# A PHREEQC-Based Tool for Planning and Control of In Situ Chemical Oxidation Treatment
## 1 Introduction  
The pollution of soil and groundwater by organic pollutants is a major problem in industrialised and urbanised areas as the use of hydrocarbons increases. It is estimated that potentially polluting activities have taken place at a total of 2.8 million sites in the member countries of the European Environment Agency (EEA) [1, 2]. Mineral oils and heavy metals are the main pollutants, accounting for around 60% of soil contamination. In 2016, only 115,000 contaminated sites were remediated in the EU, which corresponded to 8.3% of the potentially contaminated sites currently registered. In the USA, on the other hand, groundwater contamination by organic compounds was detected at 300,000 to 400,000 sites [3, 4]. Contaminated sites, even if abandoned, still pose a serious threat to the environment, and they should be revitalised. Conventional treatment methods, such as pump-and-treat technology, are often costly and only partially effective as they are difficult to implement in densely built-up areas. It is hoped that these inconveniences will lead to the introduction of more cost-effective, smaller-scale technologies. However, the introduction of a new technology requires thorough research. The calculation methods that enable prediction could significantly reduce costs and increase effectiveness. In addition, real-time numerical simulations can be routinely used to support and monitor ongoing remediation activities. Originally, the main method of treating groundwater contaminated by organic compounds was to pump it out and purify it on site, i.e., the pump-and-treat method. The application of this ex situ technique was associated with some technical and economic problems (the annual cost of pump-and-treat services reached billions of dollars in the United States [5]). In general, the removal of contaminated soil and groundwater for treatment requires a trade-off between cost, feasibility, and technical limitations. Ex situ methods are considered when the depth of contamination, the concentration of the contaminant, the type of contaminant, and the space required for treatment allow for it [6; 7; 8]. Therefore, the challenges of the pump-and-treat process have led to the development of alternative in situ methods such as in situ chemical oxidation (ISCO). However, this method also has its limitations. For example, it should be carried out in homogeneous soil. If layers with more impermeable soils are present, the oxidising agent cannot flow into the contaminated area, and the contact between the reagent and the contaminated water is limited. In addition, the mass of contaminants can be sorbed, thus changing the dose of oxidising agent required for successful remediation. In addition, ISCO is also not recommended for the treatment of sites where LNAPL is present. In these areas, it is more advisable to use the pump-and-treat method [9; 10; 11] first. During injection, oxidants degrade the contaminants of concern (COECs), i.e., chlorinated hydrocarbons, fuels, phenols, polycyclic aromatic hydrocarbons, polychlorinated biphenyl, explosives, and pesticides. Currently, there are several oxidising agents that are commonly used or tested on site. Their brief characteristics and the targeted pollutants are presented in many publications [12; 13; 14; 15; 16; 17; 18]. According to the US EPA, ISCO is the fastest growing remediation method for hazardous waste. The number of publications (over 5000) with keywords related to chemical oxidation also shows that it is an important and emerging field of research [19]. The ISCO process still needs to be tested, improved, and provided with practical guidelines [20; 21; 22] as the precise, almost surgical injection of the liquid and the accurate estimation of the oxidant dose at the time of use, as well as the rebound effect and the loss of oxidants due to reactions with the groundwater matrix [19; 23; 24; 25], represent a real challenge. As the process is very dynamic and invasive, its control is also a problem [26].