# Configuration of Non-Pumping Reactive Wells Considering Minimum Well Spacing
## 3 Results and Discussion
### Practical Configuration of NPRW
#### 3.1.2 Configurations of NPRW Considering Field Constraints  
To design the optimum configuration of the NPRW, various configurations were devised by changing the well diameter and the transverse distance between the wells in the upgradient area to minimize the number of reactive wells and reduce the contaminant plume leakage. The well diameter is important for determining the construction equipment and the amount of reactive material filled inside the non-pumping wells. Depending on the purpose, wells of various diameters can be used for NPRWs. However, it is necessary to select a well with an appropriate diameter because the construction technology is limited, and the cost increases with increasing diameter.  
Well spacing is another important factor constraining well construction. Because narrow well spacing can increase the collapse risk of the geologic media, the engineers constructing the wells are advised to provide enough space between the wells. Here, the minimum well spacing is the minimum distance between any two wells that the engineer considers technically safe and is widely applied to prevent the collision of any well [43, 44]. In our recent field work, the new wells were subject to a restriction condition that they must be at least 1 m from the existing wells. Because the close distance between the designed wells is likely to cause well instability, it can be a construction constraint at the field site.  
However, the wider the spacing between the wells, the more groundwater may bypass the reactive wells, so the closer the well spacing, the higher the decontamination efficiency [45]. Therefore, when determining the spacing between wells, structural stability must be considered as well as pollutant removal efficiency. For example, the configuration of the zigzag array could not meet the minimum well spacing constraint applied on field (Figure 3a). To prevent collapse due to adjacent wells being too close, well spacing at least must be greater than minimum well spacing. And at the same time, to reduce the bypass of groundwater, it is necessary to properly determine the effective well spacing so that the spacing between the adjacent wells is not too wide. The minimum well spacing serves to determine the well spacing so that NPRW can be realistically designed, and the effective well spacing determines the appropriate well configuration so that NPRW can treatment groundwater economically and efficiently. Depending on the determination of effective well spacing, NPRW can have various configurations, and in this work, we proposed NPRW with sawtooth array (Figure 3b). The NPRW with sawtooth array is a practical configuration that satisfies the minimum well spacing, and requires fewer wells than the previously proposed NPRW when treat contaminant plume of the same width.  
Figure 2: Capture width of a NPRW according to ratio between the hydraulic conductivities of the geologic media and the filling material (K\({}_{w}\)/K\({}_{a}\)): (a) determination of capture width from the simulation (in case of d\({}_{1}\) = 0.2m); (b) relationship between the capture width and hydraulic condition (K\({}_{w}\)/K\({}_{a}\)); (c) relationship between the capture width and well diameter.