Transverse dispersion of non-reactive tracers in porous media: a new nonlinear relationship to predict dispersion coefficients

Assessing the potential of natural attenuation in groundwater relies on the ability to predict and quantify the processes that occur in contaminant plumes. Transverse dispersion is a significant mass transfer mechanism for mixing of electron acceptors and donors and thus may control the lengths of steady state plumes. Laboratory experiments were carried out using a 2-dimensional acrylic glass tank filled with glass beads, quartz sand and field site material as porous media. Flow velocities and grain sizes were varied in order to cover a large range of Peclet numbers including typical field scenarios. The laboratory study was extended by a comprehensive literature search to compare the new results with earlier work. As a result we propose a new empirical relationship for prediction of transverse dispersion coefficients (Dt) which is based on the Peclet number (Pe). This new relationship indicates a nonlinear dependency on the flow velocity (nu a) and grain size (d), namely a relative decrease of the dispersion coefficient with increasing flow velocity in relatively fast flowing water: Dt/Daq=Dp/Daq+0.28(Pe)0.72 (with Pe=nu a d/Daq; Daq and Dp denote the aqueous and pore diffusion coefficients, resp.).     

Determining straining of Escherichia coli from breakthrough curves

Though coliform bacteria are used world wide as an indication of faecal pollution, the parameters determining the transport of Escherichia coli in aquifers are relatively unknown, especially for the period after the clean bed collision phase brought about by prolonged infiltration of waste water. In this research, the breakthrough curves of E. coli after total flushing of 50-200 pore volumes were studied for various influent concentrations in various sediments at different pore water flow velocities. The results indicated that straining in Dead End Pores (DEPs) was an important process that dominated bacteria breakthrough in fine-grained sediment (0.06-0.2 mm). The filling of the DEP space with bacteria took 5-65 pore volumes and was dependent on concentration. Column breakthrough curves were modelled and from this the DEP volumes were determined. These volumes (0.21-0.35% of total column volume) corresponded well with values calculated with a formula based on purely geometrical considerations and also with values calculated with a pore size density function. For this function the so-called Van Genuchten parameters of the sediments used in the experiments were determined. The results indicate that straining might be a dominant process affecting colloid transport in the natural environment and therefore it is concluded that proper knowledge of the pore size distribution is crucial to an understanding of the retention of bacteria.     

Transport of Escherichia coli and solutes during waste water infiltration in an urban alluvial aquifer

Recharge of waste water in an unconsolidated poorly sorted alluvial aquifer is a complex process, both physically and hydrochemically. The aim of this paper is to analyse and conceptualise vertical transport mechanisms taking place in an urban area of extensive wastewater infiltration by analysing and combining the water balance, the microbial (Escherichia coli) mass balance, and the mass balance for dissolved solutes. For this, data on sediment characteristics (grain size, organic carbon, reactive iron, and calcite), groundwater levels, and concentrations of E. coli in groundwater and waste water were collected. In the laboratory, data on E. coli decay rate coefficients, and on bacteria retention characteristics of the sediment were collected via column experiments. The results indicated that shallow groundwater, at depths of 50 m below the surface, was contaminated with E. coli concentrations as high as 10(6) CFU/100 mL. In general, E. coli concentrations decreased only 3 log units from the point of infiltration to shallow groundwater. Concentrations were lower at greater depths in the aquifer. In laboratory columns of disturbed sediments, bacteria removal was 2-5 log units/0.5 cm column sediment. Because of the relatively high E. coli concentrations in the shallow aquifer, transport had likely taken place via a connected network of pores with a diameter large enough to allow bacterial transport instead of via the sediment matrix, which was inaccessible for bacteria, as was clear from the column experiments. The decay rate coefficient was determined from laboratory microcosms to be 0.15 d(-1). Assuming that decay in the aquifer was similar to decay in the laboratory, then the pore water flow velocity between the point of infiltration and shallow groundwater, coinciding with a concentration decrease of 3 log units, was 0.38 m/d, and therefore, transport in this connected network of pores was fast. According to the water balance of the alluvial aquifer, determined from transient groundwater modelling, groundwater flow in the aquifer was mainly in vertical downward direction, and therefore, the mass balance for dissolved solutes was simulated using a 1D transport model of a 200 m column of the Quaternary Alluvium aquifer. The model, constructed with PHREEQC, included dual porosity, and was able to adequately simulate removal of E. coli, cation-exchange, and nitrification. The added value of the use of E. coli in this study was the recognition of relatively fast transport velocities occurring in the aquifer, and the necessity to use the dual porosity concept to investigate vertical transport mechanisms. Therefore, in general and if possible, microbial mass balances should be considered more systematically as an integral part of transport studies.     

Transport of Escherichia coli through variably saturated sand columns and modeling approaches

A sand column leaching system with well-controlled suction and flow rate was built to investigate the effects on bacterial transport of air-water interface effects (AWI) correlated to water content, particle size, and column length. Adsorption of Escherichia coli strain D to silica sands was measured in batch tests. The average % adsorption for coarse and fine sands was 45.9+/-7.8% and 96.9+/-3.2%, respectively. However, results from static batch adsorption experiments have limited applicability to dynamic bacterial transport in columns. The early breakthrough of E. coli relative to bromide was clear for all columns, namely c. 0.15 to 0.3 pore volume earlier. Column length had no significant effects on the E. coli peak concentration or on total recovery in leachate, indicating retention in the top layer of sands. Tailing of breakthrough curves was more prominent for all fine sand columns than their coarse sand counterparts. Bacterial recovery in leachate from coarse and saturated sand columns was significantly higher than from fine and unsaturated columns. Observed data were fitted by the convection-dispersion model, amended for one-site and two-site adsorption to particles, and for air-water interface (AWI) adsorption. Among all models, the two-site+AWI model achieved consistently high model efficiency for all experiments. Thus it is evident from experimental and modeling results that AWI adsorption plays an important role in E. coli transport in sand columns.     

Colloid transport in unsaturated porous media: the role of water content and ionic strength on particle straining

Packed column and mathematical modeling studies were conducted to explore the influence of water saturation, pore-water ionic strength, and grain size on the transport of latex microspheres (1.1 microm) in porous media. Experiments were carried out under chemically unfavorable conditions for colloid attachment to both solid-water interfaces (SWI) and air-water interfaces (AWI) using negatively charged and hydrophilic colloids and modifying the solution chemistry with a bicarbonate buffer to pH 10. Interaction energy calculations and complementary batch experiments were conducted and demonstrated that partitioning of colloids to the SWI and AWI was insignificant across the range of the ionic strengths considered. The breakthrough curve and final deposition profile were measured in each experiment indicating colloid retention was highly dependent on the suspension ionic strength, water content, and sand grain size. In contrast to conventional filtration theory, most colloids were found deposited close to the column inlet, and hyper-exponential deposition profiles were observed. A mathematical model, accounting for time- and depth-dependent straining, produced a reasonably good fit for both the breakthrough curves and final deposition profiles. Experimental and modeling results suggest that straining--the retention of colloids in low velocity regions of porous media such as grain junctions--was the primary mechanism of colloid retention under both saturated and unsaturated conditions. The extent of stagnant regions of flow within the pore structure is enhanced with decreasing water content, leading to a greater amount of retention. Ionic strength also contributes to straining, because the number of colloids that are held in the secondary energy minimum increases with ionic strength. These weakly associated colloids are prone to be translated to stagnation regions formed at grain-grain junctions, the solid-water-air triple point, and dead-end pores and then becoming trapped.     

Bacterial deposition in unsaturated porous media as related to surface properties

Transport of Escherichia coli, Pseudomonas fluorescens and Bacillus subtilis in silica sand under water-unsaturated conditions was investigated using column experiments. It was hypothesised that bacterial deposition was due to their interactions within the pore environment, which was a function of their surface physicochemical properties as well as pore water chemistry. Surface thermodynamic properties of these bacterial strains were measured independently by means of contact angle measurements under different water saturation conditions using variable lawn moisture contents. Bacterial interactions with the liquid-gas interface and the porous media were calculated based on their surface properties and were related to their transport observations.     

Effect of water content on transient nonequilibrium NAPL-gas mass transfer during soil vapor extraction

The effect of water content on the volatilization of nonaqueous phase liquid (NAPL) in unsaturated soils was characterized by one-dimensional venting experiments conducted to evaluate the lumped mass transfer coefficient. An empirical correlation based upon the modified Sherwood number, Peclet number, and normalized mean grain size was used to estimate initial lumped mass transfer coefficients over a range of water content. The effects of water content on the soil vapor extraction SVE process have been investigated through experimentation and mathematical modeling. The experimental results indicated that a rate-limited NAPL-gas mass transfer occurred in water-wet soils. A severe mass transfer limitation was observed at 61.0% water saturation where the normalized effluent gas concentrations fell below 1.0 almost immediately, declined exponentially from the initiation of venting, and showed long tailing. This result was attributed to the reduction of interfacial area between the NAPL and mobile gas phases due to the increased water content. A transient mathematical model describing the change of the lumped mass transfer coefficient was used. Simulations showed that the nonequilibrium mass transfer process could be characterized by the exponent beta, a parameter which described the reduction of the specific area available for NAPL volatilization. The nonequilibrium mass transfer limitations were controlled by the soil mean grain size and pore gas velocity, were well described by beta values below 1.0 at low water saturation, and were well predicted with beta values greater than 1.0 at high water saturation.     

Column experiments to study nonlinear removal of bacteriophages by passage through saturated dune sand

In a recent field study on dune recharge, bacteriophages MS2 and PRD1 were found to be removed 3 log10 over the first 2.4 m and only 5 log10 over the next 27 m. To understand the causes of this nonlinear removal, column experiments were carried out under conditions similar to the field: same recharge water, temperature (5 +/- 3 degrees C) and pore water velocity (1.5 m day(-1)). Soil samples were taken along a streamline between the recharge canal and the first monitoring well. Bacteriophage phiX174 was included for comparison. The high initial removal in the field was found not to be due to heterogeneity of phage suspensions but to soil heterogeneity. Phage removal rates correlated strongly positively with soil organic carbon content, and relatively strongly positively with silt content and the presence of ferric oxyhydroxides. Soil organic carbon content, silt content and the presence of ferric oxyhydroxides were found to decrease exponentially with travel distance. Removal rates of phiX174 were found to be 3-10 times higher than those of MS2 and PRD1 due to the lower electrostatic repulsion that the less negatively charged phiX174 experiences. It is suggested that the high initial removal in the field is due to the presence of favorable sites for attachment formed by ferric oxyhydroxides that decrease exponentially with travel distance. Similar removal rates may be found at both laboratory and field scale. However, due to local variations at field scale detailed knowledge on soil heterogeneity may be needed to enable a reliable prediction of removal.     

Characterization of pore scale NAPL morphology in homogeneous sands as a function of grain size and NAPL dissolution

In this study, we investigate pore scale morphology of nonaqueous phase liquids (NAPLs) trapped in different pore sizes using tracer techniques. Specific interfacial area and saturation of NAPL trapped in homogeneous sands were measured using the interfacial and partitioning tracer techniques. The observed NAPL-water interfacial areas increased in a log-linear fashion with decreasing sand grain size, but showed no clear trend with residual NAPL saturation formed in the various grain sizes. The measured values were used to calculate the NAPL morphology index, which characterizes the spatial NAPL distribution within the pore space. The NAPL morphology indices, increased exponentially with decreasing grain size, indicating that the NAPL becomes smaller, but more blobs. For a fixed grain size, the specific interfacial area and saturation of the NAPL were measured following changes caused by dissolution using alcohol. The observed interfacial areas showed a decrease linearly as a function of the NAPL saturation.     

高气泡表面积通量浮选柱气浮除藻的研究

湖泊、水库等水源的富营养化,使藻类去除成为饮用水生产的重要任务.本研究采用高气泡表面积通量浮选柱气浮除藻,考察了混凝剂、气泡表面积通量和浮选柱高度等因素的影响.试验表明,高气泡表面积通量浮选柱气浮可高效地去除绿藻、硅藻和蓝藻,叶绿素a和藻类去除率达95%以上,比传统浮选柱气浮和沉降作业有较大幅度提高.与普通气浮柱比较,高气泡表面积通量浮选柱增加了气泡与藻的碰撞几率,防止因大表观充气速率造成的紊流和扰动,使气泡/藻结合体有相对静态的浮升环境,避免了气泡/藻结合体在浮升过程中的脱落,实现对藻类的迅速捕集和转移.     

高气泡表面积通量浮选柱气浮除藻的研究

湖泊、水库等水源的富营养化，使藻类去除成为饮用水生产的重要任务。本研究采用高气泡表面积通量浮选柱气浮除藻，考察了混凝剂、气泡表面积通量和浮选柱高度等因素的影响。试验表明，高气泡表面积通量浮选柱气浮可高效地去除绿藻、硅藻和蓝藻，叶绿素a和藻类去除率达95％以上，比传统浮选柱气浮和沉降作业有较大幅度提高。与普通气浮柱比较，高气泡表面积通量浮选柱增加了气泡与藻的碰撞几率，防止因大表观充气速率造成的紊流和扰动，使气泡／藻结合体有相对静态的浮升环境，避免了气泡／藻结合体在浮升过程中的脱落，实现对藻类的迅速捕集和转移。     

Use of hydrophilic and hydrophobic microfiltration membranes to remove microorganisms and organic pollutants from primary effluents.

Primary-effluent wastewater from the Allegheny County Sanitary Authority (Pennsylvania) was used as a surrogate for combined-sewer-overflow waters contained in the sewershed. Pathogens contained in combined-primary-effluent wastewaters or combined-sewer overflows (CSOs) may pose a human-health threat to those coming in contact with such receiving waters. Polymeric ultra- and nano-membranes can be used to capture these microorganisms from CSOs. This research investigates the ability of polymeric hydrophobic and hydrophilic membranes, ranging in pore size from 0.2 to 0.8 microm, to remove pathogen-indicator organisms (Escherichia coli, enterococcus, and fecal coliforms). Membranes, with pore size 0.45 microm and smaller, were able to reduce the bacteria levels to nondetectable levels, with the sole exception of one membrane with pore size 0.3 microm.     

Kinetic modeling of virus transport at the field scale

Bacteriophage removal by soil passage in two field studies was re-analyzed with the goal to investigate differences between one- and two-dimensional modeling approaches, differences between one- and two-site kinetic sorption models, and the role of heterogeneities in the soil properties. The first study involved removal of bacteriophages MS2 and PRDI by dune recharge, while the second study represented removal of MS2 by deep well injection. In both studies, removal was higher during the first meters of soil passage than thereafter. The software packages HYDRUS-ID and HYDRUS-2D, which simulate water flow and solute transport in one- and two-dimensional variably saturated porous media, respectively, were used. The two codes were modified by incorporating reversible adsorption to two types of kinetic sites. Tracer concentrations were used first to calibrate flow and transport parameters of both models before analyzing transport of bacteriophages. The one-dimensional one-site model did not fully describe the tails of the measured breakthrough curves of MS2 and PRD1 from the dune recharge study. While the one-dimensional one-site model predicted a sudden decrease in virus concentrations immediately after the peaks, measured data displayed much smoother decline and tailing. The one-dimensional two-site model simulated the overall behavior of the breakthrough curves very well. The two-dimensional one-site model predicted a more gradual decrease in virus concentrations after the peaks than the one-dimensional one-site model, but not as good as the one-dimensional two-site model. The dimensionality of the problem hence can partly explain the smooth decrease in concentration after peak breakthrough. The two-dimensional two-site model provided the best results. Values for k(att2) and k(det2) could not be determined at the last two of four monitoring wells, thus suggesting that either a second type of kinetic sites is present in the first few meters of dune passage and not beyond the second monitoring well, or that effects of soil heterogeneity and dimensionality of the problem overshadowed this process. Variations between single collector efficiencies were relatively small, whereas collision efficiencies varied greatly. This implies that the nonlinear removal of MS2 and PRD1 is mainly caused by variations in interactions between grain and virus surfaces rather than by physical heterogeneity of the porous medium. Similarly, a two-site model performed better than the one-site model in describing MS2 concentrations for the deep well injection study. However, the concentration data were too sparse in this study to have much confidence in the fitted parameters.     

Effect of transpiration on Pb uptake by lettuce and on water soluble low molecular weight organic acids in rhizosphere

The effect of transpiration (high and low) on Pb uptake by leaf lettuce and on water soluble low molecular weight organic acids (LMWOAs) in rhizosphere has been studied. After two weeks of growth the plants were cultured in greenhouse for more four weeks and two days. Pb(NO(3))(2) solutions of different concentrations (100, 200, and 300 mg l(-1) of Pb) were then added to the quartz sand pots of different plants and studies were initiated. Blank experiments (without treating the quartz sand pots with Pb(NO(3))(2) solutions) were also run in parallel. No significant differences in the growth of the plants with the concentrations of added Pb(NO(3))(2) solutions were observed by both low and high transpirations at the end of the 0, 3rd, and 10th days of studies. The total evaporation of the volatiles during 10 days did not depend on the concentration of Pb(2+) but with high transpiration the rate of evaporation was significantly higher than with low transpiration. Uptake of Pb by shoots and roots of the plants was found to be proportional to the concentration of various Pb(NO(3))(2) solutions added and more accumulation was observed in roots than in shoots at the end of 3rd and 10th days. High transpiration created more Pb uptake than low transpiration did. One volatile acid, propionic acid and nine non-volatile acids, lactic, glycolic, oxalic, succinic, fumaric, oxalacetic, D-tartaric, trans-aconitic, and citric acids in rhizosphere quartz sands were identified and quantified by gas chromatography (GC) analysis. D-Tartaric and citric acids were major among the non-volatile acids. The amount of LMWOAs in rhizosphere quartz sands increased with the higher amount of Pb uptake and also with the duration of studies. The total quantities of the LMWOAs in the rhizosphere quartz sands were significantly higher under high transpiration with 300 mg l(-1) Pb solution addition at the end of 10th day. The present study shows prominent correlation between transpiration and uptake of heavy metal and interesting correlation between Pb contaminated level and quantity of water soluble LMWOAs in rhizosphere quartz sands. The latter thus deserves of further studies.     

基质结构对水平潜流人工湿地净化效果影响

为研究基质填充结构变化对人工湿地净化效果的影响,构建2个不同基质结构的对比水平潜流人工湿地实验系统,对城市污水处理厂出水进行深度处理。实验结果表明,1～7 mm石英砂均混填充结构人工湿地系统对COD、NH4+-N和TP的平均去除率分别为49.5%,40.1%和56.1%;比较而言,1～7 mm石英砂分层填充结构湿地系统对3种污染物的平均去除率分别为53.7%,60.4%和68.6%。方差分析表明,不同石英砂结构的人工湿地对COD、NH4+-N及TP的净化效果差异显著(p<0.05)。此外,不同水力停留时间条件下,基质填充结构不同的湿地系统对以上3种污染物的净化效果也存在显著差异(p<0.05),去除率随着水力停留时间的增加而升高,当HRT为36 h时,各系统均能取得较好的净化效果。     

Surface charge and adsorption from water onto quartz sand of humic acid

The surface charge of humic acid under different conditions of ionic strength, pH, and the presence of various cationic ions (Cu(2+), Zn(2+), Ba(2+), and Ca(2+)) was determined by a titration method using a cationic polyelectrolyte as titrant. Adsorption isotherms in batch experiments of the polymer from water onto quartz sand were determined at 20 degrees C, 40 degrees C, and 60 degrees C and under different conditions of ionic strength, pH, and the presence of various cationic ions (Cu(2+), Zn(2+), Ba(2+), and Ca(2+)). The data indicate significant decrease of humic acid surface charge by decreasing the pH value from 10.0 to 4.1. Similar decrease of humic acid surface charge was observed by increasing either the ionic strength or the affinity of the divalent cation toward the humic acid. At ambient temperature the adsorption of humic acid on the quartz sand seems to be controlled mainly by electrical interaction between the organic particle and the solid substrate. A correlation is found between the surface charge and the adsorbed amount of the polymer, the adsorbed amount increases when the surface charge of humic acid decreases. The increase of the adsorbed amount with the temperature suggests that adsorption process is endothermic.     

Transport of Escherichia coli in 25 m quartz sand columns

To help improve the prediction of bacteria travel distances in aquifers laboratory experiments were conducted to measure the distant dependent sticking efficiencies of two low attaching Escherichia coli strains (UCFL-94 and UCFL-131). The experimental set up consisted of a 25 m long helical column with a diameter of 3.2 cm packed with 99.1% pure-quartz sand saturated with a solution of magnesium sulfate and calcium chloride. Bacteria mass breakthrough at sampling distances ranging from 6 to 25.65 m were observed to quantify bacteria attachment over total transport distances (α(L)) and sticking efficiencies at large intra-column segments (α(i)) (>5m). Fractions of cells retained (F(i)) in a column segment as a function of α(i) were fitted with a power-law distribution from which the minimum sticking efficiency defined as the sticking efficiency of 0.001% bacteria fraction of the total input mass retained that results in a 5 log removal were extrapolated. Low values of α(L) in the order 10(-4) and 10(-3) were obtained for UCFL-94 and UCFL-131 respectively, while α(i)-values ranged between 10(-6) to 10(-3) for UCFL-94 and 10(-5) to 10(-4) for UCFL-131. In addition, both α(L) and α(i) reduced with increasing transport distance, and high coefficients of determination (0.99) were obtained for power-law distributions ofα(i) for the two strains. Minimum sticking efficiencies extrapolated were 10(-7) and 10(-8) for UCFL-94 and UCFL-131, respectively. Fractions of cells exiting the column were 0.19 and 0.87 for UCFL-94 and UCL-131, respectively. We concluded that environmentally realistic sticking efficiency values in the order of 10(-4) and 10(-3) and much lower sticking efficiencies in the order 10(-5) are measurable in the laboratory, Also power-law distributions in sticking efficiencies commonly observed for limited intra-column distances (<2m) are applicable at large transport distances(>6m) in columns packed with quartz grains. High fractions of bacteria populations may possess the so-called minimum sticking efficiency, thus expressing their ability to be transported over distances longer than what might be predicted using measured sticking efficiencies from experiments with both short (<1m) and long columns (>25 m). Also variable values of sticking efficiencies within and among the strains show heterogeneities possibly due to variations in cell surface characteristics of the strains. The low sticking efficiency values measured express the importance of the long columns used in the experiments and the lower values of extrapolated minimum sticking efficiencies makes the method a valuable tool in delineating protection areas in real-world scenarios.     

污染底泥及其间隙水分层特性的模拟实验研究

底泥冲刷沉降、再悬浮是作用于水环境内源污染的主要因素,而底泥间隙水的污染释放则是主要途径。通过底泥培养实验,研究了污染底泥及其间隙水的分层特性。结果表明,底泥及其污染物具有显著的分层特性。底泥间隙水中CODMn随深度的增加略有下降的趋势;底泥湿容重和干容重呈增大的趋势;含水率随底泥深度的增加而减小。底泥深度对底泥泥沙颗粒的级配有很大影响,在表层以细颗粒为主,随着深度的增加．底泥粒径增加,但趋势衰减较快。这对底泥冲刷作用下的再悬浮有很大影响。