Mobility Classification of Chemicals in Soil

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Mobility Classification of Chemicals in Soil

Little Pro on 2019-12-05

The mobility of a chemical substance is primarily measured by soil adsorption coefficient (Kd or Koc). Low Kd or Koc indicates high mobility in soil. A highly mobile substance (i.e., pesticide) is more likely to leach to groundwater or run off to surface water bodies such as lakes and rivers. It is essential that we classify the mobility of a chemical substance in soil according to unified criteria.

There are two well-recognized soil mobility classification schemes: McCall, et al. classification and FAO classification. We will compare them in this article.

McCall's Soil Mobility Classification Scheme

For a long time, McCall et al.'s classification scheme is the most frequently used soil mobility classification scheme for chemicals. Koc is the only parameter needed to determine the mobility class of a compound in soil.

Range of Koc	Mobility class
0 - 50	Very high
50 - 150	High
150 - 500	Medium
500 - 2000	Low
2000 - 5000	Slightly
> 5000	Immobile

FAO Soil Mobility Classification Criteria Based on KOC

FAO has developed its own soil mobility classification criteria for pesticides and they are recommended for use by the US EPA.

KOC (mL/g or L/kg)	Log KOC (mL/g or L/kg)	Mobility Class
< 10	< 1	Highly Mobile
10-100	1 - 2	Mobile
100-1,000	2 - 3	Moderately Mobile
1,000 - 10,000	3 - 4	Slightly Mobile
10,000 - 100,000	4 - 5	Hardly Mobile
> 100 ,000	> 5	Immobile

Reference

https://www.epa.gov/pesticide-science-and-assessing-pesticide-risks/guidance-reporting-environmental-fate-and-transport
http://www.fao.org/3/X2570E/X2570E06.htm
McCall P.J., Laskowski D.A., Swann R.L., and Dishburger H.J., (1981), “Measurement of sorption coefficients of organic chemicals and their use, in environmental fate analysis”, in Test Protocols for Environmental Fate and Movement of Toxicants. Proceedings of AOAC Symposium, AOAC.

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