What is a percolation test in Ireland?

A percolation test, commonly called a T-test, is a field procedure set out in the EPA's Code of Practice for Domestic Wastewater Treatment Systems that measures how quickly water percolates through the subsoil at a proposed site. It is carried out by a suitably qualified person as part of the wider site suitability assessment, and the result — the T-value — is central to sizing the percolation area needed to safely disperse treated effluent from a septic tank or treatment unit.

The test involves digging test holes to a specified depth, pre-soaking the soil, then timing how long the water level takes to drop a set distance, with the result expressed in minutes per millimetre (or minutes per 25 mm, depending on the method). A lower T-value means faster-draining, more free-draining soil; a higher T-value means slower drainage requiring a larger percolation area to treat the same volume of effluent.

The test is normally carried out alongside a trial hole assessment that examines the soil profile itself — texture, structure, presence of rock or clay layers, and depth to the water table — because the T-value on its own does not tell the whole story: very fast-draining gravel can actually fail a site (insufficient natural filtration, risking groundwater contamination) just as effectively as very slow clay can (insufficient dispersal capacity).