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Naturlig nedbrydning af PAH'er i jord og grundvand SummaryBackground and objective PAH-contamination Contamination by PAH (poly-aromatic hydrocarbon) is found disseminated in the environment. Heavy PAH contamination is found at e.g. the former gasworks and other industries using tar, such as bitumen factories, wood impregnation companies and tar sites (for fishing nets). Furthermore, there are a number of sources for PAH contamination of a more diffuse nature, e.g. traffic, oil- and coal-fired incineration plants, the previously much-used use of tar products for impregnation purposes and depositing of partly or poorly incinerated waste. The PAHs thus make up a substantial group of contaminating substances, which, due to their toxicity and carcinogenic effects, cause a limitation to area use at PAH-contaminated sites. In addition, the PAH from tar contamination may also to some extent percolate to the groundwater. This is a.o. things caused by the fact that tar being a liquid non-soluble in water and heavier than water, wherefore tar can sink into the groundwater basins, with consequent dissolving and spreading of PAHs in the groundwater. Objective For a number of years, in Denmark as well as internationally, there has been focus on PAHs, and extensive data material exists on the toxicity, bio-degradation and mobility in the soil environment, etc. The objective of the present project has on this basis been to assess the potential for degradation of PAHs in soil and groundwater. Within this, it has been the objective to describe the importance of the PAHs physical and chemical properties in relation to their behaviour and degradation in soil and groundwater. It is the objective that the collected information and attached assessments may be used for support in a targeted planning of field investigations and clean-up activities at PAH-contaminated sites. Desk Study The project is carried out as a desk study, based on a survey of substantial amounts of material, including scientific articles and dissertations on degradation of PAHs in soil and groundwater. Occurence and properties Hot-spot contamination and diffuse contamination On the background of experience from a large number of investigations in the past, it is evident that sources of heavy (hot-spot) PAH-contamination are typically tar contamination. This is so at e.g. former gasworks sites, contaminated sites at tar/bitumen factories and tarring sites. Besides, there are a series of sources of more diffuse character, such as traffic, incineration plants, etc. The concentration of PAHs of diffuse contamination is typically around the size of 10 - 20 mg/kg, whereas at hot-spot PAH-contamination it is not unusual to find PAH-contents around 1,000 - 10,000 mg/kg. When comparing investigations of 44 tar-contaminated localities, the groundwater concentration levels of naphthalene were 11,000 - 19,000 µg/l and benzo(a)pyrene at 140 - 280 µg/l in the groundwater in the source areas. In distances of 10 - 50 m from the source, the concentrations in the groundwater were considerably lower, for instance were the naphthalene and benzo(a)pyrene concentrations, in 90 % of the samples, lower than 630 µg/l and 5 µg/l respectively. At a distance of more than 50 m from the source sites, the concentration app. half of the samples were below detection limits. Characteristic physical-chemical properties The PAHs are characterised by being substances with a low vapour pressure, low water-solubility and marked hydrophobic properties. These characteristics have high impact on the dispersion of PAHs in the different phases in the soil environment and thus also on the degradation of PAHs in soil and groundwater. Phasedistribution Traditional phasedistribution calculations at ordinary soil types show that the PAHs will for the most part (> 99 %) be bound to fractions of organic material in the soil. Typically, heavier PAH-contamination is found at a concurrent presence of free phase contamination, such as tar (including large/small tar lumps), but also contamination of more diffuse character can be attached to free phase contamination, such as soot particles. At the presence of these free phase contaminations, which constitute a part-fraction of the organic material in the soil, the main part of PAHs will bound to the free phase. In cases where no free phase contamination exists, the main part of PAHs is bound to the soil’s natural content of organic material. Groundwater Because of its physical/chemical properties, tar-containing PAHs may be in direct contact with the groundwater, whereby PAHs can be dissolved in the groundwater. Maximum concentrations in equilibrium with tar can be estimated using Raoults law. Higher concentrations may possibly be found, as a consequence of a co-solvent effect, formation of bio-detergent and/or colloid-sorptioned PAHs. The tar phase itself, however, also changes (hardens/solidifies), during ageing, which may cause a considerable decrease in solubility of the heavier PAHs and thereby in the concentrations in the water phase. Changes in the tar’s composition and character over time can thus result in both decreasing and increasing PAH-concentrations in the water phase. Biological degradation Degradation at laboratory tests Aerobe degradation of PAHs must be characterised as well documented at laboratory tests. Correspondingly, degradation at laboratory tests under anaerobe conditions has been seen, but the documentation is not as extensive. At laboratory tests, there are typically used microorganisms inoculated on PAH-containing media. However, they will normally be a natural occurrence of microorganisms, which are isolated from PAH-contaminated soil and groundwater. Degradation rates in the laboratories At tests under aerobe conditions, half-life periods have been seen from <3 days for naphthalene, increasing to 30 - 300 days for the heavier PAHs, such as benzo(a)pyrene. Under anaerobe conditions there has typically reported a slower degradation. Importance of the PAH concentration In general at degradation tests there is an increasing degradation rate at increasing concentrations of PAHs. The lowest concentration where degradation is observed is called the threshold value. Likewise, a maximum concentration of PAHs exists, where the degradation of PAHs is inhibited because of the substance toxicity towards the microorganisms. Typically, the microorganisms will be inhibited by the NSO-compounds before the PAH-components at tar contaminations. Bioavailability The degradation rate of PAHs depends on the presence of organic material, since the an increasing amount of organic material gives an increasing sorption of PAHs and thereby may result in a lower amount of PAH being available for biological degradation. Metabolite degradation During degradation of PAHs a series of metabolites are created. However, typically a further degradation of metabolites takes place, wherefore the metabolites are not accumulated. Degradation in soil Degradation of PAHs is well documented Aerobe degradation of PAHs in soil is documented e.g. by laboratory tests. At degradation tests or field tests there are, however, a series of mechanisms that limit the degradation and the possibility of interpreting it. Bioavailability At degradation of PAHs in soil, the bioavailability is crucial. A number of conditions in the soil affect the bioavailability of the PAHs. At the occurrence of tar, PAHs are typically found as and integrated part of the product. PAHs released from tar or in other ways added to the soil, are normally bound hard to the soil’s natural content of organic material. Several theories exist for those mechanisms, which are important for the PAHs binding to the soil’s organic material. Humification Apart from the binding of PAHs to organic material at sorption, the bio-degradation of PAHs also seems to be able to lead to an incorporation of the degradation products (metabolites) in the soil’s natural content of organic material by an actual chemical binding to e.g. humus substances. This is often described as humification. The PAHs will hereby not be mineralised, but an incorporation of humus substances results in immobilisation of the metabolites. Variations Another condition that is important at degradation of PAHs, is the heterogeneous distribution typically identified for PAH-contamination in soil. The heterogeneous distribution causes large variations in determining PAH-concentrations in soil. The variations are a.o. caused by the PAHs typical connection with tar contamination, which is identified as large/small tar lumps in the soil. The variations do it difficult to interpret results from degradation tests, particularly at full-scale field tests. Degradation rate slowest in the field At field and laboratory tests with soil from PAH-contaminated sites there is a markedly slower degradation rate than at laboratory tests with added PAHs. This difference seems a.o to be ascribable to the fact that degradation in tests with soil from PAH-contaminated sites is controlled by the release rate rather than by the degradation rate for the PAHs. Degradation rates in PAH-contaminated soil At degradation studies of PAHs in soil from PAH-contaminated sites, the field tests show half-life periods of 6-16 years, against 200 - 1,700 days at degradation tests in laboratory scale. The 2-cyclic PAHs degrade naturally It is noted that some PAHs are removed from the soil to a considerable extent. The concentration of the 2-cyclic PAHs (naphthalenes and biphenyles) in aged contaminated soil from gasworks is generally very low in comparison with e.g. fresh tar, indicating that these substances to a large extent degrade/evaporate/wash out. Degradation in groundwater PAH-contamination in groundwater Knowledge of PAHs aerobe degradation under normal conditions in groundwater is small, except regarding the simplest 2- and 3-cyclic ones. Under anaerobe conditions, conflicting results have been observed with regard to naphthalene’s degradability, and knowledge of other PAHs degradability is negligible. Contamination plumes At tar-contaminated localities there is often found source areas with free phase tar, resulting in high concentrations of a complex mix of substances in the groundwater/in the upper aquifers. Aerobe biodegradation of the more easily soluble and more easily degradable tar compounds (phenols and mono-aromatic hydrocarbon) will normally mean that the oxygen is used, so that there will be anaerobe conditions in the plume. At some larger investigations of contamination plumes with PAHs and other tar compounds from tar contaminated sites, there seems to be identified a stationarity in the plume distribution, which may be caused by natural degradation of tar substances, including the PAHs. Due to the heavier PAHs low water solubility and strong hydrophobe properties, they only appear in limited concentrations and are only slowly spread outside the source area of the contaminations. In a aerobe aquifer it thus seems possible that a natural degradation can keep the spreading of a contamination plume in a stationary condition. Self-purification of the source area itself by natural degradation is most likely, however, to take an indefinitely long time. Remediation Bioremediation In literature a series of remediation methods have been described for PAH-contamination, based on biological degradation (bioremediation). Full-scale experiences, however, are limited especially to land farming, mile composting and bioventing. Generally, large removal rates are described at the given methods, but the removal is normally not well documented. In Denmark a series of test remediations have been carried out at gasworks, where a limited effect from bioremediation has been found. In comparison, there is markedly higher purification grades for PAHs when applying thermal methods, such as thermal desorption. Documentation of remediation Similar to degradation tests, the heterogeneous distribution of PAH-contamination’s in the soil leads to large variations in determining PAH-concentrations in soil at remediations. In connection with bioremediation, the soil is often homogenised by mixing, filtering, etc. Hereby the distribution of the PAHs in the soil is changed, so that the variations decreased. This may be misinterpreted to mean that a removal of the contamination has been obtained by biological degradation. The mentioned conditions illustrate that there is a need for documentation in form of an adequate number of analyses, and execution of parallel tests for control of abiotic degradation, effect of homogenising, etc, in order to statistically document to what extent the PAHs are biodegraded during remediations. Monitoring Analysis of individual components For characterising PAH-contamination there are a number of field methods, as well as traditional laboratory methods. When describing PAH-contamination, it is recommended, as a start, to investigate those PAHs for which the Danish Environmental Protection Agency (DEPA) has listed quality criteria. To follow a larger part of the contamination mass by e.g. description of possible degradation, it is proposed to supply with investigations of the 16 US/EPA PAHs. Biotests The analysis programme mentioned above may be supplied with biotests, bioavailibility test and analysis for redox parameters. Biotest can describe the toxicity of the complex mix of substances that will typically be found on tar and PAH-contaminated sites. Specifically, the biotests are considered suitable for description of toxicity of percolate, leaking from waste deposits. On the basis of the existing data, there cannot, however, be given more concrete recommendations for the use of biotests, but as a rule there will a need for a test battery of several methods to secure useful results. Determination of bioavailability There are a series of methods available for determination of PAHs’ bioavailability. These methods may be applied for assessment of the effect of natural degradation and bioremediation at PAH-contaminated sites. Yet, the methods have typically been tested on marine sedimentation, and their use at investigations of PAH-contaminated sites is therefore not well described. This is assessed so that further development and documentation of the methods is necessary. Redox parameters The degradation rate for PAHs depends primarily of the redox conditions, wherefore it can be desirable to carry out analyses for redox parameters such as oxygen, nitrate, sulphate, iron, a.o. in the groundwater, and of the oxygen level in the soil’s unsaturated zone. The concentrations of the individual redox parameters in the groundwater, as well as monitoring of the oxygen content in the soil, may also indirectly indicate whether a degradation process is/has been going on in soil and groundwater. Degradation in groundwater To document a natural degradation of PAHs in groundwater, a large number of samples are needed over a long period of time, as well as a consistent execution of the analyses, with low detection rates. |