Colloid transport and distribution in the hyporheic zone

Colloids moving from the stream into the hyporheic zone may have a negative impact on aquatic ecosystems as they are potential contaminants or carriers of contaminants. Moreover, retained colloids in the hyporheic zone could not only reduce the exchange flux between the stream and streambed but also change the conditions of the bed, affecting the habitats for aquatic organisms. Previous studies focused on the exchange flux across the sediment–water interface, but the colloid transport processes and distribution of retained colloids in the streambed have received little attention. We conducted experiments within a laboratory flume to examine these processes in a streambed driven by bedform‐induced hyporheic flow. Retained colloids measured in the bed at the end of the experiments revealed colloid retention mainly in the shallow layer of hyporheic zone (0–5 cm below the interface). The results demonstrated significant effects of particle trapping and settling on the colloid transport and distribution in the streambed. Retention leads to the formation of a colloid‐filled shallow layer in the bed. Particle paths based on model simulations showed that colloid settling in pore water modifies the direction of colloid transport and allows the colloid particles to move more deeply in the bed.     

A novel ATR‐FTIR technique for identifying colloid composition in natural waters

Although understanding colloid composition has been frequently cited as essential to predicting contaminant transport in natural waters, most current methods to collect and identify colloid composition chemically alter the colloids prior to analysis and fail to identify colloid mineralogy and organic components. This paper presents a new, low‐cost method employing attenuated total reflection Fourier transform infrared spectroscopy (ATR‐FTIR) to identify colloids including organic material in concentrated suspensions. The concentration method employing tangential ultrafiltration at a steady temperature prevents redistribution of dissolved phase and suspended sediments into the colloidal fraction through post‐sampling reactions. ATR‐FTIR allows for direct analysis of concentrated suspensions rather than requiring drying that may alter composition in the colloidal phase, for example, by precipitating carbonates in samples from karst waters. The ability of this technique to monitor variation in colloidal composition is demonstrated through the examination of colloids under two different flow conditions in a karst aquifer and the West Branch of the Susquehanna River in Central Pennsylvania. Copyright © 2014 John Wiley & Sons, Ltd.     

Coupling of a Column System with ICP‐MS for the Characterisation of Colloid‐mediated Metal(loid) Transport in Porous Media

In this work a coupling method for the characterisation of colloid‐mediated transport of the metal(loid) species in porous media was developed. For this transport experiments quartz sand was used as column packing material and the synthetic three‐layer clay mineral laponite as model colloid. The determination of colloids was conducted by means of UV detection. The quantification of the metal(loid) ions was carried out in two different ways: (1) The fractions collected at the column outlet were analysed with an inductively coupled plasma mass spectrometer (ICP‐MS) (offline measurements); (2) the column system was directly coupled with ICP‐MS (online measurements). In the column experiments the influence of laponite colloids on the transport of Cu, Pb, Zn, Pt and As species was investigated. In the offline experiments as a consequence of dilution during sample preparation no metal(loid) species at the column outlet could be found. Unlike this the breakthrough of all metal(loid)s could be detected under the same experimental column conditions in the coupling experiments. This coupling technique offers the online detection of the metal species and colloidal particles with high resolution even at low concentrations and without any time‐consuming preparation. The coupling experiments have shown that the laponite particles accelerate the transport of the cationic metals. For anionic metal(loid) species no influence of laponite on their transport behaviour was found.     

Experimental study of extreme shear stress for shallow flow under simulated rainfall

The raindrop impact and overland flow are two major factors causing soil detachment and particle transportation. In this study, the turbulent characteristics of the shallow rain‐impacted water flow were investigated using a 2‐D fibre‐optic laser Doppler velocimetry (FLDV) and an artificial rainfall simulator. The fluctuating turbulent shear stress was computed using digital data processing techniques. The experimental data showed that the Reynolds shear stress follows a probability distribution with heavy tails. The tail probability increases with an increase of rainfall intensity or raindrop diameter, and it decreases with an increase of Reynolds number. A modified empirical equation was derived using both the raindrop diameter and rainfall intensity as independent variables to provide a better prediction of the Darcy‐Weisbach friction coefficient f under rainfall conditions. Copyright © 2009 John Wiley & Sons, Ltd.     

Review of the development of laser fluorosensors for oil spill application

As laser fluorosensors provide their own source of excitation, they are known as active sensors. Being active sensors, laser fluorosensors can be employed around the clock, in daylight or in total darkness. Certain compounds, such as aromatic hydrocarbons, present in petroleum oils absorb ultraviolet laser light and become electronically excited. This excitation is quickly removed by the process of fluorescence emission, primarily in the visible region of the spectrum. By careful choice of the excitation laser wavelength and range-gated detection at selected emission wavelengths, petroleum oils can be detected and classified into three broad categories: light refined, crude or heavy refined.

This paper will review the development of laser fluorosensors for oil spill application, with emphasis on system components such as excitation laser source, and detection schemes that allow these unique sensors to be employed for the detection and classification of petroleum oils. There have been a number of laser fluorosensors developed in recent years, many of which are strictly research and development tools. Certain of these fluorosensors have been ship-borne instruments that have been mounted in aircraft for the occasional airborne mission. Other systems are mounted permanently on aircraft for use in either surveillance or spill response roles.     

天然胶体的水环境行为

胶体是一种分散相粒子具有特定粒径(通常为1 nm~1μm)的混合物,天然水体中的胶体通常是指水环境中普遍存在,而又符合“胶体”形态定义的物质的总和.因为具有特殊的表面性质以及独特的水动力学特点,天然水体中的胶体被认为是一种重要的污染物载体,尤其在促进重金属类的污染物迁移以及性质转化的过程中具有重要作用.本文归纳总结了水环境研究领域中胶体概念的提出与发展,讨论了现有研究中对胶体的定义与理解,比较了当前几种主要的胶体提取手段的特点.同时,针对胶体在水环境中的环境行为,详细归纳了河湖水体中胶体的物质组成、特征以及相应的检测手段,重点总结了胶体在水环境中对污染物的结合作用、胶体污染物的生物效应、胶体自身的团聚规律以及胶体对污染物迁移的影响机制.本文提出了胶体在水环境中复杂的组成成分与动态变化在胶体研究中的重要性,并对进一步揭示胶体环境效应的研究思路以及方法手段进行了展望.     

Effects of chemical dispersants and mineral fines on crude oil dispersion in a wave tank under breaking waves

The interaction of chemical dispersants and suspended sediments with crude oil influences the fate and transport of oil spills in coastal waters. A wave tank study was conducted to investigate the effects of chemical dispersants and mineral fines on the dispersion of oil and the formation of oil-mineral-aggregates (OMAs) in natural seawater. Results of ultraviolet spectrofluorometry and gas chromatography flame ionized detection analysis indicated that dispersants and mineral fines, alone and in combination, enhanced the dispersion of oil into the water column. Measurements taken with a laser in situ scattering and transmissometer (LISST-100X) showed that the presence of mineral fines increased the total concentration of the suspended particles from 4 to 10microl l(-1), whereas the presence of dispersants decreased the particle size (mass mean diameter) of OMAs from 50 to 10microm. Observation with an epifluorescence microscope indicated that the presence of dispersants, mineral fines, or both in combination significantly increased the number of particles dispersed into the water.     

Contaminant transport in a fracture with spatially variable aperture in the presence of monodisperse and polydisperse colloids

A quasi-three-dimensional particle tracking model is developed to characterize the spatial and temporal effects of advection, molecular diffusion, Taylor dispersion, fracture wall deposition, matrix diffusion, and co-transport processes on two discrete plumes (suspended monodisperse or polydisperse colloids and dissolved contaminants) flowing through a variable aperture fracture situated in a porous medium. Contaminants travel by advection and diffusion and may sorb onto fracture walls and colloid particles, as well as diffuse into and sorb onto the surrounding porous rock matrix. A kinetic isotherm describes contaminant sorption onto colloids and sorbed contaminants assume the unique transport properties of colloids. Sorption of the contaminants that have diffused into the matrix is governed by a first-order kinetic reaction. Colloids travel by advection and diffusion and may attach onto fracture walls; however, they do not penetrate the rock matrix. A probabilistic form of the Boltzmann law describes filtration of both colloids and contaminants on fracture walls. Ensemble-averaged breakthrough curves of many fracture realizations are used to compare arrival times of colloid and contaminant plumes at the fracture outlet. Results show that the presence of colloids enhances contaminant transport (decreased residence times) while matrix diffusion and sorption onto fracture walls retard the transport of contaminants. Model simulations with the polydisperse colloids show increased effects of co-transport processes.     

Modelling of colloidal particle and heavy metal transfer behaviours during seawater intrusion and refreshing processes

The proper management of coastal aquifers commonly requires an understanding of regional mass flow and complete seawater–freshwater circulation. In this study, time series observations of seawater intrusion and refreshing were conducted using a column experiment based on natural flow conditions in coastal groundwater and a sampled medium from a coastal sandy aquifer without chemical treatment. Ranges of hydrodynamic and hydrochemical variables were tested and analysed. The results showed that the zeta potential of suspended colloids in aqueous solution in an aquifer polluted with 0.5 g/kg of heavy metals exhibited an isoelectric point for pH values ranging from 5.70 to 6.07 when freshwater or seawater completely occupied the aquifer pores, which is representative of natural hydrochemical conditions. In this scenario, a high background concentration of heavy metals induced colloidal immobilization. Otherwise, seawater–freshwater circulation enabled colloid mobilization due to ionic strength and pH fluctuations. The migration of multiple heavy metals occurred at a characteristic time of approximately 1 pore volume after each intrusion stage began and when the peak rate of colloid release was reached. At these times, the colloid behaviour determined the quantity and pathway of heavy metal transport. On the basis of the influences of seawater and freshwater interactions, the quantity of mobilized particles generally decreased and was uniformly distributed in each fraction due to particle loss and decreased porous connectivity. We speculate that the decrease in the total surface area of the migratory colloids may cause colloid‐associated heavy metal transport to decrease. The experimental results provide a useful basis for testing coastal groundwater flow and mass transport models because these phenomena require full characterization to precisely evaluate the associated fluxes from the field scale to the microscopic dimension.     

Mathematical modeling of colloid and virus cotransport in porous media: Application to experimental data

A conceptual mathematical model was developed to describe the simultaneous transport (cotransport) of viruses and colloids in three-dimensional, water saturated, homogeneous porous media with uniform flow. The model accounts for the migration of individual virus and colloid particles as well as viruses attached onto colloids. Viruses can be suspended in the aqueous phase, attached onto suspended colloids and the solid matrix, and attached onto colloids previously attached on the solid matrix. Colloids can be suspended in the aqueous phase or attached on the solid matrix. Viruses in all four phases (suspended in the aqueous phase, attached onto suspended colloid particles, attached on the solid matrix, and attached onto colloids previously attached on the solid matrix) may undergo inactivation with different inactivation coefficients. The governing coupled partial differential equations were solved numerically using finite difference methods, which were implemented explicitly or implicitly so that both stability and speed factors were satisfied. Furthermore, the experimental data collected by Syngouna and Chrysikopoulos [1] were satisfactorily fitted by the newly developed cotransport model.     

Monodisperse and polydisperse colloid transport in water-saturated fractures with various orientations: Gravity effects

Numerical experiments are conducted to examine the effects of gravity on monodisperse and polydisperse colloid transport in water-saturated fractures with uniform aperture. Dense colloids travel in water-saturated fractures by advection and diffusion while subject to the influence of gravity. Colloids are assumed to neither attach onto the fracture walls nor penetrate the rock matrix based on the assumptions that they are inert and their size is larger than the pore size of the surrounding solid matrix. Both the size distribution of a colloid plume and colloid density are shown to be significant factors impacting their transport when gravitational forces are important. A constant-spatial-step particle-tracking code simulates colloid plumes with increasing densities transporting in water-saturated fractures while accounting for three forces acting on each particle: a deterministic advective force due to the Poiseuille flow field within the fracture, a random force caused by Brownian diffusion, and the gravitational force. Integer angles of fracture orientation with respect to the horizontal ranging from ±90° are considered: three lognormally distributed colloid plumes with mean particle size of 1 μm (averaged on a volumetric basis) and standard deviation of 0.6, 1.2 and 1.8 μm are examined. Colloid plumes are assigned densities of 1.25, 1.5, 1.75 and 2.0 g/cm³. The first four spatial moments and the first two temporal moments are estimated as functions of fracture orientation angle and colloid density. Several snapshots of colloid plumes in fractures of different orientations are presented. In all cases, larger particles tend to spread over wider sections of the fracture in the flow direction, but smaller particles can travel faster or slower than larger particles depending on fracture orientation angle.     

Determination of the particle load based on detailed suspended sediment measurements at a hydropower plant

Suspended sediment particles contained in inflows of water systems of hydropower plants (HPPs) cause hydro-abrasive erosion of the hydraulic turbines and structures leading to significant maintenance costs, efficiency reductions, and downtime. Relevant parameters such as suspended sediment concentration (SSC), particle size distribution (PSD), shape, and mineralogical composition were measured with an online multi-frequency acoustic instrument and based on manually taken samples from the end of the sand trap of the Toss HPP in the Himalayan region, India. In the laboratory, the samples were analyzed using the gravimetric method, laser diffraction, turbidity, dynamic digital image processing, scanning electron microscope, petrography analysis, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The online instrument and the samples provided measurement results at a single point. To investigate vertical gradients in concentration and particle sizes, additional samples were collected 9 times at 7 relative water depths. The SSC, most particle sizes, and particle shape were found to be evenly distributed over depth except d90, i.e. the diameter which is not exceeded by 90% of the particle mass. d90 measured at 76% of the water depth was in the range of fine sand and was multiplied by 1.05 to obtain an average value representative for the entire depth. Improved methodologies to quantify both particle shape and size in an analytical model for hydro-abrasive erosion are proposed. Also, the PSD measuring performance of laser diffraction and dynamic imaging was studied and similar values of the median particle sizes were obtained from both instruments. Further, multi-frequency acoustic, turbidity and laser diffraction techniques were found suitable for SSC measurement at the test case HPP.     

Evaluation of a laser‐assisted particle sizing/settling velocity determination technique

The use of predictive models for the understanding and management of sediment and contaminant transport generally requires knowledge of particle size and settling velocity. Particle size is often obtained by direct measurements, and the settling velocities are usually predicted using the Stokes' law (or a modification thereof) for single‐grained spherical particles. Such measurements and estimates are not satisfactory measures for cohesive sediments, which exist as agglomerated particles called flocs and whose behaviour is significantly different from that of the single‐grained particles. Direct measurement of settling velocity and size using optical methods in settling columns has also been employed to improve these predictions; however, the subjectivity in determining which particles are in focus results in unreliable size data. An out‐of‐focus particle will generally possess a larger size than in reality. This paper evaluates a laser‐assisted particle sizing/settling velocity determination technique's ability to eliminate the subjectivity and improve particle‐sizing accuracy during settling column experiments. Although the diffraction of light by the translucent standard beads (used for evaluating the technique's accuracy for determining particle size) posed a problem, the results suggest that this technique has potential for assisting researchers to obtain the most accurate settling particle size data possible. Copyright © 2005 John Wiley & Sons, Ltd.     

Deposition and re-entrainment dynamics of microbes and non-biological colloids during non-perturbed transport in porous media in the presence of an energy barrier to deposition

This paper examines the non-perturbed deposition and re-entrainment dynamics of biological and non-biological colloids in porous media in the presence of an energy barrier to deposition at the grain surface. Deposition and re-entrainment rate coefficients were determined from numerical simulation of breakthrough–elution behavior and the profiles of retained colloids. We present composite trends from original and previously published data for biological and non-biological colloids which demonstrate that hydrodynamic drag mitigates deposition and drives re-entrainment of both biological and non-biological colloids in the presence of an energy barrier under non-perturbed conditions. Original data is presented for two sizes of colloids (1.1 and 5.7 μm microspheres) under a variety of ionic strength and fluid velocity conditions to examine the torque balance governing re-entrainment of colloids attached to the grain surfaces. The analysis indicates that in the presence of an energy barrier to deposition, hydrodynamic drag may influence deposition and re-entrainment of colloids associated directly with the grain surface via primary energy minima. However, the hydrodynamic field would also be expected to influence deposition and re-entrainment of colloids associated with the surface via secondary energy minima. Hence, the observed influences of fluid velocity are consistent with colloid association via either mechanism. These results call for the development of colloid transport theories that explicitly account for the influence of the hydrodynamic field at the grain surface.     

Effects of continuous flow centrifugation on measurements of trace elements in river water: intrinsic contamination and particle fragmentation

Continuous flow centrifugation (CFC) is a well‐established technique used in natural surface water studies to collect large amounts of suspended solids, thus allowing a broad spectrum of measurements. However, a potential contamination or changes in the particle size distribution during the centrifugation may restrain the use of CFC effluents for element analysis in the colloidal and dissolved fractions. In this paper we evaluate the possibility of using the effluent of a Westfalia centrifuge (type KA2‐06‐075, 9700 rpm) for such analysis. This evaluation is based on two laboratory experiments with deionized and tap water and two field experiments in rivers. Elemental concentration changes across the CFC were assessed from the CFC influent and effluent after a filtration at 0·45 µm. Significant increases were found, mainly in the field experiments at a high suspended solids level and a slightly acid pH. A hypothesis was made on the origin of these increases as a superposition of a centrifuge intrinsic contamination and a particle fragmentation effect. A numerical model based on elemental concentration measurements (inductively coupled plasma mass spectrometry) gave a particle fragmentation level of 0·55% (mass percentage of particles broken up into smaller fragments during centrifugation). In another experiment, a direct particle counting (single particle counter) shows an excess of particles smaller than 500 nm in the CFC effluent, corresponding to a fragmentation level of 0·11%. In consequence, the use of CFC effluent for element analysis is possible in low‐turbidity river or lake waters, but should be carefully considered in waters with high suspended matter contents. Copyright © 2006 John Wiley & Sons, Ltd.     

Controls of interactions between iron hydroxide colloid and water on REE fractionations in surface waters: Experimental study on pH-controlling mechanism

The influence of pH on the partitioning behavior of REE at the water/particulate interface has been studied experimentally. At the beginning of colloid formation the adsorption of REE on iron hydroxide colloids is dominant, followed by REE desorption. Finally adsorption and desorption tend to reach equilibration. The capability of iron hydroxide colloids to adsorb the HREE is greater than that to adsorb the LREE. With increasing pH, LREE/HREE fractionations will take place between iron hydroxide colloids and water, leading to the reduction of their partition coefficient ratio (DLREE/DHREE). The DREE distribution patterns show Y anomalies (DY/DHo <1), with obvious REE tetrad effects appearing under low pH conditions. Experimental results have shown that there do exist REE tetrad effects in nature. In addition to pH, the chemical type of surface water and ion intensity are also the important factors controlling REE tetrad effects and leading to fractionations between particulate-adsorbed REE and dissolved REE.     

Changes in the saltation flux following a step‐change in macro‐roughness

The effect of a step change in macro‐roughness on the saltation process under sediment supply limited conditions was examined in the atmospheric boundary layer. For an array of roughness elements of roughness density λ = 0.045 (λ = total element frontal area/total surface area of the array) the horizontal saltation flux was reduced by 90% (±7%) at a distance of ≈150 roughness element heights into the array. This matches the value predicted using an empirical design model and provides confidence that it can be effectively used to engineer roughness arrays to meet sand flux reduction targets. Measurements of the saltation flux characteristics in the vertical dimension, including: saltation layer decay (e‐folding) height and particle size, revealed that with increasing distance into the array, the rate of mass flux change with increasing height decreased notably, and (geometric) mean particle diameter decreased. The distribution of the saltation mass flux in the vertical remains exponential in form with increasing distance into the roughness array, and the e‐folding height increases as well as increasing at a greater rate as particle diameter diminishes. The increase in e‐folding height suggests the height of saltating particles is increasing along with their mean speed. This apparent increase in mean speed is likely due to the preferential removal, or sequestration, of the slower moving particles across the size spectrum, as they travel through the roughness array. Copyright © 2018 John Wiley & Sons, Ltd.     

High spatial resolution mapping of surface velocities and depths for shallow overland flow

Point measurements of flow rate, depth or velocity are not sufficient to validate overland flow models, particularly when the interaction of the water with the soil surface creates a complex flow geometry. In this study, we present the coupling of two techniques obtaining spatial data of flow depths and surface velocity measurements for water depths as low as 1 mm. Overland flow experiments were performed in the laboratory at various flow rates and slopes on two surfaces. The first surface was 120 cm by 120 cm showing three undulations of sinusoidal shape with an amplitude of 1 cm and a wavelength of 20 cm, while the second was a 60 cm by 60 cm moulded reproduction of a seedbed with aggregates up to 2 cm in size. Large scale particle image velocimetry (LSPIV) was used for velocity measurements with a sub‐centimetre spatial resolution. An instantaneous‐profile laser scanner was used to map flow depths with a sub‐millimetre spatial resolution. A sensitivity analysis of the image processing of the LSPIV showed good robustness of the method. Comparison with measurements performed with hot film anemometer and salt velocity gauge showed that LSPIV surface velocities were representative of the flow. Water depths measured with the laser scanner were also in good agreement with single‐point measurements performed with a dial indicator. Spatially‐distributed flow rates could be computed by combining both presented techniques with a mean relative error less than 20%. Copyright © 2012 John Wiley & Sons, Ltd.     

Estimating instantaneous concentration of suspended sediment using acoustic backscatter from an ADV

The measurement of instantaneous sediment concentration remains a challenging task. In this study, a three-step procedure is proposed to estimate instantaneous sediment concentration using acoustic backscatter from Acoustic Doppler Velocimeter (ADV). The influences of acoustic noise and particle diameter on an ADV's performance was first tested in the laboratory, then the three-step procedure was verified based on field measurements in the Zhongxian and Fengjie reaches in the Three Gorges Reservoir. The first step involves reconstructing the backscatter signal time series. Due to contamination from both the noise floor and spurious spikes, the denoising-despiking method was applied instead of the traditional velocity-despiking methods, and this approach performed well based on spectrum ana-lysis. The second step involves calibrating the sediment concentration against the backscatter signal. A linear relation, whose slope and intercept were calibrated to be dependent on particle diameter, is proposed in double logarithmic coordinates. The third step involves calculating the instantaneous sediment concentration using reconstructed instantaneous backscatter based on the proposed relation. The accuracy of the proposed method was evaluated through consideration of the concentration spec-trum and sediment flux, indicating that the proposed three-step procedure is effective for the mea-surement of instantaneous sediment concentration.