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

[摘要] 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.

[发布机构] University:University of Birmingham;Department:Earth Science

[关键词] Q Science;QE Geology