GROUNDWATER - Water Quality Engineering in Natural Systems - page 169

5.6 NONAQUEOUS PHASE LIQUIDS cial, and military facilities. Commonly encountered

chlorinated hydrocarbons are tetrachloroethylene
(PCE), trichloroethylene (TCE), carbon tetrachloride
(CT), and 1,1,1-trichloroethane (TCA). Petroleum
products are typically low molecular weight hydrocar-
bons whose solubilities are similar to those of chlori-
nated hydrocarbons and are less dense than water;
hence, they are LnAPLs. Commonly encountered
petroleum products are benzene, toluene, ethylbenzene,
and xylene (collectively referred to as BTEX com-
pounds), which are among the most soluble constituents
of gasoline. Tars are by-products of coke and gas pro-
duction, and creosote is a widely used wood preserva-
tive. Tars and creosote are DnAPLs and are sparingly
soluble in water. PCBs have been used in many indus-
trial applications, including fire retardants in hydraulic
oils and electrical transformer fluids. The production of
PCBs has been banned in the United States since 1979;
however, their hydrophobic nature has led to their
occurrence predominantly in soils and sediments that
act as sources for their long-term release to the environ-
ment. PCBs are generally denser than water, and hence
are DnAPLs. The densities and solubilities of several
nAPLs are given in Table 5.12.
movement of groundwater past a nAPL trapped
in the solid matrix of a porous medium results in the

many organic compounds are only slightly soluble in
water and exist in both the dissolved and pure phase in
groundwater. Pure liquids that are not dissolved are
called nonaqueous phase liquids (nAPLs). nAPLs are
typically composed of either a single chemical or a
mixture of several chemicals. nAPLs are further classi-
fied as light nAPLs (LnAPLs) that are less dense than
water and tend to float on the water table, and as
dense nAPLs (DnAPLs) that are denser than water
and tend to sink to the bottom of the aquifer. Typical
LnAPL and DnAPL spills are illustrated in Figure 5.10.
Compounds with solubilities less than 20,000 mg/L are
likely to exist as nAPLs. The nAPLs commonly encoun-
tered in contaminated groundwater can be categorized
into four groups on the basis of their similar chemical
structures, fluid properties, and behavior in the subsur-
face: (1) chlorinated hydrocarbons, (2) petroleum prod-
ucts, (3) tars and creosote, and (4) mixtures with
polychlorinated biphenyls (PCBs) and oils.
Chlorinated hydrocarbons are low molecular weight
compounds that are sparingly soluble in water, volatile
in nature, and denser than water; hence, they are
DnAPLs. Chlorinated hydrocarbons are used predomi-
nantly as solvents and degreasers at industrial, commer-

ground surface

residual NAPL

water tabl e vadose zone

saturated zone

dissolved plume

ground water flow

confining layer

(a)

ground surface

residual NAPL

water table

low permeability
stratigraphic unit

dissolved plume

DNAPL pools

ground water flow

Clay

(b)

Figure 5.10. Typical (a) LnAPL and (b) DnAPL spills.