苯、甲苯对粒状铁去除四氯乙烯影响的柱实验研究

挥发性氯代烃和石油烃类污染是地下水中最常见的混合污染类型,而且这两类污染物毒性极强,对人类危害非常严重。文中选取具有代表性的四氯乙烯、苯和甲苯为研究对象,采用柱实验的方法研究苯和甲苯在粒状铁反应系统中吸附平衡后,对粒状铁去除四氯乙烯的机理及反应动力学的影响。在实验装置运行的过程中,苯、甲苯和四氯乙烯的浓度始终控制在2mg/L左右的水平。实验结果表明:苯或甲苯的存在对被还原的产物组成没有影响,主要氯代中间产物均为TCE、1,1-DCE、cis-1,2-DCE和VC,但组成比例略有不同。苯和甲苯的存在对去除速率有影响,即苯对四氯乙烯的去除有促进作用,去除速率平均提高13.5%;而甲苯则抑制四氯乙烯的去除,去除速率平均降低13.8%。对比控制柱,苯和甲苯存在时对出水水化学变化的影响没有明显差异。     

Halocarbons in the environment: estimates of thermodynamic properties for aqueous chloroethylene species and their stabilities in natural settings

Standard partial molal thermodynamic parameters for the aqueous chlorinated-ethylene species, perchloroethylene (PCE), trichloroethylene (TCE), 1,1-dichloroethylene (1,1-DCE), cis-1,2-dichloroethylene (cis-1,2-DCE), trans-1,2-dichloroethylene (trans-1,2,-DCE), and vinyl chloride (VC) have been estimated by using experimental gas-solubility data and correlation algorithms. The provided thermodynamic values may be used to calculate properties of reactions involving the aqueous chloroethylene species at a wide range of temperatures and pressures. Estimated values for the chloroethylenes were used, along with published values for minerals, gases, aqueous ions, and aqueous neutral organic species, to calculate the stability of chloroethylene species in equilibrium with the minerals magnetite, hematite, pyrite, and pyrrhotite in the subsurface. Estimated values for the aqueous chloroethylenes were also used to calculate reduction potentials for microbially-mediated reductive dechlorination half-reactions at elevated temperatures. Calculations indicate that all aqueous chloroethylene species are energetically favored to decompose to ethylene(aq) under a wide range of conditions in the subsurface, by both abiotic and biotic pathways. Anaerobic microbially mediated degradation is especially favored under conditions at least sufficiently reducing to promote sulfate-reduction, but not under conditions sufficient for microbial denitrification, pyrolusite reduction, or ferric-iron reduction.     

Remediation of TCE-contaminated groundwater using zero valent iron and direct current: experimental results and electron competition model

The objectives of this study are to design an optimal electro-enhanced permeable reactive barrier (E2PRB) system for the remediation of trichloroethylene (TCE)-contaminated water using zero valent iron (ZVI) and direct current (DC) and to investigate the mechanisms responsible for TCE degradation in different ZVI-DC configurations. A series of column experiments was conducted to evaluate the effect of different arrangements of electrodes and ZVI barriers in the column on the TCE removal capacity (RC). In twelve different combinations of ZVI and/or DC application in the test columns, the rate of reductive dechlorination of TCE was improved up to six times with simultaneous application of ZVI and DC compared to that using ZVI only. The most effective arrangement of electrode and ZVI for TCE removal was the column set with ZVI and a cathode installed at the down gradient. Based on the electrokinetic study for the column systems with only DC input, single acid front movement could explain different RCs. An enhanced dechlorination rate of TCE using E2PRB systems, compared to a conventional PRB system, was observed, and is considered to be attributed to more electron sources: (1) external DC, (2) electrolysis of water, (3) oxidation of ZVI, (4) oxidation of dissolved Fe²⁺, (5) oxidation of molecular hydrogen at the cathode, and (6) oxidation of Fe²⁺ in mineral precipitates. Each of these electron sources was evaluated for their potential influencing the TCE RC through the electron competition model and energy consumption. A strong correlation between the quantity of electrons generated, RC, and the energy-effectiveness was found.     

Compound-specific chlorine isotope ratios of TCE,PCE and DCE isomers by direct injection using CF-IRMS

A method for determining compound-specific Cl isotopic compositions (δ³⁷Cl) was developed for tetrachloroethene (PCE), trichloroethene (TCE), cis-dichloroethene (cis-DCE), trans-dichloroethene (trans-DCE) and 1,1-dichloroethene (1,1-DCE). The isotope ratio mass spectrometry (IRMS) used in this study has nine collectors, including two for m/z 50 and 52 (CH₃Cl) and two for m/z 94 and 96 (CH₃Br). The development of this method is based on the fact that fragments with mass ratios of 94/96, 95/97 and 96/98 are produced from PCE, TCE and DCE isomers during ion bombardment in the source of a mass spectrometer. Using continuous flow isotope ratio mass spectrometry coupled with gas chromatography (GC–CF-IRMS), it is possible to separate these compounds on-line and directly measure the Cl isotopic ratios of the fragments with the specific mass ratios.Both pure phase and aqueous samples were used for Cl isotopic analysis. For pure phase samples, a vapour phase of the chlorinated ethenes was injected directly into the GC, whereas the solid phase micro extraction (SPME) method was used to extract these compounds from aqueous solutions. The precisions of this analytical technique were ±0.12‰ (1σ, n = 30), ±0.06‰ (1σ, n = 30), and ±0.08‰ (1σ, n = 15) for PCE, TCE and DCE isomers, respectively. The limits of quantification (LOQ) for analyzing Cl isotopic composition in aqueous solutions were 20, 5, and 5 μg/L for PCE, TCE and DCE isomers, respectively. This corresponds to 6–9 nano-mole of Cl, which is approximately 80 times lower than the most sensitive existing method. Compared to methods previously available, this new development offers the following advantages: (1) The much lower LOQ make it possible to extract these compounds directly from aqueous solutions using SPME without pre-concentration; (2) The linking of a GC with an IRMS eliminates off-line separation; and (3) Because the fragments used for isotopic ratio measurement are produced during ion bombardment in the mass spectrometer, there is no need to convert chlorinated ethenes to methyl chloride. As a result, this technique greatly enhances the efficiency for isotopic analysis by eliminating procedures for pre-concentration, off-line separation and sample preparation. In addition, it also reduces the potential for isotopic fractionation introduced during these procedures.Compound-specific Cl stable isotope analysis can be used as a tool to study the sources of organic contaminants in groundwater and their behaviour in the subsurface environments. It may also assist in understanding processes such as transport, mixing, and degradation reactions.     

不同基质共代谢降解地下水中四氯乙烯的研究

文中以甲醇、乙醇、甲酸盐、乙酸盐、乳酸盐和葡萄糖作为共代谢基质,利用某污水处理厂厌氧池中的污水(泥)作为接种物,对厌氧微生物进行培养和驯化,降解四氯乙烯(PCE)。实验结果表明,在厌氧污水(泥)和土壤混合环境下培养的微生物,以COD为培养指标,11d可培养成熟。6种基质均能使PCE还原脱氯降解成三氯乙烯(TCE)和1,1-二氯乙烯(1,1-DCE),PCE的降解均符合一级反应动力学方程,反应速率常数大小依次为K乙酸盐>K葡萄糖>K乳酸盐>K乙醇>K甲酸盐>K甲醇,其中乙酸盐是最有效的共代谢基质,其反应速率常数为0·6632d-1。     

Degradation of Tetrachloroethene by Several Co-metabolism Substrates in Groundwater

Tetrachloroethene （PCE） is biodegraded by reductive dechlorination with co-metabolism substrates under anaerobic conditions. By inoculating sludge from an anaerobic pool, a biodegradation test of PCE is conducted in the anaerobic condition. In the test, several substrates including methanol, ethanol, formate, acetate, lactate and glucose, are conducive to the conversion from PCE to TCE and 1,1-DCE. The results show the microbe can be cultivated well under the anaerobic circumstances of mixture of sewage （sludge） and soil with the index of COD after eleven days. Degradation of PCE accords with one order reaction kinetics equation. The sequence of the reaction rate constant is Kacetate 〉Kglucose 〉 Klactate 〉 Kethanol 〉 Kformate 〉 Kmethanol, and acetate is an outstanding co-metabolism substratum whose reaction rate constant is 0.6632d^-1.     

Fate and transport of volatile organic compounds in glacial till and groundwater at an industrial site in Northern Ireland

Volatile organic compound (VOC) contamination of subsurface geological material and groundwater was discovered on the Nortel Monkstown industrial site, Belfast, Northern Ireland. The objectives of this study were to (1) investigate the characteristics of the geological material and its influences on contaminated groundwater flow across the site using borehole logs and hydrological evaluations, and (2) identify the contaminants and examine their distribution in the subsurface geological material and groundwater using chemical analysis. This report focuses on the eastern car park (ECP) which was a former storage area associated with trichloroethene (TCE) degreasing operations. This is where the greatest amount of volatile organic compounds (VOCs), particularly TCE, were detected. The study site is on a complex deposit of clayey glacial till with discontinuous coarser grained lenses, mainly silts, sands and gravel, which occur at 0.45–7.82 m below ground level (bgl). The lenses overall form an elongated formation that acts as a small unconfined shallow aquifer. There is a continuous low permeable stiff clayey till layer beneath the lenses that performs as an aquitard to the groundwater. Highest concentrations of VOCs, mainly TCE, in the geological material and groundwater are in these coarser lenses at ∼4.5–7 m bgl. Highest TCE measurements at 390,000 μg L⁻¹ for groundwater and at 39,000 μg kg⁻¹ at 5.7 m for geological material were in borehole GA19 in the coarse lens zone. It is assumed that TCE gained entrance to the subsurface near this borehole where the clayey till was thin to absent above coarse lenses which provided little retardation to the vertical migration of this dense non-aqueous phase liquid (DNAPL) into the groundwater. However, TCE is present in low concentrations in the geological material overlying the coarse lens zone. Additionally, VOCs appear to be associated with poorly drained layers and in peat <3.0 m bgl in the ECP. Some indication of natural attenuation as VOCs degradation products vinyl chloride (VC) and dichloromethane (DCM) also occur on the site.     

Hydrodynamic and isotopic characterization of a site contaminated by chlorinated solvents: Chienti River Valley,Central Italy

Contaminant sources have been attributed to shoe manufacturers in an alluvial aquifer located along 26 km² in the Chienti River Valley, Central Italy. During the 1980s and 1990s, the main chlorinated compound used in the study area was 1,1,1-Trichloroethane (1,1,1-TCA), which was substituted by Perchloroethene (PCE) in the last 15 years. A hydrogeological conceptual model has been developed for the alluvial aquifer taking into account the presence of low permeability lenses, forming a multilayer semi-confined aquifer. Hydrodynamic tests (pumping and flowmeter heat-pulse tests) coupled with standard and multilevel hydrochemical and isotopic samplings were performed. Flowmeter tests showed the existence of vertical flow between aquifer levels having different permeability. Physical–chemical parameter logs agreed with the existence of a multilayer aquifer. Concentration data collected in 21 wells located downgradient of the different sources revealed VOC (Volatile Organic Compound) levels lower than 100 μg/L in the upper part of the valley and levels reaching about 200 μg/L in the near shore areas. PCE is the main compound present in the aquifer. No evidence of the presence of TCA was found in the upper areas of the Chienti Valley, but in the areas near the shore, TCA and its degradation products are predominant. Data collected at multilevels located at two sites (upper and near shore areas) to refine the results obtained in the regional survey show a stratification of the VOC concentrations; values of each compound are higher than those measured in the conventional wells during the standard sampling (e.g. PCE: 150 μg/L instead of 2 μg/L). In addition, the vertical distribution of the contaminant reflects the vertical flow pattern inferred from hydrogeological data. The hydrogeological, VOC and isotope data showed that dilution is the main process controlling VOCs concentration in the aquifer. Degradation also played a role in the attenuation of the parent compounds in some localized areas of the aquifer. The role of the low permeability layers on VOC degradation was documented by the presence of cis-1,2-DCE, a main daughter product of PCE, in some zones of the upper area, and 1,1-Dichloroethene (1,1-DCE) and 1,1-Dichloroethane (1,1-DCA), byproducts of 1,1,1-TCA degradation, in the near shore areas.     

Effects of land use on the spatial distribution of trace metals and volatile organic compounds in urban groundwater, Seoul, Korea

To investigate the urban groundwater contamination by eight trace metals and 69 volatile organic compounds (VOCs) in relation to land use in Seoul, a total of 57 groundwater samples collected from wells were examined using a non-parametric statistical analysis. Land use was classified into five categories: less-developed, residential, agricultural, traffic, and industrial. A comparison of analyzed data with US EPA and Korean standards for drinking water showed that some metals and VOCs exceeded the standards in a few localities, such as Fe (N=5), Mn (N=6), Cu (N=1), TCE (N=6), PCE (N=8), 1,2-DCA (N=1), and 1,2-dichloropropane (N=1). Among the 69 investigated VOCs, 19 compounds such as some gasoline-related compounds (e.g., toluene) and chlorinated compounds (e.g., chloroform, PCE, TCE) were detected in groundwater. Non-parametric statistical analysis showed that the concentrations of most trace metals (Fe, Mn, As, Cr, Pb, Cd) and some VOCs (especially, TCE, PCE, chloroform; toluene, carbon tetrachloride, bromodichloromethane, CFC113) are significantly higher in the industrial, residential, and traffic areas (P<0.05), indicating that anthropogenic contamination of urban groundwater by those chemicals is growing. Those chemicals can be used as effective indicators of anthropogenic contamination of groundwater in urban areas and therefore a special attention is warranted for a safe water supply in those areas. The results of this study suggest that urban groundwater quality in urban areas is closely related with land use.     

Dechlorination of perchloroethylene using zero-valent metal and microbial community

This research evaluates an integrated technique for the degradation of perchloroethylene (PCE) using a combination of zero-valent metal and an anaerobic microbial community. The microbial community was obtained from river sediment through a series of cultivation and enrichment processes, and was able to successfully convert PCE to ethylene. The degradation tests were carried out in five groups of reactors, i.e. Zn (zero-valent zinc), Fe (zero-valent iron), MB (a microbial community), ZnMB (zinc and a microbial community) and FeMB (iron and a microbial community). The results suggested that the FeMB system had the highest efficiency in removing PCE (about 99.9% after 24 days), followed by Zn (98.5%), ZnMB (84.8%), Fe (76.9%) and MB (49.6%). The degradation kinetics of PCE could be described by a pseudo-first-order reaction; the apparent reaction rate constants were 0.231, 0.187, 0.135, 0.076, 0.031 days⁻¹ for the Zn, FeMB, ZnMB, Fe, and MB systems, respectively. Much more Cl⁻ was detected as a by-product in the degradation tests using FeMB or ZnMB than using Fe or Zn. The results implied that not only PCE, but also the reaction intermediates, e.g., trichloroethylene (TCE) and dichloroethylenes (DCE isomers) may have been dechlorinated in the FeMB and ZnMB systems. Results suggest that the presence of the microbial community in the FeMB and ZnMB systems may not only affect the removal efficiency of PCE, but can also change the reaction pathways in the dechlorination process. The integrated technique combining the iron and the microbial community showed better degradation efficiency than the others; it may be viewed as an environmentally desirable remediation.     

土壤类型及组分对热活化过硫酸盐氧化降解土壤中挥发性氯代烃的影响

为了探究高级氧化技术对土壤中有机氯代烃的氧化降解作用,为ISCO（in situ chemical oxidation）技术体系提供重要的理论依据和数据支撑,考察了热活化过硫酸盐（persulfate,PS）氧化降解不同类型土壤（砂类土壤、黏土类土壤）中挥发性氯代烃污染物（三氯乙烯（TCE）、三氯乙烷（TCA）、顺式-1,2-二氯乙烯（cis-1,2-DCE）、1,2-二氯乙烷（1,2-DCA））的效能;同时,通过硫酸盐与土壤相互作用过程研究,探究了不同土壤介质中有机质和无机组分在过硫酸盐消耗中所占比例。结果表明：在50℃时,热活化过硫酸盐可有效降解土壤中1,2-DCA、cis-1,2-DCE、TCA和TCE,砂类土壤介质中4种氯代烃降解效果依次为25%、89%、5%和61%,黏土类土壤介质中4种氯代烃降解效果依次为35%、86%、8%和63%;4种氯代烃的降解效果从高到低顺序依次为cis-1,2-DCE、TCE、1,2-DCA、TCA,砂类土壤中的氯代烃总体降解效果优于黏土类土壤中氯代烃的降解效果。另外,土壤中过硫酸盐氧化降解氯代烃反应研究发现,砂类和黏土类土壤介质组分中有机质消耗率分别为81.3%和72.6%,铁元素消耗率分别为80.5%和38.6%,表明土壤介质组分与过硫酸盐发生了氧化还原反应,从而导致过硫酸盐自身的大量消耗。由此可知,土壤介质中的有机质、铁元素等矿物质均参与过硫酸盐的消耗过程,且土壤有机质、铁元素与氯代烃之间在消耗过硫酸盐反应上存在竞争关系,土壤组分过多地消耗了过硫酸盐,导致了氯代烃的氧化降解效率较低。因此,针对实际有机氯代烃污染场地,采用过硫酸盐氧化技术进行修复时,过硫酸盐的实际投加量要远高于化学计量值,需充分考虑到土壤组分对过硫酸盐自身的消耗作用。     

Combining multivariate statistical analysis with geographic information systems mapping: a tool for delineating groundwater contamination

Multivariate Statistical Analysis (MSA) has successfully been coupled with geographic information system (GIS) mapping tools to delineate zones of aquifer contamination potential. While delineating contaminants is key to site remediation, it is often compromised by a poor understanding of hydrogeologic conditions, and by uncertainties in contaminant observations. MSA provides improved estimates of contamination potential by augmenting observed contaminant concentrations with auxiliary information from other water-quality parameters. GIS is useful for organizing and managing water-quality information by visually communicating large amounts of information. The proposed method first establishes appropriate areal extents, GIS coverages, and scales for displaying groundwater contamination concentrations of tritium and the volatile organic contaminants trichloroethylene (TCE) and tetrachloroethylene (PCE) at the Savannah River Site, South Carolina, USA. Principal components analysis is used to group variables that are most indicative of contamination potential. Tritium contamination potential is best represented as the combination of tritium with the cations Al, Mg, Na and total dissolved solids, while PCE contamination potential is predicted using PCE and Cl. Maps of contamination potentials for 1993–1995 geochemical data compare favorably with measured contaminant concentrations during 1999. Cluster analysis of water-quality data groups geochemical and contaminant concentrations into zones of homogeneous behavior.

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Resumen El Análisis Estadístico Multivariado (AEM) se ha acoplado con éxito con herramientas cartográficas del Sistema de Información Geográfico (SIG), para delinear zonas de potencial contaminación de acuíferos. En tanto que la delimitación de contaminantes es importante para recuperación del sitio, ella está a menudo amenazada por una comprensión pobre de las condiciones hidrogeológicas, y por las incertidumbres en las observaciones del contaminante. El AEM proporciona estimaciones mejoradas de contaminación potencial al aumentar las concentraciones observadas del contaminante, con la información auxiliar de otros parámetros de calidad de agua. El SIG es útil para organizar y manejar la información de calidad de agua, a través de la comunicación visual de cantidades grandes de información. El método propuesto establece primero magnitudes de área apropiadas, el área cubierta por el SIG, y las escalas para mostrar las concentraciones de contaminación del agua subterránea, a partir de tritio y de contaminantes orgánicos volátiles como tricloroetileno (TCE) y tetracloroetileno (PCE) en el sitio del Río al Savannah, Carolina del Sur, EE.UU. El análisis de componentes principales se usa para agrupar variables que son más indicativas de contaminación potencial. El potencial de contaminación Tritio, se representa mejor como la combinación de tritio con los cationes Al, Mg, Na y los sólidos disueltos totales, mientras la contaminación potencial de PCE se predice usando PCE y Cl. Los mapas de potenciales de contaminación para los datos geoquímicos de 1993–1995 se comparan favorablemente con las concentraciones medidas del contaminante durante 1999. El análisis de racimo hecho en datos de calidad de agua, agrupa concentraciones geoquímicas y concentraciones del contaminante en las zonas de comportamiento homogéneo.

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Résumé L’Analyse Statistique Multivariée (ASM) a été couplée avec succès avec les outils cartographiques des Sytèmes d’Information Géographique (SIG), afin de délimiter les zones à potentiel de contamination des aquifères. Alors que la connaissance de l’extension des contaminants est primordiale pour la réhabilitation des sites, elle est fréquemment compromise par une compréhension limitée du contexte hydrogéologique, et par les incertitudes sur l’observation des contaminants. L’ASM fournit une meilleure estimation du potentiel de contamination, en complétant les concentrations observées en contaminants par des informations annexes issues d’autres paramètres qualitatifs. Les SIG, par leurs capacités à conjuguer visuellement de grandes quantités d’information, sont utiles pour organiser et gérer les informations sur la qualité de l’eau. La première étape de la méthode proposée consiste à définir les limites spatiales appropriées, les couvertures SIG, ainsi que les échelles de visualisation des concentrations en contaminants des eaux souterraines, soit le tritium et les composés organiques volatiles trichloroéthylène et tetrachloroéthylène, sur le site de la rivière Savannah (Californie du Sud, USA). L’analyse en composantes principales est utilisée afin de regrouper les variables caractérisant de manière optimale le potentiel de contamination. Le potentiel de contamination en tritium est représenté le plus significativement par la combinaison du tritium, des cations Al, Mg et Na, et des solides dissous totaux (TDS). Le potentiel de contamination en tétrachloroéthylène est quant à lui estimé en utilisant le tetrachloroéthylène et Cl. Les cartes des potentiels de contamination établies sur les données géochimiques de la période 1993–1995 renvoient une image comparable aux concentrations en contaminants effectivement mesurées en 1999. Une analyse de groupement des données sur la qualité de l’eau rassemble les concentrations géochimiques et en contaminants dans des zones aux comportements homogènes.

     

Trace metals and organic compounds in sediment samples from the River Danube in Russe and Lake Srebarna (Bulgaria)

 In 1995 the contamination status of accumulated fine surface sediments and effluent material from the River Danube in Russe (Bulgaria) was analysed for trace metals (Hg, Cd, Pb, Cu, Zn, Cr, Ni, As), polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), hexachlorobenzene (HCB), 1,1,1-trichloro-2,2-bis [chlorophenyl] ethane (DDT), 1,1-dichloro-2,2-bis [chlorophenyl] ethylene (DDE), 1,1-dichloro-2,2-bis [chlorophenyl] ethane (DDD), and hexachlorocyclohexanes (α-, β-, γ- and δ-HCH) to achieve basic information about the River Danube in Bulgaria. The range of trace-metal levels came close to or below the intended quality criteria for the River Elbe in Germany. The only exceptions were Pb in a shipyard, exceeding the final criteria by a factor of 17, and Cr downflow from a metal factory. In one sample from a shipyard (D8) the highest trace metal concentrations were analysed in the fraction 125–2000 μm. The maximum measured PAH contents exhibited a contamination on the lower μg g^–1 d.w.-level (dry weight base), traces of PCBs, HCB and DDD/DDE were analysed in the ng g^–1 d.w.-range. DDT and HCHs were not detected in any sample. The trace-metal and organic-compounds level in sediments from Lake Srebarna (UNESCO-Biosphere Reserve) displayed completely background character. Received: 18 March 1997 · Accepted: 21 July 1998     

First results of operating and monitoring an innovative design of a permeable reactive barrier for the remediation of chromate contaminated groundwater

An innovative setup of a permeable reactive barrier (PRB) was installed in Willisau, Switzerland to remediate chromate contaminated groundwater. Instead of a conventional continuous barrier, this PRB consists of cylinders installed in rows: a single row for lower expected Cr^VI-concentrations and an offset double row for higher expected Cr^VI-concentrations. The cylinders are filled with reactive grey cast-Fe shavings mixed with gravel to prevent extensive precipitation of secondary phases in the pore space. The treatment of the contaminants takes place both within the cylinders and in the dissolved Fe^II plume generated downstream of the barrier. Monitoring of the contamination situation over a period of 3 a provided evidence of the mobilization, transport and behavior of the contaminants in the aquifer. Groundwater and reactive material were sampled upstream, within and downstream of the barrier by a Multi-Port Sampling System (MPSS) that revealed the geochemical processes as a function of time and space. Comprehensive chemical analyses included sensitive parameters such as Cr^VI, Fe^II/Fe^III, redox potential, dissolved O₂ and pH. Several campaigns using multiple optical tracers revealed a rather complex hydrological regime at different scales, thereby complicating the barrier performance.     

Development of modified flyash as a permeable reactive barrier medium for a former manufactured gas plant site, Northern Ireland

A sequential biological permeable reactive barrier (PRB) was determined to be the best option for remediating groundwater that has become contaminated with a wide range of organic contaminants (i.e., benzene, toluene, ethylbenzene, xylene and polyaromatic hydrocarbons), heavy metals (i.e., lead and arsenic), and cyanide at a former manufactured gas plant after 150 years of operation in Portadown, Northern Ireland. The objective of this study was to develop a modified flyash that could be used in the initial cell within a sequential biological PRB to filter complex contaminated groundwater containing ammonium. Flyash modified with lime (CaOH) and alum was subjected to a series of batch tests which investigated the modified cation exchange capacity (CEC) and rate of removal of anions and cations from the solution. These tests showed that a high flyash composition medium (80%) could remove 8.65 mol of ammonium contaminant for every kilogram of medium. The modified CEC procedure ruled out the possibility of cation exchange as the major removal mechanism. The medium could also adsorb anions as well as cations (i.e., Pb and Cr), but not with the same capacity. The initial mechanism for Pb and Cr removal is probably precipitation. This is followed by sorption, which is possibly the only mechanism for the removal of dichromate anions. Scanning electron microscopic analysis revealed very small (<1 μm) cubic highly crystalline precipitates on the flyash, although this new crystalline zeolite growth did not occur rapidly enough to enable productive zeolite formation. Surface area measurements showed that biofilm growth on the medium could be a major factor in the comparative reduction of surface area between real and synthetic contaminant groundwaters. The modified flyash was found to be a highly sorptive granular material that did not inhibit microbiological activity, however, leaching tests revealed that the medium would fail as a long-term barrier material.     

Impacts of zerovalent iron application on the adsorption behavior of alachlor and metalaxyl in water and soil systems

Alachlor and metalaxyl contaminations of environmental and agricultural water and soil systems cause potential threats to human health. However, information on the pesticide adsorption quantity–intensity (Q/I) relationships in water and soil systems is limited. Therefore, adsorption behavior and the fate of alachlor and metalaxyl in the systems as influenced by the application of zerovalent iron (ZVI, Fe⁰) were investigated using the pesticide adsorption Q/I relationships. After treating ZVI in the systems, the concentration of alachlor rapidly decreased within a few days and then it disappeared at approximately 5–7 experiment days; whereas metalaxyl concentration was reduced by approximately 40–45% during the 28 day experimental period. In particular, ZVI adsorbed more metalaxyl in the aqueous system than in the soil-solution system. The alachlor concentration in the water and soil solution drastically decreased with increasing ZVI treatment, while metalaxyl concentration was relatively slow in its decrease. Adsorption mechanism of the pesticides in the soil-solution system was shown as multiple-site adsorption Q/I fitting. Buffering capacity (BC) of the pesticides increased with ZVI treatment in all sorption sites. The BC values in sandy soil were escalated with increasing ZVI application rates, so that the values were rather higher in sandy soil than in clayey soil. In addition, changes in alachlor concentration with applying ZVI were due to both dechlorination and adsorption; whereas metalaxyl concentration was dependent upon adsorption reaction. Thus, the pesticide adsorption Q/I relationships in different soil-solution systems were critically affected by the ZVI treatment.     

Transport of dissolved methoxy-nonafluorobutane through a saturated column

Solute transport experiments were conducted in a one-dimensional saturated column using dissolved methoxy-nonafluorobutane (HFE-7100), a Novec engineered fluid developed by the 3M Corporation, as the solute. Novec engineered fluids are considered dense non-aqueous phase liquids (DNAPLs) because they are immiscible with water and have a specific gravity greater than one. The HFE-7100 fluid is safer and environmentally friendlier than common DNAPL contaminants such as tetrachloroethylene (PCE) or trichloroethylene (TCE); thus, it is an ideal substitute DNAPL for laboratory groundwater contamination research. Three sets of solute transport experiments were conducted. The first set of experiments was conducted in a glass-bead-packed column using dissolved HFE-7100 as the solute. The second set of experiments was conducted in a sand-packed column using dissolved HFE-7100 as the solute. The third set of experiments was conducted in a sand-packed column using dissolved PCE as the solute. The dissolved HFE-7100 column breakthrough concentrations were compared with dissolved PCE breakthrough concentrations. Results show that the one-dimensional solute transport equation was successful in describing the transport behavior of dissolved HFE-7100. This study demonstrates that the HFE-7100 fluid can be used as a safer substitute DNAPL for groundwater contaminant dissolution and transport research.     

Vertical variability of PCE sorption in the lacustrine clays of Mexico City

 A surficial clay aquitard extends through the urban area where Mexico City is located. It has been assumed to function as a protective layer to the underlying aquifer that provides 42 m³/s out of 63 m³/s of water used by 18 million inhabitants. To provide such protection, the aquitard must be impermeable to water flow and, ideally, have a significant capacity to sorb contaminants. The latter aspect was addressed, studying the vertical variability of sorption of perchloroethylene (PCE), a widely used organic compound considered to pose health risks in groundwater. Batch sorption tests were used and the clay-rich strata in the depth interval from 8 to 75 m were studied. The results suggest that sorption depends mainly on the fraction of organic carbon (f_oc) present in the clayey materials. The sorption data were fit to the linear and Freundlich models; many strata could be fit well by either model, while some strata were distinctly non-linear. The linear isotherms showed a mean value of 32.8 ml/g, and the Freundlich isotherm 96.6 ml/g, confirming that the clay-rich media have significant sorption capacity for PCE. From the environmental perspective the clay-rich materials are serving as protection to the groundwater system. Received: 2 November 1998 · Accepted: 15 February 1999     

Influence of surface/groundwater interaction on pollution by pesticides in farmlands of the Fucino Plain,central Italy

This paper analyses flow and transport of pesticides from the unsaturated zone to groundwater so as to predict concentration of those contaminants in the Fucino Plain’s groundwater, by site investigations and numerical simulations. Pesticides were detected in surface water (peaks of 13 μg L⁻¹) and groundwater (peaks of 0.37 μg L⁻¹). Modelling tools made it possible to identify that pattern of precipitation, organic matter content, and root thickness are the key factors involved in vertical seepage of pesticides. Numerical simulations indicated that a significant fraction of contaminants is leached from the most surficial soil layers through runoff, while only a secondary fraction is mobilised towards groundwater. Likelihood of contaminating deep groundwater is fairly low, whereas surface waters show higher susceptibility. Results of the proposed conceptual hydrogeological model show that pesticides are more likely to be entrained by mixing of stream water with shallow groundwater in periods of high water exploitation from shallow wells.     

In situ push-pull tests for the determination of TCE degradation and permanganate consumption rates

A simple, single-well push-pull test was conducted at a TCE-contaminated site to estimate the site-specific TCE degradation and permanganate (MnO₄⁻) consumption rate. Known quantities of a conservative tracer (Br⁻) and permanganate were rapidly injected into a saturated aquifer then periodically sampled during extraction from the same well. Concentrations of Br⁻, TCE, and MnO₄⁻ were measured; breakthrough curves (BTCs) for all species of solute were determined. Data analysis of BTCs for bromide and TCE showed that the first-order rate constant of TCE degradation by MnO₄⁻ is 1.67 ± 0.152 h⁻¹. Further, the in situ MnO₄⁻ demand rate by TCE and aquifer materials is estimated to be 0.54 ± 0.371 h⁻¹. This study demonstrates that in situ push-pull tests are useful and economical tools for field investigations to determine contaminant reaction and oxidant consumption rates, which may then be used to optimize groundwater remediation designs.