Excavation is generally the preferred way to address minor soil contamination problems. It is a simple technology with little uncertainties. Considering the limited amount of time required to complete the project, this technique is often relatively inexpensive. In combination with a dewatering technique, we can easily excavate below groundwater level.
However, complex soil contaminations, with chlorinated solvents for example, are generally characterized by (i) a highly concentrated, but relatively small source of contamination, and (ii) a diffuse, low-concentration plume, that can be very large. In this case, excavation can be used to eliminate the smaller concentrated source of the contamination. When eliminating the source zone, the largest part of the contaminant mass is removed. Moreover, leaching of contamination to the plume is stopped by removing the source. During removal of the source zone, the strategic planning and execution of the dewatering system are extremely important to reach the remedial objective.
Excavation is much less suited to treat the contaminant plume. Due to lower contaminant concentrations in the plume, less contaminant mass is removed and efficiency decreases (kg contaminant mass/cost €). In most cases, the plume will be treated with an in situ technique in which the soil is not excavated but is treated on site.
Soil vapour extraction and air sparging
Soil vapour extraction is a technology based on the extraction of volatile compounds in the vadose zone. The technology is mainly suited for soils with high to moderate permeability. Soil vapour is extracted from wells with a blower and subsequently treated in a treatment unit. The technology can only be used to treat the vadose zone and is often combined with other technologies for treatment of the saturated zone.
Air sparging involves the injection of compressed air into the soil to volatize the contamination in the saturated zone. It can be used for soils contaminated with volatile compounds such as BTEX and chlorinated solvents. This technology will also bring oxygen into the soil that can stimulate the biodegradation of aerobically biodegradable contaminants. It is combined with a soil vapour extraction system to capture the vapours in the vadose zone. Airsparging is sometimes only used for stimulation of the aerobic degradation of the contamination. This usually occurs at lower flow rates and is called ‘biosparging’.
Pump & treat
This technology involves the extraction of contaminated groundwater from extraction wells and the subsequent treatment in a treatment unit. The treated water can be discharged or re-injected into groundwater with injection wells. As contaminated groundwater is removed, contamination that is adsorbed on the soil particles will partition into the water phase until all contamination is removed. Because of this adsorption effect, this technology will often require a long timeframe, especially if the amount of sorbed contamination is high.
The technology can also be used to lower the water table in order to increase the vadose zone. In this way, soil can be excavated below the water table and volatile contaminants can be more easily removed by soil vapour extraction. The combination of pump & treat and soil vapour extraction is called dual phase extraction.
In situ chemical oxidation (ISCO)
ISCO is a remedial technology in which a chemical oxidant (hydrogen peroxide, persulfate, permanganate, ozone, …) is injected into the subsurface. Depending on the situation, an activator is co-injected. When the oxidant comes into contact with an organic compound, it is destroyed to CO2, water and salts.
Chemical oxidants will also react with the soil matrix. The amount of oxidant consumed by the soil is usually significantly higher than the amount of oxidants needed to destroy contamination. A determination of the soil oxidant demand in lab before field application is in a lot of cases recommended in order to determine the economic feasibility of the technology and to determine the required dosing for a site. Sodecon performs these tests in its own treatability lab.
In situ chemical reduction (ISCR)
During ISCR, a reductant is injected into the soil to chemically destroy organic compounds, to stimulate anaerobic biodegradation or to reduce metals like Cr(VI).
The most well-known application is the use of zero valent iron to destroy chlorinated solvent contaminations. The disadvantage of iron is that it is not soluble and as a consequence it can not be easily injected into the soil. The following applications are possible:
- Slurry injection as microscale iron: a particle slurry is injected into the soil to treat the contamination. During injection, fractures are created in which the slurry is injected.
- Slurry injection as nano-iron: nano-iron particles are smaller than the soil pores and can therefore be injected more easily than microscale iron. It is however more expensive than microscale iron.
- Permeable reactive barrier: granular iron is being applied in a trench perpendicular to the groundwater flow. Contaminated groundwater flows through the trench and is treated by the iron in the trench.
ISCR can also be used to reduce Cr(VI) to the less harmful Cr(III). For this purpose, zerovalent iron can be used but also other products like dithionite or polysulfide.
Stimulation of biodegradation
A lot of organic compounds can degrade naturally. Sometimes, the right circumstances are naturally present in the subsurface and the compounds degrade naturally at a sufficient velocity to remove the contamination within a reasonable timeframe. In most cases, the velocity of degradation is limited and the degradation rate can significantly be enhanced by the addition of an additive. The stimulation of biodegradation can generally be divided into 3 categories:
- Oxidative stimulation: an electron acceptor is added to stimulate the degradation of the contamination. Oxygen is the most frequently used acceptor, but also nitrate and sulfate can be injected as an electron acceptor. The most well-known application is oxidative stimulation of petroleum/BTEX compounds.
- Reductive stimulation: an electron donor is added to create reductive conditions in the soil. Under these conditions the contaminants are consumed as an electron acceptor by specialized bacteria. This method is very often used for the reductive destruction of chlorinated solvents.
- Bioaugmentation: this strategy involves the injection of bacteria into the soil with the degradation capacity to destroy the contamination. This strategy is only used when the natural degradation capacity is not present and is always combined with either the stimulation of the oxidative or the reductive processes.
Technologies like ISCO, ISCR, stimulation of biodegradation and in situ metal precipitation are all dependent on the injection of substrates into the subsurface. In order to inject a substrate into the soil, it is important to select the best injection technology for a site. Injection technologies can be categorized into 3 different groups:
- Direct injection: with this technology, a substrate is injected into the subsurface while advancing an injection rod into the soil. The injection product is injected through holes at the bottom of the injection rod. There are different methods of direct injection: direct push injection, Spin® injection,… that can be applied depending on the site specific circumstances. Compared to the other technologies, this method allows for injection in low permeability soils and for the injection of slurries.
- Injection on infiltration wells: With this method, injection substrates are injected in a screened well. Good well installation is crucial for optimal performance. Sodecon especially focuses on the installation of a good well seal. This method is recommended when high volumes need to be injected multiple times. This method is preferably used in homogeneous soils.
- Injection on drains: When access to a site is limited, drains can be installed by horizontal drilling in order to install a system in areas that are difficult to reach. Drains allow for the injection of high volumes in permeable soils.
- Recirculation: This method is a combination of injection wells/drains and extraction wells/drains. Steering is more complex as extraction pumps are needed and measures to avoid clogging need to be put in place. The advantage of this method is that no external water source is necessary and that large plumes can be treated as continuous recirculation is possible.
The selection of the right injection technology depends on different factors:
- Soil permeability: low permeability soils will require other technologies than high permeability soils
- Soil heterogeneity: a different strategy is needed for homogeneous soils compared to heterogeneous soils with an alternating sequence of high and low permeability layers.
- Groundwater velocity: the groundwater flow rate will determine when the injection substrates will be washed out of the soil and when a new injection is necessary.
- Depth of treatment: when treatment of deeper soil layers is necessary, it will be more favorable to install permanent wells instead of temporary wells.
- Injection substrate: soluble substrates or emulsions are more easy to inject. Particulate substrates are more difficult to inject and need to be injected by a fracturing method.
Sodecon has extensive experience with all injection technologies and different types of substrates and can design the most effective injection strategy for a site. Sodecon can also perform injection testing to evaluate the efficacy of the injection approach and to determine the injection design parameters like injection volume and flow rates, injection pressure and radius of influence.
LNAPL and DNAPL removal
When a lot of contamination is present in the subsurface, it can be present as pure product. When the product is lighter than water, it will appear on top of groundwater as LNAPL. When the product is heavier than water, it will sink in groundwater until a low permeability layer is encountered where it accumulates. The product can be removed by different technologies:
- Skimming: dependent on the level of groundwater and the amount of contamination, different skimming strategies can be used (periodic pumping, continuous pumping with aboveground or underground pumps).
- Surfactant flushing: a surfactant can be injected into the soil, in order to make the contamination more mobile. The surfactant/contaminant mixture is subsequently extracted. This cycle is repeated until the pure product is removed.
- Absorption socks: when the amount of pure product is small, absorbent socks can be applied into a well to sorb the contamination present.
Heating of the soil can be used to volatize contaminants or to enhance the extraction of volatile contaminants. In this way the timeframe of the remediation can be significantly reduced. Soil heating can also be used to stimulate the degradation of the contamination. By increasing temperatures to 30°C, the biodegradation velocity can be significantly increased. There are different methods to heat the soil. The most suited method depends on the goal, the geology and the site specific constraints.
- Electrical resistance heating: Electrical current is passed through a volume of soil. The soil is heated because of the resistance of the soil.
- Conductive heating: By heating a heating element in the soil, the heat diffuses from the point of heating into the soil until the required temperature is reached.
- Convective heating: This method involves the injection of steam, hot air or hot water into the soil. Therefore this method is most suited for permeable soils.
Risk management measures
In some cases, removal of the contamination is technically impossible or economically not feasible. In these circumstances, it is possible to mitigate the risks posed by the contamination. These actions are called risk management measures and can consist of:
- Groundwater pumping system at the edge of the contaminant plume
- Installation of a vapour barrier inside a building
- Impermable clay mats on the bank of a stream
It is always preferred to remove the contamination when possible as these measures need to be maintained for a very long time.
Sodecon is continually looking to improve existing remediation systems and to find new solutions for emerging contaminant problems. Sodecon is currently working on the following improvements:
- Automatic pressure control system for continuous injection on injection wells
- Application of sulfate for the treatment of BTEX compounds
- Application of persulfate for treatment of MTBE and monochlorobenzene
- Development of a new surfactant for surfactant flushing
- Thermal bioremediation of BTEX/petroleum compounds
- In situ treatment options for cyanide
We are developing new solutions for emerging contaminants like 1,4-dioxane and PFAS
- 1,4-dioxane: development of a new type of aboveground reactor as an alternative to a UV/hydrogen peroxide reactor
- PFAS: development of new technologies for water and soil treatment