Polycyclic Aromatic Hydrocarbon (PAH) Contamination in Urban Soils: Testing and Management
Melissa Chilinski, Paul Grossl, and Melanie Stock
Polycyclic aromatic hydrocarbons (PAHs) are a large class of chemicals and common environmental pollutants. Understanding which PAHs and soil test levels may impact human health is an important aspect of gardening and micro-farming, particularly in urban environments that are at increased risk of soil contamination. Land use histories, such as sites associated with vehicle and industrial emissions, burning, and dumping, can elevate concentrations of PAHs in soils. This fact sheet provides instructions on assessing your site for PAHs that may present health risks to humans, testing the soil, and first steps for interpretation and management.
Common Contamination Sources
Polycyclic aromatic hydrocarbons (PAHs) can occur naturally from volcanic eruptions and wildfires, but in urban or industrial areas, soil contamination is most often caused by human activity. Wind can deposit PAHs onto the soil where PAHs can persist on the surface for 1 to 2 years until they degrade fully (Gan et al , 2009; CRC CARE, 2017). The most common and widespread, continuous (i.e., repeated) sources of PAHs are industrial and vehicle emissions (Table 1) Particles in exhaust and fumes created during engine combustion can enter the air and can be deposited onto the soil. Areas within 500 feet of a busy road, highway, train track, or industry (e.g., incinerators, refineries, power plants) can be at increased risk for elevated contamination levels. Other, smaller-scale continuous sources include grilling and burning wood or garbage.
One-time events or noncontinuous point sources can also cause soil contamination (Table 1). Examples include
refuse from previous dumping, i.e., spent coal and oil, chunks of asphalt, charred wood, or other refuse left in the soil, and particles in fumes emitted during repaving, especially when using coal tar-based pavement sealers (U.S. Geological Survey [USGS], 2019).
Land-use histories from previous owners are often unknown. The United States Environmental Protection Agency’s (US EPA) Superfund: National Priorities List (NPL) map is a helpful resource for determining areas with greater risk of contamination Testing for PAHs (Figure 1) is highly recommended if any potential risk factors from Table 1 are identified on the property, the history is unknown, and/or food crops will be grown.
Table 1. Sources that elevate the risk of polycyclic aromatic hydrocarbon
(PAH) contamination in the soil.
Source1 Reason for Potential Soil Contamination Continuous
Emissions from industry and vehicles (proximity within 500 ft)
Emissions from home activities
Noncontinuous
Dumping sites
Paving and resealing emissions
Industries like waste incinerators, manufacturing, refineries, power production, and/or any facility that will emit fumes into the atmosphere can deposit PAHs. Particles in exhaust released from cars, trains, and off-road vehicles can deposit PAHs onto the soil over time.
Household activities, such as garbage, wood, or coal burning.
Areas with a history of dumping, such as piles of coal, garbage, oil, or charred wood.
Asphalt fumes and asphalt itself. Coal-tar based pavement sealers have an increased concentration of PAHs compared to asphalt and asphalt-based sealers 2
1Sources are grouped as continuous that present repeated risk of exposure or non-continuous that are one-time-event sources for exposure. 2USGS (2019).
Composition and Classification
Polycyclic aromatic hydrocarbons are a class of chemicals composed of fused aromatic rings of carbon and hydrogen atoms (Figure 2). Although PAHs are only composed of hydrogen and carbon atoms, they exhibit different qualities and levels of toxicity depending on their size and how their atoms are arranged at a molecular level. The PAHs that are more harmful to human health and persistent for longer in the environment often have greater molecular weights (Kanaly and Harayama, 2000). Larger, more complex PAHs with greater molecular weights can attach (sorb) more strongly to soil organic matter, are slower to degrade (have longer half-lives), are less able to dissolve in water (increased hydrophobicity), and have low risk for plant uptake (low bioavailability). In general, during lower temperature processes, like wood burning, lower molecular weight PAHs are formed, whereas during hightemperature processes, such as combustion, higher molecular weight PAHs are formed (Ukalska-Jaruga et al , 2018)
Because PAHs are generally not soluble or bioavailable, and thus are not likely to accumulate in plant tissues or transfer to water, primary human exposure in urban gardens and farms occurs through direct contact with bare soils by digging, planting, playing, or eating unwashed vegetables. Root crops present the greatest risk for exposure if they are not washed and/or peeled correctly.
Guidelines for PAH Soil Sampling
Screening urban soils for garden and farm suitability is important for human and crop health. Chemtech-Ford
Laboratories in Sandy, Utah, offers Soil Test 8270, or the semi-volatile test, for PAH soil concentrations.
Unlike other soil tests that can be sent in the mail, collecting samples for the semi-volatile test is time sensitive and requires specific vials for sampling (Figure 1). Obtain these vials from Chemtech-Ford Laboratory during business hours prior to collecting soil samples. Soil must be collected, stored in a cooler with ice, and delivered to Chemtech-Ford Laboratories on the same day.
Methods for collecting samples for PAH testing are almost identical to procedures for the Routine Soil Test (#S28) at the Utah State University Analytical Laboratories (USUAL). For one garden area, or “zone,” collect multiple subsamples with a shovel from the soil surface to a 6-inch depth, mix the subsamples in a clean bucket, and fill the laboratory vial to the top with the mixed soil. For further step-by-step instructions on how to properly zone a property for testing, refer to pages 23 of Urban Garden Soils: Testing and Management
Depending on previous test results and the presence of continuous-type risk factors, retest for PAHs every 5 to 10 years.
Screening Levels and Carcinogenic Qualities
The US EPA has set Regional Screening Levels (RSLs) to standardize human health exposure limits for PAHs. Values relevant to urban farming and gardening are found under “Resident Soil” and include an exposure limit in mg/kg, or equivalent units of ppm (Table 2). The more toxic the PAH, the lower the EPA RSL.
Table 2. The name and molecular formula of key polycyclic aromatic hydrocarbons (PAHs) with the US EPA Residential Screening Level (RSL) for soil in mg/kg, designation on the US EPA priority pollutant list, and the Toxicity classification and likelihood based on International Agency for Research on Cancer (IARC). Although all PAHs are structurally related and/or their chemical formulas can be the same, qualities vary by the arrangement of aromatic rings of carbon and hydrogen.
Though more than 100 PAHs have been identified, the US EPA targets 16 PAHs as High Priority Pollutants that pose the most risk to human health. Seven of those priority pollutants, along with Benzo(j)fluoranthene, are highlighted with toxicity levels given by the International Agency for Research on Cancer that include (Table 2):
• Group 1, carcinogenic
• Group 2A, probably carcinogenic
• Group 2B, possibly carcinogenic
The most toxic PAHs are benzo(a)pyrene (B(a)P), a known carcinogen, and dibenz(a,h)athracene, a probable carcinogen. Both have low RSLs of 0.11 mg/kg.
dissolves in water (is hydrophobic), exists only in the solid (particulate) phase, and strongly sorbs to clays and organic matter, which reduces its ability to degrade (increases its half-life). Half-lives of B(a)P can range from 14 months to almost 2 years. More weathered and strongly sorbed B(a)P can degrade more slowly, while freshly contaminated areas can degrade more quickly (CRC CARE, 2017).
Test Results and Remediation Strategies
Test results from sampling will show total concentrations in mg/kg for 26 PAH compounds. Of the compounds in the testing report, 8 of the 26 are particularly important, as they may be carcinogenic; these are also listed in Table 2 and include benzo(a)anthracene, benzo(a)pyrene, benzo(b)fluoranthene, benzo(j)fluoranthene, benzo(k)fluoranthene, chrysene, dibenz(a,h)anthracene, dibenz(a,h)anthracene, and indeno(1,2,3-cd)pyrene.
Figure 2. Chemical structure of Benzo(a)pyrene, one of the most carcinogenic PAHs. There are five fused hexagonal rings arranged in an angular, or non-linear structure
Benzo(a)pyrene
Benzo(a)pyrene (B(a)P) is the most extensively studied PAH because it is a known carcinogen and contamination is widespread. It is primarily created from incomplete engine combustion and the processing of coal, tar, oil, and gas; vehicle emissions; cooking; and heating with coal and woodburning stoves and fireplaces. With its greater molecular weight, or larger size, B(a)P rarely
If a test result is close to a screening level for any of the PAHs in Table 2, or one of the other 18 PAHs on the test report is flagged, consider remediation strategies that decrease exposure. Recommendations include building raised beds with uncontaminated topsoil and covering the surrounding native soil with mulch, turfgrass, or rock to decrease airborne exposure. Consider starting a perennial garden that needs minimal maintenance or disturbance, and mulch the surface to avoid bare soils For in-ground plantings, dilute the native soil with clean, uncontaminated topsoil to decrease contamination levels. For example, if a test result comes back with chrysene at 150 mg/kg, add 1/3 more topsoil to your garden to decrease chrysene levels by 1/3 (33%) to 100 ppm. Remember to measure garden areas on a volume
basis, or length × width × grow depth (at least 12-inches deep).
If test results for any of the highlighted polycyclic aromatic hydrocarbons from Table 2, especially benzo(a)pyrene and dibenz(a,h)athracene, surpass the US EPA’s RSL, stop growing practices and contact your state’s Department of Environmental Quality for remediation suggestions.
Additional Resources
Andersson, J and Achten, C. 2014. Time to Say Goodbye to the 16 EPA PAHs? Towards an Up-to-Date Use of PACs for Environmental Purposes. Polycyclic Aromatic Compounds 35(2-4): 330-354.
Centers for Disease Control and Prevention (CDC). 2022. Polycyclic Aromatic Hydrocarbons (PAHs) Factsheet
Collins, J., Brown, J., Alexeef, G., and Salmon, A. 1998. Potency Equivalency Factors for Some Polycyclic Aromatic Hydrocarbons and Polycyclic Aromatic Hydrocarbon Derivatives. Regul Toxicol Pharmacol 28(1): 45-54.
CRC CARE. 2017. Risk-Based Management and Remediation Guidance for Benzo(a)pyrene. Proceedings of the 7th International Contaminated Site Remediation Conference 52(46): 633.
Dybing, E., Schwarze, P., Nafstad, P., Victorin, K., and Penning, T. 2013. Chapter 7 Polycyclic Aromatic Hydrocarbons in Ambient Air and Cancer. IARC Sci Pub 161: 75-94.
Gan, S., Lau, E., and Ng, H. 2009. Remediation of Soils Contaminated with Polycyclic Aromatic Hydrocarbons (PAHs). J Hazardous Materials 172(2-3): 532-549.
International Agency for Research on Cancer (IARC). 2010. Some NonHeterocyclic Polycyclic Aromatic Hydrocarbons and Some Related Exposures IARC Monographs on the Evaluation of Carcinogenic Risks to Humans: 92: 1-853.
Jameson, C. 2019. Chapter 7 Polycyclic Aromatic Hydrocarbons and Associated Occupational Exposures. IARC Sci Pub, 165, Chapter 7.
Kanaly, R., and Harayama, S. 2000. Biodegradation of High-Molecular Weight Polycyclic Aromatic Hydrocarbons by Bacteria. J Bacteriol 182(8): 2059-2067.
Kariyawasam, T., Doran, G., Howitt, J., and Prenzler, P. 2002. Polycyclic Aromatic Hydrocarbon Contamination in Soils and Sediments: Sustainable Approaches for Extraction and Remediation. Chemosphere 291(3): 132981.
Lukic, B., Panico, A., Huguenot, D., Fabbricino, M., van Hullebusch, E., and Esposito, G. 2017. A Review on the Efficiency of Landfarming Integrated with Compositing as a Soil Remediation Treatment Environ Tech Reviews 6(1): 94-116.
Marchal, G., Smith, K., Rein, A., Winding, A., Trapp, S., and Karlson, U. 2013. Comparing the Desorption and Biodegradation of Low
Concentrations of Phenanthrene Sorbed to Activated Carbon, Biochar, and Compost. Chemosphere 90(6): 1767-1778.
Massachusetts Department of Environmental Protection. 2002. Background Levels of Polycyclic Aromatic Hydrocarbons and Metals in Soil
Mohan, S., Kisa, T., Ohkuma, T., Kanaly, R., and Shimizu, Y. 2006. Bioremediation Technologies for Treatment of PAH-Contaminated Soil and Strategies to Enhance Process Efficiency. Rev Environ Sci and Biotechnol 5: 347-374.
Obrycki, J., Basta, N., and Culman, S. 2017. Management Options for Contaminated Urban Soils to Reduce Public Exposure and Maintain Soil Health. J Environ Qual 46: 420-430.
Roslund, M., Gronross, M., Rantalainen, A., Jumpponen, A., Romantschuk, M., Parajuli, A., Hyoty, H., Laitinen, O., and Sinkkonen, A. 2018. Half-lives of PAHs and Temporal Microbiota Changes in Commonly Used Landscaping Materials. PeerJ 6: e4508. Smith, M., Lethbridge, G., and Burns, R. 1997. Bioavailability and Biodegradation of Polycyclic Aromatic Hydrocarbons in Soil. FEMS Microbiology Letters 152: 141-147.
Sushkova, S., Deryabkina, I., Atonenko, E., Kizilkaya, R., Rajput, V., Vasilyeva, G. 2018. Benzo(a)pyrene Degradation and Bioaccumulation in Soil-Plant System Under Artificial Contamination. Sci of Tot Environ 633: 1386-1391.
Ukalska-Jaruga, A., Debaene, G., and Smreczak, B. 2020. Dissipation and Sorption Processes of Polycyclic Aromatic Hydrocarbons (PAHs) to Organic Matter Amended by Exogenous Rich-Carbon Material. J Soils Sediments 20: 836-849.
Ukalska-Jaruga, A., Smreczak, B., and Klimkowicz-Pawklas, A. 2018. Soil Organic Matter Composition as a Factor Affecting the Accumulation of Polycyclic Aromatic Hydrocarbons. J Soils Sediments 19: 1890-1900.
Ukalska-Jaruga and Smreczak, B. 2020. The Impact of Organic Matter on Polycyclic Aromatic Hydrocarbons (PAH) Availability ad Persistance in Soils. Molecules 25(11): 2470.
US EPA. 2014. Priority Pollutant List
US EPA. 2021. Regional Screening Level (RSL) Summary Table
US EPA. 2017. Toxicological Review of Benzo(a)pyrene
USGS. 2019. Coal-Tar-Based Pavement Sealcoat, PAHs, and Environmental Health.
Utah Department of Health. 2022. Health Indicator Report of Community Design: Proximity of Population and Schools to Highways
Disclaimers
Using figures, tables, and photos without written permission from the authors is prohibited.
In its programs and activities, including in admissions and employment, Utah State University does not discriminate or tolerate discrimination, including harassment, based on race, colo r, religion, sex, national origin, age, genetic information, sexual orientation, gender identity or expression, disability, status as a protected veteran, or any other status protected by University policy, Title IX, or any other federal, state, or local law. Utah State University is an equal opportunity employer and does not discriminate or tolerate discrimination including harassment in employment including in hiring, promotion, transfer, or termination based on race, color, religion, sex, national origin, age, genetic information, sexual orientation, gender identity or expression, disability, status as a protected veteran, or any other status protected by University policy or any other federal, state, or local law. Utah State University does not discriminate in its housing offerings and will treat all persons fairly and equally without regard to race, color, religion, sex, familial status, disability, national origin, source of income, sexual orientation, or gender identity. Additionally, the University endeavors to provide reasonable accommodations when necessary and to ensure equal access to qualified persons with disabilities. The following individuals have been designated to handle inquiries regarding the application of Title IX and its implementing regulations and/or USU’s non-discrimination policies: Executive Director of the Office of Equity, Matt Pinner, JD, matthew.pinner@usu.edu, Title IX Coordinator, Hilary Renshaw, hilary.renshaw@usu.edu, Old Main Rm. 161, 435-797- 1266. For further information regarding non-discrimination, please visit equity.usu.edu, or contact: U.S. Department of Education, Office of Assistant Secretary for Civil Rights, 800-421-3481, ocr@ed.gov or U.S. Department of Education, Denver Regional Office, 303-844-5695 ocr.denver@ed.gov. Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Kenneth L. White, Vice President for Extension and Agriculture, Utah State University.
