3:45 PM - 4:00 PM
[HRE12-02] Application of Reactive Transport Modeling to Surplus Soil Processing
★Invited Papers
Keywords:Reactive Transport Modeling, Surplus Soil Processing, Dissolution Kinetics Law, The Soil Contamination Countermeasures Act, Water-Rock Interactions
In last two decades, there has been concern over the elution of heavy metals, acidic water, and other toxic components contained in the surplus soils generated during tunnel construction, and the large amount of cost and labor required to dispose of this material has been a major obstacle to the smooth progress of construction. The Soil Contamination Countermeasures Act revised in 2010 also covers soil containing hazardous elements from natural causes, and the evaluation and treatment of surplus soil from construction is now conducted in accordance with the act. In other words, the characteristics of surplus soil are evaluated only by leaching characteristics, and the degree of impact is evaluated only by advection and dispersion of leached substances, without considering the leaching adsorption mechanisms of elements. On the other hand, Reactive Transport Modeling, which models water-rock interactions and advection-dispersion, is being used overseas in fields such as groundwater quality assessment and prediction, underground storage of carbon dioxide, and geological disposal of nuclear waste (Maher and Mayer, 2019).
The effectiveness of Reactive Transport Modeling has been confirmed in various fields, and I believe it is useful for understanding the phenomenon of elemental leaching from construction surplus soil and for designing countermeasures. Therefore, I attempted to simulate the changes in soil water and leachate quality within the test embankment by one-dimensional Reactive Transport Modeling using PHREEQC.
The Reaction Model for the analysis was set up as follows. The mineral content of the surplus soil was calculated from the bulk composition by the method of Ohta et al. (2013). The diameter of the contained minerals was assumed to be 0.1 mm for silicate minerals, 0.001 mm for sulfide and carbonate minerals, and spherical in shape. Mineral dissolution kinetics laws were determined according to Sverdrup and Warfvinge (1995), Williamson and Rimstidt (1994), Zhang et al. (2004) and Plummer et al. (1978).
The Transport Model was set up as follows. The test embankment was modeled as a one-dimensional column with a height of 2 m and a cross-sectional area of 40 m2. The flow rate was set to 4.778 x 10-8 m/s based on the measured leachate volume. The dispersion length and diffusion coefficient were set to 0.5 and 1×10-9m2/s, respectively. The initial condition of the column was assumed to be saturated with precipitation in equilibrium with the atmosphere. The molar volume and surface area of each mineral in the column were calculated assuming a specific gravity of 2.0 and a porosity of 0.5 for the surplus soil.
In the soil water in the test embankment, pH showed an increasing trend after the start of the test and remained constant at around 8.0 after about one year. In the simulation, the pH tended to increase immediately after the start of the test and then converged to a constant value while slightly increasing. The As concentration in the soil water showed an increasing trend for 2.5 to 3 years after the start, and then reached a constant. As concentrations in the leachate were continuously low immediately from the start of the test. The results of simulation for As concentrations show that the soil water above the middle of the embankment initially rises and then begins to decrease, and the concentration in the lower part decreases rapidly immediately after the start and remains at a low level. The results of simulation, in which the Reactive Transport Model was applied to the embankment of construction surplus soil, can be considered to reproduce to some extent the actual changes in the quality of soil water and leachate. Therefore, the application of the Reactive Transport Modeling to the leaching of elements from construction surplus soil is considered to be effective in understanding the phenomena and designing countermeasures.
The effectiveness of Reactive Transport Modeling has been confirmed in various fields, and I believe it is useful for understanding the phenomenon of elemental leaching from construction surplus soil and for designing countermeasures. Therefore, I attempted to simulate the changes in soil water and leachate quality within the test embankment by one-dimensional Reactive Transport Modeling using PHREEQC.
The Reaction Model for the analysis was set up as follows. The mineral content of the surplus soil was calculated from the bulk composition by the method of Ohta et al. (2013). The diameter of the contained minerals was assumed to be 0.1 mm for silicate minerals, 0.001 mm for sulfide and carbonate minerals, and spherical in shape. Mineral dissolution kinetics laws were determined according to Sverdrup and Warfvinge (1995), Williamson and Rimstidt (1994), Zhang et al. (2004) and Plummer et al. (1978).
The Transport Model was set up as follows. The test embankment was modeled as a one-dimensional column with a height of 2 m and a cross-sectional area of 40 m2. The flow rate was set to 4.778 x 10-8 m/s based on the measured leachate volume. The dispersion length and diffusion coefficient were set to 0.5 and 1×10-9m2/s, respectively. The initial condition of the column was assumed to be saturated with precipitation in equilibrium with the atmosphere. The molar volume and surface area of each mineral in the column were calculated assuming a specific gravity of 2.0 and a porosity of 0.5 for the surplus soil.
In the soil water in the test embankment, pH showed an increasing trend after the start of the test and remained constant at around 8.0 after about one year. In the simulation, the pH tended to increase immediately after the start of the test and then converged to a constant value while slightly increasing. The As concentration in the soil water showed an increasing trend for 2.5 to 3 years after the start, and then reached a constant. As concentrations in the leachate were continuously low immediately from the start of the test. The results of simulation for As concentrations show that the soil water above the middle of the embankment initially rises and then begins to decrease, and the concentration in the lower part decreases rapidly immediately after the start and remains at a low level. The results of simulation, in which the Reactive Transport Model was applied to the embankment of construction surplus soil, can be considered to reproduce to some extent the actual changes in the quality of soil water and leachate. Therefore, the application of the Reactive Transport Modeling to the leaching of elements from construction surplus soil is considered to be effective in understanding the phenomena and designing countermeasures.