# Activation of Persulfate for Groundwater Remediation:
## 4 Factors Affecting the Application of Persulfate in ISCO
### Radius of Influence of Persulfate  
The radius of influence (ROI) of persulfate is significantly related to the soil permeability and transport ability of persulfate in the soil, which affects the transport length of the persulfate around the injection well, and thus determines the placement of injection wells. In laboratory studies, ROI of persulfate is mainly evaluated from the breakthrough curves of persulfate in the flow field column. Even more, the convective and dispersion coefficients can be also obtained and used to calculate the ROI. In practical remediation sites, ROI is usually calculated from groundwater persulfate concentrations in observation wells constructed at different distances from the injection well in different directions.  
The main processes associated with persulfate transport include advection and diffusion. Several studies in the literature have tested the diffusion process of persulfate in column experiments. One-dimensional column experiments combined with tracer tests after persulfate injection were performed in a relatively high hydraulic conductivity medium by [PERSON] (2016) and the longitudinal dispersity (\(\alpha_{\mathrm{L}}=2.431\pm 0.002\) cm) and retardation factor (R = 1.10) were obtained [131]. [PERSON] et al. (2010) tested the persulfate transportability in two low-permeability soils (kaolin and Palouse loess). The results showed that persulfate diffused into both soil types under each treatment condition and the transport distance of persulfate in the kaolin (4 cm after 82 days) was relatively shorter than that in the Palouse loess (12 cm after 70 days). In addition, the higher persulfate concentrations and the longer transport time enhanced the persulfate diffusion process [132]. Persulfate diffusion can be enhanced by electrokinetic delivery [133]. The longest delivery distance reached 10 cm after 10 days when electrokinetic delivery was coupled with alkaline activation [133].  
ROI of persulfate in practical field applications can be calculated by measuring persulfate concentrations in monitoring wells placed in various directions at the injection site. The transport length of persulfate was estimated by [PERSON] et al. (2010) and ranged from 0.1 m to 23 m, however very little data were used, even without mentioning the kind of soil media [44]. [PERSON] et al. (2017) estimated the ROI of persulfate was 3.6 m to 4.5 m in a loamy sand remediation site wherein the groundwater velocity was 0.5 m per day [120]. [PERSON] et al. (2014) reported that a slight rise of persulfate was observed in downstream monitor wells which was 4.8 m far from the injection well [134]. [PERSON] et al. (2010) reviewed 29 ISCO cases and concluded the median value of the observed ROI was 7.62 m and the design ROI was 4.27 m, which indicated that the spread of oxidants in the soil was wider than anticipated [128]. Groundwater quality such as ORP, DO, temperature, and conductivity are sometimes employed as indirect indicators of oxidant ROI. After the oxidant has fully reacted, these parameter values can spike in groundwater, but the residual effects of the oxidant can still be determined, resulting in ROI false positives [128]. The hydraulic conductivity (K) profile can vary by an order of magnitude over brief vertical sections when measured across screened intervals in groundwater wells [135]. Under these circumstances, an ROI artifact and an excessive amount of oxidant may be delivered into high K layers [128]. For a more precise evaluation of oxidant transport and ROI, injection and monitoring well screens that are representative of discrete vertical intervals would be helpful in diagnosing this condition.