Contaminant Mass Research Articles

Intra-workday fluctuations of airborne contaminant concentration and the time-weighted average

Air contaminant concentrations vary between and within workdays and are often measured across a workday by passing a known air volume through a collection device. Laboratory analysis determines the contaminant mass trapped, providing a time-weighted average air concentration (C TWA). This approach was driven by the best technologies available as exposure measurement processes developed and accuracy and measurement precision were sought. However, all integrated concentration•time (C•t) values determining C TWA are equally weighted in assessing exposures, intra-workday concentration variability is unknown, and results are available days later. At times inappropriately, an occupational exposure limit (OEL) expressed as a C TWA also requires equal weighting of all C•t values across an exposure period following concepts of Haber’s law. Continuous monitoring (real-time detection) informs both the C TWA and the variability of C during sampling, which are needed for stressors where a ceiling or peak OEL exists, for dangerous exposures to permanent gas-type contaminants, and for immediately dangerous to life or health (IDLH) conditions. Selective and accurate real-time detection instruments are not available for all air contaminants, but exposure magnitude information may be provided. The large amounts of data from continuous monitoring and the ability to correlate exposure maxima to specific tasks are also important. An exposure assessment role exists for selective and nonselective monitors, and in some cases, similar accuracy and precision are provided compared to laboratory analyses. Continuous monitoring may be of value when the alternative is the collection of a few C TWA data points. Digitized personal monitor data can support the automation of some exposure control decisions or allow such decisions to be made by people in near real-time. The emerging Internet of Things (IoT) offers opportunities to integrate digital exposure data into decision-making to increase both efficiency and safety. The perceived and real uncertainty associated with real-time exposure assessments may be lessened with work to rule out the presence of know interferents and confirm the presence of target analytes.

PFOS Mass Flux Reduction/Mass Removal: Impacts of a Lower-Permeability Sand Lens within Otherwise Homogeneous Systems.

Perfluorooctane sulfonic acid (PFOS) is one of the most common per- and polyfluoroalkyl substances (PFAS) and is a significant risk driver for these emerging contaminants of concern. A series of two-dimensional flow cell experiments was conducted to investigate the impact of flow field heterogeneity on the transport, attenuation, and mass removal of PFOS. A simplified model heterogeneous system was employed consisting of a lower-permeability fine sand lens placed within a higher-permeability coarse sand matrix. Three nonreactive tracers with different aqueous diffusion coefficients, sodium chloride, pentafluorobenzoic acid, and β-cyclodextrin, were used to characterize the influence of diffusive mass transfer on transport and for comparison to PFOS results. The results confirm that the attenuation and subsequent mass removal of the nonreactive tracers and PFOS were influenced by mass transfer between the hydraulically less accessible zone and the coarser matrix (i.e., back diffusion). A mathematical model was used to simulate flow and transport, with the values for all input parameters determined independently. The model predictions provided good matches to the measured breakthrough curves, as well as to plots of reductions in mass flux as a function of mass removed. These results reveal the importance of molecular diffusion and pore water velocity variability even for systems with relatively minor hydraulic conductivity heterogeneity. The impacts of the diffusive mass transfer limitation were quantified using an empirical function relating reductions in contaminant mass flux (MFR) to mass removal (MR). Multi-step regression was used to quantify the nonlinear, multi-stage MFR/MR behavior observed for the heterogeneous experiments. The MFR/MR function adequately reproduced the measured data, which suggests that the MFR/MR approach can be used to evaluate PFOS removal from heterogeneous media.

Determining effective diffusion coefficients of chlorohydrocarbons in natural clays: Unique results from highly resolved controlled release field experiments

This study aims to precisely determine the effective diffusion coefficients of chlorohydrocarbons in low permeable units under in-situ field conditions. To this end, two controlled release field experiments using TCE and PCE as dense non-aqueous phase liquids (DNAPLs) were conducted in two natural clayey deposits. Several months to years after the controlled DNAPL release, highly resolved concentration profiles were determined for the chlorohydrocarbons that had diffused into the clayey deposits. Effective diffusion coefficients for TCE and PCE were then determined by calibrating a 3D numerical and 1D analytical model, respectively, to the measured high-resolution concentration profiles. The simulations revealed that the effective diffusion coefficients vary by as much as a factor of four within the same low permeability unit being consistent with observed small-scale heterogeneities. The determined chlorohydrocarbon effective diffusion coefficients were further used to determine the equivalent thickness of DNAPL that would completely dissolve in an idealized, parallel-plate fracture by diffusion transport into clayey deposits for the time periods of the controlled release field experiments. The equivalent TCE and PCE DNAPL film thicknesses ranged between 36 and 581 μm, respectively, comparable and exceeding fracture apertures measured in naturally fractured clay rich deposits. Hence, films of DNAPL initially contained within fractures in clay-rich deposits can completely dissolve away within a few months to a few years due to diffusion. This stored contaminant mass poses a risk to adjacent aquifers if it is re-released due to diffusion out of the matrix after source depletion or remediation.

Contaminant Mass–Removal Behavior during Chemical Oxidation in a Three-Dimensional, Fractured Sandstone Network Experiment

Contaminant Mass–Removal Behavior during Chemical Oxidation in a Three-Dimensional, Fractured Sandstone Network Experiment

Enhanced semi-volatile DNAPL accessibility at sub-boiling temperature during electrical resistance heating in heterogeneous porous media

Successful remediation of semi-volatile contaminants using electrical resistance heating (ERH) coupled technologies requires a deep understanding of contaminant migration and accessibility, especially with stratigraphic heterogeneity and dense nonaqueous phase liquid (DNAPL) occurrence. Here, we chose nitrobenzene (NB) as a model contaminant of semi-volatile DNAPL and uniquely demonstrated that temperature variation during ERH could induce NB DNAPL migration out of the low permeability zone (LPZ) even below water boiling temperature. When heating the system using alternating current (AC) of 140 V to a temperature range of 50–79 °C, obvious DNAPL migration was visually observed. The upward migration of DNAPL would considerably increase the mass of accessible contaminant by other remediation measures. The downstream cumulative NB mass of 1092 mg in 140 V system raised 56-folds compared to that of 19 mg in the control experiment with only groundwater flow. This migration was mainly attributed to a complex natural convection caused by temperature gradient. Comparing with traditional AC heating, ERH powered by pulsed direct current (PDC-ERH) showed a higher and more uneven heating pattern, resulting in a stronger convection at the same voltage that enhanced the DNAPL migration out of LPZ. These results revealed the importance of natural convection in the ERH process, which could be further optimized to improve the energy efficiency of remediation.

Comparison of Radiocarbon‐ and Background Location‐Corrections on Soil‐Gas CO2 Flux‐Based NSZD Rate Measurements at Petroleum Impacted Sites

AbstractThe measurement of contaminant natural source zone depletion (NSZD) rates has become an important tool to manage petroleum contaminated sites. Most NSZD rate measurement methods rely on a balance on the biodegradation by‐products (either carbon or heat). Carbon balance‐based methods stoichiometrically convert measured soil‐gas CO2 flux related to contaminant degradation to equivalent contaminant mass loses. CO2 flux‐based methods require separating the fraction of the total CO2 flux produced by NSZD from the fraction of CO2 flux produced by natural soil processes (due to modern carbon turnover). Two method corrections are available to distinguish the nature of the measured CO2 flux: (1) The background location correction subtracts a measurement from an unimpacted location from measurements collected at impacted locations, and (2) a location‐specific radiocarbon (14C) analysis that differentiates the modern and old carbon fractions of each total CO2 measurement. This work evaluates both correction approaches using 36 measurements at impacted locations from five sites. The 14C‐corrected data shows that the magnitude and variability of the modern carbon‐related fraction (noise) and the NSZD related fossil fuel signal are similar, suggesting a location‐specific correction is more valid. Only one sparsely vegetated arid site showed close agreement between both corrections (14C‐ and background location). These results highlight the strong effect of the correction method used on NSZD rate measurements, and the importance of considering data quality on the subsequent data management process.

Predicting first-order evaporation rate constant alpha (α) from small spills of organic solvents in a controlled environment

Exposures to vapors generated by small spills of organic solvents are common in the occupational hygiene practice. In these scenarios, contaminant mass release is exponentially decreasing, driven by an evaporation rate constant alpha (α). Knowing α is fundamental for adequately modeling peak concentrations and/or short-term exposures that occur and for achieving efficient occupational risk analysis and management. The purpose of this study was to measure alpha experimentally using a gravimetric approach in a controlled environment during solvent evaporation tests designed to simulate small spills of solvents. The effects of several factors on α were evaluated. Equations based on regression models derived from the experimental data were proposed for predicting α. Predictions were externally validated against experimental data. A total of 183 tests was performed. Data analyses found that alpha (α) values increased with vapor pressure, spill surface area-to-spill volume ratio, and air speed across the spill. Larger α were associated with petri dish containers compared to watch glasses. Three regression models were created for predicting α. They had four variables in common, namely vapor pressure, molecular weight, air speed above the liquid, and surface tension of the liquid. The fifth variable was either spill volume, spill surface area, or spill surface area-to-spill volume ratio. The R2 of the regression models were equal to 0.98. External validation showed mean relative errors of −32.9, −32.0, and −25.5%, respectively, with associated standard deviations of the relative errors of 17.7, 33.3, and 26.0%, respectively, and associated R2 of 0.92, 0.65, and 0.87, respectively. The proposed equations can be used for estimating α in exposure scenarios similar to those evaluated in this study. Moreover, these models constitute a step further in the improvement of knowledge on estimating evaporation rates for small spills of organic solvents.

Concentrations of Tire Additive Chemicals and Tire Road Wear Particles in an Australian Urban Tributary.

Tire road wear particles (TRWPs) are one of the largest sources of microplastics to the urban environment with recent concerns as they also provide a pathway for additive chemicals to leach into the environment. Stormwater is a major source of TRWPs and associated additives to urban surface water, with additives including the antioxidant derivative N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-quinone) demonstrating links to aquatic toxicity at environmentally relevant concentrations. The present study used complementary analysis methods to quantify both TRWPs and a suite of known tire additive chemicals (including 6PPD-quinone) to an urban tributary in Australia during severe storm events. Concentrations of additives increased more than 40 times during storms, with a maximum concentration of 2760 ng/L for ∑15additives, 88 ng/L for 6PPD-quinone, and a similar profile observed in each storm. TRWPs were detected during storm peaks with a maximum concentration between 6.4 and 18 mg/L, and concentrations of TRWPs and all additives were highly correlated. Contaminant mass loads to this catchment were estimated as up to 100 g/storm for ∑15additives, 3 g/storm for 6PPD-quinone, and between 252 and 730 kg of TRWPs/storm. While 6PPD-quinone concentrations in this catchment were lower than previous studies, elevated concentrations post storm suggest prolonged aquatic exposure.

Per- and poly-fluoroalkyl substances in sediments from the water-level-fluctuation zone of the Three Gorges Reservoir, China: Contamination characteristics, source apportionment, and mass inventory and loadings

Sixteen paired surface sediment samples (0–5 cm, n = 32) covering upstream to downstream of water-level-fluctuation zone of Three Gorges Reservoir, China were collected in March 2018 (following six months of submergence) and September 2018 (after six months of exposure). Seventeen per- and poly-fluoroalkyl substances (PFASs) were quantified to evaluate contamination characteristics, apportion source categories and estimate mass inventory and loadings. The concentration of ΣPFASs ranged from 0.26 to 0.82 ng·g−1 at high water-level (HWL) and 0.46–1.53 ng·g−1 at low water-level (LWL). Perfluorooctanoic acid (PFOA, mean: 0.32 ng·g−1) and perfluorooctane sulfonate (PFOS, mean: 0.12 ng·g−1) dominated, accounting 44.9% and 16.3% of the total PFASs, respectively. The distribution of PFASs was more influenced by anthropogenic activities than physicochemical parameters of the sediments. Positive matrix factorization (PMF) identified PFOA-based products was the major sources (40.1% and 38.6%, respectively). Besides, the direct sources of PFOA-, PFOS-, PFNA-and PFBA-based products played the predominant role, while the indirect degradation of precursors contributed relatively little. The sediment (0–5 cm) mass inventory of PFASs at LWL (57.5 kg) was higher than HWL (39.3 kg). The annual mass loadings of the total PFASs, PFOA, PFOS, perfluoroundecanoic acid (PFUdA) and perfluorononanoic acid (PFNA) from the upstream to the middle-lower reaches of Yangtze River were 27.4 kg, 11.1 kg, 4.63 kg, 2.89 kg and 2.57 kg, respectively. This study could provide the basic datasets of PFASs in surface sediments of the TGR, and also indicate an important transport of PFASs from upstream to the lower reaches, which should be further studied as well.

Analysis of environmental dispersion in wetland flows with floating vegetation islands

The fate of the contaminant transport process within wetland flows considering contaminant depletion via a floating vegetation island (FVI) is the focus of this work. To identify the feature of contaminant cloud expansion under an FVI, multiscale theory is extended to obtain a two-dimensional spatial concentration distribution analytical solution. The characteristics of two-dimensional concentration distributions under upper FVI absorption and damping factor within wetland flows are illustrated. The performance of the contaminant depletion process via an FVI is also examined in this work, with discussions on removal efficiency, the absorption ratio, and the nonuniformity of contaminant mass. As FVI removal intensity changes along the stream direction, the analytical solution of removal efficiency is obtained to illustrate the longitudinal variation in the FVI removal intensity. Furthermore, the effects of removal intensity and damping factor on FVI removal efficiency are expressed separately. In addition, the removal intensity variance in the vertical direction is discussed using the defined absorption ratio. Moreover, as residual mass is a fatal issue in the contaminant removal process, temporal residual mass is discussed, and the residual mass proportion within different layers is depicted. Finally, the difference of the mass proportion between layers is employed to explain the transformation of the contaminant cloud.

Contamination and ecological risks of steroid metabolites require more attention in the environment: Evidence from the fishing ports

Due to strong endocrine disrupting effects, steroids in the environment have attracted substantial attention, with studies mostly focusing on the parent steroids. Here, we conducted the first investigation on the contamination profiles, possible sources, mass inventories, and ecological risks of 27 steroids and their metabolites in 15 typical fishing ports in Southeast China. Twelve steroids were detectable in the sediment samples with the total mean concentrations of 4.6–35 ng/g. High proportions of steroid metabolites were measured in the sediments and five metabolites were newly observed. Untreated municipal sewage and aquaculture wastes constitute the possible steroid sources in the studied fishing ports. The total inventories of steroids in fishing ports ranged from 2.1–16 mg/m2, with their metabolites being important contributors. The ecological risk analysis indicated high risks across all sampling sites mainly due to the contributions of parent steroids. Furthermore, our results found that progesterone is an acceptable chemical indicator for various steroids in sediments. This study provides the first evidence of steroid metabolites in the marine environment, calling for more studies in environmental behavior and ecotoxicology of steroid metabolites.

Evaluating the impact of back diffusion on groundwater cleanup time

Back diffusion of groundwater contaminants from low permeability (K) zones can be a major factor controlling the time to reach cleanup goals in downgradient monitor wells. We identify the aquifer and contaminant characteristics that have the greatest influence on the time (TOoM) after complete source removal for contaminant concentrations to decline by 1, 2 and 3 Orders-of-Magnitude (T1, T2 and T3). Two aquifer configurations are evaluated: (a) layered geometry (LG) with finite thickness low K layers; and (b) boundary geometry (BG) with thick semi-infinite low K boundaries. A semi-analytical modeling approach (Muskus and Falta, 2018) is used to simulate the concentration decline following source removal for a range of conditions and generate ≈21,000 independent values of T1, T2 and T3. Linear regression is applied to interpret this large dataset and develop simple relationships to estimate TOoM from three characteristic parameters – the mass residence time (TM), diffusion time (TD), and ratio of low K to high K mass storage (γ). TM is most important predictor of T1, T2 and T3 for both geometries and is equal to the combined high and low K contaminant mass divided by the mass flux, at the end of the loading period (TL). For LG, T3 is strongly influenced by TD = RLLD2/(4D*), where RL is the low K retardation factor, LD is the half-thickness of the embedded low K layers, and D* is the effective diffusion coefficient. For BG, T3 is strongly influenced by γ. Contaminant decay in low K zones can significantly reduce cleanup times when λLTD > 0.01, where λL is the effective first order decay rate in the low K zone. The 1st Damköhler (Da), equal to TM/TD, provides a useful indicator of the relative importance of back diffusion on TOoM. Back diffusion impacts are greatest on T3 when 0.01 > Da > 0.1, then decrease with increasing Da. Back diffusion has less impacts on T2, with limited influence on T1. The results are summarized in a simple conceptual model to aid in evaluating the impact of back diffusion on the time for concentrations to decline by 1–3 OoM.

Contamination characteristics, mass concentration, and source analysis of metal elements in PM2.5 in Lanzhou, China

PM2.5 and its bound metals pose a serious threat to human health. Understanding their contamination characteristics and source could provide implication for controlling their spreading and ensuring air quality. In this article, 1,600 of PM2.5 samples were collected from 5 urban sites in Lanzhou, China. The contamination characteristics of PM2.5, its relationship with meteorological factors, and the source of its bound metals were studied based on multiple linear regression analysis, enrichment factor (EF), principal component analysis and correlation analysis. The outcomes show that the PM2.5 concentrations in winter (0.117 mg·m–3) and spring (0.083 mg·m–3) are higher than those in summer (0.043 mg·m–3) and autumn (0.048 mg·m–3). The influence degree of meteorological factors on PM2.5 concentration is in the order of wind speed > atmospheric pressure > temperature > humidity. The major source of Fe and Cu in PM2.5 is construction dust, Pb and As is industrial, and Hg is coal combustion. In addition, Cd, V, Co, and Mn are mainly derived from dust produced by weathering of soil or rock. In general, the spatiotemporal distribution of PM2.5 and its bound metals are different, which is closely related to geographical location, source, and meteorological factors. The results in this article could provide support for the scientific formulation to prevent air pollution in Lanzhou.

A K-Sensor correlation-based evolutionary optimization algorithm to cluster contamination events and place sensors in water distribution systems

Contaminants that are introduced to drinking water systems can threaten large populations, and the potential for catastrophic consequences accentuates the need for efficient post-disaster strategies, including optimal hydrant flushing. Efficient hydrant flushing can significantly reduce impacts on public health, but performance relies on information about the propagation of a contaminant and the affected regions in a water network. While observations from water quality sensors are useful in timely detections of contaminants, little information on its source, propagation, and affected regions can be inferred. In the absence of such information, opening or closing hydrants might not help discharge contaminants but could accelerate propagation of a plume through the water network due to drops in pressure. To address this limitation of sensor layout optimization models, this research has developed a new model to identify the optimal location of sensors to effectively support hydrant flushing mechanisms. The model has been developed in three steps: (1) contamination events were simulated in a water network; (2) spatially similar propagating contamination events were identified; and (3) the layout of water quality sensors was optimized. In the first step, a representative number of potential contamination events were simulated using a hydraulic model. The second step clustered contamination events based on spatial similarity in their propagation regimes. Finally, the last step identified locations for placing water quality sensors within clusters (identified in the previous step) while minimizing detection time and maximizing probability of detection. This model ensures that when a sensor alarm is activated, contaminated region where hydrants should be opened or closed are spatially restricted. The approach developed in this research was applied to design a sensor network for a benchmark case study, Mesopolis. The layout of 10 water quality sensors was optimized over a set of 9161 contamination events, leading to 76% probability of detection with an average detection time of 8.2 h. The solution was compared with sensor layouts based on existing approaches, and it was found that the new approach could improve the mass of contaminant that was removed from the pipe network through hydrant flushing strategies. The new approach model improves the effectiveness of hydrant flushing strategies by restricting the area where hydrants are flushed to predefined zones based on the activation of sensors.

Compound-specific carbon isotope analysis of volatile organic compounds in complex soil extracts using purge and trap concentration coupled to heart-cutting two-dimensional gas chromatography–isotope ratio mass spectrometry

Compound-specific carbon isotope analysis (CSIA) is a powerful tool to track the origin and fate of organic subsurface contaminants including petroleum and chlorinated hydrocarbons and is typically applied to water samples. However, soil can form a significant contaminant reservoir. In soil samples, it can be challenging to recover sufficient amounts of volatile organic compounds (VOC) to perform CSIA. Soil samples often contain complex contaminant mixtures and gas chromatography combustion isotope ratio mass spectrometry (GC-C-IRMS) is highly dependent on good chromatographic separation due to the conversion to a single analyte. To extend the applicability of CSIA to complex volatile organic compound mixtures in soil samples, and to recover sufficient amounts of target compounds for carbon CSIA, we compared two soil extraction solvents, tetraglyme (TGDE) and methanol, and developed a heart-cutting two-dimensional GC-GC-C-IRMS method. We used purge & trap concentration of solvent-water mixtures to increase the amount of analyte delivered to the column and thus lower method detection limits. We optimized purge & trap and chromatographic parameters for twelve target compounds, including one suffering from poor purge efficiency. By using a 30 m thick-film non-polar column in the first and a 15 m polar column in the second dimension, we achieved good chromatographic separation for the target compounds in difficult matrices and high accuracy (trueness and precision) for carbon isotopic analysis. Tetraglyme extraction was shown to offer advantages over methanol for purge & trap concentration, leading to lower target compound method detection limits for CSIA of soil samples. The applicability of the developed method was demonstrated for a case study on soil extracts from a former manufacturing facility. Our approach extends the applicability of CSIA to an important matrix that often controls the long-term fate of contaminants in the subsurface.

A novel two-step approach for optimal groundwater remediation by coupling extreme learning machine with evolutionary hunting strategy based metaheuristics

We propose a simulation-optimization (SO) model based on a novel two-step strategy for the optimal design of groundwater remediation systems. The SO models are developed by coupling simulation models directly or through the extreme learning machine (ELM) with evolutionary hunting strategy based metaheuristics (EHSMs). In the first step, EHSMs with a combinatorial optimization technique are used to obtain optimal pumping locations by minimizing the percentage of contaminant mass that remained in the aquifer while keeping the pumping strategy as constant. In the second step, the optimal pumping locations are directly used as input, and a composite function is employed to minimize the sum of the water extraction rates and the percentage of extracted contaminant mass by constraining hydraulic heads and contaminant concentrations. The performance of the two-step strategy is found to be slightly better and computationally more efficient than the alternate approach. Moreover, various statistical measures suggest the superiority of EHSMs over other metaheuristics for groundwater remediation.

Semiquantitative Risk Assessment Method for Groundwater Source Protection Using a Process-based Interdisciplinary Approach

Potable groundwater resources are at risk as a result of industrial development and an increase in fertilizer use. Protection of the groundwater supply may require the establishment of groundwater source protection zones (GSPZs) to allow the implementation of protective measures. The aim of this study was to develop a new approach to groundwater source risk assessment (GSRA). The risk was defined as the highest of the risks identified for individual potential sources of contamination (PSCs). The risk resulting from a given PSC is the combination of its adverse impact on groundwater, unwanted event probability, and adverse effect and annoyance to the population. A multicriteria assessment method was designed to estimate indices of the potential groundwater impact of industrial facilities and noninert waste landfills with the range and weight method. Application of the proposed approach was tested considering an idealized model and three scenarios involving various industrial PSCs and fertilization practices. The overall nitrogen load was compared to the maximum effective nitrogen load for the crops in question. Sensitivity analysis of this methodology revealed that the main factors influencing the risk to the water supply include the nonreactive contaminant mass load in PSC leachate and the ratio of the groundwater volume abstracted from wells to the amount of water flowing from PSCs into wells, which determines the dilution degree of the contaminant mass. This proposed interdisciplinary approach to GSRA provides a robust basis to reach decisions on GSPZ establishment and the development of a groundwater risk analysis methodology.

Vertical Hydraulic Gradient Estimation in Clay Till, Using MiHPT Advanced Direct‐Push Technology

AbstractThe vertical transport of contaminants from source areas is employed in many risk assessment models and screening tools in order to estimate the contaminant mass discharge (CMD) into underlying aquifers. The key parameters for estimating CMD are the contaminant source area and concentration, and the vertical water flux, the latter of which depends on the vertical hydraulic conductivity and the vertical hydraulic gradient in the subsurface. This study focuses on advancing the use of the combined membrane interface probe hydraulic profiling tool (MiHPT) to investigate the vertical hydraulic gradient across a clay till overlying a sandy aquifer at a contaminated site in Denmark. Only the HPT is necessary for the estimate of vertical hydraulic gradient. The hydraulic head, clay till thickness, and resulting vertical hydraulic gradients found using the MiHPT compared well with observations from nearby nested wells. The parameter with the largest discrepancy was the thickness of the clay till. The advantage of the MiHPT is its relatively quick depth discrete access to information regarding subsurface permeability, vertical hydraulic gradients and contaminant distribution across a site. In this case study, performance of additional dissipationtests during the HPT log to acquire determination of the vertical hydraulic gradient increased the cost by 3% compared to standard HPT logs.

A field-scale remediation of residual light non-aqueous phase liquid (LNAPL): chemical enhancers for pump and treat

The remediation of petroleum-contaminated soil and groundwater is a challenging task. The petroleum hydrocarbons have a long persistence in both the vadose zone and in the aquifer and potentially represent secondary and residual sources of contamination. This is particularly evident in the presence of residual free-phase. Pump-and-treat is the most common hydrocarbon decontamination strategy. Besides, it acts primarily on the water dissolved phase and reduces concentrations of contaminants to an asymptotic trend. This study presents a case of enhanced light non-aqueous phase liquid (LNAPL) remediation monitored using noninvasive techniques. A pilot-scale field experiment was conducted through the injection of reagents into the subsoil to stimulate the desorption and the oxidation of residual hydrocarbons. Geophysical and groundwater monitoring during pilot testing controlled the effectiveness of the intervention, both in terms of product diffusion capacity and in terms of effective reduction of pollutant concentrations. In particular, non-invasive monitoring of the reagent migration and its capability to reach the target areas is a major add-on to the remediation technique. Most of the organic contaminants were decomposed, mobilized, and subsequently removed using physical recovery techniques. A considerable mass of contaminant was recovered resulting in the reduction of concentrations in the intervention areas.

Thermal-Hydrological-Chemical Modeling of a Covered Waste Rock Pile in a Permafrost Region

In order to reduce contaminant mass loadings, thermal cover systems may be incorporated in the design of waste rock piles located in regions of continuous permafrost. In this study, reactive transport modeling was used to improve the understanding of coupled thermo-hydrological and chemical processes controlling the evolution of a covered waste rock pile located in Northern Canada. Material properties from previous field and laboratory tests were incorporated into the model to constrain the simulations. Good agreement between simulated and observational temperature data indicates that the model is capable of capturing the coupled thermo-hydrological processes occurring within the pile. Simulations were also useful for forecasting the pile’s long-term evolution with an emphasis on water flow and heat transport mechanisms, but also including geochemical weathering processes and sulfate mass loadings as an indicator for the release of contaminated drainage. An uncertainty analysis was carried out to address different scenarios of the cover’s performance as a function of the applied infiltration rate, accounting for the impacts of evaporation, runoff, and snow ablation. The model results indicate that the cover performance is insensitive to the magnitude of recharge rates, except for limited changes of the flow regime in the shallow active layer. The model was expanded by performing an additional sensitivity analysis to assess the role of cover thicknesses. The simulated results reveal that a cover design with an appropriate thickness can effectively minimize mass loadings in drainage by maintaining the active layer completely within the cover.