Composition and Dissolution of a Migratory, Weathered Coal Tar Creosote DNAPL

Kerstin E Scherr,Viktoriya Vasilieva,Wolfgang Lantschbauer,Manfred Nahold

doi:10.3389/fenvs.2016.00061

Abstract

Opaque, viscous tars derived from the carbonization of fossile carbon feedstocks, such coal tars and creosote, are long-term sources of groundwater contamination, predominantly with poly- and heterocyclic aromatic hydrocarbons (PAH). The dissolution, ageing and migratory behavior of dense, non aqueous phase liquid (DNAPL) coal tar blobs and pools forming at the aquitard is not sufficiently understood to estimate the risk and adequately design groundwater treatment measures at a contaminated site. In this study, we investigate the composition and dissolution of a migrated, aged creosote DNAPL and corresponding experimental and groundwater profiles using comprehensive two-dimensional gas chromatography (GCxGC-MS). GC-FID unresolved compounds were attributed to methylated homocyclic species using GCxGC-MS in the Methylanthracene weight range. Equilibrium concentrations were estimated using Raoult’s law, assuming non-ideal behavior. Low molecular weight compounds were found to be prevalent even after decades of weathering, with Naphthalene (8% by mass) representing the most abundant identified compound, contrary to the expected preferential depletion of hydrophilic compounds. Morevoer, dimethylnaphthalenes were relatively more abundant in the aqueous boundary layer than in the DNAPL. DNAPL migration over 400m with the groundwater flow effected lower viscosity and specific gravity of the migrated phase body in a superposition of weathering, transport and aquifer chromatography effects. Based on a decomposition of analysed and estimated constituents using the group contribution approach, reference DNAPL values for activity coefficients γi were used to model aqueous solubilities for selected compounds. Anthracene was close to its theoretical precipitation limit in the bulk DNAPL. While laboratory and modelled DNAPL dissolution behavior agree well, field data imply the presence of specific interfacial in situ processes significantly impacting dissolution processes. Based on aqueous GCxGC-MS profiles over the DNAPL, a hypothetical interfacial in situ film was calculated to be composed primarily of Phenanthrene, with minor contribution by Naphthalene, possibly forming a viscous barrier for the dissolution of lower molecular weight PAH. The main advances and gaps in electron donor DNAPL understanding are discussed regarding our conception of weathered, migrating hydrophobic DNAPL bodies in the aquifer of historic contaminated sites for the adequate treatment of contaminated water.

Highlights

Hydrophobic non-aqueous phase liquids (NAPLs), including light—typically petroleum products, LNAPLs; specific gravity below unity—and dense (DNAPLs) varieties are common forms of hydrocarbons released from industrial processes to the environment
Subsurface chromatography effects have been observed for high molecular weight hydrocarbons in crude oil reservoirs (Bastow et al, 2007), and a similar depletion may have occurred during DNAPL migration presently
Our calculations indicate that the composition of the water/DNAPL interface may be substantially different, i.e., composed of higher molecular weight components, from the bulk DNAPL composition

Summary

Introduction

Hydrophobic non-aqueous phase liquids (NAPLs), including light—typically petroleum products, LNAPLs; specific gravity below unity—and dense (DNAPLs) varieties are common forms of hydrocarbons released from industrial processes to the environment. Tarderived DNAPLs are a source of groundwater contamination at many manufactured gas plant sites, coke plants, and wood impregnation facilities. They may be present as DNAPL blobs or collect as pools on the aquitard, where they may dissolve into and contaminate groundwater over centuries to millennia (Eberhardt and Grathwohl, 2002). Such sites are difficult to manage due to the poor source accessibility and the complexity of factors determining mass transfer controlling groundwater pollution and the quality of associated waste water streams. DNAPLs have been observed to migrate with groundwater flow, but associated phenomena related to physical-chemical properties were not comprehensively analyzed

Methods

Results

Conclusion