Hydrocarbons are some of the most common pollutants in soil. Groups of hydrocarbons have been classified as toxic, carcinogenic, and/or mutagenic and they pose an existing threat to human health and the environment. However, prohibition of hydrocarbons is not an option as the pollution
sources are an integrated part of our everyday life. Hydrocarbons derive as follows primarily from the combustion of orga ic materials such as fossil fuels (pyrogenic sources) and from oil-related products such as crude oil and refined petroleum products (petrogenic sources) and hydrocarbons are found everywhere in our environment - in the air, food and soil. This study focuses on hydrocarbons in soil; I exploit the complex fingerprints of hydrocarbons in soil and compare with them with the soil quality criteria and I look into the fate of hydrocarbons and degradation products in the soil.
Thus, although hydrocarbons are dangerous, we cannot avoid them – they are everywhere. Instead we have tried to limit the exposure of hydrocarbons to humans and to the environment by establishing quality criteria for hydrocarbons in soil. Soil quality criteria vary from country to country and even within countries, but in general we choose to assess the quality of soil based on the content of total petroleum hydro arbons (TPH) and of a number of un-substituted polycyclic aromatic hydrocarbons (PAHs). Hydrocarbon contamination in soil is complex. Often hydrocarbon contamination is a mixture of thousands of compounds in varying concentrations and of varying
compounds, depending on the pollution sources and the extent to which contaminants are weathered and degraded. In our work with the complex chemical composition and source identification of hydrocarbon contamination in soil, we have asked the same questions over and over again: are the
quality criteria for hydrocarbons in soil representative of the often complex chemical composition of hydrocarbons in the soil? And is the soil clean when quality criteria are met? In my thesis, I try to find answers to these questions by examining a range of soils, all of which exceed the Danish quality criteria for TPH and/or individual compounds of PAHs. I found that the selected PAHs in the quality criteria are not representative of hydrocarbon contamination derived from petrogenic sources as the selected PAHs are all un-substituted and petrogenic PAHs are dominated by alkylsubstituted
PAHs. In the studied soils with petrogenic sources more than 94% of the total content of PAHs was alkyl-substituted and we therefore speculate that the concentration of PAHs in soils with petrogenic sources may be heavily underestimated.
Source identification based on TPH and a number of un-substituted PAHs gives only a weak indication of potential sources. I used a variety of source identification techniques: The chemical composition of both un-substituted and alkyl-substituted PAHs, diagnostic ratios of isomeric PAHs or groups of PAHs, qualitative screening of the TPH chromatograms, and multivariate data analysis (PCA). The selected diagnostic ratios were only partially useful for identifying sources. For a robust identification of hydrocarbons in soil, the composition of PAHs and the screening of TPH chromatograms cannot stand alone and a combination of techniques is required. The multivariate data analysis, however, was highly applicable for describing all the hydrocarbon sources in the soils.
With the phrase "we find what we are looking for" in mind, I chose to extend the horizon and examine the hydrocarbon contaminated soils for oxygenated PAHs (O-PAHs). O-PAHs are formed along with PAHs in the sources or by subsequent degradation of PAHs for example in situ in the soil during remediation of hydrocarbon-contaminated soil. O-PAHs are as toxic as PAHs and are also persistent in the soil. I quantified 13 O-PAHs in the soil and found that O-PAHs ranged between 6 and 18 % of the total content of PAHs in pyrogenic contaminated soils; the soil quality criteria do not reckon with the O-PAHs. Soils with petrogenic sources, however, showed very low concentrations of O-PAHs. This may very well be because I only "find what I'm looking for". The 13 O-PAHs I chose as target compounds are probably derived from primarily un-substituted PAHs and as petrogenic sources predominantly contains alkyl-substituted PAHs, derived O-PAHs from
alkyl-substituted PAHs are more likely to be found.
It is important to know the fate of hydrocarbons and PAHs in soil in order to make a proper risk assessment. The PAHs generally have low water solubility and sorb very strongly to soil. When PAHs degrade, a wide variety of metabolites with physico-chemical properties very different from the PAHs are formed. Oxidation and subsequent conjugation of a polar substance group to a PAH significantly increases the water solubility of the PAH metabolite. Unlike PAHs, the polar metabolites are highly water soluble and have a preference for pore water in soil. This increases the potential for leaching into surface water and groundwater. I chose to focus on a biological degradation pathway of PAHs that includes an initial oxidation of PAHs and a subsequent
conjugation of a polar group. I have worked with two test organisms that utilize this degradation pathway, the earthworm Eisenia Fetida and the terrestrial fungus Cunninghamella elegans. E. fetida was exposed to the two PAHs, phenanthrene and pyrene, in a hydroponic culture. E. fetida was
extensively able to transform both phenanthrene and pyrene into O-PAHs and conjugated PAH metabolites in the form of sulfate, glucoronide, and glucoside conjugates. Glucoronide and glucoside-conjugated PAHs were mainly found in the worm tissue, while the more polar sulfate conjugated PAHs were found predominantly in the water after excretion from the worm. The choice of C. elegans as a test organism had a dual purpose: To investigate the etabolites
formed by C. elegans after exposure to three un-substituted PAHs, two alkyl-substituted PAHs, and dibenzothiophene (a heterocyclic polyaromatic compound (PAC) containing sulfur), and to use C.elegans to produce conjugated PAH metabolites to test sorption in soil (only a few conjugated PAH metabolites are commercially available). I found a total of 58 different metabolites, many of which have not previously been identified. The conjugated PAHs derived from un-substituted PAHs were all oxidized and conjugated with a sulfate group. The alkyl-substituted PAHs were either oxidized or carboxylated and likewise conjugated with a sulfate group. In contrary to the un-substituted PAHs, I found that the conjugated PAHs only to a very limited extent sorbed to soil. An example is 1-methyl pyrene which was carboxylated and hydroxylated and then conjugated with a sulfate group by C. elegans. From the parent PAHs being strongly sorbed to soil (the expected amount of sorbed 1-methyl pyrene to the solid phase in soil is 2000 times higher than what is expected to be in the aqueous phase), the degradation products were almost exclusively in the soil solution (Kd coefficients between 0 (no sorption) and 2.8). This change in preference between soil and soil
solution was general for all metabolites of C. elegans. However, the metabolites possessing both a carboxylic acid and a conjugated sulfate had the highest preference for the aqueous phase. These findings confirmed that pore water is largely a means of exposure for the conjugated PAHs and a
potential risk of leaching into surface and groundwater exists.
One comprehensive challenge is to identify PAH metabolites in the laboratory and to establish the potential mobility of the metabolites in the soil pore water; another great challenge is to recover PAH metabolites in the environment. To test whether the polar metabolites of PAHs leached from hydrocarbon contaminated soils, a field trial with two lysimeters was set up. The two lysimeters were filled with approximately 5 tons of two hydrocarbon contaminated soils, respectively, and drainage water from the lysimeters was collected and analysed for hydrocarbons and PAH metabolites during a 1½ years period. The leachates revealed a high content of substances of the
same size and of the same polarity as the conjugated PAHs. Soil, however, contains a large amount of organic matter including compounds with the same size and polarity as the target PAH metabolites, and I was not able to identify specific metabolites of PAHs in the drainage water.
In my thesis I contribute to knowledge about the complex chemical composition of hydrocarbons in soil and of the fate of PAHs in soil. I ask a series of questions, and in the work underlying this thesis, I have found some answers:
The quality criteria for hydrocarbons in soil are not representative of the complex chemical composition of hydrocarbons in soil, since petrogenic sources of hydrocarbons are not taking into account in the target group of PAHs. This can be remedied by including alkyl homologous series of
PAHs in the risk assessment. Inclusion of alkyl-substituted PAHs as well as un-substituted PAHs will likewise improve the identification of sources of hydrocarbon contamination in soil. However, it is recommended to combine a number of source identification techniques and especially to use multivariate data analysis to achieve robust source identification.
Significant levels of O-PAHs were found in pyrogenic contaminated soils and conjugated PAH metabolites are formed by the soil living organisms, E. fetida and C. elegans: Soil is not necessarily clean, where the quality criteria are met, as we do not look for these compounds. PAH metabolites are an important niche to study as some of the metabolites are as toxic as their parent substances, they are persistent in the soil, and the metabolites are potentially mobile in soil.