First time in Belgium: Implementation of zerovalent iron for source zone treatment via soil mixing
The soil and groundwater at a textile manufacturing site in Flanders is polluted with chlorinated solvents. Trichloroethylene is found in the groundwater in very high concentrations (426 mg/l). Much less 1,2-dichloroethylene and vinyl chloride has been detected in the groundwater. The pollution in the soil is only trichloroethylene. A pure product layer has been detected at a depth between 7,2 and 7,6 m bgl. The soil consists of sandy clay and the permeability is very low.
Practice has shown that the source zone can’t be removed by traditional techniques such as pump and treat. Excavation would be complicated because the pollution is situated near a building and at a great depth. To excavate the pure product layer expensive measurements are required. Because of this, excavation is not BATNEEC. In situ chemical reduction of the DNAPL by injection of ZVI (zero valent iron) may be a solution.
The chemical destruction of the chlorinated solvents by addition of ZVI was examined by VITO in lab tests. These tests proved a degradation of more than 95% at the dose that was selected for field application. After a period of 8 weeks, a carbon source to stimulate biodegradation was added. With the C-source, a further decomposition of the chlorinated solvents was achieved.
On large scale, it is often a problem to keep the iron in suspension during injection and to distribute the iron equally over the polluted zone because of the great density of iron and permeability limitations of the soil. To counter this problem, the iron was suspended in a guar gum slurry which was distributed in the subsurface by soil mixing. During drilling, the ZVI-slurry is injected under high pressure and mixed with the soil at the same time, creating a soil mix pile.
For this particular case, a total of 14 soil mix piles were executed successfully until 8,4 m bgl, whereby 3500 kg of fine sized micro scale ZVI was applied. The guar gum will be biodegraded in time with release of simple sugars. This is expected to stimulate the anaerobic biodegradation of the chlorinated solvents, which would complement the chemical reduction by the ZVI.
The soil in the soilmix columns was sampled 12 months after soil mixing. The concentration decreased from 43700 mg/kg dm to 81,5 mg/kg dm. The concentration of iron in the soil is still high.
The groundwater concentrations measured until 12 months after soil mixing prove that trichloroethene is converted to the degradation products cis+trans 1,2-dichloroethene and vinyl chloride by the ZVI. Relatively high concentrations of ethene are measured. The groundwater and soil concentrations will be further monitored.
This pilot has already shown that soil mixing may be a promising alternative to injection of ZVI by direct push or by injection in wells. The soil mixing can be a solution for the treatment of chlorinated solvents in high concentrations in dense soils and at great depth without removing a lot of soil.