Summary
Depth interval: 1,0 – 7,0 m-bgl
Context
In a metal processing plant, soil and groundwater pollution was detected near the chromium plating baths, up to 10 m deep in a clayey silt.
The most appropriate remediation technique was in situ remediation by injection of reagents because of the low mobility of the pollutants and the great depth reached.
Reaction
The reagents were proposed to obtain fast reducing conditions, especially in the most permeable layers of the site, but also to ensure a deep treatment with the combination of the ELS® solution (Lecithin-based substrate of food-grade carbon used to enhance anaerobic bioremediation) having a guaranteed longevity of 3 years and to allow time for the reagents to reach the less permeable parts of the area in order to optimize the treatment results.
Reagents
The site to be remediated is extensive and its geology is characterized by a high horizontal and vertical heterogeneity. For the low-permeability and heterogeneous zones, the remediation contractor selected the SPIN® injection technology as the most suitable injection technique.
The selection of the most suitable injection technologies was based on a preliminary study by the remediation company. For this purpose, they developed a method to monitor injection distribution using electrical tomography, placing electrodes inside monitoring wells located around and within the injection zone. These electrodes allowed for the detection of changes in electrical resistivity before and after injection.
Since sodium persulfate is an ionic salt, its presence in the groundwater alters the soil’s electrical resistance. This enables, through interpolation and graphical representation (including 3D visualization), the assessment of the injection’s impact, radius of influence and reagent distribution and allowed adjustment of the injection grid after each injection phase. This ensures the most homogeneous treatment of the area and maximizes the efficient usage of the injected reagents by optimizing the direct contact between the oxidant and the contaminants. The combination of accurate injections with high resolution monitoring technologies significantly enhances remediation effectiveness and helps achieve better outcomes.
Accessibility
Another challenge on this site was the limited accessibility, namely the presence of the pollution underneath an underground parking lot. The source and plume zone were only accessible via a car elevator with a maximum height of 2,0 m. For these conditions, Injectis has developed the Micro-SPIN® injection rig. This rig has an overall driving height of 1,95 m and a width of 78 cm, which makes the rig fit through a normal door opening, or in this case the car elevator.
