Contaminant Fate and Reactive Transport in Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 853

The main migration pathway of contaminants in aquifer systems implies their
transport as dissolved species forming a plume in the groundwater flow direc-
tion. The contaminant plumes can impact some sensitive receptors such as surface
water bodies or water supply wells. Different physical and biogeochemical pro-
cesses affect the migration of contaminant plumes including the dissolution from
NAPL (Non-Aqueous Phase Liquid) sources, advective transport, longitudinal and
transverse dispersion, sorption to the solid matrix of the aquifer and biological
degradation.
19.2.1 Contamination Sources and Plume Formation
At the majority of contaminated groundwater sites, organic contaminants, such as
fuels and chlorinated solvents, are present in the form of NAPLs that have migrated
into the subsurface. Dissolution of NAPLs, the process by which contaminants
transfer from the NAPL into the aqueous phase, is one of the most important pro-
cesses that determines the formation of dissolved contaminant plumes. NAPLs are
organic, non-miscible liquids in the subsurface. The dynamic of NAPLs migration
in the subsurface can be very complex (Pankow and Cherry 1996); a schematic
representation is illustrated in Fig. 19.1 . When introduced into the vadose zone,
they initially stay in a connected body of continuous NAPL (also called free-phase
NAPL, or mobile NAPL, or non-residual NAPL or free product) that migrates
through the subsurface, invading pore spaces, fractures and any preferential path-
ways in the subsurface by displacing water or air in the unsaturated zone. At the
water table a major difference in behavior is observed between organic liquids less
dense than water (LNAPLs) and those denser than water (DNAPLs). LNAPLs will
float on the top of the water table, forming a thin, pancake-like layer that spreads
across the water surface. Being denser than water, DNAPLs will penetrate the water
table, displace the water from the pores of the aquifer and migrate into the saturated
zone and finally will spread laterally where changes in the matrix permeability (low
permeability barriers) inhibit vertical NAPL migration. Once the contaminant sup-
ply ceases, the NAPL begins to spread laterally and discrete, isolated blobs start to
form (residual NAPL).
Although NAPLs can be present as a single contaminant (e.g. trichloroethene),
they usually are composed of a mixture of contaminants. Typical LNAPLs such as
gasoline and diesel (density in the range 0.8-0.9 kg L − 1 ) are composed of hundreds
of different contaminants with a wide variety of chemical and physical properties.
Many DNAPLs like industrial solvents, coal tar, creosote, et cetera (density range
1.01-1.2 kg L − 1 ) are complex mixtures of different contaminants as well.
NAPL dissolution in the groundwater depends on different factors such as the
interfacial area between the organic phase and water, the extent and morphology of
the source, the groundwater flow velocity, the solubility of individual contaminants
and the composition of the NAPL source. Eberhardt and Grathwohl ( 2002 )showed
that for complex, multicomponent coal tar sources the equilibrium or saturation