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.