Coupling chemical transport modeling and diagnostic ratio analysis for predicting PAH fingerprint in coal tar and creosote contaminated soils

ABSTRACT

This article illustrates the use of a fate and transport model and a diagnostic ratio analysis to determine chemical signatures of polyaromatic hydrocarbons (PAHs) in coal tar and creosote residues in soil. The fate and transport model, based on diffusion, convection – advection of PAHs, helps determine their weathering rates in two soil types and provides an insight on the air – soil exchange of PAHs. Time-averaged volatilisation flux, estimated from the model, was used to determine the residual composition of the 16 priority PAH (non-alkylated PAHs) in coal tar and creosote contaminated soils. Expectedly, lower molecular weight (MW) PAHs volatilised faster than higher MW PAHs. In the absence of water movement towards the surface, vapour diffusion is the primary mechanism of transport, and the flux was observed to be high ranging from 8.52–2167.3 x 10⁻⁷ kg m⁻² day⁻¹. Higher flux was observed for clayey soils. At high values of organic carbon (f_oc), the flux was observed to be lower due to the binding of PAHs to the soil. The mass flux was higher with water movement (0.6–1036.5 x 10⁻³ kg m⁻² day⁻¹) indicating that diffusion along with wicking were the key mechanisms for PAH losses from soil residues. The modelled volatilisation flux for the PAHs was observed to be within the range of the flux values (for soils and sediment) reported in literature. Double diagnostic ratio analysis was used to compare chemical signatures of the modelled residues to PAH data in actual PAH contaminated soils. The diagnostic PAH ratios of the modelled weathered coal and creosote residues and the actual soil samples correlated well, especially for 5- and 6-ringed PAHs. A good correlation between the modelled and actual diagnostic ratios suggested that the mathematical model can be used to supplement physical sampling and analysis, thereby providing one additional line of evidence for source apportionment.

KEYWORDS:

• PAHs
• weathering
• air-soil exchange
• diagnostic ratios
• fingerprinting

Acknowledgement

The authors are grateful to Department of Science & Technology, New Delhi for support on this project under the Water Technology Initiative Scheme (WTI, DST/TMD/EWO/WTI/2K19/EWFH/2019/98(G)). We also acknowledge SASTRA Deemed to be University, Thanjavur for extending the necessary infrastructure support to carry out the study.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the Department of Science and Technology, Ministry of Science and Technology [DST/TMD/EWO/WTI/2K19/EWFH/2019/98(G)]