Kilder til jordforurening med tjære, herunder benzo(a)pyren i Danmark
Summary and conclusions
This review report concerns the assessment of known sources of soil contamination with tars with special reference to benzo(a)pyrene as well as atypical sources not normally considered as propagators of pollution.
Tar constituents have frequently been registered in connection with soil investigations in recent years. These compounds are not easily degraded and a number of them are considered as carcinogens. In this report, the term tar is interpreted solely with relation to PAH.
The objective of the report is to identify and assess possible sources of PAH in soils. PAH profiles for different sources are identified and profiles in soils differentiated with respect to source as well as to temporal changes are evaluated. Furthermore, the report considers the possibility of transformations that might produce PAH and identifies other possible indicators for soils contaminated with PAH.
The report also incorporates the information on PAH profiles (composition) in the preparation of a sampling and analytical program that can be used in a future monitoring program to define the background levels for PAH in soils in Denmark. The report is therefore primarily concerned with a detailed literature study on diffuse sources of soil contamination with PAH.
In the introduction, a description of the physical and chemical properties, and the effects on the environment and health are given. The significance of benzo(a)pyren for environmental and health assessments is explained and related to the history of development for soil quality criteria for PAH in Denmark. National soil quality criteria from other countries are quoted.
Degradation of PAH is described as degradation affects the levels of soil contamination and the temporal changes in the PAH profile, since some compounds degrade more readily than others. Degradation of the lower PAH (few rings) is quicker than degradation of the higher PAH (many rings), and the process is quicker under aerobic conditions than under anaerobic conditions. It is noted that there is a significant discrepancy between degradation constants measured under field conditions as compared with laboratory experiments, presumably due to the differences in the availability of PAH in soil under different conditions. Degradation for a number of PAH occurs by a co-metabolic process, which requires the presence of other, more degradable compounds, while other compounds (NSO-compounds) are shown to inhibit degradation of certain PAH. Neither toxic intermediates that accumulate or are formed during degradation, nor intermediates that could be included in the monitoring program as an indicator for degradation could be identified in the literature study.
Four approaches are used for the assessment of sources of pollution for PAH:
 | PAH in products and processes |
| Sources (diffuse or point) for soil contamination with PAH |
| Background levels in soil without known sources of pollution |
| Natural formation of PAH |
Products and processes
PAH are found naturally in a range of fossil fuels such as coal and oil, and are released from them by heating. PAH are also formed as a consequence of incomplete combustion of all forms of organic material such as coal, oil, wood, and vegetation. In other words, fossil fuels are sources of PAH, both as PAH-containing products and in connection with incomplete combustion.
Among the many PAH-containing products, coal tar is of great importance as it has been used in many production processes such as road building, wood preservation and roof felts, and is incorporated in many products such as shampoos, skin preparations, ointments, dyes, medicines, plastics, floor covering, smokeless fuels, enamels and ship paints. Distribution of PAH in wastes and in wastewater also occurs. PAH is found in sediments and sludge and often ends in sludge fertilisers on agricultural lands.
Other PAH-containing materials such as crude oil, oil products, wood tars and coke contain a lower content of PAH and therefore, in theory, are not so significant as sources of PAH. However oil products are used for many purposes in such large amounts that they must still be considered as an important source of PAH to the soil environment. Petrol, diesel oil, heating oil lubricants and heavy fuel oil are some of the oil products that throughout the last 100 years have been used for transport, heating and in industry. Crude oil is also used in the production of bitumen that has been heavily used in more recent years as an alternative to tar in road building and for roof felts.
Sources of soil pollution with PAH
Distribution of PAH in the environment can occur naturally due to forest fires but especially human activities such as oil refinery, tar distillation, asphalt production, waste incineration, the former production of town gas and the use and storage of fuels (power stations, domestic heating, traffic), as well as emission from different forms of transport, heating, and industry (cars, lorries, planes, trains, domestic heating, wood burning stoves, power plants and industrial plant) contribute to PAH loads in the environment. These sources of pollution can be defined as either a known or a diffuse source, although the division between these is never well defined.
| Known sources (point sources, line and surface sources); discharge, spill, dumping and waste from PAH-containing products and processes. |
| Diffuse sources; typically industrial emission or traffic, leading to air-borne pollution. |
At production plants, PAH-containing products are often the source of relatively high concentrations of PAH (hotspots), but PAH are also found everywhere in the environment as PAH-containing compounds have been used or produced in innumerable processes for hundreds of years.
Among the many sources of pollution are waste disposal, the use of creosote for wood preservation, run-off from roads, dust from wear and tear of car tires on asphalt roads, oil spills, industrial waste water as well as the utilisation of compost and waste water sludge in agriculture. Coke ovens, gas works, oil refineries, wood preservation, municipal domestic waste, disposal of roof felts, coke cinders and PAH-containing industrial waste are also sources of soil contamination with PAH.
Gardens, especially in towns, are often contaminated with PAH due to the influence of compost (PAH content is due to atmospheric deposition on vegetation), waste, ashes and cinders from domestic heating, bricks, roof felt as well as bonfires of organic waste (vegetation). Cinders from the incineration of coal are also a source of PAH in soils in gardens. PAH from the atmosphere pollutes the topsoil by dry and wet deposition.
PAH contamination in the atmosphere is primarily caused by incomplete combustion of organic materials such as wood or fossil fuels (oils and coal) and constitutes an important diffuse source of soil contamination, both locally close to sources of pollution (roads, railways, industrial emissions, incineration plants), but also by transport from distant sources.
It is concluded that PAH are ubiquitous, but are especially prevalent in areas affected by human activities.
Background levels
Based on a review of the literature, typical concentration levels for PAH in soils without point sources of pollution have been collected for a number of situations and in a number of countries. It is estimated that background levels for Danish soils without point sources or influence from traffic are less than 0,05 - 0,1 mg total PAH/kg as based on measurements in rural areas and in areas without long-term influence from traffic. On the other hand, it is clear that soils in towns and close to traffic have a higher "background level".
Natural formation of PAH
Natural formation of perylene has been determined in soils under anaerobic conditions. No literature sources have indicated the formation of PAH during natural decomposition of vegetation. Low concentrations of PAH are also found in connection with forest fires. Furthermore, analysis of soils with high content of organic material often indicates a natural background of hydrocarbons, and these can be confused with PAH unless a GC-MS analysis is carried out to confirm identification of the hydrocarbons.
PAH profiles and sources of pollution
PAH profiles for a number of sources of pollution such as oil products, tars, exhaust from cars and combustion of wood vary considerably. The physical and chemical properties and degradation rates for PAH are, however, instrumental in reducing dissimilarities in PAH profiles. Therefore contaminated soils from different diffuse sources show relatively similar PAH profiles when compared. It can be concluded that there is no convincing indication for the use of simple PAH profiles to characterise sources of pollution in the case of soil pollution from diffuse sources.
However, it is proposed that certain selected typical compounds such as the methylated PAH especially the 2 and 3-ringed PAH, which primarily originate from oil products, can be used as indicators for the source of origin of PAH pollution. As these compounds are relatively rapidly exposed to degradation, volatilisation and leaching, the picture can change significantly for soil pollution of a less recent date. However, some more stabile methylated PAH (methylated 3 and 4-ringed PAH and methylated thiophenes) have been identified as likely constituents in aged soil pollution and can be the key to the source of pollution. Constituents such as perylene and retene can also be used as indicators for biological formation and combustion of coniferous trees. Finally some PAH ratios that can be of use for identification of sources of PAH pollution are listed.
Sampling and analytical program
A suggestion for a sampling and analytical program, with a view to the establishment of realistic background levels for PAH in soils for use by the environmental authorities is proposed in this report.
Concluding remarks
It is concluded that there is good agreement between the levels of PAH generally found in soils from urban areas and in soils beside roads, when compared with the identified sources of soil pollution for PAH and the estimated deposition over many years based on actual measurements of emission, of atmospheric deposition and of levels in runoff water.
PAH in the soil is considered to be the result of either pollution with PAH containing waste (slag, cinders, etc.) or due to atmospheric deposition directly to the top soil or indirectly to plants followed by composting of the organic material in the soil humus layer.