08/27/2026
A percolation test measures how quickly water moves into the soil. The stabilized result is recorded as MPI (minutes per inch).
That measured rate can be converted into a clean-water hydraulic loading rate expressed in GPD/ft² (gallons per day per square foot):
Clean-water rate = 897 ÷ MPI
The constant 897 comes from multiplying 1,440 minutes per day by 0.623 gallons, the volume of one inch of water covering one square foot.
That result is not automatically the LTAR (long-term acceptance rate). It represents water movement under the conditions of the test. LTAR must account for the quality of the effluent entering the drainfield and the soil’s ability to maintain that loading over time.
When pretreatment lowers the concentration of the material responsible for clogging the infiltrative surface, the standard LTAR can be adjusted using:
LTAR treated ≈ LTAR standard × (C standard ÷ C treated)
Where:
LTAR standard is the established long-term acceptance rate for ordinary septic-tank effluent.
C standard is the concentration of the controlling constituent in ordinary effluent.
C treated is the measured concentration of that same constituent after pretreatment.
The controlling constituent may be BOD (biochemical oxygen demand), TSS (total suspended solids) or another parameter used to represent the remaining clogging load. The same constituent and units must be used on both sides of the ratio.
If pretreatment reduces the controlling concentration, less organic and suspended material reaches the soil interface. This reduces the loading responsible for biomat development and allows the soil’s hydraulic capacity to become the controlling factor.
The adjusted LTAR still cannot exceed the soil’s sustainable hydraulic rate:
LTAR treated = min [LTAR standard × (C standard ÷ C treated), (897 ÷ MPI) × Fh]
Fh (long-term hydraulic factor) is applied to the clean-water test rate. It accounts for changing moisture conditions, soil variability and the difference between a short field test and continuous long-term operation.
The lowest calculated value controls.
This calculation separates two limits:
1. The amount of residual material the infiltrative surface can receive without excessive clogging.
2. The amount of water the soil can sustainably transmit.
Pretreatment can reduce the first limit. It cannot remove the second.
A soil classification estimates these conditions from observed soil properties. A percolation test physically measures water movement. They provide different information and should not be represented as the same test.
