## 3 Results and Discussion
### In Situ Remediation with a Field-Scale Study  
In this study, actual pollution cases in Taiwan (Taoyuan City) were simulated using laboratory adsorption experiments and the adsorption equations of the Langmuir and Freundlich models. This pollution incident is the RCA pollution case, which occurred in Taoyuan, Taiwan in 1994. Due to the long-term digging of wells by the manufacturer to dump organic solvents and other toxic wastes, the soil and groundwater in the factory area were seriously polluted. Therefore, the Taiwan Environmental Protection Agency announced in 2004 that the RCA plant is a groundwater pollution remediation site, and the main pollutants in this area are volatile chlorine-containing organic compounds. The pollution area of this pollution remediation site is approximately 7.2 hectares; the TCE concentration in groundwater is 4.54 mg L\({}^{-1}\), which is 90.8 times the control standard; and the PCE concentration is 7.22 mg L\({}^{-1}\), which is 144 times the control standard. After years of remediation treatment, the remediation agency pointed out that the pollution concentration exceeding the standard has been greatly improved. Therefore, the method of estimation in this study is based on when the situation where the pollution situation was relatively serious at the beginning. The quality of the adsorbents (silica gel and activated carbon) required to remove the local organic pollutants and the required removal costs were predicted. The depth of the groundwater contaminated with organic matter is 15-30 m [45]. The initial pollution concentration and water volume in the pollution case were substituted into the Langmuir and Freundlich models (Equations (9) and (13)) to calculate the mass and amount of silica gel and activated carbon required to use as the adsorbents. At present, the prices of industrial-grade activated carbon (high-carbon, powder, top grade, Huiheng Activated Carbon Co., Ltd., Ningxia, China) and silica gel (purity 99.99%, powder, Mantanghong Abrasives Co., Ltd., Zhengzhou City, China) on the market are 2.5 USD and 0.55 USD per kilogram, respectively, and the price of activated carbon is approximately 5 times higher than that of silica gel. [PERSON] [46] used activated carbon as the adsorbent for the permeable adsorptive barrier, applied it to the remediation of groundwater, and obtained a good removal efficiency. Therefore, in this study, the TCE and PCE concentrations were reduced to low concentrations by silica gel, and then activated carbon was added. In the process of adding silica gel for adsorption, it was found that, if the TCE concentration was lower than 1.362 mg L\({}^{-1}\), the adsorption effect of silica gel on the TCE would decrease from 0.408 mg L\({}^{-1}\) per kg to 0.280 mg L\({}^{-1}\). When the PCE concentration was lower than 2.166 mg L\({}^{-1}\), the adsorption effect decreased from 0.530 mg L\({}^{-1}\) to 0.366 mg L\({}^{-1}\) per kg. The best adsorption effect of the silica gel occurred when the TCE and PCE concentrations decreased from 4.54 and 7.22 mg L\({}^{-1}\) to 1.362 and 2.166 mg L\({}^{-1}\), respectively. Therefore, these two absorbent concentrations were used to convert from the silica gel to the concentration point of adding the activated carbon to achieve the best adsorption efficiency and adsorption cost for the TCE and PCE. If silica gel is used to adsorb TCE and PCE to a lower concentration, then activated carbon is added to adsorb to the maximum allowable concentration of TCE and PCE in drinking water established by the US Environmental Protection Agency (0.005 mg L\({}^{-1}\)). The masses of silica gel and activated carbon required to adsorb the TCE were 6427 \(\pm\) 172 tons and 343 \(\pm\) 21 tons, respectively, and the total cost of the adsorbent was approximately 4,390,000 USD. The adsorption of PCE required 7553 \(\pm\) 57 tons of silica gel and 350 \(\pm\) 68 tons of activated carbon, and the total cost of the adsorbent was approximately 5,030,000 USD (Figure 8).