Large-Scale Physical Models of Thermal Remediation of DNAPL Source Zones in Aquifers

Based on the findings of the R&D project THERIS and the successful field application of the thermal in-situ remediation technique for the remediation of the unsaturated zone, the SERDP project has created the basics for applying "thermal wells" (electrically operated heaters) for thermal remediation of the saturated zone (aquifer).

The experiments on small and technical scale (column, flume, and large tank) as well as the accompanying numerical simulations (accomplished by the project partners from the USA), have led to a good understanding of the relevant processes. For the investigations on a technical ("pilot") scale, an experimental setup similar to the earlier studies of the THERIS project was selected. In two large VEGAS containers a two layered confined medium permeable aquifer (permeability kf ~ 10^-6 to 10^-5 m/s) overlaid by a highly permeable unsatu-rated zone was installed. Several well defined contaminant sources (tetrachloroethene (PCE)) were introduced into the aquifer. The aim of the investigations was to specify the remediation boundary conditions and opportunities.

It was shown that even low permeable, saturated soil zones can be remediated effectively using thermal wells if the contaminant source zone is heated from the outside to the inside and the heated area is controlled by a soil vapour extraction system.

In addition, special attention should be paid to a reasonably high energy density, e.g. at least 8 kW per m³ of treated soil. The faster the warming takes place, the more secure the transport of the gaseous pollutant will be. In case of the contaminant source zone being ahead of the steam front an accumulation of pollutants in the condensed steam front can result in an undesirable displacement of pollutants (in case of DNAPL this displacement will mainly be downwards). In that case the organic phase is displaced by the vapour front rather than evaporated, thus an efficient cleaning by soil vapour extraction can no longer be guaranteed. Although this effect has been observed in the tank experiment, it can be handled in field applications by the correct technical setup. The heated zone needs to be much bigger than the contaminant source zone, also the contaminant source zone needs to be heated from the outside to the inside. Therefore it is important to design the location of the heating elements in such a way, that the heating starts outside the contaminant source zone and progresses rapidly.

Overall, the studies showed that application of thermal wells for the remediation of low per-meable saturated zones is promising.
This is a follow-up project to: Technologieentwicklung zur thermischen In-situ-Sanierung gering durchlässiger Böden (THERIS)

Koschitzky, Hans-Peter

Hiester, Uwe

VEGAS

From: 2005-06-01

To: 2008-06-30

SERDP USA, Strategic Environment Research and Development Program, Project ER-1423

TerraTherm, Inc. USA

MK Tech Solutions, Inc. USA