Good for the Earth, Good for Your Septic Tank
In almost all areas, you have to get a permit from the county or city to rebuild a septic tank or leach field - permit cost typically $150-600, though can reach $2000 in some areas for a new installation. The reason is they will then come out and inspect the installation and review the test results from the civil engineer, from whom a design and soil percolation test and construction inspection report will be required before they issue an operating permit.
If you have a question on the process, the county or city most likely has a permit requirements info sheet on their website describing the process and requirements - googel your locale name combined with a search phrase like this - leach field test
The real reason a permit is required is to be sure an engineer designs the field, that the leach field is designed to match the existing soil conditions, and that it is built to the design so hopefully it will neither break through to the surface, contaminate the groundwater, nor contaminate nearby wells or waterways, thereby creating a public nuisance and health hazard.
The reason for the initial presale infiltration capacity test you had done is to provide some assurance to the buyer that the system is likely to work as needed, at least for the forseeable future, and is based on the number of bedrooms and sometimes number of bathrooms in the house as the basis for estimating the "load" the system is likely to see. There are different tests in different locales - some use a minimal criteria of 3 or 4 gpm flow for 20 minutes, other use the more common design guide of 150gpd (gallons per day) per bedroom in the house- some prorated for a 1 hour period, some far more strict by requiring a 24 hour test. 300 gallons in 45 minutes is an extremely stringent criteria - I would check with the buyilding department that this is right, because this adds up to a rate of 9600 gallon sper day - of about 64 bedrooms worth of capacity ! I am sure I have never ever seen that strict a criteria for a residential (as opposed to resort/ hotel/ commercial) septic system, so I would check that the tester did not somewhere add a zero to the volume to be pumped or something.
Also, returning water is not a viable basis for failing the test - the proper way to conduct a test is to pump the specified amount of water, and verify that the water level at the discharge pipe has returned to the original level by the end of the specified time - usually to the invert (bottom) of the pipe, but generally a conditional or "warning" approval will be given to one that does not raise the water table above the top of the 4" discharge pipe during the test time. You might discuss this with the civil engineer before going ahead with a new leach field construction - to me it sounds like the test may have been rigged to fail,, because in-service leach fields would not take 300 gallons of water without raising the water level to the top of the pipe for a period, even though they could take that flow for an extended period ot time.
I would guess that the wrong term was used to describe your problem. What was probably meant was that the leach field was saturated. This is to say that the soil around your leach field is saturated with solids from the septic tank and you may have to have it replaced. If you do not have the tank pumped often enough this can happen as well as just aging of the system. Many people ignore the septic till there is a problem do to out of sight out of mind or the false belief they are saving money. If you have a large family you may have to have the tank pumped every 2 or three years. Some of the towns around here have passed laws to that effect to avoid such problems.