For soil washing, contaminants sorbed onto fine soil particles are separated from bulk soil in a water-based system on the basis of particle size. The wash water may be augmented with a basic leaching agent, surfactant, or chelating agent or by adjustment of pH to help remove organics and heavy metals. Soils and wash water are mixed ex situ in a tank or other treatment unit. The wash water and various soil fractions are usually separated using gravity settling.

4.19 Soil Washing (Ex Situ Soil Remediation Technology)
In Remediation Technologies Screening Matrix and Reference Guide, Version 3.0. Federal Remediation Technologies Roundtable.

Understanding Soil Washing
CL:AIRE Technical Bulleting TB 13, 4 pp, 2007