Environmental Engineering Reference
In-Depth Information
spread above it, resulting in so-called floating NAPL slick pollution. This floating
slick may laterally move afterwards due to local hydraulic gradients and appear
somewhere else, either in collecting water or in new excavation works. This was the
case during the construction of the Stade de France for the 1998 FIFA World Cup in
Paris. The groundwater lowering carried out prior to the excavation in the stadium
area attracted a floating light NAPL slick from an accident that occurred in an oil
refinery located in the vicinity some 40 years earlier. We should also note that
seasonal fluctuations of the water table may result in NAPL being trapped below the
water table, making decontamination significantly more difficult. Note also that,
beside the possible movement of pollution, the dilution of some soluble
hydrocarbon components may affect the quality of the water. BTEX solubility is
higher. The volatile organic compounds (VOC) emitted by the NAPL (and
particularly by BTEX) can cross the vadose zone and result in hazardous
emanations at the soil surface. Going back to the Paris Stade de France, a pumping
system had been installed under the football field and was working during the 1998
World Cup so as to avoid such emanations and allow for good sanitary air quality
during the Cup. Pumping had to continue for several years after 1998. It was
stopped when the quality of the gas pumped was estimated satisfactory.
Figure 14.1 also describes the infiltration of dense NAPL (DNAPL) in the
vadose zone. As described in the report [BRG 05], DNAPL includes heavy oils,
organochlorinated solvents, polycyclic aromatic hydrocarbons, polychlorinated
biphenyls
(PCB) and phenol compounds (see Chapter 12). In the case of DNAPL
infiltration, the liquid also passes down the aquifer to finally reach the impermeable
substratum. In the case of phenol compounds, note that the solubility of DNAPL in
water may be significant. DNAPL aquifer contamination is much more difficult to
remedy because the immersed pollutant is difficult to reach. The potential impact of
pollution on the aquifer is higher if the DNAPL contains more soluble and toxic
compounds.
Some examples of molecular structures of aromatic and polycyclic hydrocarbons
are given in Figure 14.2. We observe the typical benzene cycle structure of some
aromatic hydrocarbons (benzene and toluene, from the BTEX category) together
with the scheme of some polycyclic aromatic hydrocarbons (PAH, see Chapter 12)
including the biphenyl radical that is typical of PCBs.
[YON 91] describes some physicochemical interaction mechanisms between
hydrocarbon compounds and clays (see also [YON 92]). Retention and transfer
mechanisms of NAPL in soils are defined by a retention potential similar to that of
water in unsaturated soils. This potential, called the matrix potential, includes both
capillary and physicochemical interactions. Few experimental data are currently
available on the retention and transfer parameters of NAPL in the vadose zone [CUI
93] in spite of their obvious potential interest in decontamination techniques.
