This document discusses constraints on achieving temperature goals in the vadose zone during in situ thermal remediation (ISTR) projects. It explains that evaporative cooling effects from vaporization of water and volatile organic compounds in subsurface soils can lower the boiling point and achievable temperatures below 100°C. Models are presented relating evaporative cooling and vapor extraction rates to subsurface temperature profiles during ISTR. The presentation provides evidence that contaminant removal is unaffected by lowered temperatures from evaporative cooling, and argues for incorporating such effects into setting ISTR contract performance metrics above 80°C in the vadose zone.

1. Constraints on Achieving Vadose Zone Temperature Performance Goals
Julie Christiansen (Global Remediation Solutions LLC., Longview, WA, USA)
Michael Dodson (Global Remediation Solutions LLC., Longview, WA, USA)
Gary Healea (Global Remediation Solutions LLC., Longview, WA, USA)
Background/Objectives. The limited understanding of processes and mechanisms occurring in
the subsurface during ISTR operations is an impediment in the current application of our in situ
heating technology and industry standard methodologies. Furthermore, this understanding is
paramount to the contract performance metrics being set and the heating strategies developed
for ISTR projects. Within this presentation we will discuss the concept of evaporative cooling (ie.
the psychrometric effect) on subsurface temperatures in the vadose zone, and develop
theoretical models relating these phenomena to the efficiency of contaminant removal and
subsurface temperature profiles during ISTR operations.
Approach/Rationale. A commonly recognized phenomena which occurs during ISTR heating is
the inability to reach and maintain a temperature of 100o
C throughout the vadose zone. ISTR
technologies heat the subsurface in a particular zone to the boiling point of the complex mixture
of contaminants and groundwater occurring in that zone. This complex mixture often reaches a
state of volatilization at temperatures well below the boiling point of pure water (100o
C), and
therefore lowers the temperatures achievable by ISTR technology in that zone.
Volatilization of a chemical compound is controlled by its vapor pressure, which increases with
temperature until boiling occurs as the vapor pressure exceeds atmospheric pressures. Since
the total vapor pressure is the sum of partial pressures of all of the components of a mixture
(Dalton’s Law), the boiling point of that mixture (eutectic point of an azeotropic mixture) can be
achieved at a lower temperature than any separate components’ boiling point. In the vadose
zone of ISTR sites, volatilization in the form of vaporization can occur at temperatures below
100o
C for a few reasons. The presence of VOCs can cause soil moisture to occur as an
isotropic mix, and in appreciable enough concentrations this exerts enough partial pressure on
the overall vapor pressure of the system to decrease the co-boiling point of the soil moisture
(Raoult’s Law). However, VOC concentrations are rarely found in such appreciable quantities.
Conversely, the effects of transmuting air:steam ratios found in the infilled void spaces within a
vadose zone’s lithology is capable of significantly lower boiling points, effectively changing the
temperature achievable by ISTR technologies in that zone. Since volatilization of a chemical
compound is controlled by the pressure exerted by the gas phase in equilibrium with its liquid or
solid phase, as ISTR progresses, the vapor-filled pore spaces in the vadose zone’s lithology will
contain a decreasing ratio of steam and air, decreasing the water vapor pressure of that
mixture, and effectively decreasing the boiling point temperature – a process known as the
psychrometric effect or “evaporative cooling”.
Results/Lessons Learned. This presentation details the effect of evaporative cooling on
temperatures achievable by ISTR technologies in varying subsurface conditions. The effects of
vadose zone lithology and ISTR vapor extraction rates on vadose zone boiling temperatures will
be discussed in detail, and possible means of controlling the extent of evaporative cooling
occurring at a given site will be presented. Additionally, the material presented here will
substantiate why contaminant removal is unaffected by the lowered boiling points and vadose
zone temperatures that are being defined by the effects of evaporative cooling. Ultimately, this
presentation will provide a strong rational for incorporating subsurface processes such as
evaporative cooling into contract performance metrics in excess of 80o
C in the vadose zone.