Natural Attenuation - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 992

Petroleum hydrocarbons have densities ranging from 0.7 g/ml (gasoline) to
0.9 g/ml (lube oil), i.e., they are lighter than water. Due to their low solubility in
water, petroleum hydrocarbons constitute a separate phase in groundwater, termed
lighter than water non-aqueous phase liquids (LNAPL).
After being released into the environment, petroleum hydrocarbons will be sub-
ject to weathering processes, i.e., dissolution in water, volatilization and biodegra-
dation. Each process affects the component groups differently. As alkanes are less
soluble but more volatile than aromatics of comparable molecular weight, alka-
nes will be found primarily in the gas phase, while aromatics constitute the main
contaminants in the water phase. Generally, solubility and volatility decrease with
increasing number of carbon atoms, i.e., with increasing molecular weight. Thus, as
the more volatile and more soluble constituents disappear first from the petroleum
mixture, a shift towards heavier constituents in the remaining NAPL occurs. The rate
of dissolution and volatilization of a certain component is, according to Raoult's
law, proportional to its solubility, respectively vapour pressure and its mole frac-
tion within the NAPL mixture (see Chapter 19 by Rolle et al., this topic). Gasoline
contains for example only about 1% of benzene, which results in a saturation con-
centration of benzene in water of about 20 mg/L, due to its high maximum water
solubility of 1,800 mg/L.
After dissolution and/or volatilization from the NAPL phase, the single con-
stituents are subject to biodegradation especially in the water and/or to sorption
onto the solid phase, i.e., the sediments. Most petroleum hydrocarbon components
apparent in the water phase are readily biodegradable, especially under aerobic
conditions. Biodegradability of the different petroleum hydrocarbon constituents
decrease in the following order: straight-chained aliphatics, branched aliphatics,
aromatics, cyclic aliphatics, MTBE. Anaerobic conditions occur after oxygen is
depleted, leading to a thermodynamically steered order of electron acceptor con-
sumption: if nitrate is present, it will be the next favourable electron acceptor after
oxygen, before iron(III)/manganese(IV) and sulphate. After complete depletion of
these electron acceptors, methanogenesis will eventually take place, where the sub-
strate acts as both electron acceptor and donor. It is thermodynamically the least
favourable degradation pathway, exhibiting thus the slowest degradation kinetics.
Under anaerobic conditions MTBE, benzene and trimethylbenzenes are less
degradable, which may cause long groundwater contaminant plumes. Typical plume
lengths of most petroleum hydrocarbon constituents are less than 300 m (Rice et al.
1995 ; Teutsch et al. 1997 )), except those of MTBE, showing plume lengths of up to
1,800 m (Stupp 2007 ).
Sorption of petroleum hydrocarbons takes place on the organic material in the
sediment. Thus, the adsorbed amount of a single constituent depends on the content
of organic material in the sediment as well as on the contaminant's partition-
ing coefficient between water and the organic material ( k oc ). Several empirical
correlations exist to derive the k oc coefficient from the better measurable octanol-
water partition coefficient k ow (Grathwohl 1998 ;see Chapter 19 by Rolle et al.,
this topic). Generally, the partitioning coefficient k ow increases with increasing