Groundwater treatment

It is important that the groundwater in the catchment area is protected so that sustainable abstraction can be maintained. Protection requires that resources be mapped and monitored and that conditions and restrictions be set on utilization. Other measures may include protection zones around abstraction wells, limiting the use of pesticides and fertilizers, and minimizing activities in general that carry a risk of polluting groundwater.

Groundwater treatment typically includes aeration for the addition of oxygen and the stripping of volatile compounds, sand filtration for the removal of particles and support for biological processes, and UV disinfection as a hygienic barrier. Other methods may include softening for reduction of calcium precipitation potential (CCP) and the use of adsorptive media (e.g., Granular Activated Carbon - GAC) for removal of micro-contaminants.

Although simple treatment of groundwater greatly supports a more decentralized supply structure, areas with high population density can also benefit from groundwater resources. Of course, this will often require the water to be transported further from the catchment area to the treatment plant. In densely populated countries such as Denmark and the Netherlands, the water supply is largely based on groundwater.

For volatile organic compounds (VOCs), concentrations are reduced to less than 1 mg/kg and 0.1 mg/l in soil and groundwater, respectively. Where desired, the system may be operated long enough to achieve maximum concentration limit (MCL) concentrations in the groundwater, and non-detect in soils. For semi-volatile organic compounds (SVOCs) treatment in soils, non-detect concentrations have been achieved by treating at 300-350°C for a period of several weeks. Basically, the in-situ thermal desorption (ISTD) system can be designed for the desired remedial efficiency.

Nearly any organic compounds or a combination of organic compounds can be treated with in-situ thermal desorption (ISTD) /  In-Pile Thermal Desorption (IPTD).
Contaminants to be treated with ISTD/IPTD are:

• Polychlorinated biphenyls (PCBs), dioxins and dibenzofuran
• Polycyclic aromatic hydrocarbons (PAHs) and coal tar
• Trichloroethene (TCE), tetrachloroethene (PCE), 1,2-dichloroethene (1,2-DCE), trichloroethanes (TCA), and other chlorinated solvents
• Pesticides and herbicides
• Petroleum and petroleum products
• Benzene, toluene, ethylbenzene, xylenes (BTEX)
• Methyl tertiary butyl ether (MTBE)
• Any other volatile or semi-volatile hydrocarbon
• Dense and light non-aqueous phase liquids (DNAPLs and LNAPLs)
• Mercury

Contaminants not to be treated are:

• Heavy metals
• Inorganics
   

Thermal wells can be used to treat contaminants to theoretically hundreds of meters, as well as under structures and roads. The deepest full-scale application to date is 50 meters. However, thermal conduction heaters are also used for thermally enhanced oil recovery at depths of more than 300 meters.

Many types of buried utilities, such as concrete sewage lines and steel water lines can be left in place and/or protected during heating through placement of heaters and insulation. Some utilities (e.g., gas lines, PVC pipes) may need to be rerouted or decommissioned.
Since the heat front drops off sharply adjacent to the heated zone, experience has shown that heating adjacent to foundations typically has no effect on the foundations. However, measures can be taken to further protect structures, if necessary.
Several ISTD remediation projects have now been accomplished adjacent to or beneath buildings both in residential and industrial settings.

ISTD is quite different from ex-situ thermal desorption or incineration. With these aboveground thermal technologies, the soil or sludge being treated is exposed to high temperatures only briefly - typically for seconds or minutes. Thus, there can be cool spots where the soil does not get fully treated and where compounds such as dioxins can sometimes be created. By contrast, with ISTD the entire treatment zone is heated to target temperatures for days, at a minimum. Most (approximately 95-99%) of the organic contaminants are destroyed in-situ. Not only are dioxins not created, treatability and field data indicate they too are destroyed, typically to below background levels. Dioxins that are extracted are treated in the air pollution control system.

Immediately after ISTD treatment the soil is sterile, but experience shows that recovery will be rapid. After the soil is disked, fertilized and seeded, following normal revegetation practices, regrowth during the first growing season after treatment should be as good as with other soil. Microbiota residing outside the target treatment zone may see mildly elevated temperatures, which are likely to promote rather than hinder their growth and attenuative capacity.

A major reason for the effectiveness of ISTD is its application of heat to the soil using thermal conduction. During conductive heating, heat moves out through the soil and waste material in a highly predictable fashion, regardless of how heterogeneous the soil is or its permeability. This is in sharp contrast to the movement of a fluid through the soil, which is the basis for nearly all other in-situ remediation technologies (e.g., groundwater pump-and-treat, soil vapor extraction, air sparging, steam injection, solvent and surfactant injection or chemical oxidant injection). Rates of fluid flow can vary over many orders of magnitude, depending on how permeable the soil is and on the degree of heterogeneity. Fluid-based technologies thus tend to bypass some contaminated zones, leading to poor efficiency, diffusion-limited mass transport and an extended duration of remediation. By contrast, the thermal conductivity of a wide range of soil types varies over less than a factor of plus or minus two.

ISTD has a wider applicability than other in-situ thermal technologies. For example, the process of electrical resistivity heating, also known as Six-Phase Heating or joule heating, relies on the flow of an electrical current through soil. Electrical conductivity can vary over two orders of magnitude. Since electrical currents ceases to flow in soils once the water has boiled off, moreover, electrical resistivity heating cannot heat the soil above the boiling point of water. As a result, it is not suitable in thetreatment of high boiling-point compounds such as pesticides, PCBs, and PAHs that require stringent soil cleanup levels. Similarly, steam injection in the shallow subsurface is limited to heating approximately to the boiling point of water.