Planning and Ecology

Infiltration and percolation testing

Percolation testing is now a requirement as per the latest Natural Resources Wales (NRW) advice (opens in a new tab) for planning applications proposing private sewage treatment systems that discharge (less than 2 m³ per day) to a drainage field. This is to ensure that the waste effluent from the private sewage treatment system can infiltrate the soil matrix (to avoid pooling) and percolate at an appropriate rate to minimise risk of groundwater table and/or surface water contamination.

Below is the recommended method as outlined in Natural Resources Wales Environmental Permitting Regulations guidance (opens in a new tab) (Part B6.5, Version 3, March 2017) to determine the potential drainage field’s percolation value (Vp). Please complete infiltration and percolation testing using the following guidance:

• Avoid carrying out this test in extreme weather conditions such as drought, frost and heavy rain.
• Excavate at least two holes 300 mm square to a depth 300 mm below the proposed invert level (bottom of pipe) of the infiltration pipe and space them evenly along the proposed line of the subsurface irrigation system.
• Fill each hole with water to a depth of at least 300 mm and allow to seep away overnight.
• Next day, refill each hole with water to a depth of at least 300 mm and observe the time in seconds for the water to seep away from 75% full to 25% full (i.e. a depth of 150mm).
• Divide this time by 150. This answer gives the average time in seconds (Vp) required for the water to drop 1 mm.
• The test should be carried out at least three times with at least two trial holes. The average figure from the tests should be taken.

This is the percolation value Vp (in seconds)

• The average figure for the percolation value (Vp) is obtained by summing all the values and dividing by the number of values used.
• Drainage field disposals should only be used when percolation tests indicate average values of Vp between 15 and 100 and the preliminary assessment of the trial hole tests has been favourable.
• The minimum value of 15 ensures that untreated effluent cannot percolate too rapidly into groundwater.
• Where Vp is above the limit of 100, effective treatment is unlikely to take place in a drainage field as there will be inefficient soakage in this location which may lead to sewage ponding on the surface.

Provide the surface area of your infiltration system. For domestic premises, the floor area of the drainage field (A in square metres) required may be calculated from:

• A = p × Vp × 0.25 for septic tanks
• A = p × Vp × 0.20 for package sewage treatment plants where p is the number of people served by the tank (this should be the maximum number of people that could live in the house).

Vp is the percolation value described above. If in doubt, consult your professional advisor or local authority building control officer for advice.

Use the following calculations:

• For sewage treatment plant:
• Vp (percolation value) × P (number of inhabitants) × 0.20 = surface area
• For septic tank:
• Vp (percolation value) × P (number of inhabitants) × 0.25 = surface area