Contaminant Flux Research Articles

Contaminant transport in a largely-deformed aquitard affected by delayed drainage

This paper employed a one-dimensional large-deformation model in consideration of the coupling of mechanical consolidation and solute transport, to study the transport of contaminants in a largely-deformed aquitard. An analytical solution has been derived to describe the drawdown variation in a largely-deformed aquitard which is subjected to abrupt hydraulic head decline in adjacent confined aquifers. The pore water flux and void ratio variation were obtained on the basis of the analytical solution. The equation for transient contaminant flux was solved by the finite difference method. A hypothetical case study was done to explore the effect of consolidation on the contaminant transport in a largely-deformed aquitard. The transit time of contaminant transport in the aquitard is mainly determined by the hydraulic conductivity, thickness, partitioning coefficient, void ratio and effective diffusion coefficients of aquitard, as well as the drawdown in the adjacent confined aquifer. The impact of delayed drainage on the contaminant transport in the largely-deformed aquitard is mainly controlled by two factors: the transient water flow and the decrease of aquitard thickness, in the process of aquitard consolidation. The former increases the breakthrough time of contaminant transport in the aquitard, and the latter gives rise to an opposite case with its effect decreasing with increasing contaminant partitioning coefficient for soil particles sorption. A larger deformation, which may be induced by a larger thickness, higher specific storativity of aquitard or a larger drawdown of the adjacent confined aquifer, and a lower hydraulic conductivity of aquitard cause a more significant impact of delayed drainage on the contaminant transport in an aquitard.

Increased organic contaminant residence times in the urban riverbed due to the presence of highly sorbing sediments of the Anthropocene

This study evaluates the hypothesis that increased organic-contaminant sorption and hence residence time in urban riverbed deposits may occur due to the presence of highly sorbing sediment components of anthropogenic origin that contain thermally altered carbonaceous materials (TACM). The hypothesis was confirmed through laboratory-batch studies examining tetrachloroethene (PCE) sorption on riverbed sediment samples from the highly urbanised River Tame headwaters catchment (Birmingham, UK) and the River Tern (Shropshire) providing a nearby rural benchmark. Urban-riverbed sorption was elevated with observed organic-carbon partition coefficient (Koc) values over an order of magnitude greater than rural values. The latter compare to the widely used US EPA geometric-mean Koc and attributed to absorption-dominated partitioning into (recent) natural organic matter. Use of that mean is inappropriate for the urban riverbed, potentially underestimating retardation by an order of magnitude. Organic petrography confirmed the urban riverbed contained a high proportion of TACM-rich organic-matter particles to which significant adsorption might be expected; furthermore, elevated urban Koc values were consistent with TACM-rich sediment literature estimates. The TACM source appears to be riverine transport of particulate matter from historical industrial/urban or coal-mining/processing activities prevalent upstream since the Industrial Revolution. Consequences of the increased urban riverbed residence time on attenuation and bioavailability assumption sensitivities were modelled. Under the standard assumption of sorbed contaminant being non-bioavailable, rapid rates of aqueous-phase biodegradation are necessary for complete attenuation within a riverbed. Conversely, if adsorbed contaminant is bioavailable, then complete attenuation is probable where highly adsorbing (Anthropocene urban) riverbed sediments are present, even for long half-lives. Therefore, Anthropocene-sediment occurrence may significantly influence modern-day contaminant fate within the urban riverbed; the principal implication being greatly increased sorption and residence times may occur in urban riverbeds than conventionally assumed. Further research should focus upon whether adsorbed contaminants have any bioavailability causing increased contaminant flux attenuation and surface-water quality protection.

Impacts of land use/land cover change on stream flow and sediment yield of Gojeb watershed, Omo-Gibe basin, Ethiopia

Land use/land cover (LULC) change is an important landscape process capable of altering the fluxes of water, sediment, contaminants and energy. In this study, Soil and Water Assessment Tool (SWAT) model was used to examine the effects of LULC on the hydrological process of Gojeb Watershed in the Omo-Gibe basin. The performance of the model was evaluated through sensitivity, uncertainty analysis, calibration and validation. The LULC change analyses for three periods (1989, 2000 and 2013) were performed using ERDAS Imagine 2014 with a maximum likelihood classifier. The time series LULC changes results were used for estimation of stream flow and sediment yield. The result has shown that during the study period most parts of the forest land were converted into cultivated land with an increase of cultivated land by 14.97% over the study periods which resulted in an increase of stream flow and sediment yield by 8.6 m3/s and 41.07ton/km2 respectively. The Nash sutcliff efficiency (NSE), coefficient of determination (R2) and percent of bias (PBIAS) were used for evaluating the model performance. The model results have shown a good agreement with the observed values with NSE > 0.75, R2 > 0.78, and PBIAS < 0.5 values. Based on the validated sediment yield results of 2013 land use, high potential source of spatial variability of sediment was found at middle and downstream of the watershed. Hence, for the critical sub-watersheds, the design and development of three best management practices (BMPs) were performed. These best management scenarios include: S1 (filter strip), S2 (terrace/ bund) and S3 (reforestation). Based on these scenarios, the findings have shown a sediment yield reductions by 22.9%, 29.4% and 32.3% with the implementation of S1, S2 and S3 respectively. Therefore, based on the findings, implementing S3 is the best management strategy as compared to other options and hence, such intervention should be encouraged at a wider scale for efficient sediment reductions of Gojeb watershed.

Slow-release permanganate versus unactivated persulfate for long-term in situ chemical oxidation of 1,4-dioxane and chlorinated solvents

The objective of this research was to evaluate slow-release permanganate and unactivated persulfate for in situ treatment of dioxane and associated chlorinated solvents. Laboratory batch studies with unactivated persulfate in deionized water or in soil and groundwater demonstrated dioxane removal with pseudo second-order rate constants ranging from 10−5 to 10−3 M−1 s−1. Flow-through column studies demonstrated over 99% dioxane removal with slow-release unactivated persulfate but not with slow-release permanganate. The slow-release permanganate cylinders became coated with a rind that limited oxidant mass transfer and dioxane oxidation. A field study was conducted with slow-release persulfate cylinders transverse to groundwater flow. Over 99% removal of dioxane and chlorinated solvents was observed 2.5 m downgradient of the cylinders. Density-driven flow associated with the released persulfate was observed and was attributed to a low horizontal hydraulic gradient. Thus, most of the contaminant and persulfate flux was thought to be isolated to a deep aquifer zone that was bound by an underlying silt aquitard. Contaminant reductions were also observed in shallow groundwater samples, albeit at a lesser extent. The longevity of the persulfate oxidant cylinders was estimated to be 6–12 months. Results of this study demonstrate that dioxane and co-mingled chlorinated solvents can be effectively treated using slow-release persulfate cylinders. Careful consideration to cylinder placement during the design phase is essential to prevent the contaminant plume from bypassing and not coming into contact with the released oxidant.

Evaluation of contaminant retention in the soil of sustainable drainage systems: methodological reflections on the determination of sorption isotherms

Abstract Runoff infiltration in Sustainable Drainage Systems enables the interception of a part of urban contaminant fluxes owing to several processes. The soil's ability to retain dissolved pollutants is generally assessed via sorption isotherms obtained from batch studies; however, the experimental points are not always in the same range as runoff concentrations. The present work (i) explores the consequences of modelling runoff–soil interactions from out-of-range equilibrium concentrations and (ii) proposes an improved method to ensure that experimental points fall within the desired range. Uncertainty analysis demonstrates that for a non-linear isotherm, using an extrapolated relationship may introduce significant biases in the ensuing estimations. Therefore, the proposed method consists of anticipating the equilibrium state of batch tests to accurately set the experimental conditions and reach appropriate concentrations. It is successfully applied to the determination of the sorption properties of copper and zinc onto three soils with different electrolyte solutions, as well as those of bisphenol A and three alkylphenols onto one soil. The contrasting affinities between the studied species and the soil materials could be related to their intrinsic properties and the soils' pedological parameters, as well as the presence of salt or dissolved organic ligands which partially inhibited metal sorption.

Sn-doped and porogen-modified TiO2 photocatalyst for solar light elimination of sulfure diethyle as a model for chemical warfare agent

In the context of the increase in chemical threat due to warfare agents, the development of efficient methods for destruction of Chemical Warfare Agents are of first importance both for civilian and military purposes. Here, we demonstrate that optimized Sn-TiO2 doped and PEG-modified photocatalysts allow increased and high performances under UVA and solar-light irradiations leading to total elimination of highly contaminated environments containing Diethylsulfide (DES) used as a model molecule mimicking Yperite (Mustard Gas). It has been shown that Sn doping induces significant modifications on the structural, morphological, surface, electronic and optical properties of TiO2. For example, the addition of 1% Sn increases significantly the surface area from 30 to 80 m2/g and decreases the particle size, while Sn-doping results in a reduction of the anatase band-gap from 3.2 to 2.95 eV. Total DES elimination could be reached for 90 and 120 min under continuous contaminant flux under UVA and solar light activation, respectively accompanied with limited deactivation phenomenon. Correlations between the resulting physico-chemical properties of the doped and PEG-modified materials and the photocatalytic activities were carried out. The results open up extremely promising way for the decontamination of highly contaminated environments containing real warfare agents under UVA but also under solar light illumination.

Neutron Contamination Unexpected Dose in Varian LINAC

Introduction: Neutron contamination is very harmful for the patients receiving radiation therapy due to the high relative biological effect of neutrons. Medical linear accelerators head are made up of materials with high atomic numbers, and High-energy photons interaction with heavy materials can produce neutron contamination. In this study, neutron equivalent dose, neutron spectrum, and the effect of field size on neutron contamination dose is calculated by the FLUKA Monte Carlo code system. Materials and Methods: In this study, all parts of the Varian Clinac 2100 accelerator head’s elements, such as target, flattening filter, jaws, MLC, shields, primary and secondary collimators are simulated by the FLUKA code system. LINAC performance energy was considered 18MV and a water phantom with dimension of 140×140×35 cm3 was simulated for neutron dose calculation. To investigate the effect of field size on neutron contamination, larger treatment fields such as 10×10, 20×20, 30×30 and 40×40 was formed by the jaws, and the smaller ones like 1×1, 3×3 and 5×5 was formed by the MLC. Results: The results show that the Neutron contamination dose can be up to 2.54 mSv per 1Gy photon dose at isocenter. Neutron to photon dose ratio is 0.004 at isocenter and more than 8.6 in out-of-field areas. Furthermore, the neutron dose changes at the surface of the water phantom is only 15% that show the protective elements of the LINAC head are not able to control the neutron contamination flux. The most of neutron dose distribution is at the surface of the water phantom, So it can increases the secondary cancer risk in patients, including skin cancer. The effect of treatment field size on neutron contamination shows that the larger field size formed by the jaws contribute to higher in-field neutron dose, and using of smaller field size formed by the MLC, cause the lower in-field neutron dose. Conclusion: The neutron contamination dose can not be neglected, and the effective parameters in neutron contamination exasperation should be thoroughly investigated. Furthermore, the harmful effects of the neutron contamination should be minimized by the appropriate protective shields

Rare Earth Element fluxes over 15 years into a major European Estuary (Garonne-Gironde, SW France): Hospital effluents as a source of increasing gadolinium anomalies

New and rapidly developing technologies imply the emission of emerging potentially toxic contaminants such as Rare Earth Elements (REEs). Yet, the lithology-derived quantities and anthropogenic contributions, especially from urban areas, to annual REE fluxes into fluvial-estuarine systems remain widely unknown. The Garonne River drains water from ~20% of the French land surface hosting about 5,200,000 inhabitants and two large cities. Based on long-term monitoring (2003–2017) of water discharges and dissolved REEs concentrations at the outlet of the Garonne Watershed upstream from Bordeaux, this study aims at assessing REE anomalies and evaluating temporal evolution of annual dissolved REE fluxes into the Gironde Estuary. Additionally, potential urban sources (e.g. domestic, medical) in the urban area of Bordeaux (1,190,000 inhab.) were analyzed to evaluate respective signatures and contributions. Gadolinium (Gd) showed clear anomalies in all samples, with annual average anthropogenic concentrations ranging from 1.8 to 7.2 ng·L−1 (0.011 to 0.046 nmol·L−1) in the Garonne River. If variations in annual Gd fluxes depend on hydrology, anthropogenic Gd fluxes have shown an overall increasing trend from 32 kg·year−1 (204 mol·year−1) in 2003 to 75 kg·year−1 (475 mol·year−1) in 2017. Sewer waters from the third largest hospital complex of France, the hospital group Pellegrin, contributed 25% to the incoming daily Gd flux into Bordeaux major Waste Water Treatment Plant (WWTP), owed to Gd use as contrast agent for Magnetic Resonance Imaging (MRI). Due to weak removal efficiency in the WWTP, the Bordeaux Metropole significantly contributes (>27 kg·year−1; 172 mol·year−1) to Gd fluxes in the Gironde Estuary. The temporal evolution of anthropogenic Gd fluxes in the Garonne River may be related with the growing regional population and the increasing number of MRI instruments, highlighting the importance of new high-tech applications in urban areas on contaminant fluxes and their potential harmful effects in fluvial-estuarine systems in the future.

The aqueous solubility of common organic groundwater contaminants as a function of temperature between 5 and 70 °C

High-temperature thermal energy storage in shallow aquifers can potentially increase ambient groundwater temperatures up to 70 °C or even more. Since an increase in temperature is expected to influence contaminant mass flux into groundwater monitoring the spreading of organic contaminants located in the subsurface is crucial. In numerous former studies, the NAPL solubility, one major parameter controlling mass flux on field scale, was measured at temperatures up to 70 °C for a broad spectrum of organic substances. However, quantitative calculations of solubilities as a function of temperature considering a compiled database are largely missing. Aiming to examine the reliability of existing solubility-temperature relationships, to describe them functionally and further to identify knowledge gaps, previously published data on solubilities of 42 different organic groundwater contaminants were evaluated in this study. By using a common temperature regression function, the calculated solubility curves from compiled solubility data for 5–70 °C show relative changes between a few percent (CHCs and BTEX) and up to 2000% (PAHs). As published temperature-dependent solubilities for chlorinated ethylenes are contradictory in parts, solubilities of tetrachloroethylene, trichloroethylene, 1,2-cis-dichloroethylene and 1,2-trans-dichloroethylene were additionally investigated in more detail using batch experiments between 5 and 70 °C. The results show distinctive solubility minima at medium temperatures (20–40 °C) with concentrations decreasing from 5 °C to the minimum by 10–20%. The measured and calculated temperature-dependent solubilities enable a more reliable assessment of thermal energy storage at contaminated sites, of existing thermal remediation approaches and of combinations of underground heat storage with groundwater remediation.

The horizontal reactive media treatment well (HRX Well®) for passive in situ remediation: Design, implementation, and sustainability considerations

AbstractA new in situ remediation concept termed a Horizontal Reactive Media Treatment Well (HRX Well®) is presented that utilizes a horizontal well filled with reactive media to passively treat contaminated groundwater in situ. The approach involves the use of a large‐diameter directionally drilled horizontal well filled with solid reactive media installed parallel to the direction of groundwater flow. The engineered contrast in hydraulic conductivity between the high in‐well reactive media and the ambient aquifer hydraulic conductivity results in the passive capture, treatment, and discharge back to the aquifer of proportionally large volumes of groundwater. Capture and treatment widths of up to tens of feet can be achieved for many aquifer settings, and reductions in downgradient concentrations and contaminant mass flux are nearly immediate. Many different types of solid‐phase reactive treatment media are already available (zero valent iron, granular activated carbon, biodegradable particulate organic matter, slow‐release oxidants, ion exchange resins, zeolite, apatite, etc.). Therefore, this concept could be used to address a wide range of contaminants. Laboratory and pilot‐scale test results and numerical flow and transport model simulations are presented that validate the concept. The HRX Well can access contaminants not accessible by conventional vertical drilling and requires no aboveground treatment or footprint and requires limited ongoing maintenance. A focused feasibility evaluation and alternatives analysis highlights the potential cost and sustainability advantages of the HRX Well compared to groundwater extraction and treatment systems or funnel and gate permeable reactive barrier technologies for long‐term plume treatment. This paper also presents considerations for design and implementation for a planned upcoming field installation.

The Negro River (Ancash-Peru): A unique case of water pollution, three environmental scenarios and an unresolved issue

This paper is focused on the hydrogeochemical characterization of the Negro River along its course, as well as in the proposal of a functioning model for the contamination processes in order to establish potential cause-effect relationships between water quality, geology (ARD), mining activities (AMD) and the tectonic framework as transmission vector of acidity, metals and sulphates. The scenario shows a heavily-contaminated river compared to the unaffected regional background. By graphical and statistical treatments of physico-chemical data of Negro River and the unaffected values of regional background and other AMD/ARD representative rivers' it is possible to conclude that Antamina Mine, is not the cause of the Negro River contamination, without the need of isotopic tracers, but just through the inexistent concentrations of Cu, Bi and Mo found in the waters. In the proposed contamination model, climatic factors (glacial retreat) activate geological (ARD) processes. The tectonic scenario (faults) intervenes as a transport medium of the contamination flux from the sulphide oxidation surface in upper altitudes until the spring in lower altitudes. At the end, it is concluded that this contamination comes from the recent glacial retreat in areas near the Cordillera Blanca that has left massive amounts of sulphide materials exposed to weathering conditions, oxidizing naturally (ARD processes) and finally contributing to the contamination of the Negro River through faults. In this case, we would face an ARD process in the strict sense, which is the direct oxidation of sulphides outcropping in the upper part of the mountain with the generation of sulphates, the release of hydrogen ions and the consequent generation of acid and the dissolution of the metals. This ARD process would come from the glacial retreat, which, through the faults, transports contaminated water until the spring.

Numerical analyses and experimental validations on transport and control of carbon in Czochralski silicon crystal growth

Czochralski silicon (CZ-Si) crystal growth is invariably accompanied by the generation and transport of impurities, such as carbon (C) and oxygen (O), from chemical reactions in the high-temperature range. Reduction of C contamination in the grown crystal is required for the production of a high-quality Si wafer. Therefore, we systematically performed the transient global simulations and in situ measurements of CO concentrations in argon (Ar) gas domain. Parametric studies on the furnace pressure, flow rate of argon gas, and gap width were conducted for the transport of CO and accumulation of C during the melting process of Si feedstock. SiO etching reactions on graphite and SiC coating are the major sources of CO production. The contact reaction between the crucible and susceptor is an additional source of CO and SiO. Furthermore, the control mechanisms of gas flow on the incorporation of the generated CO and the accumulation of C in Si feedstock were clarified by comparison of different parameter settings. According to the mechanisms of C transport, the final C content of the melting process depends on the contamination flux at the gas/melt interface which could be controlled by the Péclet number of the gas flow and diffusion distance of CO.

Impact of young stellar components on quiescent galaxies: deconstructing cosmic chronometers

Context. Cosmic chronometers may be used to measure the age difference between passively evolving galaxy populations to calculate the Hubble parameter H(z) as a function of redshift z. The age estimator emerges from the relationship between the amplitude of the rest frame Balmer break at 4000 Å and the age of a galaxy, assuming that there is one single stellar population within each galaxy. Aims. First, we analyze the effect on the age estimates from the possible contamination (&lt; 2.4% of the stellar mass in our high-redshift sample) of a young component of ≲ 100 Myr embedded within the predominantly old population of the quiescent galaxy. Recent literature has shown this combination to be present in very massive passively evolving galaxies. Second, we evaluate how the available data compare with the predictions of nine different cosmological models. Methods. For the first task, we calculated the average flux contamination due to a young component in the Balmer break from the data of 20 galaxies at z &gt; 2 that included photometry from the far-ultraviolet to near-infrared at rest. For the second task, we compared the data with the predictions of each model, using a new approach of distinguishing between systematic and statistical errors. In previous work with cosmic chronometers, these have simply been added in quadrature. We also evaluated the effects of contamination by a young stellar component. Results. The ages inferred using cosmic chronometers represent a galaxy-wide average rather than a characteristic of the oldest population alone. The average contribution from the young component to the rest luminosity at 4000 Å may constitute a third of the luminosity in some samples, which means that this is far from negligible. This ratio is significantly dependent on stellar mass, proportional to M−07. Consequently, the measurements of the absolute value of the age or the differential age between different redshifts are at least partially incorrect and make the calculation of H(z) very inaccurate. Some cosmological models, such as the Einstein-de Sitter model or quasi-steady state cosmology, which are rejected under the assumption of a purely old population, can be made compatible with the predicted ages of the Universe as a function of redshift if we take this contamination into account. However, the static Universe models are rejected by these H(z) measurements, even when this contamination is taken into account.

Chemostatic behaviour of major ions and contaminants in a semiarid spring and stream system near Los Alamos, NM, USA

AbstractRecent studies have focused on the relationship between solute concentrations and discharge in streams, demonstrating that concentrations can vary little relative to changes in discharge (chemostatic behaviour). Chemostatic behaviour is dependent on catchment characteristics (e.g., lithology, geomorphology, and vegetation) and chemical characteristics of the solute (e.g., availability, reactivity, and mobility). An investigation of 3 springs and a stream near Los Alamos, NM, reveals that springs can behave in a chemostatic fashion as stream systems tend to do. Another unique finding of this study is that the anthropogenic contaminants barium and the high explosive RDX (hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine) can also behave chemostatically. The chemostatic behaviour of a contaminant has important implications for the residence time of contaminants in a system as well as having a major control on contaminant flux and mass transport. Redox (reduction–oxidation) and biogeochemically sensitive analytes (e.g., Fe, SO4, and NO3) display a combination of chemostatic and chemodynamic behaviour, showing the influence of temporally variable conditions on stream and spring chemistries.

Field demonstration of foam injection to confine a chlorinated solvent source zone

A novel approach using foam to manage hazardous waste was successfully demonstrated under active site conditions. The purpose of the foam was to divert groundwater flow, that would normally enter the source zone area, to reduce dissolved contaminant release to the aquifer. During the demonstration, foam was pre generated and directly injected surrounding the chlorinated solvent source zone. Despite the constraints related to the industrial activities and non-optimal position of the injection points, the applicability and effectiveness of the approach have been highlighted using multiple metrics. A combination of measurements and modelling allowed definition of the foam extent surrounding each injection point, and this appears to be the critical metric to define the success of the foam injection approach. Information on the transport of chlorinated solvents in groundwater showed a decrease of contaminant flux by a factor of 4.4 downstream of the confined area. The effective permeability reduction was maintained over a period of three months. The successful containment provides evidence for consideration of the use of foam to improve traditional flushing techniques, by increasing the targeting of contaminants by remedial agents.

Empirical Modeling of the Redshift Evolution of the [ N II ] /Hα Ratio for Galaxy Redshift Surveys

Abstract We present an empirical parameterization of the [N ii]/Hα flux ratio as a function of stellar mass and redshift valid at 0 &lt; z &lt; 2.7 and 8.5 &lt; log ( M / M ⊙ ) &lt; 11.0 . This description can (i) easily be applied to simulations for modeling [N ii]λ6584 line emission, (ii) deblend [N ii] and Hα in current low-resolution grism and narrow-band observations to derive intrinsic Hα fluxes, and (iii) reliably forecast the number counts of Hα emission-line galaxies for future surveys, such as those planned for Euclid and the Wide Field Infrared Survey Telescope (WFIRST). Our model combines the evolution of the locus on the Baldwin, Phillips &amp; Terlevich (BPT) diagram measured in spectroscopic data out to z ∼ 2.5 with the strong dependence of [N ii]/Hα on stellar mass and [O iii]/Hβ observed in local galaxy samples. We find large variations in the [N ii]/Hα flux ratio at a fixed redshift due to its dependency on stellar mass; hence, the assumption of a constant [N ii] flux contamination fraction can lead to a significant under- or overestimate of Hα luminosities. Specifically, measurements of the intrinsic Hα luminosity function derived from current low-resolution grism spectroscopy assuming a constant 29% contamination of [N ii] can be overestimated by factors of ∼8 at log ( L ) &gt; 43.0 for galaxies at redshifts z ∼ 1.5. This has implications for the prediction of Hα emitters for Euclid and WFIRST. We also study the impact of blended Hα and [N ii] on the accuracy of measured spectroscopic redshifts.

Condutividade hidráulica da formação rio claro a partir de ensaios granulométricos

A condutividade hidráulica (K) é um fator determinante em relação às características de um aquífero, permitindo (ou não) a transmissão de fluidos e influenciando, assim, na velocidade de fluxo em diferentes camadas geológicas. Dessa forma, constitui fator-chave na compreensão do fluxo de água e de contaminantes. Neste trabalho, a granulometria do Aquífero Rio Claro é utilizada para mais bem entender a distribuição de suas propriedades hidráulicas, em particular da condutividade hidráulica, obtida pelos métodos de Hazen, Shepherd e Krumbein &amp; Monk. Os resultados foram comparados a testes de slug (Hvorslev) efetuados em poços existentes que exploram o aquífero. Verificou-se que a Formação Rio Claro apresenta grande variabilidade na relação areia/argila, sendo constituída por areias limpas, areias argilosas, até lentes silto-argilosas. A variabilidade granulométrica é grande próximo ao contato com a Formação Corumbataí, com mudanças marcantes nas quantidades de argila, silte e areia fina, além de contribuição expressiva de grânulos. Essa variabilidade litológica influencia na distribuição da condutividade hidráulica, cujos valores variam entre 8 x 10-3 cm/s e 2 x 10-3 cm/s em arenitos e são cerca de duas a três ordens de grandeza menores em lamitos. Os arenitos da Formação Rio Claro, apesar de espessuras restritas, possuem alta condutividade hidráulica, constituindo um ótimo reservatório de águas subterrâneas; porém, a presença de espessas lentes de argila pode atuar como barreiras hidráulicas. Desse modo, esta unidade aquífera tem o seu potencial de explotação subordinado à presença e continuidade espacial de camadas de arenitos com pouca matriz.

Decoupled Advection-Dispersion Method for Determining Wall Thickness of Slurry Trench Cutoff Walls

Low-permeability slurry trench cutoff walls are commonly constructed as barriers for containment of subsurface point-source pollution or as part of seepage-control systems on contaminated sites. A method to estimate wall thickness in slurry wall design is proposed based on decoupling the advective and dispersive components of contaminant fluxes through the wall. The relative error of the result obtained by the proposed method compared with that by an analytical solution was found to increase as the ratio of the specified breakthrough exit concentration (c*) to the source concentration (c0) increased. For c*/c0 of less than 0.1, which covers common practical situations, the relative error was not greater than 4% and was always conservative, indicating that the proposed method provides sufficient accuracy for design. For a given breakthrough criterion (i.e., c*/c0), the relative error was low for the scenarios having either a low or high column Peclet number, where either dispersion or advection dominates the contaminant migration, respectively, and the relative error was high for the scenario having an intermediate column Peclet number, in which case the coupling effect of advective and dispersive migrations is relatively high.

Monitoring well utility in a heterogeneous DNAPL source zone area: Insights from proximal multilevel sampler wells and sampling capture-zone modelling

Groundwater-quality assessment at contaminated sites often involves the use of short-screen (1.5 to 3 m) monitoring wells. However, even over these intervals considerable variation may occur in contaminant concentrations in groundwater adjacent to the well screen. This is especially true in heterogeneous dense non-aqueous phase liquid (DNAPL) source zones, where cm-scale contamination variability may call into question the effectiveness of monitoring wells to deliver representative data. The utility of monitoring wells in such settings is evaluated by reference to high-resolution multilevel sampler (MLS) wells located proximally to short-screen wells, together with sampling capture-zone modelling to explore controls upon well sample provenance and sensitivity to monitoring protocols. Field data are analysed from the highly instrumented SABRE research site that contained an old trichloroethene source zone within a shallow alluvial aquifer at a UK industrial facility. With increased purging, monitoring-well samples tend to a flow-weighted average concentration but may exhibit sensitivity to the implemented protocol and degree of purging. Formation heterogeneity adjacent to the well-screen particularly, alongside pump-intake position and water level, influence this sensitivity. Purging of low volumes is vulnerable to poor reproducibility arising from concentration variability predicted over the initial 1 to 2 screen volumes purged. Marked heterogeneity may also result in limited long-term sample concentration stabilization. Development of bespoke monitoring protocols, that consider screen volumes purged, alongside water-quality indicator parameter stabilization, is recommended to validate and reduce uncertainty when interpreting monitoring-well data within source zone areas. Generalised recommendations on monitoring well based protocols are also developed. A key monitoring well utility is their proportionately greater sample draw from permeable horizons constituting a significant contaminant flux pathway and hence representative fraction of source mass flux. Acquisition of complementary, high-resolution, site monitoring data, however, vitally underpins optimal interpretation of monitoring-well datasets and appropriate advancement of a site conceptual model and remedial implementation.

Examination of effects of acid and metals on benthic algae in streams using chemical diffusing substrata

Acid mine drainage environments have multiple stressors, making it difficult to isolate effects of individual contaminants on benthic algal communities in situ in order to develop more specific indicators. Additionally, responses to AMD contaminants may be dependent on the abiotic and biotic conditions within the affected stream. We examined the efficacy of using chemical diffusing substrata (CDS) to determine how acid and metals interact to affect benthic algae in 2 streams with different trophy. CDS containing Mn, Fe, Al, acid, Mn + acid, Fe + acid, and Al + acid were deployed in a flow-through system to determine flux from the liquid media. Flux values for metals ranged from 1.0 to 18.1 µg/cm2/d for 5 days, and 0.5–7.4 µg/cm2/d for 21 days. Flux values for H+ were 467 (5 day) and 212 (21 day) µg/cm2/d. Corresponding water column concentrations that would produce the same flux as CDS metal treatments were estimated to be in the low to mid toxicity range for Al, low range for Fe, and nontoxic range for Mn. Acid treatments were estimated to be equivalent to a stream pH of 2.1–2.5. Five replicates of each CDS treatment, and a water control, were placed into a high-alkalinity, eutrophic stream (Wolf Creek) for 5 days and a low-alkalinity, oligotrophic stream (Silver Creek) for 21 days, respectively. In Silver Creek, all treatments except circumneutral Fe and circumneutral Mn had significantly lower algal cell densities than control treatments. Cell densities were significantly lower on Al + acid and Mn + acid than on the acid-only treatment, indicating synergistic effects. Cell densities were not significantly different among treatments in Wolf Creek. Higher alkalinity, nutrients, and productivity in this stream likely increased neutralization and detoxification processes within the biofilm. However, significantly lower chlorophyll a and higher phaeophytin concentrations of communities on treatments containing acid indicated sublethal effects. Significantly more acid tolerant taxa occurred on treatments with acid in Silver Creek, and significantly more metal-tolerant taxa occurred on metal treatments in Wolf Creek. However, treatment effects on taxonomic composition appeared to be constrained by the species pool within each stream, as these indicator taxa made up less than 4% of any community. Although equating CDS flux of contaminants to water column concentrations requires assumptions, and flux decreases over the exposure period, this study indicates that in situ experiments using CDS can separate out effects of individual toxic components on benthic algae, and determine context-dependent responses.