Natural Attenuation - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 993

molecular weight, thus polycyclic aromatic hydrocarbons tend to adsorb much more
strongly than mono-aromatics or MTBE.

22.3.2 Natural Attenuation Potential and Challenges at Petroleum
Hydrocarbon Contaminated Sites

Besides the specific experience gained using a variety of methods to investigate
the efficiency of NA at petroleum hydrocarbon contaminated sites (see Table 22.1 ),
some general conclusions concerning the NA potential and its limits can be drawn
for petroleum hydrocarbon contaminated sites:
Concerning the s ource zone , the following experiences have been made at various
test sites:

• At most petroleum hydrocarbon contaminated sites LNAPLs are present, which

act as a long-lasting source for contaminants in groundwater and soil air.
However, once dissolved in groundwater, most petroleum hydrocarbon con-
stituents are readily biodegradable.

• Unlike DNAPLs, where it is barely possible to investigate the source zone archi-

tecture accurately in the field, several methods exist to delineate the lateral
spreading of an LNAPL (see Table 22.1 ). However, assessing the LNAPL mass
in the subsurface remains a challenge.

• Emission, i.e. the mass flow rate from an LNAPL source into groundwater

or soil gas, from a typical petroleum hydrocarbon LNAPL differs for each
component and is time-dependent, among other things due to temporally vary-
ing mole fractions within the LNAPL. Thus, mole fractions of relatively low
soluble components (e.g., trimethylbenzenes) increase with time due to disso-
lution and depletion of more soluble components (e.g., BTEX), thus emission
of the first mentioned constituents may increase with time for a certain time
period. Generally, alkanes constitute the main contaminants in the soil air, while
aromatics are the main groundwater contaminants. Besides BTEX and PAH,
trimethylbenzenes are important contaminants in groundwater, which should be
analysed at each petroleum hydrocarbon contaminated site.

•

Emission from a petroleum hydrocarbon LNAPL is difficult to measure in the
field, therefore alternative methods like analytical or numerical calculations
and/or laboratory experiments have to be applied. The emission from an LNAPL
source depends on parameters that characterize the unsaturated and the satu-
rated zone as well as the NAPL. As an LNAPL in and below the capillary fringe
constitutes a multiphase system (water-NAPL-air), parameters to mathematically
describe this multiphase system are needed: these are VanGenuchten or Brooks
Corey parameters for the phase saturation - pressure relationship and the satura-
tion - relative conductivity relationship, as well as scaling factors depending on
the surface tensions of the present fluids. Furthermore, groundwater flow veloci-
ties, porosities and recharge rates are needed, that all are highly site-specific (see,
for example, Peter et al. 2008 ).