# Modeling and Parametric Simulation of Microplastic Transport in Groundwater Environments
## 3 Results and Discussion  
Figure 3 shows the simulation results for the transport of MPs in the reference case. MPs are supplied upstream at the 2 min mark and transported downstream over time, as shown in Figure 3a. As the concentration C increases at each location, so does the concentration S (Figure 3c). After the upstream supply of MPs stops at the 10 min mark, the concentration C decreases at each location. Here, Equation (6), which concerns only the occurrence of attachment, is used instead of Equation (5), which concerns both attachment and detachment. The MPs are therefore not detached from the surface of the soil once they become attached (Figure 3d). Figure 4 shows the simulation results for the transport of MPs with increased MP diffusivity. As the transport rate of MPs increased, the downstream concentration increased more rapidly (Figure 4a,b). The attachment rate and amount of attached MPs also increased (Figure 4c,d). These simulation results were obtained using a constant upstream concentration. Therefore, these results correspond to the conditions in which sufficient MPs are supplied upstream. Under the boundary condition in which the amount of supplied MPs is constant, the surface values at the upstream boundary (x = 0) may be lower than C = 10,000 and S = 1000. The boundary condition in this case can be modeled using the time-dependent boundary condition of C. However, to simulate situations such as the accidental bulk leakage of MPs into groundwater environments, the current boundary condition of a constant upstream concentration of C is more appropriate. Therefore, the diffusivity is an important parameter for MP transport in groundwater environments because a higher diffusivity means greater and faster leakage into groundwater environments. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Simulation ID** & **Condition** & **Comment** \\ \hline S0 & Reference case & See Table 1. \\ S1 & \(D=4\;D_{0}\) & \\ S2 & \(v=2\;v_{0}\) & \\ S3 & \(\theta=2\;\theta_{0}\) & \\ S4 & \(k_{a}=0.5\;k_{a0}\) & \(k_{a0}=\frac{3(1-\theta_{0})}{2\beta_{500}}\eta_{0}\alpha_{0}v_{0}\) (by setting \(d_{50}=2\;d_{50,0}\)) \\ S5 & \(\rho=2\;\rho_{0}\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Simulation condition. Figure 2: MP transport simulation. Figure 5 shows the simulation results for the transport of MPs with an increased water flow velocity through a pore space. A noticeable difference compared to the reference case, S0, is that the concentration S almost doubles. This is attributed to the increased influx of MPs with a higher flow velocity, as well as a sufficient upstream supply of MPs (see  
Figure 4: Simulation results for case S1 (\(D\) = 4 \(D_{0}\)): (**a**) distribution of C at various times, (**b**) variation in C at various locations, (**c**) distribution of S at various times, and (**d**) variation in S at various locations. Figure 3: Simulation results for reference case S0: (**a**) distribution of C at various times, (**b**) variation in C at various locations, (**c**) distribution of S at various times, and (**d**) variation in S at various locations. Equation (3)). The rate at which S grows over time increases with an increase in ka, as shown in Equation (6). In summary, an increase in the flow velocity of water leads to an increase in the amount of attached MPs. Considering water management and leakage accidents, the pressurization of the contaminated soil surface area may be utilized such that the leaked MPs may be contained within the soil. The contaminated soil could then be removed later. Many geotechnical engineering solutions for achieving such pressurization have been developed for application [45]. Figure 6 shows the simulation results for the transport of MPs with an increased porosity. Similar to the previous case, wherein an increased water flow velocity is investigated, the concentration of the attached MPs, S, increased noticeably compared to the reference case, S0.