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Thermal Remediation of DNAPL

In Situ Thermal Remediation (ISTR) technologies achieve excellent results in remediation of Dense Non-Aqueous Phase Liquid (DNAPL) source zones in soil and groundwater.

Over the past decade, In Situ Thermal Desorption (ISTD), Steam Enhanced Extraction (SEE), and Electrical Resistance Heating (ERH), have been employed alone and in combinations to effectively treat DNAPL sites in a wide variety of geological settings.

While SEE has its greatest applicability to higher permeability zones beneath the water table through which injected steam can flow, ERH and ISTD are most often applied within low and moderate permeability zones and in heterogeneous geologic settings. The invariance of thermal conductivity across a wide range of soil types means that ISTD is an excellent fit in lower-permeability and heterogeneous formations for treatment of DNAPL. ERH and SEE can effectively address DNAPL containing chlorinated solvents, but are limited to achieving the boiling point of water. They are therefore not well-suited to treating higher-boiling types of DNAPL such as coal tar or creosote. ISTD alone has applications for contaminants with moderate to high boiling points (see SVOCs).


Coal tar, creosote, and No. 6 fuel oil are common examples of types of DNAPL that do not contain CVOCs. The constituent compounds in coal tar, creosote, and No. 6 fuel oil are predominantly higher-boiling compounds such as PAHs. Thus, when thorough removal of these higher-boiling constituents is required, heating to the boiling point of water to accomplish steam distillation will not be effective. Often, removal of mobile product may be a sufficient goal.

Low temperature heating can be employed to lower the viscosity of the DNAPL and enhance free product recovery. Depending on the geologic setting, ISTD, ERH , or SEE may be selected for this purpose. Moderate temperatures can also be employed to accomplish In Situ Thermochemical Solidification of coal tar (refer to the accompanying SVOC and MGP web pages).

Using Combined Technologies to Treat DNAPL

The combination of steam and ERH, termed Dynamic Underground Stripping, was successfully utilized at the Visalia Pole Yard site in Visalia, CA, and the Young-Rainey STAR Center, Area A site in Largo, FL.

Another example is the combination of SEE and ISTD, employed at the Knullen site in Denmark to successfully treat a CVOC DNAPL source zone. SEE was needed to heat a high velocity aquifer zone, while ISTD was needed to heat the adjacent low permeability zones beneath an active dry cleaning facility. TerraTherm is currently utilizing this SEE-ISTD combination in other full-scale projects.

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