## 4 Discussion  
### The Screened Technologies for the Case Site  
The screened technologies include institutional control, risk monitoring, emergency response, multiphase extraction, interception ditch, monitoring of natural attenuation, and hydrodynamic control. These technologies have been selected and widely used in managing various contaminated sites [33; 34; 35; 36], but combining these technologies together to remediate one site is seldom performed.  
Previous studies always aimed at a certain layer of the aquifer or considered several layers as a whole [37; 38; 39]. Therefore, there was always only one optimal technology for each location. In the present study site, two aquifer layers have been contaminated. According to the present developed method, the different layers should be treated discriminately, which resulted in the two optimal technologies at one location. For example, for blocking the transportation of contaminates, interception ditch and hydrodynamic control were both selected, but the former is for the upper (phreatic) aquifer and the latter is for the lower (micro-confined) aquifer. The interception ditch is hardly operated when the depth is greater than 15 m and, therefore, was not suitable for the lower aquifer, while in the shallow aquifer, it is easily developed, operated, and maintained [40; 41; 42], and, therefore, was selected for upper aquifer in the study. The other technology, hydrodynamic control, is not suitable for the clay medium [43] and, therefore, was not been selected for the upper aquifer, but is suitable for the sandy lower (micro-confined) aquifer. Separately treating for different characteristic stratum or contaminant concentrations is the precondition for precise remediation and can avoid ineffective remediation.  
In practice, combining more than one technology to remediate contaminated soil and aquifer is not rare [44]. For example, Fenton processes and biotereatment were combined for soil remediation [45]; microbial combined methods, including microbe-biochar, microbe-nutrition, and microbe-plant technologies were used to remediate the petroleum-contaminated soils [46]; a plant-microbial combined bioremediation of polychlorinated naphthalene-contaminated soil was established using the intelligent integration of analytic hierarchy process and formula evaluation methods [47]; a sustainability assessment methodology for prioritizing the technologies of groundwater contamination remediation was established [48]. In comparison, combining multiple techniques to remedy a contaminated site is rare. A classic case is combining five technologies to remediate a PHC site, which was a former gasoline service station located in southwest Ohio [49]. At first, the remediation plan consisted of a combined groundwater pump and treat and soil vapor extraction. However, accompanied by the remediation processes, to meet the requirements, the plan had to be adjusted, and natural attenuation needed to be monitored, so surfactant-based soil washing and catalytic oxidation had been added empirically. The case represents most remediation projects that choosing the techniques is experiential and lacking systematicness. In our study, the contaminated soil and groundwater, as well as the receptors, were deemed as a system, and the techniques can be chosen systematically. Combining with the assessment methods and compatibility evaluation, an effective and reasonable approach is expected to be found.