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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 [[heavy metals|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 material.  

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 [[heavy metals|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), [[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.

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]].

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]].