An analytical model for chemical diffusion in layered contaminated sediment systems with bioreactive caps

An analytical model for contaminant transport in multilayered capped contaminated sediments including the degradation of organic contaminant is presented. The effect of benthic boundary layer was treated as a Robin-type boundary condition. The results of the proposed analytical model agree well with experimental data. The biodegradation of contaminant in bioturbation layer shows a significant influence on the flux at the surface of system. The maximum flux for the case with t_1/2,bio = 0.07 year can be 4.5 times less than that of the case without considering the effect of biodegradation. The thickness of bioturbation layer has a significant effect on the performance of the capped contaminated sediment. The maximum flux for the case with l_bio = 15 cm can be 17 times larger than that of the case without bioturbation layer. This may be because the effective diffusion coefficient of sand cap can be 28 times lower than D_bio. The mass transfer coefficient should be considered for the design of the capping system as the contaminant concentration at the top of system for the case with k_bl = 2.5 × 10⁻⁵ cm/s can be 13 times greater than that of the case with k_bl = 10⁻⁴ cm/s. The proposed analytical model can be used for verification of complicated numerical methods, evaluation of experimental data, and design of the capping contaminated sediment systems with reactive cap layers.     

Centrifuge simulation of the consolidation characteristics of capped marine sediment beds

Marine sediment capping is a technique where clean sand or sediment is placed over contaminated sediment to reduce the migration of contaminants to the environment. Environmental regulations have limited the use of in situ sediment capping due to concerns about the contaminant migration through the cap. A series of centrifuge tests were conducted to simulate the effects of consolidation settlement of capped marine sediment. This study describes the testing and monitoring of the centrifuge tests. The results from the centrifuge tests are interpreted and compared to predictions made by the PSDDF computer program, which can qualitatively estimate the consolidation settlement of capped marine sediment.

Centrifuge tests were utilized to predict the consolidation of marine sediment caused by the placement of a capping layer. The centrifuge tests used the modeling of models technique to verify that correct modeling procedures were utilized. In this study, the maximum deviation between the centrifuge test results and PSDDF prediction was 20%. Thus, designers should utilize PSDDF consolidation settlement results with caution. Dye tracer studies showed the importance of consolidation-induced advective transport of contaminants. Thus, the capping layer must be appropriately designed to reduce the effects of consolidation-induced advective transport. This may be accomplished by adding a reactive barrier or geosynthetic barrier layer to the cap design.     

Reactive Transport Modeling of Subaqueous Sediment Caps and Implications for the Long-Term Fate of Arsenic, Mercury, and Methylmercury

A 1-D biogeochemical reactive transport model with a full set of equilibrium and kinetic biogeochemical reactions was developed to simulate the fate and transport of arsenic and mercury in subaqueous sediment caps. Model simulations (50?years) were performed for freshwater and estuarine scenarios with an anaerobic porewater and either a diffusion-only or a diffusion plus 0.1-m/year upward advective flux through the cap. A biological habitat layer in the top 0.15?m of the cap was simulated with the addition of organic carbon. For arsenic, the generation of sulfate-reducing conditions limits the formation of iron oxide phases available for adsorption. As a result, subaqueous sediment caps may be relatively ineffective for mitigating contaminant arsenic migration when influent concentrations are high and sorption capacity is insufficient. For mercury, sulfate reduction promotes the precipitation of metacinnabar (HgS) below the habitat layer, and associated fluxes across the sediment–water interface are low. As such, cap thickness is a key design parameter that can be adjusted to control the depth below the sediment–water interface at which mercury sulfide precipitates. The highest dissolved methylmercury concentrations occur in the habitat layer in estuarine environments under conditions of advecting porewater, but the highest sediment concentrations are predicted to occur in freshwater environments due to sorption on sediment organic matter. Site-specific reactive transport simulations are a powerful tool for identifying the major controls on sediment- and porewater-contaminant arsenic and mercury concentrations that result from coupling between physical conditions and biologically mediated chemical reactions.     

Analytical solutions for advective–dispersive solute transport in double‐layered finite porous media

Analytical solutions for advection and dispersion of a conservative solute in a one‐dimensional double‐layered finite porous media are presented. The solutions are applicable to five scenarios that have various combinations of fixed concentration, fixed flux and zero concentration gradient conditions at the inlet and outlet boundaries that provide a wide number of options. Arbitrary initial solute concentration distributions throughout the media can be considered via explicit formulations or numerical integration. The analytical solutions presented have been verified against numerical solutions from a finite‐element‐based approach and an existing closed‐form solution for double‐layered media with an excellent correlation being found in both cases. A practical application pertaining to advective transport induced by consolidation of underlying sediment layers on contaminant movement within a capped contaminated sediment system is presented. Comparison of the calculated concentrations and fluxes with alternative approaches clearly illustrates the need to consider advection processes. Consideration of the different features of contaminant transport due to varying pore‐water velocity fields in primary consolidation and secondary consolidation stages is achieved via the use of non‐uniform initial concentration distributions within the proposed analytical solutions. Copyright © 2010 John Wiley & Sons, Ltd.     

Numerical modeling of contaminant transport resulting from dissolution of a coal-tar pool in an experimental aquifer

A previously developed two-dimensional numerical model is further developed for simulating the transport of dissolved contaminants originating from dissolution of a coal tar pool in a stratified, saturated porous medium. The model is used to simulate contaminant transport resulting from a rectangular-shaped coal-tar-pool dissolution experiment conducted in a large-scale experimental aquifer. The experimental porous medium consists of two sand strata, a high-hydraulic-conductivity upper stratum and a low-hydraulic-conductivity bottom stratum. The experiment was conducted to a time of 354 days and the groundwater velocity was changed several times within this duration. Model simulations show good agreement against observed contaminant concentrations, and simulations show that dissolved solute below the pool migrated deeper into the bottom stratum as compared to the upper stratum. Furthermore, simulations also suggest that contaminant concentrations in the lower stratum never reached quasi steady-state during the experimental time frame.     

Simulation of groundwater flow and contaminant transport at a landfill site using models

Numerical and analytical modelling studies were conducted for the analysis of groundwater flow and contaminant transport at the Innisfil landfill site in the Town of Innisfil, County of Simcoe, in Ontario, Canada. Previously conducted field studies categorized the upper stratigraphy at the site into three units: upper sand unit, upper silt/clay unit and Intermediate Sand unit. Essentially horizontal groundwater movement in the two sand units and vertical downward flow in the silt/clay unit were reported by the field hydrogeologists. In the following, application of three computer models (FLOWPATH, USGS MOC and POLLUTE) for the simulation of the groundwater flow and contaminant transport processes at the Innisfil landfill site is described. The paper focuses on the calibration of groundwater flow and contaminant transport systems, and demonstrates how the insight gained during the contaminant transport calibration was used to improve the initial groundwater flow characterization of the hydrogeological system.     

大变形黏土防渗层中的污染物迁移和转化规律研究

国内湖泊疏浚污染底泥堆场一般以较厚的黏土层作为主要防渗层,由于在上覆底泥作用下黏土层会发生较大的固结变形,因此,在研究黏土防渗层中的污染物运移和转化规律时,应该考虑土体变形的影响。基于Gibson一维大变形固结理论和饱和多孔介质中的污染物对流扩散方程,建立了二者耦合的可变形多孔介质中污染物的运移和转化模型,其中首次考虑了土体自重和生物降解作用的影响。利用所建立模型的数值解,研究了在可变形黏土防渗层中的污染物运移和转化规律,同时分析了模型中不同项和主要参数的作用和影响。研究结果表明,土体大变形对黏土防渗层中污染物的运移有着较复杂的影响,一方面土体变形会加速污染物的运移;另一方面土体固结带来的渗透性减小会增加污染物的穿透时间,二者的不同作用取决于众多的影响因素,如土层厚度和吸附作用等。研究结果对于评估天然黏土防渗层对污染物的阻隔作用有重要的指导意义。     

考虑土体固结变形的污染物运移模型

土体的固结压缩变形对污染物的运移具有重要的影响,而目前国内关于污染物在变形多孔介质中运移规律的研究尚处于空白,在国外也是近几年才有人开始这方面的研究工作。在Biot固结理论和污染物运移理论相结合的基础上,提出了污染物在黏土防渗层中迁移转化的一维数学模型,该模型的最大特点是考虑了土体受力变形对污染物运移的影响,在合理简化的基础上给出了模型的解析解,并将计算结果与太湖疏浚污染底泥堆场的实测结果进行了比较分析,模拟计算的结果在一定程度上能够反映实际土层中污染物的运移情况。     

Flow dynamics at the origin of thin clayey sand lacustrine turbidites: Examples from Lake Hazar,Turkey

Turbidity currents and their deposits can be investigated using several methods, i.e. direct monitoring, physical and numerical modelling, sediment cores and outcrops. The present study focused on thin clayey sand turbidites found in Lake Hazar (Turkey) occurring in eleven clusters of closely spaced thin beds. Depositional processes and sources for three of those eleven clusters are studied at three coring sites. Bathymetrical data and seismic reflection profiles are used to understand the specific geomorphology of each site. X‐ray, thin sections and CT scan imagery combined with grain‐size, geochemical and mineralogical measurements on the cores allow characterization of the turbidites. Turbidites included in each cluster were produced by remobilization of surficial slope sediment, a process identified in very few studies worldwide. Three types of turbidites are distinguished and compared with deposits obtained in flume studies published in the literature. Type 1 is made of an ungraded clayey silt layer issued from a cohesive flow. Type 2 is composed of a partially graded clayey sand layer overlain by a mud cap, attributed to a transitional flow. Type 3 corresponds to a graded clayey sand layer overlain by a mud cap issued from a turbulence‐dominated flow. While the published experimental studies show that turbulence is damped by cohesion for low clay content, type 3 deposits of this study show evidence for a turbulence‐dominated mechanism despite their high clay content. This divergence may in part relate to input variables, such as water chemistry and clay mineralogy, that are not routinely considered in experimental studies. Furthermore, the large sedimentological variety observed in the turbidites from one coring site to another is related to the evolution of a sediment flow within a field‐scale basin made of a complex physiography that cannot be tackled by flume experiments.     

A general approach to the modelling of contaminant transport through composite landfill liners with intact or leaking geomembranes

Finite‐element models of contaminant transport through composite landfill liners require highly refined meshes around the interface between the geomembrane and the clay layer, especially if leakage through holes in the geomembrane is considered. In addition, no general formulation for transport through leaking geomembranes can be found in the literature. The paper develops a general approach to time‐dependent contaminant migration through composite liners with intact or leaking geomembranes. Equations are derived for various combinations of system conditions including Dirichlet and Neumann boundary conditions in the waste, constant mass of contaminants in the waste, steady state or transient transport in the geomembrane, and steady state or transient seepage velocities in the mineral liner. The effect of the geomembrane on transport in the soil is converted into an equivalent boundary condition applicable at the top of the clay layer. Hence, only the media underlying the top geomembrane are explicitly represented in the numerical model, yielding a computationally efficient algorithm. The new formulation is validated in conjunction with finite‐layer, finite‐element and boundary‐element methods, by comparing its predictions to those of more conventional approaches which represent the geomembrane explicitly. The scope of the method is illustrated by modelling a landfill liner with a geomembrane leaking in five locations. Copyright © 2007 John Wiley & Sons, Ltd.     

Groundwater residence time downgradient of Trench No. 22 at the Chernobyl Pilot Site: Constraints on hydrogeological aquifer functioning

Following the explosion of reactor 4 at the Chernobyl power plant in northern Ukraine in 1986, contaminated soil and vegetation were buried in shallow trenches dug directly on-site in an Aeolian sand deposit. These trenches are sources of radionuclide (RN) pollution. The objective of the present study is to provide constraints for the Chernobyl flow and RN transport models by characterising groundwater residence time. A radiochronometer ³H/³He method (t_1/2 = 12.3 a) and anthropogenic tracers including CFC and SF₆ are investigated along with the water mass natural tracers Na, Cl, ¹⁸O and ²H.     

Natural acid rock drainage associated with black shale in the Yukon Territory,Canada

Investigated herein are water and sediment geochemistry, and metal attenuation processes associated with natural acid rock drainage originating from black shale formations in the Macmillan Pass area, Clear Lake prospect and Engineer Creek by the Dempster Highway in the Yukon Territory, Canada. The most metalliferous water having pH 3.0, 150 mg/L Zn, 39 mg/L Ni, 2.8 mg/L Cu and 9.1 mg/L As was found in a tributary stream of Engineer Creek with no known mineral deposits occurring in the vicinity. For all three study areas, the water and sediment geochemistry is significantly affected by the local lithology and prevailing metal attenuation processes. Despite their anomalous acidity and metal contents, the natural acid streams contribute only a small fraction of the contaminant loadings to the major water courses because of their low flows. Dilution, neutralization, sorption and co-precipitation are identified as the major mechanisms attenuating aqueous transport of potentially deleterious metals. However, microbial mediation in metal attenuation is also evident in low-flow systems. The wide variation of water and sediment geochemistry along a flow path renders the establishment of background metal values difficult. In assessing environmental impacts, it may be more practical to consider metal loadings on a watershed scale than to rely on a comparison with operationally defined background concentrations.     

Evaluation of on-line pyrolysis two-dimensional gas chromatography time-of-flight mass spectrometry (Py-GC × GC-ToFMS) on whole sediments from a Mediterranean sapropel sequence

Analysis of the molecular composition of the organic matter (OM) from whole sediment samples can avoid analytical bias that might result from isolation of components from the sediment matrix, but has its own analytical challenges. We evaluated the use of GC × GC-ToFMS to analyze the pyrolysis products of six whole sediment samples obtained from above, within and below a 1 million year old OM-rich Mediterranean sapropel layer. We found differences in pyrolysis products <n-C₂₂ between the OM-rich sapropel samples and the OM-poor background marls. The presence of alkyl pyrroles, probably derived from chlorophyll, in pyrolysates of the sapropels but not in those of the marls suggests that higher marine productivity and greater OM preservation accompanied deposition of the sapropels. Detection of tetramethyl benzenes considered to be pyrolysis products of isorenieratene in the sapropel samples is evidence that nitrogen-fixing green sulfur bacteria contributed to the high productivity. Greater abundances of shorter chain aliphatic hydrocarbons, pyrroles, furans and alkyl aromatics in the pyrolysates of sapropel samples relative to the marls confirm better preservation of marine OM in the sapropels. In addition, the presence of greater amounts of thiophenes in the sapropels than in the marls is consistent with the existence of euxinic conditions during sapropel deposition. The combination of whole sediment pyrolysis and GC × GC-ToFMS is promising, but the procedure requires careful selection of its multiple analytical variables, particularly the pyrolysis temperature and the operational features of the GC columns.     

Knowledge based systems for risk assessment of the subsurface of contaminated sites

Computer-based risk evaluation methods are practical tools to compare automatically and evaluate contaminated sites. HYDRISK is an example of adequate knowledge-based systems: HYDRISK evaluates hydrogeological properties and chemical criteria relevant to contaminant transport with respect to the necessary remedial actions. It works at a high investigative level and enables geologists and engineers to draw differentiated conclusions for selecting remediation methods for the contaminated site. HYDRISK emphasizes space and time dependent aspects of the contaminant transport. It is supported by a geographic information system (GIS) to display the evaluation results. Some ideas on considering the spatiotemporal variability of relevant parameters by means of geostatistical methods and numerical models will be given.     

Development of Unsteady Windblown Sand Transport

The threshold condition and mass flux of aeolian sediment transport are the essential quantities for wind erosion prediction, dust storm modeling and geomorphological evolution, as well as the sand control engineering design. As a consequence, they have long been the key issues of windblown sand physics. Early researches on aeolian sediment transport focus mainly on steady transport process. While recently, synchronous, high frequency measurements show that wind field in atmospheric boundary layer is always unsteady, showing up as intense fluctuation of wind speed, which thus results in the intense spatial-temporal variability of aeolian sand transport. It has been proven that unsteady sand/dust transport is closely related with boundary layer turbulence and affects significantly the determination of threshold condition and the prediction of aeolian transport rate. The researches of experiment, theory analysis and numerical simulation on unsteady sand/dust transport in recent two decades are reviewed. Finally, open questions and future developments are suggested.     

Investigations of clay colloid aggregates by scanning transmission X-ray microspectroscopy of suspensions

The environmental behaviour of colloidal clay in aquatic systems is linked to the properties of their aggregates. Earlier investigations of clay colloids were performed with electron microscope techniques which caused de-hydration of the particles. Information on the structure of colloid aggregates is needed for understanding their sedimentation behaviour, as well as colloid contaminant transport properties in natural systems. Scanning transmission X-ray microspectroscopy successfully produced images of montmorillonite colloid aggregates in a pseudo-equilibrium state in 1 mM NaCl suspensions equilibrated for more than a year. These clay aggregates were revealed at photon energies below the O absorption edges of clay and water. They were spherical or ellipsoidal with diameters of the order of 100–800 nm. The aggregates are porous and gel like with lower densities than the clay mineral. These investigations are important for modelling the occurrence of clay aggregates in aqueous environments.     

Evaluating methods for quantifying cation exchange in mildly calcareous sediments in Northern Alberta

Cation exchange capacity (CEC) and exchangeable cation composition are important properties in defining water–sediment interactions in contaminant transport scenarios, such as the seepage of process-affected water from oil sands tailings impoundments. At present, detailed characterization of these properties has been overlooked in the surficial Pleistocene sediments of the Athabasca Oil Sands region in Canada. Although a great variety of methods for determining CEC exist and continue to be developed, there is a deficiency in detailed comparisons amongst these methods in the literature, and consequently uncertainty still remains as to which extractants/methods are most suitable for use with sediments containing partially soluble carbonate salts. The present study (1) conducts an up-to-date evaluation of several distinct methods for their suitability in characterizing cation exchange properties in mildly calcareous sediments; using them to (2) evaluate the sorptive capacity and easily displaceable cations of mildly calcareous, surficial sediments near Fort McMurray, Alberta. Four conceptually different CEC methods were chosen: (a) a summation method (Lithium Chloride extractant); (b) a method that displaces an index cation after intermediary washes (Sodium Acetate–Ammonium Acetate); (c) a method using a high affinity cation complex (Silver Thiourea); and (d) a titration method using an organic cationic dye (Methylene Blue). Exchangeable cations were characterized using the Lithium Chloride and Silver Thiourea methods. Analysis suggests that the Methylene Blue test is least impacted by the presence of carbonate mineral phases. Results from field sediments suggest that the exchangeable cations, in decreasing abundance, consist of Ca > Mg > K > NH₄ > Na; while the CEC was found to be 4.7 ± 2.7 meq 100 g⁻¹ in the clay-rich glacial till, 0.7 ± 0.0 meq 100 g⁻¹ in the underlying sand and 4.1 ± 2.3 meq 100 g⁻¹ in the till-sand transition zone.     

Mercury and other metal and metalloid soil contamination near a Pb/Zn smelter in east Hunan province, China

The spatial distributions of contaminant metals (Hg, Pb, Zn, Cd, Cu) and a metalloid (As) in vegetable plots and paddy fields located near a large scale Pb/Zn smelter in Hunan province, China, were investigated. Soil located 4 km from the smelter was severely contaminated, with maximum concentrations of Hg, Pb, Zn, Cd, Cu and As as high as 2.89, 1200, 3350, 41.1, 157 and 93 mg kg⁻¹, respectively. Concentrations of soil metal and As decreased with distance in the dominant wind direction. Single-factor assessment indicated pollution levels of Hg, Pb, Zn and Cd were most severe, while pollution levels for Cu and As were less severe. Results from a potential ecological risk assessment indicated high risk associated with the soil within a 4 km radius, with the contribution for each contaminant calculated as follows: Cd (70.0%), Hg (19.4%), Pb (4.8%), As (3.0%), Cu (1.7%) and Zn (1.1%). The forest soil in the nearby city park was also affected by the atmospheric depositions from smelting activities. Soil profiles demonstrated the pollutants were mostly accumulated in the upper 20 cm layer. Contamination of the topsoil with Hg, Pb, Zn, Cd, Cu and As indicated remediation should be considered.     

Sediment supply: The main driver of shelf-margin growth

Despite the obvious importance of sediment supply to shelf-margin architecture and to the potential of margins to contain and bypass deep-water sands, the role of supply in shelf-margin growth has received limited attention. High cross-shelf sediment flux is critically important for the occurrence of deep-water sands, not least on Greenhouse or rapidly subsiding margins where the impact of eustatic sea-level fall may be insufficient to drive sediment delivery out across the shelf into deep-water areas. To draw greater attention to the supply parameter we review a number of shelf margins that have grown chiefly through supply by shelf-edge deltas and associated sediment-gravity flows. Based on structural style and water depth, we recognize two broad types of shelf-margin. Moderately deep-water margins produce clinoforms < 1000 m high and show rates of shelf-edge progradation < 60 km/My and aggradation < 270 m/My, and consequently, infill their basins relatively rapidly, and develop more progradational architectures with morphologically smooth and relatively undeformed slopes. Very deep-water margins produce clinoforms > 1000 m high and generally show rates of shelf-edge progradation < 40 km/My and aggradation < 2500 m/My, and therefore infill their basins more slowly and develop more aggradational architectures with much gravity-driven slope deformation, proneness to failure and ponded architectures (salt or shale driven). On both margin types, long-term (> 1 My) rates of shelf-edge progradation of several tens of km/My tend to be linked to the delivery of relatively large volumes of sand into the deep-water basin. Delivery of this sand beyond the shelf-edge happens despite Greenhouse conditions and is likely recurrent and periodic (delivery cycles in the order of 100′s ky). Such prominent margin growth is a strong indication that sediment influx was relatively high and we refer to these margins as “supply-dominated” shelf margins. The Gulf of Mexico margin is a well-known and data-rich example of a “supply-dominated” shelf-margin during certain times (e.g., Paleocene). In contrast, on both margin types, low rates of shelf-edge progradation are linked to diminished (or even non-existent) and less frequently recurrent deep-water sediment delivery suggestive of relatively low sediment influx. Occurrence of deep-water sand delivery under low sediment influx probably requires fall of relative sea level. The differences between rapidly and slowly prograding margins indicate that sediment supply (and not sea level) is likely to be the key limiting factor on the growth of shelf margins and that sediment supply, as interpreted through progradation rate, can therefore be used to make a first-order prediction of relative amounts of sand passed to deep-water areas.