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Summary:

Interface between Water Table and Unsaturated Zone
- Assumes Henry's Law equilibrium at the interface between the saturated and unsaturated zone
- Assumes constant flux from the soil element below the interface to the center of the soil element above the interface

Coefficients for Soil Element Above Interface:
- Equation 2.25.18 defines the coefficient D_p1/Δx_p1
- Equation 2.25.18 defines the variable b

Equation 2.25.18 for Soil Element Above Interface:
- Describes the formulation for the variable r

Coefficients for Soil Element Below Interface: 
- Equation 2.25.19 defines the coefficient D_m1/Δx_m1
- Equation 2.25.19 defines the variable a

Equation 2.25.19 for Soil Element Below Interface:
- Describes the formulation for the variable r

The summary concisely covers the key points about the interface between the water table and unsaturated zone, the equations and coefficients for the soil elements above and below the interface, and the variables defined in those equations.