Design and implementation of remediation of the old ecological load in the area of the industrial complex contaminated with Cr⁶⁺

The Cr⁶⁺-contaminated area, on which remediation was taking place, was located in the area of what was originally a machine and then an automobile plant located in the South Moravian Region, Brno-town. Specifically, it concerned the area under the building of the former chrome plant, where more significant indications of subsoil pollution were recorded already in 1995. Confirmation of significant contamination took place in 2006 as part of processing an updated risk analysis. In 2015 and 2016, there was an additional survey and update of the risk analysis in the given area, which concluded that during the operation of the chrome plant, massive Cr⁶⁺ contamination occurred not only in the building itself but also in the surrounding bedrock. Through precipitation infiltration, Cr⁶⁺ then migrated to the lower parts of the rock profile, where the saturated zone located at a depth of about 20 m below ground level was contaminated. As a result of the groundwater flow, the contamination cloud subsequently shifted and expanded. Remediation in the form of complete removal of contaminated soil down to the groundwater level to a depth of approx. 20 m would not be realistic, therefore, based on the information obtained, such measures were implemented, which on the one hand prevented further contamination of the saturated zone and on the other hand eliminated the spread of the toxic form of chromium to more distant areas areas of its reduction and immobilization in the rock environment. The remediation itself was preceded by a set of laboratory experiments, which proved that the reduction of toxic Cr⁶⁺ to Cr³⁺ by nanoparticles of zero-valent iron (nZVI) leads to its rapid sorption to the rock environment and the resulting hydrated forms of iron oxides, and as a result to stable immobilization in the rock environment. From the middle of 2017 to June 2018, an effective remediation intervention was carried out in the area of the automobile plant, consisting of the removal of the building of the former chrome plant, the extraction of contaminated soil, the creation of an impermeable layer, the construction of a network of application wells and subsequent full-scale remediation of the saturated zone through the pressure application of nZVI. After the pressure injection of the activated form of nZVI, a rapid and effective reduction of the toxic form Cr⁶⁺ to Cr³⁺ and its immobilization in the rock environment due to sorption to the surrounding soil and the resulting hydrated iron oxides were observed. There was also the creation of long-lasting areas with a reducing environment, which are capable of still reducing Cr⁶⁺ flowing through these places together with the groundwater long after the application. (authors)