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Organic contaminant transport through a thin clay aquitard influenced by palaeo-heterogeneities

White, Rachel A. (2007)
Ph.D. thesis, University of Birmingham.

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Abstract

Processes controlling the transport of dissolved-phase organic solutes through clay aquitards have been investigated. The study was centred upon a former UK industrial facility at which dissolved-phase aromatic solutes contaminated, and in areas penetrated, a discrete clay bed underlying the site. The lacustrine clay stratum (1-2 m thick) at 6 m bgs located in a sand aquifer was cored in 13 locations and intensively sampled with depth (primarily benzene, ethylbenzene, toluene and styrene). Two types of hydrocarbon invasion profiles were identified; (i) diffusion-based invasion and (ii) advection-dominated invasion. The latter has been shown from extensive physicochemical analysis of the clay cores in the laboratory to be primarily through connected “palaeo-root” holes. Root connectivity has been shown at various scales (serial sectioning, x-ray tomography). The hydraulic conductivity in the case of the advection-based profiles was ~0.04 m/d, whereas in the case of the diffusion-based profiles it was much lower at approximately 3 x 10-5 m/d. Sorption characteristics of the clay have been investigated yielding Kd over 2.98 – 6.95 l/kg and Kf over 2.27- 6.89 μgkg-1/ μgl-1 for PCE and Kd over 0.49 l/kg and Kf over 0.57 μgkg-1/ μgl-1 for benzene. Freundlich isotherms over 3-4 orders of magnitude concentration were found to be near-linear, a phenomenon likely attributed to the occurrence of modern organic matter within the sediments. A 2-D (Fractran) numerical model confirmed that where the clay deposits are homogeneous or contain partially penetrating root holes, slow diffusion dominated invasion will offer significant protection to the lower aquifer with breakthrough times through 1 m of clay of 40 years. Conversely, where the clays contain fully penetrating root holes, advection through the root holes causes much faster contaminant penetration. Matrix diffusion from preferential flowpaths causes contaminant to become distributed completely through the clay stratum. Discharge from the bottom of the clay stratum through root holes will be associated with a significant flux of dissolved-phase contaminant (where 30% of clay is rooted with 7.5 cm spaced root holes, ~0.25 mm aperture, contaminant flux is 0.134 g/m2/d) causing contamination of the lower aquifer. The accumulation of mass in the clay stratum is likely to cause prolonged contamination of the adjacent aquifers should the original source concentrations decrease. Indeed, evidence for such reverse diffusion at this site has been observed in one of the profiles. A 3-D code using high resolution data from x-ray tomography was developed to enable modelling of contaminant transport in finely characterised root holes.

Type of Work:Ph.D. thesis.
Supervisor(s):Rivett, M. O. (Mike O.) and Tellam, J. H. (John H.)
School/Faculty:Schools (1998 to 2008) > School of Geography, Earth & Environmental Sciences
Department:Earth Science
Keywords:clay, benzene, diffusion, advection, preferential flow, root holes, sorption
Subjects:QE Geology
GE Environmental Sciences
Institution:University of Birmingham
Library Catalogue:Check for printed version of this thesis
ID Code:104
Refereed:YES
This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder.
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