Soil Types for Septic Systems

Soil Type Comparison Table

How Soil Affects Septic System Design

When wastewater effluent leaves your septic tank and enters the drain field, it percolates downward through the soil where a complex biological and physical filtration process removes pathogens, nutrients, and contaminants. This process requires a specific balance: the soil must drain fast enough to accept the daily volume of wastewater without backing up, but slow enough to allow adequate contact time between the effluent and soil organisms for effective treatment. Sandy soils drain quickly but may not provide sufficient treatment time, while clay soils provide excellent treatment but may not drain fast enough to handle the daily volume.

The percolation test measures this balance by timing how fast water drops in test holes dug at the proposed drain field site. Most states accept percolation rates between 1 and 60 minutes per inch for conventional drain field systems. Soils outside this range require alternative designs: very fast soils may need a sand filter to slow percolation, while very slow soils may need a mound system built with imported fill material above the native soil. The absorption rate derived from the perc test determines the required drain field area using the formula: drain field square footage equals daily wastewater flow divided by absorption rate.

The drain field multiplier in the table above indicates how much larger the drain field needs to be compared to the ideal sandy loam baseline. A multiplier of 1.0 means standard sizing, while a multiplier of 3.0 means the drain field must be three times larger. This multiplier directly impacts installation costs since a larger drain field requires more excavation, more gravel, more pipe, and a larger area of your property dedicated to the system. For poor and unsuitable soils, the cost increase from alternative system designs can double or triple the total installation cost compared to a conventional system in suitable soil.

Soil Cards