SUITMA 2003 Nancy - Release, transport and transformation of PAH and mobile sorbents from contaminated anthropogenic soils

Soils of Urban, Industrial, Traffic, Mining and Military Areas
SUITMA 2003 Nancy

Release, transport and transformation of PAH and mobile sorbents from contaminated anthropogenic soils

Field and lab study and modelling

SUITMA

This abstract is about the keynote lecture 2 of the SUITMA 2003 Nancy symposium.

Kai Uwe Totsche.i

• i - Lehrstuhl für Bodenkunde, Technische Universität München, Freising Weihenstephan (Allemagne).

Hydrophobic organic contaminants are released into the environment in significant amounts since begin of the industrialisation. According to national and european laws on soil protection, soils have to be effectively protected and noxious changes have to be averted or, in the case of actual damage, even remediated. The contamination with HOC and persistent organic pollutants depicts thereby a specific challenge for remediation of the unsaturated soil zone. Within the group of the hydrophobic contaminants, the polycyclic aromatic hydrocarbons (PAH) play a prominent role because of their human- and eco-toxicity and their persistence. The PAH comprise more than hundred single compounds, all of which are composed of at least two annealed benzene rings. The group of PAH features high recalcitrance against microbial and chemical transformation, very low aqueous solubility and hydrophobicity. Due to these properties, PAH are considered immobile within the unsaturated soil zone at contaminated sites. Transport with the seepage water is supposed to be of minor importance, as PAH have a high affinity to the immobile solid phase, in particular to the organic fraction of the soil matrix. This results in strong sorption and such in an effective retardation in the unsaturated soil zone. However, the presence of colloidal phase inorganic and organic materials may increases the apparent solubility of PAH and thus result in increased PAH mobility.

PAH contamination is frequently observed at tar oil contaminated sites. Such contamination may be caused by instantaneous spills or by continuous leakage of defective storage tanks, tar basins and pipelines. Tar-oils originating from coal-tars are chemically divers and viscous non-aqueous phase liquids (NAPL) formed during brown coal smouldering, coke and gas production. Besides phenols and N-, C-, S-hetereocyclic aromatic compounds, the major group are PAH, which can comprise of up to 85% of the NAPL. Compared to water, flow of NAPL is limited due to the higher viscosity and immiscibility with water. Flow and transport in the unsaturated zone is thought to be driven by gravity with a dominant vertical flow direction and lateral spreading controlled by the gradient of the relative permeabilities. The effect of soil profile build-up, preferential flow, aging or groundwater level fluctuations on the spreading of NAPL and NAPL-born contaminants within the unsaturated zone is neglected.

Remediation of tar-oil contaminated sites presumes the knowledge on the location and extent of the source zone and steady state of the plume. The latter requires in-depth information on the effectiveness of attenuation processes and on the prevailing flow and transport pathways. Most of the experience on flow, transport, and transformation of PAH and tar oils results from investigations and case studies conducted for the saturated zone in groundwater environments. The unsaturated soil zone, however, as the primary contaminated compartment, may extent to very large vertical dimensions. Spilled contaminants might not reach the groundwater aquifer but remain in soil were NAPL and PAH are transformed or degraded. Experimental and field survey results obtained for contaminated groundwater can therefore not be directly conferred to the unsaturated soil zone, as soils are characterised by high temporal and spatial fluctuations of physicochemical and biological parameters and boundary conditions, e.g. temperature, precipitation, water content, electron acceptors, etc. The fluctuations of those in combination with the heterogeneity of soils affect both the transport and transformation of PAH.

To advice soil remediation strategy, in-depth knowledge is therefore required on the physicochemical and biological parameters and processes prevailing in the unsaturated zone of contaminated sites.

Within this presentation, field and lab data on the fate of PAH in the unsaturated zone of contaminated sites will be presented. Most of the data were collected at gas work sites, while some are from former fuel production facilities in Germany. The effect of temporal fluctuations of confined and unconfined regions on NAPL entrapment within the aquifer will be shown. Special consideration will be put on the effect of • site heterogeneity • ageing of PAH source materials • release of PAH from NAPL imbibed soils • preferential flow of NAPL • dissolved and colloidal phase organic carriers on the release, transport and retardation of PAH. Based on these findings, a conceptual model will be presented which allows understanding the fate of PAH in the unsaturated soil zone of contaminated sites.

References

• Weigand H., Totsche K. U. & Kögel-Knabner I. (1998). Effect of Fluctuating Input of Dissolved Organic Matter (DOM) on Long-Term Mobility of Polycyclic Aromatic Hydrocarbons(PAH) in Soils. Phys. Chem. Earth.23(2):211-214.
• Kögel-Knabner I. & Totsche K. U. (1998). Influence of dissolved and colloidal phase humic substances on the transport of hydrophobic organic contaminants in soils. Phys. Chem. Earth.23(2):179-185.
• Kögel-Knabner I., Totsche K. U. & Raber B. (2000): Desorption of PAH from Soil in the Presence of Dissolved Organic Matter: effect of solution composition and aging. J. Environ. Qual. 29:906-916.
• Weigand H., Totsche K. U., Huwe B., Kögel-Knabner I. (2001): PAH mobility in contaminated industrial soils: a Markov chain approach to the spatial variability of soil properties and PAH levels. Geoderma, 102:371-389.
• Weigand H., Totsche K. U., Kögel-Knabner I., Annweiler E., Richnow H.H., Michaelis W. (2002): Fate of anthracene in contaminated soil - transport and biodegradation under unsaturated flow conditions. Eur. J. Soil Sci. 53(1):71-82.
• Weigand H., Totsche K. U., Mansfeldt T., Kögel-Knabner I. (2001): Slow release and mobility of polycyclic aromatic hydrocarbons and iron-cyanide complexes in contaminated soil. J. Plant Nutr. Soil Sci. 164:643-649.
• Totsche K. U., I. Kögel-Knabner und Harald Weigand (2002) Modelling contaminant transport in anthropogenic soil: Reconstruction of spatial heterogeneity by analysing the relations of adjacent pedofacies. In: Schulz, H.D. und Teutsch, G. (Hrsg). Geochemical Processes: Conceptual Models for Reactive Transport in Soil and Groundwater. 1-18. Wiley VCH, Weinheim.
• Totsche K. U., I. Kögel-Knabner, B. Haas, R. Scheibke und S. Geisen (2003): Evidence for Preferential Flow and Ageing of NAPL and NAPL borne Contaminants in the Unsaturated Soil Zone of a Creosote Contaminated Site: A Field Study. J. Plant Nutr. Soil Sci. 166:102-110.
• Totsche K. U., I. Kögel-Knabner, H. Weigand und D. Hensel (2003): Fate of PAH at contaminated sites: Facts and concepts for the unsaturated soil zone. Proceedings GeoProc 2002. in press.
• Wehrer, M. und Totsche K. U. (2003): Detection of non-equilibrium in soil columns: Delineation of experimental conditions by numerical simulations. J. Plant Nutr. Soil Sci.. Angenommen.
• Totsche, K. U. & Kögel-Knabner, I. (2003): Mobile organic sorbent affected contaminant transport in soil: Numerical case studies for enhanced and reduced mobility. Vadose Zone Journal, submitted.