4.4 Remediation methods for soil and groundwater

           Based on the toxic level of contaminants and the risk it pose to the environment, a suitable remediation method is selected. It must be noted that the remediation does not aim for entire decontamination. The major focus is to bring the contamination level well below the regulatory toxic limit. This is done by removing the toxic contaminants and/or immobilizing the contaminant that prevents its movement through subsurface geoenvironment. The remediation methods are broadly classified as physico-chemical, biological, electrical, thermal and combination of these methods.

4.4.1 Physico-chemical methods
Removal and treatment of contaminated soil
           One of the simplest physical methods for remediation is by removing the contaminated soil and replacing it with clean soil.  Essentially it is a dig, dump and replace procedure. Such a method is practically possible only if the spatial extent and depth of the contaminated region is small. The dug out contaminated soil can be either disposed off in an engineered landfill or subjected to simple washing as shown in Fig. 4.2.

Fig. 4.2 Soil washing for granular soils contaminated with inorganic pollutant

However, washing procedure is mostly suitable for granular soils with less clay content and contaminated with inorganic pollutants. For clay dominated soils, a chemical dispersion agent need to be added to deflocculate and then chemical washing is employed to break the retention of contaminants with the clay surface. Incineration is suggested for soils contaminated with organic pollutants. In case, it is necessary to remove organic pollutants then certain solvents or surfactants are used as washing agents.
            The method is directly applied in situ where solvent, surfactant solution or water mixed with additives is used to wash the contaminants from the saturated zone by injection and recovery system. The additives are used to enhance contaminant release and mobility resulting in increased recovery and hence decreased soil contamination.  
Vacuum extraction
           This method is one of the most widely used in situ treatment technologies. The method is cost-effective but time consuming and ineffective in water saturated soil. The technique, as depicted in Fig. 4.3, is useful for extracting contaminated groundwater and soil vapour from a limited subsurface depth. The contaminated water is then subjected to standard chemical and biological treatment techniques. Vacuum technique is also useful when soil-water is contaminated with volatile organic compound (VOC). The method is then termed as “air sparging”. Sometimes biodegradation is clubbed with air sparging for enhanced removal of VOC. Such a technique is then termed as biosparging.

Fig. 4.3 A schematic diagram for vacuum extraction procedure (Reddi and Inyang, 2000)