Coupled Flow and Transport of Ions

3 DIRECT AND COUPLED FLOW AND TRANSPORT OF IONS 26.3.1 Flow due to Hydraulic Gradient 

The water flow through clay soils under a hydraulic gradient is governed by Darcy s law as shown in Equation 26.1  

The hydrauhc conductivity of soils varies over several orders of magnitude ranging from 1 x 10 m/s for coarse sand to 1 x 10 m/s for fine-grained dense clays. In fine-grained soils, very high hydraulic gradients need to be generated to cause even a small flux. Therefore, the pump and treat method of cleanup or bioremediation is not effective in clayey soils. 

The electroosmotic flux is analogous to Darcy s law and can be formulated as shown in Equation 26.2  

The electroosmotic conductivity does not vary much between different soils and ranges between 1 x 10 and 1 x 10 °m /(V s) (Mitchell, 1993). The electroosmotic conductivity is dependent on many different factors. Casagrande (1949) derived the following relationship  

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