# A PHREEQC-Based Tool for Planning and Control of In Situ Chemical Oxidation Treatment
## 2 Model Concept
### Visualisation of Data (A3)
A visualisation can be performed for selected parameters for a certain time so that the user can easily follow an immediate reaction to the injected oxidant. A comparison of maps created for parameters that indicate the successful arrival of the oxidising agent could help to distinguish areas where oxidation is occurring from those that are more resistant to remediation. An ISCO operator can then plan further injections, taking into account areas where contaminant levels are still unsatisfactorily high and where the water does not meet quality standards. Estimation of the Next Injection Based on Oxidant (Permananganate) Consumption and Groundwater Flow Velocity (A4)  
The consumption of permanganate and the oxidation of COEC is simulated by advective-dispersive transport through a 1D column. The injection is represented by a solution of the Mn(7) species with a pE of 15. The oxidising agent is then transported through the contaminated medium, whereby the degree of contamination varies as the COEC concentrations at the individual measuring points representing the column are different. The transport of the oxidant also depends on the local hydrogeological conditions, which are determined by both the forward flow of the oxidant and the backward flow of the water stagnating in the micropores in the user-defined phase after injection. In both cases, some crucial parameters must be included in the models, such as the dispersivity and the diffusion coefficient. In order to reproduce the transport of the oxidising agent to the observation wells as accurately as possible, as well as the basic parameter describing the flow, the velocity should be specified. After importing the data from the loggers (via defining the path for data storage), you can use it to define the initial conditions for the geochemical model. Based on the imported data, the following parameters can be updated and used in the PHREEQC modelling:  
* Flow velocity;
* Temperature;
* Oxidation-reduction potential, which is expressed as pE;
* pH. If the \"Flow velocity\" option is selected, the velocity of the water, which is changed by the pressure of the injection, is calculated. This is estimated based on changes in the real-time temperature measurements. Based on field tests, it has been assumed that an increase of 2 \({}^{\circ}\)C over a period of 1 h indicates the inflow of an oxidising agent. When such a rise occurs, the distance between the injection point and the monitoring well is divided by the time of arrival of the oxidising agent in the monitoring well (Equation (2)). This gives the actual water velocity:  
\[\mathrm{V_{ir}}=\frac{L_{i}}{(T_{ai}-T_{0})} \tag{2}\]  
where \(\mathrm{V_{ir}}\) is the actual groundwater velocity between the injection point and the \(i\)th monitoring well, \(L_{i}\) is the distance between the injection point and the \(i\)th monitoring well, \(T_{ai}\) is the time at which the arrival of the oxidant is observed in the \(i\)th monitoring well, and \(T_{0}\) is the time at which the injection was started. If the temperature increase is negligible (less than 2 \({}^{\circ}\)C), it can be assumed that the oxidising agent has not reached the monitoring well and the user will be informed. In such a case, the user can maintain the theoretical value of the velocity previously calculated based on the hydraulic conductivity of the porous medium (Table S1). Therefore, the data from the sensors/loggers can provide important feedback on the actual flow conditions and subsequently feed into the hydrogeochemical model. The hydrogeochemical model again provides two very important pieces of information: the permanganate consumption rate, which accounts for the reactions with COECs and NOD, and the subsequent period of oxidant depletion, which allows the user to predict the decline in COEC concentrations. These two critical processes can be modelled in two phases (i.e., when the background water enters the system and ion exchange with microporous water can be added): the injection (the time to reach ROI) and the post-injection phase.