A dense non-aqueous phase liquid or DNAPL is a liquid that is denser than water and does not dissolve in water. The term DNAPL is used primarily by hydrogeologists to describe contaminants in groundwater. Many chlorinated solvents (or organochlorides), such as trichloroethylene, are DNAPLs. DNAPLs may contain other halogens

Typical for chlorinated solvent pollution in the soil is the heterogeneous pattern. Pollution is spread unevenly over the soil matrix, making it very difficult to detect and characterize the source zones, especially in an urban environment. Considering soil sampling for chlorinated solvents, the difficulty is to avoid the volatilisation of the (very volatile) solvents during the sampling process. A few innovative techniques have been developed but at this point it is not clear which technique would produce the best result in a certain situation. CityChlor will test certain techniques.

Tools and sensors are "pushed" into the ground without the use of drilling to remove soil or to make a path for the tool. With these tools or sensors groundwater samples or vapour samples can be obtained in an urban environment.

This is the summarizing, three-dimensional and simultaneous quantitative and qualitative recording and description of the hydro geological conditions in the investigation area. This test is a powerful measure to investigate source – plume interactions in the groundwater, which is the basis for the assessment of the actual impact of a source of pollution on the groundwater.

Chlorinated solvent pollutions are characterized by a relatively small core zone (the ‘source’) with a plume whose extent depends on groundwater flow and conditions for biological degradation. A cost-effective remediation of this dual situation differentiates between the core zone and the plume. Remediation usually consists of an active treatment of the core zones combined with a management of the risks of the plume by an active containment or mere control.

Several techniques are being used like pump and treat and new ones like injection of iron particles in the groundwater will be tested in pilot sites. In a remediation project authorities will ask for the 'Best Available Technology Not entailing Excessive Economical Costs '– the BATNEEC principle.

Best Available Technology Not entailing Excessive Economical Costs 'In the European Union directive 96/61/EC emission limit values were to be based on the best available techniques, described as "Whereas emission limit values, parameters or equivalent technical measures should be based on the best available techniques, without prescribing the use of one specific technique or technology and taking into consideration the technical characteristics of the installation concerned, its geographical location and local environmental conditions; whereas in all cases the authorization conditions will lay down provisions on minimizing long-distance or transfrontier pollution and ensure a high level of protection for the environment as a whole.

Pump and treat involves pumping out contaminated groundwater with the use of a submersible or vacuum pump, and allowing the extracted groundwater to be purified by slowly proceeding through a series of vessels that contain materials designed to adsorb the contaminants from the groundwater. For petroleum-contaminated sites this material is usually activated carbon in granular form. Chemical reagents such as flocculants followed by sand filters may also be used to decrease the contamination of groundwater. Air stripping is a method that can be effective for volatile pollutants such as BTEX compounds found in gasoline.

Soil vapour extraction (SVE) is an effective remediation technology for soil. "Multi Phase Extraction" (MPE) is also an effective remediation technology when soil and groundwater are to be remediated coincidentally. SVE and MPE utilize different technologies to treat the off-gas volatile organic compounds (VOCs) generated after vacuum removal of air and vapours (and VOCs) from the subsurface and include granular activated carbon (most commonly used historically), thermal and/or catalytic oxidation and vapour condensation. Generally, carbon is used for low (<500ppmV) VOC concentration vapour streams, oxidation is used for moderate (up to 4,000 ppmV) VOC concentration streams, and vapour condensation is used for high (>4,000 ppmV) VOC concentration vapour streams. Below is a brief summary of each technology.

Granular activated carbon (GAC) is used as a filter for air or water. It is commonly used to filter tap water in household sinks. GAC is a highly porous adsorbent material, produced by heating organic matter, such as coal, wood and coconut shell, in the absence of air, which is then crushed into granules. Activated carbon is positively charged and therefore able to remove negative ions from the water such as organic ions, ozone, chlorine, fluorides and dissolved organic solutes by adsorption onto the activated carbon. The activated carbon must be replaced periodically as it may become saturated and unable to adsorb (i.e. reduced absorption efficiency with loading). Activated carbon is not effective in removing heavy metals.

Thermal oxidation (or incineration) can also be an effective remediation technology. This approach is somewhat controversial because of the risks of dioxins released in the atmosphere through the exhaust gases or effluent off-gas. Controlled, high temperature incineration with filtering of exhaust gases however should not pose any risks. Two different technologies can be employed to oxidize the contaminants of an extracted vapour stream. The selection of either thermal or catalytic depends on the type and concentration in parts per million by volume of constituent in the vapour stream. Thermal oxidation is more useful for higher concentration (~4,000 ppmV) influent vapour streams (which require less natural gas usage) than catalytic oxidation at ~2,000 ppmV.

1. Thermal oxidation which uses a system that acts as a furnace and maintains temperatures ranging from 1350°F to 1500°F (730°C-815°C).
2. Catalytic oxidation which uses a catalyst on a support to facilitate a lower temperature oxidation. This system usually maintains temperatures ranging from 600°F to 800°F (315°C-430°C).

Vapour condensation is the most effective off-gas treatment technology for high (>4,000 ppmV) VOC concentration vapour streams. The process involves cryogenically cooling the vapour stream to below 40 degrees C such that the VOCs condensate out of the vapour stream and into liquid form where it is collected in steel containers. The liquid form of the VOCs is referred to as dense non-aqueous phase liquids (DNAPL) when the source of the liquid consists predominantly of solvents or light non-aqueous phase liquids (LNAPL) when the source of the liquid consists predominantly of petroleum or fuel products. This recovered chemical can then be reused or recycled in a more environmentally sustainable or green manner than the alternatives described above. This technology is also known as cryogenic cooling and compression (C3-Technology).

Either naturally occurring or specially bred bacteria are used to consume contaminants from extracted groundwater.

ATES functions for heating and cooling of buildings. In CityChlor we will test the possibility to use this as a remediation tool. During summer, cold water is pumped from the aquifer with a closed piping and heat exchanging system into the buildings that have to be cooled. After heat exchange the warm water is transferred to a second location in the aquifer which will rise in temperature up to a maximum of 40 oC. In winter, the process is reversed leading to heating of the building and lowering of the temperature in the aquifer to the initial conditions. During summer and autumn the elevated temperature in the aquifer will stimulate the microbial degradation rate of chlorinated solvents.

An approach that combines all aspects that are relevant to tackle the problems that VOC in urban environment causes

Combines: depending on area, site, context you can use different aspects together or next to each other

All aspects: socio-economical aspects (like urban development, communication, financial and legal aspects), techniques, time, space, environment, actors (active & passive) and contexts.

A technical approach which makes it possible to remediate, monitor and control multiple groundwater sources and plumes within a fixed area:

• area can be to extend the border around the plumes;
• area can be location based;
• area can be in a city (CityChlor), brownfield or even natural area (i.e. former landfill)

A chlorinated solvent is an organic compound containing at least one covalently bonded chlorine atom. Their wide structural variety and divergent chemical properties lead to a broad range of applications. Many derivatives are controversial because of the effects of these compounds on the environment.

Chlorinated solvents are amongst the most common soil and groundwater contaminants due to their widespread use as solvents and degreasing agents. Because of their physicochemical properties, they produce large-plumes of pollution in the groundwater.