What is a Percolation

Percolation is the process by which water moves downward through the pores and spaces in soil under the influence of gravity. In the context of drainage and wastewater management, it refers specifically to the rate at which water filters through the ground. This rate is a critical factor in determining the suitability of land for infiltration-based drainage solutions such as soakaways, drainage fields, infiltration trenches and Sustainable Drainage Systems (SuDS).

A soil’s percolation capacity directly influences whether it can safely and efficiently absorb and disperse surface water or treated effluent. Without adequate percolation, water may pool at the surface, cause flooding, create health risks or lead to system failure.

Why percolation matters

Percolation plays a central role in the design and operation of many drainage systems. It determines:

  • Whether a site is suitable for soakaways or drainage fields

  • The required size and layout of infiltration-based systems

  • The rate at which water will leave storage features and return to the soil

  • The likelihood of ponding, surface water accumulation or soil saturation

  • The potential for groundwater recharge or contamination

Correctly assessing percolation ensures that water is managed sustainably and that the system will continue to function under various weather conditions.

How percolation is measured

Percolation is typically measured through on-site percolation tests, which are conducted in trial holes excavated in the area where the soakaway or drainage field is planned. These tests are designed to simulate real-world conditions and measure how long it takes for water to infiltrate the soil.

In the UK, standard methods include:

  • Percolation test for drainage fields (BS 6297)
    A test hole is dug to the required depth, filled with water, and the time taken for the water level to drop is recorded. This is repeated multiple times to determine an average percolation time in seconds per millimetre of fall.

  • BRE Digest 365 method for soakaways
    Used to assess infiltration potential for surface water soakaways. The method calculates soil infiltration rate and soakaway volume based on test results.

  • Field infiltration testing for SuDS
    Larger-scale or multi-point testing used in complex developments, often involving permeameters or infiltration rings.

These methods give quantitative data used in system design, planning applications and environmental risk assessments.

Factors affecting percolation rate

Several factors influence how quickly water percolates through soil:

  • Soil texture and structure
    Sandy soils have high percolation rates due to large, interconnected pores. Clay soils have low rates because of their fine particles and compact structure. Loams fall somewhere in between.

  • Soil compaction
    Compacted or disturbed soils have reduced pore space and slower infiltration.

  • Organic content
    High levels of organic matter can retain moisture and affect flow behaviour.

  • Moisture content
    Soils that are already saturated will percolate more slowly than dry soils.

  • Presence of impermeable layers
    Hardpans or bedrock close to the surface may prevent vertical flow and cause waterlogging.

  • Biological activity
    Root systems, earthworms and other organisms can increase porosity and improve infiltration.

Site-specific conditions must always be assessed to determine real percolation performance.

Applications in drainage design

Percolation rates guide the choice, sizing and layout of various drainage systems, including:

  • Soakaways
    Require moderately permeable soils to allow water from roofs, roads or yards to soak into the ground without overflowing or stagnating.

  • Drainage fields (leach fields)
    Used for dispersing treated effluent from septic tanks. Percolation tests determine trench length, spacing and overall footprint.

  • Infiltration trenches and basins
    Designed to collect and gradually release runoff. Adequate percolation ensures these features remain functional and do not cause nuisance flooding.

  • Rain gardens and bioretention areas
    Often rely on natural or engineered soils with known percolation rates to manage stormwater sustainably.

Where percolation is too slow, alternative solutions such as attenuation tanks or pumped discharge may be required.

Regulatory and planning considerations

In the UK, Building Regulations Part H and guidance from the Environment Agency or local planning authorities require percolation tests for all proposed infiltration systems. This includes:

  • New septic tank or package treatment plant installations

  • Replacement of existing soakaways

  • SuDS integration for new developments

  • Flood risk assessments for sensitive sites

Planning approval may be refused if percolation is inadequate or if discharge poses a risk to groundwater or nearby properties.

Problems linked to poor percolation

If percolation is too slow, several issues can arise:

  • Surface ponding and waterlogging
    Standing water on lawns, roads or building foundations can damage property and pose safety risks.

  • Soakaway failure
    Poor infiltration causes water to back up into gullies, pipes or even into buildings.

  • Contamination risk
    Stagnant water near treatment system outflows can leach pathogens or nutrients into surrounding soil or watercourses.

  • Structural damage
    Repeated wetting and drying cycles may affect sub-base stability or cause erosion in poorly drained areas.

Conversely, soils that percolate too quickly may allow untreated water to pass into groundwater before sufficient treatment has occurred.

Improving percolation

In some cases, percolation can be improved or managed through interventions:

  • Avoiding compaction during construction

  • Incorporating soil conditioners or sand to improve texture

  • Installing subsoil drainage to manage groundwater levels

  • Dividing flows across multiple smaller soakaways

  • Using pre-treatment such as silt traps or filters to prevent clogging

However, such measures must be carefully evaluated, as forced improvement of unsuitable soils may not meet long-term performance or regulatory requirements.

Conclusion

Percolation is the rate at which water filters through the soil and is a fundamental factor in the planning and design of infiltration-based drainage systems. It determines whether soakaways, drainage fields and other sustainable solutions are feasible and how they must be configured to work effectively.

Accurate percolation testing, consideration of local soil conditions, and appropriate design responses are essential to ensure long-term system function, environmental protection and regulatory compliance. Whether dealing with a domestic soakaway or a complex urban SuDS scheme, understanding percolation is key to managing water where it falls.