Cationic exchange capacity(CEC)

Cationic exchange capacity (CEC) is an indicator of the capability of the soil 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 particles. Positively charged dissolved metals ions (i.e., cations) are attracted to the negatively charged soil 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), organic matter 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.

Testing

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 (ASTM International, 2010). Descriptions of acceptable laboratory equipment for measuring CEC are described in the Soil Fertility Handbook (OMAFRA, 2006).

Evaluation

Construction inspections

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 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 growing media should be prescribed by the designer, product vendor or media supplier.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

Decayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil.

A thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens.