Cationic exchange capacity(CEC)
Cationic exchange capacity (CEC) is an indicator of the capability of the soil, filter media or green roof media, to retain dissolved, positively charged elements such as metals, which are a common pollutant in stormwater runoff. Soil has the ability to retain dissolved metals due to the negative charge of clay and organic matter (OM).
Positively charged dissolved metal ions (i.e. cations) are attracted to the negatively charged particles which can cause them to be removed from solution and retained in the soil. CEC is influenced by soil texture (higher in fine textured soil), OM content (higher in organic soil), and pH (lower in acidic soil). Soils with high CEC are able to retain a larger proportion of dissolved metals and other positively charged pollutants, while soils with low CEC will retain less. The CEC of a soil sample is the sum of the exchangeable cations in the sample and expressed in milliequivalents (meq) of positive charge per 100 grams (g) of soil.
Design specifications for the soil component of LID BMPs pertaining to CEC are intended to ensure the media has adequate capacity to remove positively charged dissolved pollutants from the stormwater they receive. Where CEC is too low, dissolved metals and other positively charged pollutants may not be well retained and the BMP will not provide the targeted water treatment performance. Causes of low CEC in the media component of LID BMPs can include excessively coarse texture, deficient organic matter content or that the soil has become saturated with positively charged ions (i.e., dissolved metal retention capacity has been exhausted). To ensure LID BMPs will provide the targeted water treatment performance, soil sampling and submission for laboratory testing of CEC by a soil testing laboratory accredited in the province of Ontario should be done as part of construction, assumption and verification inspections.
A commonly used laboratory test method is to saturate a sample of the soil with a known quantity of cations and measure the amount retained by the soil. An acceptable test method is provided in ASTM D7503-10 Standard Test Method for Measuring the Exchange Complex and Cation Exchange Capacity of Inorganic Fine-Grained Soils. Descriptions of acceptable laboratory equipment for measuring CEC are described in the Soil Fertility Handbook .
If laboratory testing indicates soil CEC is not within the design or product specification range, notify the media or topsoil supplier, issue a “do not install” order to the construction site supervisor and contact the design professionals and property owner or project manager to determine corrective actions.
Assumption and Verification inspections
If laboratory testing indicates soil/media CEC is not within the design or product specification range, or acceptance criteria range, schedule investigative work to do further sampling and testing to determine the affected area and depth and decide on corrective actions. Corrective action could involve amendment of the soil with compost or removal and replacement of an uppermost portion of the soil with material that is within the design or product specification range. Corrective actions to address CEC deficiency in green roof media should be prescribed by the designer, product vendor or media supplier.
- Ontario Ministry of Agriculture F and RA. Soil Fertility Handbook. (Reid K, ed.). OMAFRA; 2006. http://www.omafra.gov.on.ca/english/crops/pub611/pub611.pdf. Accessed October 17, 2017.