Soakaway Water Quality

As the soakaway will discharge runoff into the soil and ultimately into the groundwater, it is important that the runoff does not contain any pollutants or contaminants which may adversely affect the quality of the groundwater.

The soakaway can have a treatment effect on the runoff if there is sufficient depth of soil between the soakaway base and the groundwater level. As the runoff percolates through the soil some contaminants are removed by the soil instead of travelling through to the groundwater.

There are many potential processes can take place once the runoff reaches the unsaturated zone of soils surrounding the soakaway which can remove or reduce the quantity of contaminants in the soil. These include the following;

  • Adsorption – This is the process whereby contaminants are retained within the solid matrix of the soil. Typically better adsorption is achieved through soils of mixed mineralogy, particularly where the soil includes a proportion of clay minerals. Soils made principally of silica such as sands and sandstones and those principally made of calcium carbonate such as limestones or chalk achieve poorer rates of adsorption.
  • Filtration – This is the process whereby contaminants are trapped within the pore space of the soil. The filtration rates achieved by the soil depends on the size of the pores in the soil which is generally proportional to the grain size and the depth of the soil in the unsaturated zone. This can also be affected by any fissures or fractures in the soil or rock which will allow contaminated runoff to enter the groundwater without any filtration effects.
  • Volatisation – This is when volatile organic compounds are able to vent to the atmosphere through a porous soil.
  • Biodegradation – This is the oxidation-reduction reaction of organic compounds which can occur under either aerobic or anaerobic conditions. This process is dependant on the chemistry of the soils and groundwater, microbial population and the contaminant properties. Under certain conditions this process can result in the total degradation of some organic compounds.
  • Abiotic Degradation – This is the process of the degradation of contaminants through chemical reactions. This is dependant on the soil and groundwater chemistry and the nature of the contaminants. This is typically less significant effect than biodegradation.

The assessment of the above factors will allow the designer to evaluate the effectiveness of the soils surrounding the soakaway in removing contaminants before they can reach the groundwater. In some cases this assessment change the design of the soakaway to limit infiltration through some soils which offer poor protection and maximise infiltration through other soils which provide better groundwater protection.

This assessment should also be used when determining the minimum required distance between the base of the soakaway and the highest groundwater level. This is explained in detail elsewhere in this document.

Some typical guidance on the removal of contaminants by soakaways taken from the SUDS Manual is shown below.

The potential contaminants in the runoff depends almost entirely on the type of surfaces which the runoff comes into contact with before it enters the soakaway. A few common catchment types are described below;

  • Roofs - Runoff from roofs which are allowed to proceed directly to piped systems does not have much chance of contamination and it can usually be assumed that any contaminants present in the runoff will either remain in the soakaway or be filtered by the surrounding soils.
  • Residential Areas - Similarly runoff from small domestic drives or paths would not generally require any additional treatment before being directed to the soakaway for discharge.
  • Car Parks – Runoff from small car parks does also not generally carry a risk of groundwater contamination from particulates. They will however require some method of intercepting oils from spillages such as an oil separator.
  • Minor Roads – Likelihood of contamination is generally low but an assessment taking account of site specific factors may be required. Again an oil separator will usually be required.
  • Major Roads – Similar to minor roads but a more detail assessment may be required depending on site specific factors.
  • Lorry Parks & Garage Forecourts – Detailed site specific assessment will be required.
  • Industrial or Major Commercial Sites - Detailed site specific assessment will be required.

Where a site specific risk assessment is required this should include an assessment of the risks, potential quantity and nature of any potential contaminants. A suitable assessment may take the following form;

  • Evaluate the runoff surfaces and type of catchment
  • Assess the risks and likely types and quantities of potential contaminants
  • Assess the possible impacts on groundwater resources
  • Check relevant legislation and groundwater quality standards
  • Design appropriate contaminant control methods
  • Create suitable maintenance plan for contaminant control measures
  • Design soakaway taking into account contaminant controls such as depth to groundwater and infiltration rate of surrounding soils

Some guidance has been issued in the past by the Environment Agency on the likelihood of soakaways being approved near to source protection of aquifer protection zones. This is shown in the below table taken from the SUDS Manual.

Where specific contaminants are expected these can sometimes be mitigated using physical or biological treatment processes such as separators, sedimentation basins or wetlands, filter strips, sand filters, swales or reed beds. These measures will require frequent maintenance to operate effectively and this should be considered at the design stage.

Sediment traps have proven to be very effective at removing large particulates from runoff if adequately maintained. Around 95% of particulates larger than 300μm can be removed. This rate however falls steeply as the particle size drops. Only around 15% of particles smaller than 60μm are removed.

The effectiveness of detention basins is dependent on the residency time achieved. Typical removal rates are around 90% total suspended solids, 70-80% lead, 40-50% zinc and 20-40% BOD or COD.

Biological processes such as wetlands, reed beds and swales can have a significant reduction in BOD, suspended solids and heavy metals. Where biological methods are employed an oil separator should be used upstream to prevent oil contamination. The efficiency of biological systems can also vary through the year when plant growth cycles are taken into consideration.

Related Spreadsheets from CivilWeb;

Soakaway Design Spreadsheet

This spreadsheet calculates the requirements for a soakaway system and assists the user to design a suitable system.

Attenuation Design Spreadsheet

This spreadsheet calculates the requirements for a attenuation system and assists the user to design a suitable system.

Runoff Calculator Spreadsheet

This spreadsheet calculates the design runoff flow for a site in accordance with the a number of different methods including the Wallingford Procedure.

Full Drainage Design Suite

Full drainage design suite (50% Discount) including 7 spreadsheets;

  • Colebrook White Pipe Design
  • Manning Pipe Design
  • Manning Open Channel Design
  • Linear Drainage Design
  • Runoff Calculator
  • Attenuation Design
  • Soakaway Design