Natural Attenuation - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 1000

During the 19th century the knowledge of organic chemistry increased and over
several decades tar oil was a very important raw material for production of dye (coal
tar dyes), pharmaceutics, pesticides, resigns, and lacquer. Later it was replaced by
crude oil. Creosote, composed of coal tar distillates, is used to treat wood products
(preservation) such as railroad sleepers and telegraph poles. It has strong antifungal
properties and operates as a impregnate.
Decontamination of such sites is a challenging issue because remediation is time
consuming due to various reasons and requires high costs. Tar oil based contami-
nants were chosen in this Natural Attenuation chapter as a third important class of
chemicals. These contaminants are known to be (in contrast to most of the petroleum
hydrocarbons and the CVOCs) highly persistent.

22.5.2 Characteristics of Tar Oil

The water solubility of most contaminants is very low, leading to formation of a sep-
arate phase when tar oil comes into contact with water. Due to the - in most cases -
higher density of tar oil compared to water, the organic phase is able to move down-
wards through the aquifer and will eventually reach the basis of the aquifer. Such
organic liquids having a higher density than water are called dense non-aqueous
phase liquids (DNAPL). Depending on the production process the variety of the
composition of tar oil is high and some products may even have a lower density
than water and therefore form LNAPL (Collin and Höke 2005 ).
Due to low water solubility and slow dissolution kinetics, NAPLs are long-lasting
in groundwater environments. Eberhardt and Grathwohl ( 2002 ) calculated, based on
laboratory experiments, the time scale for dissolution of tar oils in saturated zones
under natural conditions. According to these calculations, tar oil distributed in an
aquifer as blobs will release PAHs from decades up to hundreds of years while the
dissolution from pools will take hundreds up to thousands of years.

22.5.2.1 Tar Oil Components

Usually, the so-called 16 US EPA PAHs are investigated at tar oil contami-
nated sites. Additionally, two methylnaphthalenes (1-methylnaphthtalene and 2-
methylnaphthalene) are nowadays often included in monitoring programs, because
they were found in high quantities in groundwater samples. These contaminants
represent a large number of different polycyclic aromatic contaminants with very
different physico-chemical and toxic properties. For instance the water solubility
varies from 30 mg/L for naphthalene to 3 10 − 3 mg/L for benzo(a)pyrene. For

details about the group of PAH see also Table 22.2 .
Recently the NSO-heterocyclic (NSO-HET) constituents have been a focus
of investigation (Blotevogel et al. 2008 , Werner et al. 2008a ; Zamfirescu and
Grathwohl 2001 ). The chemical structure of NSO-HET is similar to the PAHs with
the exception that one carbon atom of the aromatic ring structure in these organic
molecules is replaced by a nitrogen (N), sulfur (S) or oxygen (O) atom. Their

×