Contaminant Transport Mechanisms Research Articles

Research on Passive Contaminant Transport in a Vegetated Channel

Vegetation plays an important role in the physical, ecological, and hydraulic functions of streams, rivers and many other water bodies and can affect the transport of water, sediment and nutrients both within the channel and to or between the riparian zones. This research studied the momentum and passive contaminant transport mechanism in a vegetated channel with a floodplain (river bank). Three dimensional numerical simulation and physical experimentations were conducted for turbulent flow in the presence of model vegetation. A non-linear k-Iµ model with a vegetation model for turbulent flow and an algebraic flux model proposed by Daly and Harlow (1970) (DH model) for contaminant transport were adopted. An in-house code developed by the authors was implemented for numerical simulations. Model vegetation zones were prepared in the channel to predict the mixing mechanism. The numerical results were compared with corresponding experimental observations. A good agreement between the simulated and experimental results was observed in terms of pollutant concentration.

Contaminant transport in the sub-surface soil of an uncontrolled landfill site in China: site investigation and two-dimensional numerical analysis.

A field investigation of contaminant transport beneath and around an uncontrolled landfill site in Huainan in China is presented in this paper. The research aimed at studying the migration of some chemicals present in the landfill leachate into the surrounding clayey soils after 17 years of landfill operation. The concentrations of chloride and sodium ions in the pore water of soil samples collected at depths up to 15 m were obtained through an extensive site investigation. The contents of organic matter in the soil samples were also determined. A two-dimensional numerical study of the reactive transport of sodium and chloride ion in the soil strata beneath and outside the landfill is also presented. The numerical modelling approach adopted is based on finite element/finite difference techniques. The domain size of approximately 300 × 30 m has been analysed and major chemical transport parameters/mechanisms are established via a series of calibration exercises. Numerical simulations were then performed to predict the long-term behaviour of the landfill in relation to the chemicals studied. The lateral migration distance of the chloride ions was more than 40 m which indicates that the advection and mechanical dispersion are the dominant mechanism controlling the contaminant transport at this site. The results obtained from the analysis of chloride and sodium migration also indicated a non-uniform advective flow regime of ions with depth, which were localised in the first few metres of the soil beneath the disposal site. The results of long-term simulations of contaminant transport indicated that the concentrations of ions can be 10 to 30 times larger than that related to the allowable limit of concentration values. The results of this study may be of application and interest in the assessment of potential groundwater and soil contamination at this site with a late Pleistocene clayey soil. The obtained transport properties of the soils and the contaminant transport mechanisms can also be used for the design of engineered barriers for the control of the long-term pollution of the site.

Transport mechanisms of contaminants released from fine sediment in rivers

Contaminants released from sediment into rivers are one of the main problems to study in environmental hydrodynamics. For contaminants released into the overlying water under different hydrodynamic conditions, the mechanical mechanisms involved can be roughly divided into convective diffusion, molecular diffusion, and adsorption/desorption. Because of the obvious environmental influence of fine sediment ( 0.06 mm), non-cohesive fine sediment, and cohesive fine sediment are researched in this paper, and phosphorus is chosen for a typical adsorption of a contaminant. Through theoretical analysis of the contaminant release process, according to different hydraulic conditions, the contaminant release coupling mathematical model can be established by the N-S equation, the Darcy equation, the solute transport equation, and the adsorption/desorption equation. Then, the experiments are completed in an open water flume. The simulation results and experimental results show that convective diffusion dominates the contaminant release both in non-cohesive and cohesive fine sediment after their suspension, and that they contribute more than 90 % of the total release. Molecular diffusion and desorption have more of a contribution for contaminant release from unsuspended sediment. In unsuspension sediment, convective diffusion is about 10-50 times larger than molecular diffusion during the initial stages under high velocity; it is close to molecular diffusion in the later stages. Convective diffusion is about 6 times larger than molecular diffusion during the initial stages under low velocity, it is about a quarter of molecular diffusion in later stages, and has a similar level with desorption/adsorption. In unsuspended sediment, a seepage boundary layer exists below the water-sediment interface, and various release mechanisms in that layer mostly dominate the contaminant release process. In non-cohesive fine sediment, the depth of that layer increases linearly with shear stress. In cohesive fine sediment, the range seepage boundary is different from that in non-cohesive sediment, and that phenomenon is more obvious under a lower shear stress.

Effect of sips sorption isotherm on contaminant transport mechanism in fractured porous media

A numerical model has been developed in this study for analysing the influence of Sips sorption isotherm on the contaminant transport mechanism in a coupled fracture matrix system. The implicit finite difference numerical technique has been used to solve the coupled non-linear governing equations. A varying grid is adopted at the fracture and rock matrix interface to capture the mass transfer at the interface. A constant continuous source of contaminants is assumed at the inlet of the fracture and the fracture is assumed to be saturated. Results suggest that the magnitude of the Sip's equilibrium constant in the fracture is extremely sensitive in deciding the resultant transport behaviour within the high permeable fracture. The magnitude of the Sip's model exponent within the high permeable fracture is highly sensitive in deciding the timing of zero concentration (exhaustion) within the high permeable fracture.

Contaminant transport mechanisms at an abandoned sulfide mine

An investigation at an abandoned, open-pit mine (Kokkinopezoula, Cyprus) concluded that environmental contamination has been transported by both mechanical and chemical mechanisms. The conclusion is based on several data sets, which were analysed using hierarchical clustering. Water samples were collected from the open pit during two different seasons of the year and from a creek located to the southeast of the pit and were analysed for pH and metal content. The results showed low pH and high metal content for the water samples collected from the open pit. Soil samples were collected from 32 locations (two depths each) chosen in relation to the geology and topography of the area, as well as to the mineralogy and alteration intensity of the waste dumps. The soil samples were analysed for their pH and element content with an analytical method that combines inductively coupled plasma-atomic emission spectroscopy and inductively coupled plasma-mass spectroscopy, after complete digestion of the samples. Similarities in composition were used to conclude that the dominant contaminant transport mechanism is fluvial in nature, as well as a degree of chemical weathering/precipitation, but not Aeolian. This conclusion is important in the future design of a remediation strategy for the site.

Contaminant Diffusion along uniform flow velocity with pulse type input sources in finite porous medium

Solute transport inside pore system occurs due to advection and diffusion which are the important mechanisms of contaminant transport in porous medium. Analytical solutions of one-dimensional advection-diffusion equation (the coefficient of second order space derivative being temporally dependent) are obtained in a finite domain for two sets of pulse type input boundary conditions. Initially the domain is not solute free. It is supposed uniformly distributed at the initial stage. The Laplace transform technique is used with the help of new space and time variables. The solutions are graphically illustrated and compared solute distribution for finite and semi-infinite domain.

Passive sampling of nonpolar contaminants at three deep-ocean sites

Concentrations of polychlorinated biphenyls, polyaromatic hydrocarbons, hexachlorobenzene, and DDE were determined by passive sampling (semipermeable membrane devices) with exposure times of 1–1.5 years at 0.1–5 km depth in the Irminger Sea, the Canary Basin (both North Atlantic Ocean), and the Mozambique Channel (Indian Ocean). The dissipation of performance reference compounds revealed a pronounced effect of hydrostatic pressure on the sampler-water partition coefficients. Concentrations in the Irminger Sea were uniform over the entire water column (0.1–3 km). At the Canary Basin site, concentrations were 2–25 times lower near the bottom (5 km) than at 1.4 km. Concentrations in the Mozambique Channel (0.6–2.5 km) were lower than at the other two locations, and showed a near-bottom maximum. The data suggest that advection of surface waters down to a depth of about 1 km is an important mechanism of contaminant transport into the deep ocean.

Factors affecting the presence of human-associated and fecal indicator real-time quantitative PCR genetic markers in urban-impacted recreational beaches

Urban runoff can carry a variety of pollutants into recreational beaches, often including bacterial pathogens and indicators of fecal contamination. To develop complete recreational criteria and risk assessments, it is necessary to understand conditions under which human contamination could be present at beaches solely impacted by urban runoff. Accurately estimating risk requires understanding sources, concentrations, and transport mechanisms of microbial contaminants in these environments. By applying microbial source tracking methods and empirical modeling, we assessed the presence and level of human contamination at urban runoff impacted recreational beaches. We also identified environmental parameters and pollution sources that can influence the concentration and transport of culturable and molecular fecal indicator bacteria (FIB) in systems impacted solely by urban runoff. Water samples and physico–chemical parameters were collected from shoreline locations from three South Carolina (SC) beaches (five locations per beach) and two Florida (FL) beaches (three locations per beach). Each SC beach was directly impacted by swashes or tidal creeks receiving stormwater runoff from the urbanized area and therefore were designated as swash drain associated (SDA) beaches, while FL beaches were designated as non-swash drain associated (NSDA). Sampling in swash drains (SD; three sites per SD) directly impacting each SC beach was also conducted. Results indicate that although culturable (enterococci) and real-time quantitative polymerase chain reaction (qPCR) (EC23S857, Entero1, and GenBac3) FIB concentrations were, on average, higher at SD locations, SDA beaches did not have consistently higher molecular FIB signals compared to NSDA beaches. Both human-associated markers (HF183 and HumM2) were concomitantly found only at SDA beaches. Bacteroidales species-specific qPCR markers (BsteriF1 and BuniF2) identified differences in the Bacteroidales community, depending on beach type. The marker for general Bacteroidales was most abundant at SD locations and exhibited a high correlation with both culturable and other molecular markers. Combining molecular information with predictive modeling allowed us to identify both alongshore movement of currents and SD outflow as significant influences on the concentration of molecular and culturable indicators in the bathing zone. Data also suggests that combining methodologies is a useful and cost effective approach to help understand transport dynamics of fecal contamination and identify potential sources of contamination at marine beaches.

Effect of fracture-skin formation in clay fractured porous media

The contaminant transport mechanism in fractured clay porous media has been analysed using numerical modelling. Implicit finite difference has been used to develop the numerical model. A constant continuous source of contaminants has been assumed at the inlet of the fracture. The domain is considered to be saturated. A varying grid is considered in the fracture skin to account for the flux transfer at the interface of the fracture and the skin. The effect of constant dispersion as well as distance-dependent dispersion on the contaminant transport mechanism has been analysed in the presence of fracture skin. The contaminants are assumed to follow Freundlich non-linear sorption isotherm. Sensitivity analysis has been conducted to study the effect of different fracture-skin porosities, skin diffusion coefficients, half fracture apertures, fluid velocities and skin thicknesses on the solute transport mechanism within the fractured clay porous media. Results suggest that amount of solute diffusion into the clay matrix is directly proportional to the fracture-skin porosity and fracture-skin diffusion coefficient and inversely proportional to thickness of the half fracture aperture and the fluid velocity. The role of non-linear sorption is insignificant and scale-dependent dispersivity results in enhanced mass diffusion from the fracture.

Effect of time varying fracture-skin thickness on contaminant transport in fracture-matrix coupled system

A numerical model is developed for analyzing the effect of time varying fracture skin thickness on contaminant transport mechanism in the fractured porous media. Recent investigations by researchers in subsurface media have brought out the importance of fracture-skin formation and its effect of contaminant transport in fracture-matrix coupled system. Implicit finite difference numerical technique has been used to solve the coupled non-linear governing equations for the triple continuum model consisting of fracture, fracture-skin and the rock-matrix. A varying grid is adopted at the fracture and fracture-skin interface to capture the mass transfer at the interface. The fracture-skin thickness is assumed to increase with time and the effect of time varying fracture skin thickness on the contaminant transport mechanism is analyzed. Results suggest that the mass transfer from the fracture to the fracture-skin is high when the fracture-skin thickness is small. The contaminant concentration for both constant as well as time varying fracture-skin thickness remains the same during the initial simulation time and varies as time progresses. The time varying fracture-skin thickness has a significant effect on the contaminant concentration at larger simulation periods. The effect of time varying fracture-skin thickness on contaminant concentration reduces with increment in the initial fracture-skin thickness.

Geotechnical Centrifuge Modeling of Chloride Diffusion through Soils

AbstractGeotechnical centrifuge modeling provides a means of accelerating the transport of contaminants through a soil matrix. Contaminant transport mechanisms occurring in the geotechnical centrifuge model can be used to verify various mathematical and analytical models. This technical note presents modeling of chloride (Cl) diffusion through soils in a geotechnical centrifuge. Acceleration level, N, of 100 g was used for centrifuge modeling of Cl migration through 7.5-m-deep soil strata for a period of 150 and 600 days. From the centrifuge test results, the Reynolds number, R, and Peclet number, P, were found to be several orders less than unity. This indicates that the dominant Cl transport mechanism in the soil is diffusion. Finite-element modeling was performed, using SEEP/W and CTRAN/W, to validate the experimental results, and excellent matching has been obtained. The study also highlights that geotechnical centrifuge modeling can be used as a viable alternative to field scale experimentation.

Tracer tests and the structure of permeability in the Corallian limestone aquifer of northern England, UK

The Corallian limestone of northern England (UK) is widely exploited for water supplies and exhibits the karstic phenomena of sinking rivers, conduit development and groundwater velocities of several kilometres per day. To test a number of model-derived source protection zones and elucidate contaminant transport mechanisms in the aquifer, three tracer tests were conducted from a set of swallow-holes draining the River Derwent toward public water supply wells in the eastern part of the aquifer. Tracers used included: Enterobacter cloacae (bacteriophage), Photine C (optical brightener), sodium fluorescein (fluorescent dye) and sulphur hexafluoride (dissolved gas), the varying properties of which make them suitable analogues for different types of potential contaminant. Observed tracer transport times and arrival patterns indicate that tracer transport occurs through karstic channels embedded in a network of primary fissures which exert control over tracer concentrations once initial tracer plumes have passed. A dipole flow system is observed between the swallow-holes and the closest abstraction well, whilst previously modelled source protection zones do not accurately reflect either groundwater velocity or those areas of the aquifer supplying the wells. These findings imply that managing such aquifers for potential contamination should rely upon empirical tracer evidence for source-protection zone modelling.

Arsenic and uranium transport in sediments near abandoned uranium mines in Harding County, South Dakota

Abstract Sediment samples were analyzed as part of ongoing environmental investigations of historical U mining impacts within Custer National Forest in Harding County, South Dakota. Correlations between As and U content, grain size and soil mineralogy were determined to identify contaminant fate and transport mechanisms. Soil samples collected near the mining source zone and up to 61 km downgradient of the minesites were analyzed. Samples were homogenized and wet sieved through polymer screens, and metal(loid) concentrations were determined using inductively coupled plasma mass spectrometry (ICP-MS). Powder X-ray diffraction (XRD) analysis identified quartz as the primary mineral for all size fractions, with varying amounts of analcime, indicative of volcanic origin. Selected samples were examined for trace mineral composition using scanning electron microscopy (SEM). The presence of Fe sulfides and Fe (hydr)oxides indicate heterogeneity in redox potentials on a microscopic scale. Elevated metal(loid) concentrations were associated with trace concentrations of Fe sulfide, indicating an influence on metal transport during weathering. Sequential chemical extractions (SCE) performed on source sediment fractions demonstrated that most As and U was adsorbed to Fe- and Mn-oxides and carbonates with lesser amounts bound by ion exchange, organics and Fe sulfides. Large changes in U/Th and As/Th ratios were observed to coincide with geochemical changes in the watershed, suggesting that metal(loid)–Th ratios may be used in environmental investigations to identify geochemically-significant watershed conditions.

Investigation of migration of pollutant at the base of Suzhou Qizishan landfill without a liner system

We investigated migration of pollutant at the base of the Suzhou landfill after it had been operated for 13 years. The investigation was carried out by performing chemical analyses on the soil samples taken from the silty clay deposit. Concentrations of chloride, chemical oxygen demand (COD) and the heavy metals in the soil samples were determined using the standard methods. The experimental data showed that the maximum migration depth of chloride was more than 10 m, while the maximum migration depth of COD varied between 1 and 3.5 m. It is believed that the difference is attributed to the variation in diffusion rate and leachate-soil interaction. The chloride profiles also indicated that advection may be the dominant contaminant transport mechanism at this site. The total contents of Cu, Pb and Cr are very close to the background levels and the concentration values of these metals mainly are lower than the threshold values specified by the Chinese soil quality standard and the European one. The water-extractable concentrations of COD in the surface of the silty clay generally exceed the limit value specified by the Chinese standard. The concentrations of copper and chromium in pore water are 1∼2 orders of magnitude less than the total concentrations of these heavy metals within the soils, implying that heavy metals are mainly adsorbed by the soil particles. Finally, remediation methods were suggested for this landfill site.

Determination of Diffusion Characteristics of Intact Rock Mass: A Critical Evaluation

AbstractIntact rock mass is the most suitable geological formation for disposing of hazardous wastes. Hence, for safety assessment, establishment of contaminant transport mechanism(s) in/through the rock mass becomes essential. Conventional diffusion cells have been employed by earlier researchers for establishing diffusion characteristics of the intact rock mass. However, due to extremely low porosity of the intact rock mass, the time required for obtaining the “break-through curve” runs into a few months, during which maintaining the ambient conditions becomes extremely difficult. To overcome these limitations, researchers have resorted to accelerated diffusion tests, in the recent past. Several analytical models have been developed by the earlier researchers, which can be employed for determining diffusion characteristics of the intact rock mass. This calls for a comparative study of the diffusion characteristics obtained from these models so as to establish their suitability. With this in view, diffusion characteristics of the intact rock mass of different types were established by conducting accelerated diffusion tests and employing these models. In addition, diffusion characteristics of the intact rock mass were determined based on the concept of cumulative concentration accumulation of ionic species (CCAIS) in the measuring reservoir. It has been found that the models reported in the literature over-predict the diffusion coefficients for the intact rock mass. The superiority of CCAIS model, has been demonstrated by computing the hydraulic conductivity of the intact rock mass and comparing it with the centrifuge modeling results.

A novel technique for studying diffusion of contaminants in fine-grained soils

Diffusion is one of the predominant contaminant transport mechanisms in fine-grained soils. Usually, conventional methods such as half-cell and column tests are adopted for establishing diffusion characteristics of such soils. However, these techniques are destructive in nature, quite time-consuming, and require cumbersome chemical analysis of the soil and its pore-solution. Hence, development of a technique that would overcome these limitations and would yield precise results, in extremely short duration, becomes essential. With this in view, a modified Perspex diffusion cell with provision for inserting a series of stainless steel electrodes has been fabricated to monitor the diffusive contaminant transport, indirectly, based on the variation of electrical impedance of the soil. An LCR meter is employed to measure the electrical impedance of the soil across each electrode over a period. Further, the electrical impedance variation is indirectly correlated to the diffusion of contaminants that occurs in the soil. Details of the study and the test setup developed for this purpose are presented in this paper. It has been demonstrated that the technique based on electrical impedance measurement is quite useful and efficient for establishing diffusion characteristics of the fine-grained soil.

Scaling Laws and Experimental Modelling of Contaminant Transport Mechanism through Soils in a Geotechnical Centrifuge

This paper presents the results of a research program conducted on the geotechnical centrifuge to investigate contaminant transport mechanism through the soil mass. The mechanism that governs contaminant transport through soil mass is discussed, the principles of geotechnical centrifuge modelling are outlined, and relevant scaling laws that govern the relationship between a centrifuge model and the prototype, with respect to the problem of contaminant transport, are presented. Modelling of models has been established to validate the experimental results. It has been concluded that the geotechnical centrifuge can be used as an experimental tool to simulate field scale problems.

An Experimental Methodology for Monitoring Contaminant Transport Through Geotechnical Centrifuge Models

In this paper, an attempt has been made to highlight an experimental methodology for monitoring contaminant transport through locally available silty soil and commercially available clay in geotechnical centrifuge models, for different compaction states. Use of multiple depth sensors to determine depth distribution of sodium chloride in the soil column has been detailed. The obtained results have been compared with argentometric method. To validate the centrifuge modelling, modelling of models has been used. The test setup developed can simulate contaminant transport mechanisms through the soil mass, which is approximately 10 m deep, over a period of 600 days. R (e) and P (e) are found to be N times higher in the centrifuge models. These numbers are found to be several orders less than unity. This indicates that laminar flow prevails and the dominating Cl(-) transport mechanism in centrifuge is diffusion. The study also highlights the fact that the geotechnical centrifuge modelling can be used as a viable alternative to field scale experimentation.

A Novel Technique for Monitoring Contaminant Transport Through Soils

An experimental setup which is capable of simulation as well as monitoring contaminant transport through soil mass has been developed. Efficiency of the experimental setup has been demonstrated by comparing the results obtained from the argentometric method. Reynolds number (R(e)) and the Peclet number (P(e)) have been found to be less than unity. This indicates that flow of the solute through soil mass is laminar and the dominant contaminant transport mechanism is diffusion.

Chloride Transport in Layered Soil Systems with Hydraulic Trap Effect

The natural and engineered hydraulic trap systems in sanitary-engineered solid waste landfills were investigated using three layer one dimensional laboratory models. The models consisted of a top reservoir containing a sodium chloride source solution, a compacted upper silt layer as a primary liner, a coarse sand layer as a secondary leachate collection system or a hydraulic control layer, a compacted lower silt layer as a secondary liner, and a bottom water reservoir as a groundwater aquifer. In the first test, the natural hydraulic trap system (upward flow through the lower silt layer) was modeled. In this case, the contaminant transport mechanisms through the upper silt layer were downward advection and diffusion, and through the lower silt layer, diffusion was downward and advection was upward. The results showed that the implementation of the natural hydraulic control system could effectively reduce chloride transport to the bottom reservoir. In the second test, the natural and engineering hydraulic trap systems were simulated (upward flow from the bottom reservoir to the upper reservoir). In the third test, the engineered hydraulic trap system (downward flow through the upper silt layer and upward flow through the lower silt layer) was modeled. The results showed that the natural and engineered hydraulic trap systems have an important effect in reducing chloride migration toward the underlying aquifer. In all experiments the chloride concentrations in the silt and coarse sand layers and top and bottom reservoirs were measured and the observed concentrations were compared with concentrations calculated by a theoretical model. A good agreement was obtained between the observed and theoretical data confirming the acceptable accuracy of the experimental methodologies, observations, and the theoretical model.