Passive and hydraulicContainment
Containment treatments are often performed to prevent, or significantly reduce, the migration of contaminants in soils or ground water.
For contaminated soil, sediment, bedrock and sludge, containment is necessary whenever those materials are to be buried or left in place at a site. In general, containment is performed when extensive subsurface contamination at a site precludes excavation and removal of wastes because of potential hazards, unrealistic cost, or lack of adequate treatment technologies. Containment treatments offer quick installation times and are typically a low to moderate cost treatment group. Unlike ex situ treatment groups, containment does not require excavation of soils, that lead to increased costs from engineering design of equipment, possible permitting, and material handling. However, these treatments require periodical inspections for settlement, ponding of liquids, erosion, and naturally occurring invasion by deep-rooted vegetation. Additionally, ground water monitoring wells, associated with the treatments, need to be periodically sampled and maintained. Even with these long-term requirements containment treatments usually are considerably more economical than excavation and removal of the wastes. Containment treatments for contaminated soil, sediment, bedrock and sludge include passive or active barriers and landfill cap and landfill cap enhancements. In the passive application no physical and/or chemical processes are involved to insulate contaminated media as containment is achieved by simply introducing impermeable barriers (e.g. clay, cement, plastic materials, etc.) that prevent the flow of water (and/or gases), while in the active application specific materials are used (or added to impermeable filling) in order to induce physical and/or chemical processes that prevent the spreading of contamination, e.g. sorption, precipitation, degradation, etc. of pollutants.
Hydraulic containment (pump and treat) is often performed when dealing with groudwater contamination and relies on pumping the contaminated groundwater to the surface using a series of extraction wells, treating it at the surface to remove the contaminants, and then either reinjecting the water underground or disposing of it off site. According to the type and extent of contamination, contaminated water can be treated by different physical, chemical or biological methods usually applied in waste water treatment facilities. Hydraulic containment systems are used: a) to control the movement of contaminated ground water, preventing the continued expansion of the contaminated zone; b) to reduce the dissolved contaminant concentrations in ground water sufficiently that the aquifer meets the cleanup goals. Although hydraulic containment and cleanup can represent separate objectives, more typically, remediation efforts are undertaken to achieve a combination of both.
Containment barriers are structures installed around the contaminated soil in order to avoid the possible spreading of contamination of water sources close to the site or to avoid the dispersion of contaminants displacing from the contaminated site. The specific filling chosen for a wall could be impermeable, semi-permeable or permeable barrier depending on the kind of contaminant found in a site. There are:
1) Sorption barriers which contain filling that remove contaminants from groundwater by physically grabbing the contaminants out of the water matrix and holding them on the barrier surface, with zeolites or activated carbon
2) Precipitation barriers which contain fillings that react with contaminants in ground water
3) Degradation barriers which degrade the contaminants into harmless products
This technology can be applied to any kind of soil when there is a risk of dispersion to uncontaminated soils.
v The major advantage is the treatment of contaminants in place
v No need for electrical energy supply
v Barrier technology coupled with other soil remediation technologies could enhance the capacity of cleaning of contaminated sites (treatment train)
v Low-cost treatment of contaminated water, often occurring at sites with small flow rates.
v Is not a direct technology to treat soils, just treat the water involved in the contaminated site
Technology development status:
Containment barrier is a commercial technology.
LANDFILL CAP AND ENHANCEMENTS
This technology, also referred to as landfill cover, is used to reduce or eliminate contaminant migration. An impermeable layer is installed over the contaminated site, effectively isolating it from further contact. The layer may be cement, asphalt or more complex membrane/barrier-layer system. As the pollutants are neither removed nor remediated, a long term monitoring scheme is usually required.
Landfill cap and enhancements can range from a one-layer system of vegetated soil to a complex multi-layer system of soils and geosynthetics. In general, less complex systems are required in dry climates and more complex ones are required in wet climates. The materials used in the construction of landfill caps include low-permeability and high-permeability soils and low-permeability geosynthetic products. The low-permeability materials divert water and prevent its passage into the contaminated medium. The high permeability materials carry water away that percolates into the cap. Other materials may be used to increase slope stability.
Cap performance varies, depending on its function and where it is used. For example, compacted clay liners are effective if they retain certain moisture content, but are susceptible to cracking if the clay material is dried out. Some of the alternative designs improve moisture content of the top layers and allow for evaporation and increased transpiration. This will keep the clay layers intact. Temporary caps can be installed before final closure to minimize generation of leachate until a better remedy is selected.
Landfills are generally used to contain a variety of contaminants and may be temporary or permanent.
v Landfill caps are generally inexpensive
v Can be used to quickly isolate dangerous locations
v It is simple in design, easy to install over an existing landfill cover and easy to remove if other uses of the land emerge in the future
v Monitoring of the site may be required indefinitely- this option severely restricts the potential future uses for the site
v Water treatment must be done on the leachate
v Landfill caps do not lessen toxicity, mobility or volume of hazardous wastes but only control migration
v Dry barrier enhancement has difficulty when the cover is subjected to sustained precipitation
Technology development status:
Landfill caps are commercially available worldwide. Alternatives, such as caps designed for dry climates and enhancements, such as dry barriers, are being field tested.
Hydraulic containment systems are used to control the movement of contaminated groundwater, preventing the continued expansion of the contaminated zone. Three major configurations for accomplishing hydraulic containment: are generally used, i.e.: a pumping well alone, a subsurface drain combined with a pump well and a well within a barrier wall system. In the pump & treat technology contaminated groundwater is first pumped from the subsurface and then treated before it is discharged. The wells set-up and the pumping system are dependent on the site characteristics and contaminant type and need an effective design and operation to meed cleanup goals. According to the type and extent of contamination, contaminated water can be treated by different physical, chemical or biological methods usually applied in waste water treatment facilities.
Pump-and-treat systems can be applied to groundwater contaminated with different types dissolved materials, including VOCs, SVOCs, fuels, explosives and dissolved metals.
v Can use conventional waste treatment methods
v Water is relatively easy to handle
v May (re)use water when treated
v Pump-and-treat systems often take decades to meet cleanup goals.
v Pumping depresses the groundwater level, leaving residuals sorbed to the soil, which can re-dissolve after the groundwater level returns to its normal level ("rebound" effect)
v Operating costs can be very high due to the labor-intensive requirements of the method.
Technology Development Status:
Pump-and-treat is a mature technology.