4.5 Some examples of in-situ remediation
Harbottle et al. (2006) have compared the technical and environmental impacts of taking no remedial action with those of two remediation technologies. The main objective of this study is to verify the sustainability of remediation technologies. The two remediation technologies evaluated in this study are solidification/ stabilization (S/S) and landfilling. In both these methods contaminants are contained rather than destroyed.
Therefore, it is extremely important to analyze the long-term effect to avoid any potential problems in future. In this study, sustainable remediation project is defined as the one that satisfies the five criteria listed as follows:
Criterion 1: Future benefits outweigh the cost of remediation.
Criterion 2: Overall environmental impact of the remediation method is less than the impact of leaving the land untreated.
Criterion 3: Environmental impact of remediation process is minimal and measurable.
Criterion 4: The time-scale over which the environmental consequences occur is part of the decision-making process.
Criterion 5: The decision making process
The site selected in this study was an industrial location polluted by BTEX (benzene, toluene, ethylbenzene and xylene) and TPH (total petroleum hydrocarbon). About 4400 m3 of contaminated soil has been remediated. The stabilization mix used was cement:bentonite of 2.5:1 and water:dry grout of 3.8:1. It was found that due to S/S, groundwater pollution reduced by 98 percent and the leachate from S/S sample was well within the limit. S/S process resulted in the increase in strength, reduction in permeability and increase in pH of the soil. The same quantity of contaminated soil has been landfilled at a distance of 96 km from the source. In long term, S/S has been found to perform better than landfill and no action taken for remediation. Other advantages of S/S are low material usage, low off-site waste disposal, potential ground improvement for immediate re-use, and lesser impact on the local community. However, the contaminants remain on the site which increases the level of uncertainty in long term. In the case of landfilling, long term impacts are less due to the fact that contaminated soil is removed from the site. The resources that need to be mobilized for landfill are more than S/S.
Ludwig et al. (2011) have explained the use of permeable reactive barrier (PRB) for the treatment of Cr6 in groundwater. PRB in the form of trench and fill system, chemical redox curtain or organic carbon based biotic treatment zone induce reduction condition for converting Cr6 to relatively immobile and non-toxic Cr3. The most efficient trench and fill application is granular zero valent Iron (ZVI) fillings, which rapidly converts Cr6 to Cr3. Alternatively, organic mulch and compost has been used to initiate microbially active Cr6 reduction. However, the use of organic matter as well as organic carbon does not have the longevity of ZVI. The study quotes an example of ZVI based PRB installed at North Carolina in 1996. This PRB is of 10 m depth, 0.6 m wide and 46 m long. This PRB is found to treat groundwater containing Cr6 (approximately 15 mg/l concentration) for more than 15 years. This study also quotes the use of chemical reducing agent such as sodium dithionite at US department of energy, Hanford, site for treating large Cr6 containing groundwater plume.
Asquith and Geary (2011) have compared bioremediation of petroleum contaminated soil by three methods, namely, biostimulation, bioaugmentation and surfactant addition. Bioremediation process depends on microbial activity for biodegrading petroleum hydrocarbons. Since it is a natural process, it is a slow reaction. The above mentioned three methods are used for increasing the rate of bioremediation reaction. Biostimulation enhances the growth and activity of microorganisms by the addition of nutrients and/or additives. Bioaugmentation is the addition of hydrocarbon degrading microbial cultures. Surfactant addition would enhance solubility, emulsify and disperse hydrophobic contaminants to overcome the problem of low contaminant bioavailability. Sandy loam soil with total petroleum hydrocarbon (TPH) > 30000mg/kg has been used to evaluate the three methods. It was noted from this study that biostimulation with nutrients enhanced bioremediation process. Organic amendments provided a better bioremediation than inorganic amendments. Surfactant addition was found to increase bioavailability of hydrocarbon and hence enhance bioremediation.
Ascenco (2009) has discussed about contaminated site characterization and clean up based on two case studies. The first case study pertains to the excavation and washing of soil in an industrial estate site of 0.12 km2. Preliminary investigation of the site revealed contamination upto a depth of 6m with TPH, volatile aromatics such as toluene, ethylbenzene and xylene. Soil was found to be free of heavy metals. A quantitative risk assessment indicated the need for remediation. 40000 tonnes of soil was excavated from the affected site and subjected to soil washing. Washing has been performed in a unit with a capacity of 70 tonnes/ hour. Washed soil has been declared safe after adequate laboratory testing and the clean soil reused in the site. The soil has been first homogenized and sieved. The required surfactant and extracting agents were mixed with water and used for soil washing. The waste water which comes out after washing has been treated and reused. Contaminated sludge and fines after waste water treatment and oversized soil mass rejected during sieving have been transferred to landfills.
The second case study is another industrial area of 3 km2 near Lisbon. The industrial site comprised mainly of organic and inorganic chemistry industries producing pesticides, acid, copper, lead, zinc, iron pyrites etc. The site consists of 52000 tones of hazardous sludge from zinc metallurgy and iron pyrite ashes. The site required investigation and remediation due to the placement of an airport in the vicinity of this site. The groundwater exhibited high levels of arsenic, lead, mercury, cadmium, copper, zinc, cobalt. In some areas the pH was as low as 1, which increased metal mobility. The investigations were mainly focused on developing a conceptual site model and environmental risk analysis for defining remediation options. The efforts are still on for this particular site.