### 2.25.7.6. Interface between water table and unsaturated zone[¶](#interface-between-water-table-and-unsaturated-zone "Permalink to this headline")

We assume Henry’s Law equilibrium at the interface between the saturated and unsaturated zone and constant flux from the soil element below the interface to the center of the soil element above the interface. In this case, the coefficients are the same as described above, except for the soil element above the interface:

\\\[\\frac{D\_{p1} }{\\Delta x\_{p1} } =\\left\[K\_{H} \\frac{\\Delta x\_{j} }{2D\_{j} } +\\frac{\\Delta x\_{j+1} }{2D\_{j+1} } \\right\]^{-1}\\\]

\\\[b=\\left\[\\frac{R\_{j}^{n+1} }{\\Delta t} +\\frac{1}{2\\Delta x\_{j} } \\left(K\_{H} \\frac{D\_{p1}^{} }{\\Delta x\_{p1} } +\\frac{D\_{m1}^{} }{\\Delta x\_{m1} } \\right)\\right\]\\\]

(2.25.18)[¶](#equation-24-18 "Permalink to this equation")\\\[r=\\frac{R\_{j}^{n} }{\\Delta t} C\_{j}^{n} +\\frac{1}{2\\Delta x\_{j} } \\left\[\\frac{D\_{p1}^{} }{\\Delta x\_{p1} } \\left(C\_{j+1}^{n} -K\_{H} C\_{j}^{n} \\right)-\\frac{D\_{m1}^{} }{\\Delta x\_{m1} } \\left(C\_{j}^{n} -C\_{j-1}^{n} \\right)\\right\]+\\frac{1}{2} \\left\[S\_{j}^{n} +S\_{j}^{n+1} \\right\]\\\]

and the soil element below the interface:

\\\[\\frac{D\_{m1} }{\\Delta x\_{m1} } =\\left\[K\_{H} \\frac{\\Delta x\_{j-1} }{2D\_{j-1} } +\\frac{\\Delta x\_{j} }{2D\_{j} } \\right\]^{-1}\\\]

\\\[a=-K\_{H} \\frac{1}{2\\Delta x\_{j} } \\frac{D\_{m1}^{} }{\\Delta x\_{m1} }\\\]

(2.25.19)[¶](#equation-24-19 "Permalink to this equation")\\\[r=\\frac{R\_{j}^{n} }{\\Delta t} +C\_{j}^{n} +\\frac{1}{2\\Delta x\_{j} } \\left\[\\frac{D\_{p1}^{} }{\\Delta x\_{p1} } \\left(C\_{j+1}^{n} -C\_{j}^{n} \\right)-\\frac{D\_{m1}^{} }{\\Delta x\_{m1} } \\left(C\_{j}^{n} -K\_{H} C\_{j-1}^{n} \\right)\\right\]+\\frac{1}{2} \\left\[S\_{j}^{n} +S\_{j}^{n+1} \\right\]\\\]

