ECOPROBE 5 - rs dynamics

SOIL VAPOR SURVEY
INTRODUCTION
The gases found in pore spaces in soil and rock formations are known as soil vapor. The composition of this
gaseous phase, in particular the presence of organic compounds, reveals vital information about the pollution
of the subsurface environment.
Organic compounds constitute a group of contaminants which impact severely on groundwater resources.
The presence and distribution of such pollutants can be detected using a technique known as Soil Vapor
Survey (SVS) which is based on an analysis of the soil vapor.
The following types of contaminants can be monitored:
1. Liquid hydrocarbons - mainly petroleum products, and chlorinated hydrocarbons such as
industrial solvents and pesticides.
2. Organic vapors from landfills and from gas leaks.
In situ soil vapor measurements are affected by several subsurface factors of which the most important are:
1. Presence of methane: As a product of natural biodegradation, methane occurs in varying concentrations
everywhere in the subsurface soil environment.
2. Soil permeability: Soils with differing permeability create differing soil vapor flux conditions. Due to their
texture, relatively impermeable soils such as clay produce soil vapor for a much shorter period of time
compared to highly permeable soils such as sand.
Other factors are:
3. Zoning: Differences in the physical properties such as vapor pressure, solubility in water, mobility in the
subsurface, resistance to degradation etc. of the various components of a complex contaminant, may give a
heterogeneous character to a pollution plume.
4. Surface contamination: A pollutant occurring in the surface layer will seriously distort the distribution and
concentration patterns of an underlying, deep source of contamination.
5. Age of a contaminant: All petroleum products experience some degree of natural biodegradation in the
subsurface environment. Lighter HC’s are more readily degraded than the heavier compounds. After a
certain period of time the spectral composition of contaminants will dramatically change; moreover, some
contaminants may become morphologized, that is composed of non-toxic aromatic compounds which are
outside the basic hydrocarbon spectrum.
6. Soil temperature: With increasing soil temperature, vapor density decreases which, in turn, promotes the
vapor diffusion rate.
7. Soil moisture: The presence of moisture reduces the effective porosity, which has a negative effect on
the migration and volume of soil vapor available for measuring.
8. Type of contaminant: For the SVS to be applied successfully, the contaminant in question must manifest
a sufficiently high vapor pressure. Some compounds such as transformer oils, PCB’s and certain phenols,
have a very low vapor pressure and require a special sampling technique.
The presence of methane and widely differing soil permeability are the two crucial distorting factors
in soil vapor surveys. Both parameters dramatically affect the measured values of contaminant
concentrations. Without the benefit of the ECOPROBE 5 method, these misleading results can
cause severe misinterpretation of both the in situ and also the laboratory data.
11