as to the approved solution to the problem. Surfacing septic tank effluent poses an environmental, and public health treat, so take care of this problem promptly. JWA