# Configuration of Non-Pumping Reactive Wells Considering Minimum Well Spacing
## 1 Introduction  
Humans are the major geological and geomorphological agents in the Anthropocene [1]. The geologic media provides the foundation of human activity and variety of resources for sustaining human life, including groundwater. Groundwater is continuously replenished in the geologic media during the natural water cycle and has adequate water quality as drinking water by the removal of pollutants during the movement through the geologic media [2]. The groundwater is developed from the alluvial terrain as well as hard rock terrain and used as a major water resource in regions such as islands and mountains around the world [3]. The contamination of geologic media by industrial activities has been a serious problem threatening the stable supply of groundwater resources and human safety. Especially, in the case of the contamination of hard rock aquifer, it is a more serious problem due to the difficulties of recognition and remediation of contaminated deep subsurface environment. The current technologies for treating subsurface contamination can be divided into active methods such as pump-and-treat and passive methods such as natural attenuation. Active remediation systems typically require equipment, regular operation and management, and continuous chemical and power input [4]. Passive methods are a much more cost-effective in terms of the labor force and energy than active methods [5]. Permeable reactive barriers (PRB) enable prevention of contaminant spread in groundwater through passive treatment. Groundwater containing soluble contaminants encounters the reactive material in the PRB and passes through it, resulting in the removal of the contaminants. PRB technology has been applied to treat groundwater contaminated with heavy metals [6; 7; 8], nutrients [9; 10; 11], radionuclide [12], petroleum hydrocarbons [13; 14; 15], and chlorinated solvents [16; 17; 18]. The performance of PRB for groundwater contaminant removal is dependent on the performance of the reactive material, structure and configuration of the barrier, and aquifer conditions. The reactive material should be physically stable, chemically reactive, and hydraulically permeable for long periods in subsurface environments. Various studies have been conducted on nano-zero-valent iron [19], sulfidized nanoscale zerovalent iron [20], zeolite [21], aminated electrospun nanofiber membrane [22], sodium alginate/graphene oxide [23], functionalized silica [12], natural pyrite [24], etc. as reactive materials. The structure of the reactive barrier should be designed to remove all contaminants within the width and depth of the contaminant plume. PRBs are typically limited to depths lower than approximately 20 m, and consequently are not viable for greater depth of the contamination [25]. As an alternative to PRBs with continuous-barrier structures, an array of non-pumping wells was developed for contaminant treatment in deep aquifers [26; 27; 28]. The array of non-pumping wells was filled with highly permeable reactive materials, which would enable preferential flow of groundwater through them. This type of PRB could remove contaminants from groundwater flowing through its spaced wells. The deep aquifer remediation tool (DART) is a passive treatment technique where reactive material in the form of a cartridge is inserted into non-pumping wells [29]. This is cost-effective because easy retrieval, replacement, and disposal of the reactive material are possible after contaminant breakthrough [30]. The advantages of arrays of non-pumping wells and DARTs are that they can be customized depending on contaminants and site characteristics and that many configurations can be designed due to the advanced drilling technologies currently available. The array of non-pumping wells can be more easily adapted to variations in concentration and flow direction over time, compared to common PRBs and can be extended laterally in case of observed bypass of the reactive well front [25].