Context
As part of the redevelopment of a former industrial site to a new residential neighborhood, a preliminary phase of remediation is to be carried out. The remediation company contacted INJECTIS to focus on low-permeability zones contaminated with chlorinated solvents (PCE, TCE, cis-1,2-DCE) in groundwater.
The site is divided into sub-zones, and the overall objectives are based on a 60% reduction of contaminant mass, with target concentrations of chlorinated solvents below 100 µg/L. The technical approaches for each zone were developed by the remediation company. Prior to injection, pumping tests and a pilot test using TAM injection were conducted, but both techniques didn’t yield satisfying results. Consequently, the zones were treated using direct injection, with a combination of traditional Direct Push (DP) technology and, for the less permeable zones, our SPIN® injection technology. In a zone, after two DP phases, it was decided to also inject a specific sub-zone using the SPIN® injection technology, as DP injections showed poor efficiency due to pollutant concentration rebound and heterogeneous distribution of injected reagents, as evidenced by electrical conductivity (EC) analyses performed by the remediation company (see below).
Reaction and reagents
Based on the results of preliminary laboratory and on-site pilot tests, the remediation contractor selected sodium persulfate as the most suitable oxidant for the in situ chemical oxidation (ISCO) of the chlorinated ethenes. As preparing and injecting persulfate necessitates all equipment composed of materials that are resistant to corrosion and oxidation, the SPIN® injection technology of INJECTIS was selected as the most suitable injection technique and subcontractor for this job. Injectis is one of the only injection companies that is fully equipped to inject such corrosive reagents and has the capacity to operate with multiple injection rigs simultaneously on full-scale projects of this size.
In ISCO treatment, chemical reaction and hence physical contact is the key parameter to guarantee successful remediation results. In addition to using the SPIN® injection technology, the remediation company opted for electrical tomography measurements to verify the quality of the reagent distribution.
Geology and distribution monitoring
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.
Location: Bretagne, France
Geology: Sandy Silt and clayed layer
Pollutant(s): Average sum COVH: 60.000 μg/l. Max PCE : 115.000 µg/L
Reaction: ISCO
Reagent(s): Sodium Persulfate
Application type: Grid application
Surface/length: 1.000 m²
Number of points: 150 (around 50 /phase)
Depth interval: 3,0 – 12,0 m-bgl
Dosage: 200 l/m