W11 AM Environmental Chemical Forensics
Wednesday, 16 November 2005: 8:00 AM - 11:40 AM in 343-344

(COS-1117-650979) Differentiating HMW PAHs in sediments from coal tar, creosote, and combustion-related background sources.

Costa, H¹, Sauer, T¹, ¹ Blasland, Bouck and Lee, Inc., New Bedford, MA, 02740

ABSTRACT- Former manufactured gas plant (MGP) and wood-treating site remedial investigations frequently require delineating site-related wastes and dense non-aqueous-phase liquid (DNAPL) residues in adjacent rivers and streams. Natural physical-chemical weathering and biodegradation in surficial sediments alters the composition of pyrogenic tars (e.g., coal tar, carburetted water gas tar, creosote) by depleting the 2- and 3-ring polycyclic aromatic hydrocarbons (PAHs) relatively rapidly, and the high-molecular-weight (HMW) 4- through 6-ring PAHs slowly. As a result, weathered/biodegraded pyrogenic tar residues are compositionally similar to typical urban/industrial background PAH residues, which originate primarily from incomplete combustion of fuel, coal and wood. The presence of these pyrogenic PAHs, which can accumulate from background sources at concentrations on the order of up to 100 mg/kg total PAHs in sediments, is frequently mistaken for site releases. Because background determinations are a key component common to all sediment remedial investigations, it is necessary to distinguish site-related HMW PAHs from background, especially in rivers and streams with high background PAH concentrations. Resolving the compositional similarity of weathered/biodegraded pyrogenic tar residues and pyrogenic background PAH residues calls for a fingerprinting approach directed at the HMW PAH components of these residues. A forensic interpretation approach is presented using multiple double ratios of HMW PAH pairs (non-alkylated constituents) of similar physical properties to establish a weight of evidence in source differentiation. Case study results are presented for several DNAPL tar sites where this approach has successfully distinguished residuals in sediments from elevated background PAH concentrations. The case study results include validation of double ratios from conventional PAH delineation data (Method 8270) by comparison with comprehensive alkylated PAH analytical data.

Key words: PAHs, fingerprinting, coal tar, creosote