# Study on Fluorine Pollution in a Slag Yard
## 5 Numerical Modelling Investigation
### Boundary and Model Parameters for Flow Simulation  
The seepage field must be simulated as accurately as possible to obtain reasonable simulation results of contaminant migration. Here, the seepage field was simulated as saturated-unsaturated stable seepage. For the model in Figure 5, the boundary conditions were set as follows [22]. The upper surface accepted rainfall infiltration and belonged to the flux boundary. The flux at the slag surface was 3.2 \(\times\) 10\({}^{-8}\) m/s, which was equal to the local average annual rainfall of 1012 mm. The flux at the clay surface was 1.6 \(\times\) 10\({}^{-9}\) m/s, which was 5% of the rainfall. The flux of the slag surfaces was 3.2 \(\times\) 10\({}^{-8}\) m/s because of the following reasons. First, the saturated permeability coefficients of the slag (0.0001 m/s and 0.0001 m/s) were very high so the rain water on the slag surface easily flowed into the slag and could be stored in the lower slag. Second, the clay surface under the slag was sunken so it could keep the water from the slag surface and some water from the clay surface. Finally, the water stored in the lower slag was covered by thick slag, so the evaporation of the rain water to atmosphere was very little. The flux of the clay surfaces was 1.6 \(\times\) 10\({}^{-9}\) m/s because of the following reasons. The saturated permeability coefficients of the clay were very low, so most of the rain water on the clay surface was easy to run off. Some of the water evaporated off, so the water penetration through clay surface would be very low. The two border surfaces vertical to the x-axis were set as boundaries with constant hydraulic head: one of the constant hydraulic heads was the hydraulic head in the exploratory hole; the other was the hydraulic head of the lake at the boundary. The bottom surface was set as a boundary with a constant flux of zero. The seepage field of the 3D model in Figure 6 was also simulated as saturated-unsaturated stable seepage, and the following boundary conditions of seepage simulation were used [22]. The upper surface accepted rainfall infiltration, and the border belonged to flux boundary. The flux of the slag surfaces was 3.2 \(\times\) 10\({}^{-8}\) m/s while that of the clay surfaces was 1.6 \(\times\) 10\({}^{-9}\) m/s. The two border surfaces vertical to the x-axis were set as symmetric boundaries because they belonged to watersheds. The two border surfaces vertical to the y-axis were set as boundaries with constant hydraulic head: one was the hydraulic head in the exploratory hole on the boundary; the other was \(-\)61 m based on the simulation results of 2D FEM model. The bottom surface was set as a boundary with a constant flux of zero. Figure 6: 3D compact numerical simulation model: (**a**) the whole geometric model and (**b**) the FEM model. Soil-water characteristic curves for material 1 and material 2 were obtained from a volumetric pressure plate test, and the pore volume fractions were estimated from the corresponding soil-water characteristic curves. The residual water volume fractions for material 1 and material 2 were obtained from the drying method. The saturated permeability coefficients for material 1 and material 4 were estimated from a water injection test. Other parameters were determined according to the experience values in geotechnical engineering. All the saturated permeability coefficients of the materials, except those of material 2, are shown in Table 6 for simulation. The permeability coefficients of material 2 were important for determining the seepage field. Therefore, they were determined by inverse analysis based on the test results. Materials 1 and 2, whose soil-water characteristic curves were obtained from the test results, were set to be unsaturated materials. The soil-water characteristic curves of material 2 are shown in Figure 7, and the experimental curve of sample 82 was used for simulation.