Excavation is generally THE way to address minor soil remediation. Relatively simple, it is also the least uncertain. Considering the limited amount of time required to complete the work, this technique is often relatively inexpensive. With a skilfully developed groundwater dredging technique, we can easily excavate below the natural level of the groundwater.
However, complex soil contamination, with chlorinated solvents (VOCl) for example, is generally characterised by (i) a highly concentrated, but relatively small pocket of contamination, and (ii) a diffuse, low-concentration plume, which can sometimes be of immeasurable size. To get to the heart of complex soil contaminations, we can carry out excavation work. When eliminating the pocket, we often extract most of the pollutant load with the excavation of a limited volume of highly contaminated material. Moreover, the removal of the pocket helps ensure that the plume is no longer fed by the contaminating factors. During removal of such pockets, the strategic development and location of the dredging are extremely important to reach the target objective.
In the case of complex soil contamination, excavation is always the least appropriate to absorb the contamination plume. Due to lower contaminant concentrations, the removal of the pollutant load as well as the efficacy of the means implemented decrease (kg pollutant load/cost €). In numerous cases, an in situ technique that does not require soil excavation is used to absorb the plume, but with which the contaminant is eliminated or destroyed on site.
Air sparging involves the injection of compressed air and the extraction of the air from the soil. This in situ technique is based on the principle of contaminant volatilization and can therefore be used for soil contaminated with volatile compounds such as BTEX, VOCl, benzine
During air sparging, the compressed air is injected in the saturated soil beneath the existing contamination. The injected air vaporizes as it comes to the surface bringing the volatile compounds with it. The volatile compounds are thus captured with the injected air in an air extraction system. The air collected is then transported to an air decontamination facility to remove the contamination. When the compounds are easily biodegraded in the presence of oxygen, the injection of compressed air stimulates the biodegradation of the soil contamination further.
Air sparging is more particularly used for soil contaminated with extremely volatile organic compounds with limited affinity to organic matter. Given that the technique is based on movement through the ground, a permeable and preferably homogeneous soil is necessary for the effective implementation of this technique.
Pump & treat
The pump and treat technique consists in pumping the contaminated groundwater and treating it in a water decontamination facility at the surface. The decontaminated groundwater can then be totally re-infiltrated, or partially or totally evacuated. During the pumping of the contaminated groundwater, a new equilibrium is continuously established in the soil between the part absorbed by the soil matrix and the quantity dissolved in the groundwater. As this equilibrium is constantly shifting, the concentrations progressively decrease in the groundwater. The lower the affinity of the contaminants with the soil matrix (organic matter), the faster the concentrations decrease in the groundwater.
As with the injection of compressed air, pump & treat is based on the movement of substances through the soil matrix, so the permeability of the soil is a determining factor in the efficacy of this technique. The affinity of the contaminating factors with the soil matrix, combined with the soil characteristics, is also a determining factor in the efficacy of operation.
Pump & treat can also be implemented to eliminate pockets of contamination. High quality, detailed information regarding the pocket of contamination, and the judicious installation of extraction filters are also crucial.
In situ chemical oxidation (ISCO)
chemical oxidationIn-situ chemische oxidatie (ISCO) is een bodemsaneringstechniek waarbij een chemisch oxidans (waterstofperoxide, kaliumpersulfaat, kaliumpermanganaat, ozon, ozon+waterstofperoxide,…) eventueel in combinatie met een katalysator in de bodem wordt ingebracht. Wanneer het chemisch oxidans in contact komt met de organische verontreiniging, wordt deze laatste afgebroken tot CO2
, water en zouten.
(ISCO) is a soil remediation technique in which a chemical oxidant (hydrogen peroxide, potassium persulphate, potassium permanganate, ozone, ozone + hydrogen peroxide...) is injected in the ground, possibly combined with a catalyst. When the chemical oxidant comes into contact with the organic contaminant, the latter is broken down into CO2
, water and salts.
As the oxidants are extremely reactive and can react with other substances naturally present, they have a limited lifespan in the soil, which is why the oxidant must be injected and come into physical contact with the contaminants within the average lifespan of the oxidant. Detailed knowledge of the location and the characteristics of the soil contamination, as well as good soil permeability are of paramount importance here for the successful implementation of this soil remediation technique.
When using ISCO at very high concentrations (pure product), it is recommended to assess the quantity of oxidant required based on the calculation of the pollutant load. Through oxidoreduction, the soil matrix can also contribute significantly to high, unrealistic demand for oxidant. The right expertise is required to undertake and interpret laboratory tests and pilot trials.
In situ chemical reduction (ISCR)
During in situ chemical reduction, a reducer (e.g. zerovalent ion) is injected in the soil and slowly releases electrons in the soil, which can then directly break down the organic compounds or indirectly stimulate anaerobic biodegradation. In situ chemical reduction is used primarily for chlorinated hydrocarbon (VOCl) contaminations.
As the reducer is not very mobile in the groundwater, it must come in contact with the contamination when injected in the soil. Again, detailed knowledge of the location and characteristics of the contamination as well as good soil permeability are very important for the successful implementation of this soil remediation technique.
In practice, ISCR is generally used in combination with anaerobic biodegradation stimulation to create a suitable environment, or to boost biodegradation
Most organic compounds are biodegradable in specific circumstances and at a specific speed. Some, for example, are biodegradable in the presence of enough oxygen, so-called aerobic degradation. Other organic compounds only break down in the absence of oxygen, so-called anaerobic degradation.
For biodegradation to occur, a certain number of conditions must be met:
- The right micro-organisms must be present;
- The soil environment must be suitable;
- There must be enough nutrients (nutrients and source of carbon/electron donor) for micro-organisms to be present;
- There must be enough (or as little as possible) electron acceptors.
If these circumstances are not met naturally, spontaneous biodegradation will not occur. Biodegradation can however be induced by offsetting the missing condition. For example, we can add the right microorganisms, create a favourable soil environment, or add the correct dose of nutrients and/or electron acceptors.
Aerobic degradation is generally stimulated by addition of an oxygen-free product, or injection of oxygen/compressed air. Anaerobic degradation is stimulated by administrating an appropriate source of carbon, possibly combined with other (micro)-nutrients. Again, with this technique the microorganisms, the contamination, and the stimulating product must come into contact, so a certain degree of soil permeability is necessary. Stimulation of biodegradation is highly suitable for large, moderately contaminated areas, or contamination plumes.
Soil heating is a soil remediation technique that heats the soil to accelerate certain processes. There are different ways of heating the soil, including vapour injection, conductive heating, and heating the soil matrix with an electrical resistance... Soil heating is generally used in combination with another technique such as air sparging or biodegradation stimulation.
In conjunction with air sparging, soil heating is used to accelerate the volatilization of the organic contamination. The soil is heated to near the boiling point of the contaminants. As this technique uses a lot of energy, it is above all considered for eliminating concentrations and pollutant loads containing very high levels of volatile compounds (elimination of pockets).
Reheating the soil is also used to stimulate biodegradation. The soil and the groundwater can be heated to approximately 30 °C. This gentle heating increases the bioavailability of the contamination and significantly accelerates biodegradation. However, it is always important to assess if the additional costs linked to soil heating are offset by the time saved in relation to biodegradation without heating.
Due to their often large and deep locations, only in situ soil remediation techniques are suitable for the treatment of complex VOCl contamination. All the conventional in situ techniques such as pump & treat, air sparging, or chemical oxidation require a good, quite permeable, and preferably sandy soil structure. Unfortunately, heavy soils such as the silty soils in Brussels or the clay soils in West Flanders, have not been spared VOCl contamination. However, up until now conventional soil remediation techniques have been used for these types of soil for lack of better alternatives. As these techniques are used on the boundary of or even outside their optimal scope of application, their efficacy leaves a lot to be desired.
SODECON SARL is working on the development of an innovative soil remediation technique for VOCl in heavy soils. The first results show that the technique already appears to work on a medium-scale, in the laboratory. A pilot trial will be starting soon to verify if the technique can also be implemented in "normal" full-scale conditions.
Direct injection is often used today to incorporate substrates, nutrients or oxidants directly in the soil for the in situ treatment of soil contaminations such as VOCl. To do so, a drilling machine is almost always used to create a hollow well by percussion or pressure, so-called direct-push injections. This direct-push method does however have some major disadvantages:
(i) very heavy machinery is required to carry out the injections;
(ii) it causes considerable damage and compression of the soil around the injection site, which complicates the penetration of the product injected in the soil;
(iii) a preferential channel created along the hollow well through which the product is injected regrettably rises to the surface (blowout).
SODECON SARL is working on the development of an alternative technique for the direct injection of nutrients or oxidants that will resolve the aforementioned disadvantages. We expect this new technology to have a broader impact than traditional direct push injections, and to offer the potential of being used for direct injections in heavier soils. This technique is currently under development, and will soon undergo extensive testing.
Risk management measures
In some cases, carrying out soil remediation works - in the strict sense - is impossible, unnecessary, or simply technically unrealisable, but the risks posed by the existing soil contamination must be managed. So, for example, we can prevent evaporation of volatile organic compounds in the air inside a building by applying an impermeable vapour barrier, leaking contaminated groundwater can be retained with an impermeable clay layer... The cost of such works is often a fraction of the cost of soil remediation work. These measures are detailed in the section "risk management measures".
LNAPL recovery by skimming (DYNASKIM®)
Skimming is a groundwater remediation technique to recover free phase LNAPL. For the application of skimming, Sodecon cooperates with ODS International. They have developed the DYNASKIM® skimming system. This solution enables the pumping of the product with a minimum volume of water. The system is adapted to deal with groundwater fluctuations of 2,5m. The installation requires very low maintenance (1 visit every 4 weeks) and consumes very little energy (<1KWH).