Xiaoqing Shi

Numerical Simulation of Viscoelastoplastic Land Subsidence due to Groundwater Overdrafting in Shanghai, China

Land subsidence caused by groundwater overpumping in Shanghai is becoming a serious geological hazard. Due to its important economic position, the field data, including compression of individual stratum from extensometer groups and the groundwater levels from observation wells, have been collected over the past 45 years. Considering the fact that different hydrostratigraphic units have different kinds of deformation and that an identical unit may also present different deformation characteristics, such as elasticity, elastoplasticity, and viscoelastoplasticity, at different sites of the cone of depression or in different periods, a nonlinear coupled regional land subsidence model is developed. The coupled model consists of a three-dimensional groundwater flow model and a one-dimensional vertical deformation model, both based on a viscoelastoplastic constitutive laws (called modified Merchant model), and then solved using a multiscale iterative finite-element method. The model is calibrated using 28,184 hy...

Influence of flow velocity and spatial heterogeneity on DNAPL migration in porous media: insights from laboratory experiments and numerical modelling

Understanding the migration of dense non-aqueous phase liquids (DNAPLs) in complex subsurface systems is important for evaluating contamination source zones and designing remediation schemes after spill events. Six sandbox experiments were performed to explore the individual effect of flow velocity, and the combined effect of flow velocity and layered lenses on a DNAPL (PCE) migration in porous media. DNAPL saturation was measured using a light transmission system, and saturation distribution was quantified by spatial moments. The experimental results show that large flow velocity significantly promotes lateral and vertical migration of the low-viscosity DNAPL, while when layered lenses exist, the infiltration rate decreases and horizontal spread increases. Migration processes were numerically simulated, and the modelling results tested against experimental results. Furthermore, migration of DNAPLs with different viscosities was simulated to explore the combined effects of flow velocity and geological heterogeneity. Simulation results show that enhanced heterogeneity makes low-viscosity DNAPLs migrate along preferential pathways, resulting in irregular DNAPL morphology. Layered lenses combined with heterogeneity complicate the effect of flow velocity on the migration of low-viscosity DNAPLs by changing percolation paths. Results also demonstrate that flow velocity exhibits relatively little influence on the migration of medium/high-viscosity DNAPLs, which is predominantly controlled by viscosity and heterogeneity. Enhanced heterogeneity has a larger effect on migration behavior. Findings indicate that the migration paths and position of the source zone could change significantly, due to the combined effect of groundwater flow velocity and geological heterogeneity; thus, comprehensive hydrogeological investigation is needed to characterize the source zone.RésuméComprendre la migration des phases liquides denses non-aqueuse (DNAPL) dans les systèmes souterrains complexes est important pour évaluer les zones de la source de contamination et la conception de systèmes d’assainissement après les incidents de déversements. Six expériences dans des bacs à sables ont été réalisées pour étudier l’effet individuel de la vitesse d’écoulement et l’effet combiné de la vitesse d’écoulement et des lentilles sur la migration du DNAPL (PCE) dans un milieu poreux. La saturation en DNAPL est mesurée en utilisant un système de transmission de la lumière, et la distribution de saturation a été quantifiée par les moments spatiaux. Les résultats expérimentaux montrent que la grande vitesse d’écoulement favorise considérablement la migration latérale et verticale du DNAPL de faible viscosité, tandis que lorsque des lentilles en couches existent, le taux d’infiltration diminue et la propagation horizontale augmente. Les processus de migration ont été simulés numériquement et les résultats de la modélisation ont été comparés aux résultats expérimentaux. En outre, la migration des DNAPLs avec différentes viscosités a été simulée pour explorer les effets combinés de la vitesse d’écoulement et de l’hétérogénéité géologique. Les résultats de simulation montrent qu’une hétérogénéité accrue favorise la migration de DNAPL de faible viscosité le long de cheminements préférentiels, entraînant une morphologie irrégulière du DNAPL. Des lentilles en couches combinées avec une hétérogénéité compliquent l’effet de la vitesse d’écoulement sur la migration des DNAPLs de faible viscosité en modifiant les chemins de percolation. Les résultats démontrent également que la vitesse d’écoulement provoque une petite influence sur la migration des DNAPLs de viscosité moyenne à élevée, qui est principalement contrôlée par la viscosité et l’hétérogénéité. L’hétérogénéité accrue a un effet plus important sur le comportement de la migration. Les résultats indiquent que les chemins de migration et la position de la zone source pourraient changer de manière significative, en raison de l’effet combiné de la vitesse d’écoulement d’eaux souterraines et de l’hétérogénéité géologique ; ainsi une étude hydrogéologique globale est nécessaire pour caractériser la zone source.ResumenLa comprensión de la migración de líquidos densos en fase no acuosa (DNAPLs) en sistemas subterráneos complejos es importante para evaluar las zonas de las fuentes de la contaminación y para diseñar planes de remediación después de los eventos de derrames. Se llevaron a cabo seis experimentos en recintos de seguridad para explorar el efecto individual de la velocidad de flujo, y el efecto combinado de la velocidad de flujo y lentes estratificadas en la migración del DNAPL (PCE) en el medio poroso. La saturación del DNAPL se midió utilizando un sistema de transmisión de luz, y la distribución de saturación se cuantificó por momentos espaciales. Los resultados experimentales muestran que una gran velocidad del flujo favorece significativamente la migración lateral y vertical del DNAPL de baja viscosidad, mientras que cuando existen lentes estratificadas, disminuye la tasa de infiltración y aumenta la propagación horizontal. Se simularon numéricamente los procesos de migración, y los resultados de los modelos se testearon contra resultados experimentales. Además, se simuló la migración de DNAPLs con diferentes viscosidades para explorar los efectos combinados de la velocidad de flujo y la heterogeneidad geológica. Los resultados de la simulación muestran que el aumento en la heterogeneidad hace que los DNAPLs de baja viscosidad migren a lo largo de las vías preferenciales, lo que resulta en una morfología irregular del DNAPL. Las lentes estratificadas combinadas con la heterogeneidad complican el efecto de la velocidad de flujo de la migración de los DNAPLs de baja viscosidad cambiando las trayectorias de la percolación. Los resultados también demuestran que la velocidad de flujo exhibe una influencia relativamente pequeña en la migración de los DNAPLs de media / alta viscosidad, que son controlados predominantemente por la viscosidad y la heterogeneidad. Una acentuada heterogeneidad tiene un mayor efecto sobre el comportamiento de la migración. Los hallazgos indican que las trayectorias de la migración y la posición de la zona de la fuente podrían cambiar significativamente, debido al efecto combinado de la velocidad del flujo de agua subterránea y la heterogeneidad geológica; por lo tanto, se necesita una investigación hidrogeológica integral para caracterizar la zona de la fuente.摘要了解复杂的地下系统中重非水相流体的运移对于出现污染事件时评估污染源区和设计修复方案非常重要。本文进行了6个砂箱实验以探索流速对孔隙介质中重非水相流体运移的单一影响以及流速和层状透镜体对重非水相流体运移的综合影响。采用光透射系统对重非水相流体饱和度进行的测量,通过空间矩量化了饱和分布。实验结果显示,较大的流速促进低粘度重非水相流体的侧向和垂向运移,而存在层状透晶体时,重非水相流体的垂向入渗率降低,水平扩散范围 增加。对运移过程进行了数值模拟,模拟结果验证了实验结果。此外,还模拟了不同粘性的重非水相流体运移,以探索流速和地质非均质性性的综合影响。模拟结果显示非均质性增强使低粘性重非水相流体沿优先通道运移,造成不规则的分布形态。层状透镜体结合非均质性改变了渗流通路,使流速对低粘性重非水相流体的影响更加复杂。结果还显示,流速对中/高粘性重非水相流体运移的影响相对较小,主要受控于粘性和非均质性。非均质性增强对运移行为有较大的影响。上述结果表明,由于地下水流速和地质非均质性的综合影响,运移通道和污染源区的位置变化会很大。因此,需要进行水文地质调查来描述污染源区的特征。ResumoEntender a migração de compostos de fase líquida densa não aquosa (DNAPLs) em sistemas subsuperficiais complexos é importante para avaliar zonas de fontes contaminantes e projetar esquemas de remediação após eventos de derramamento. Seis experimentos de caixa de areia foram executados para explorar o efeito individual da velocidade de fluxo, e o efeito combinado da velocidade de fluxo e lentes sobrepostas na migração DNAPL (PCE) em meios porosos. A saturação por DNAPL foi medida utilizando um sistema de transmissão de luz, e a distribuição de saturação foi quantificada por momentos espaciais. Os resultados experimentais mostram que grandes velocidades de fluxo promovem migração lateral e vertical significante de DNAPL de baixa densidade, porém quando existem lentes sobrepostas, a taxa de infiltração diminui e a propagação horizontal aumenta. Processos de migração foram simulados numericamente, e os resultados dos modelos testados contra os resultados experimentais. Além do mais, as migrações de DNAPL com viscosidades diferentes foram simuladas para explorar os efeitos combinados de velocidade de fluxo e heterogeneidade geológica. Os resultados da simulação mostram que a heterogeneidade acentuada faz com que DNAPLs de baixa viscosidade migrem por caminhos preferenciais, resultando em uma morfologia DNAPL irregular. Lentes sobrepostas combinadas com heterogeneidade complicam o efeito da velocidade de fluxo na migração de DNAPLs de baixa viscosidade pela mudança nos caminhos de percolação. Os resultados também demonstram que a velocidade de fluxo exibe relativamente pouca influência na migração de DNAPLs de média e alta viscosidade, que é predominantemente controlada pela viscosidade e heterogeneidade. A heterogeneidade acentuada tem maior efeito no comportamento da migração. As descobertas indicam que os caminhos de migração e a posição da zona de fontes podem mudar significantemente pelo efeito combinado da velocidade de fluxo das águas subterrâneas e heterogeneidade geológica, assim uma investigação hidrogeológica abrangente é necessária para caracterizar as zonas de fontes

Retention and transport of graphene oxide in water-saturated limestone media

In this work, column experiments were conducted to investigate the transport characteristics of graphene oxide (GO) nanoparticles in limestone media under various electrolytes, solution pH, and humic acid (HA) concentration conditions. In the limestone media, GO exhibited relatively low mobility with the mass recovery rate lower than 65.2%, even when solution ionic strength was low. The presence of HA enhanced its mobility. In addition, the presence of S2-, a divalent anion, also promoted GO transport in limestone media compared to Cl- under similar ionic strength conditions through neutralizing more positive charge and thus diminishing the cation bridging. Solution pH showed slight effect on the transport of GO in limestone with the mass recovery range from 40.3% to 51.7%. Over all, decreases in solution pH, HA concentration and increases in solution ionic strength reduced the mobility of GO in the limestone media under the tested conditions. These results indicated both environmental conditions and media characteristics played important roles in controlling GO fate and transport in porous media. The one-site kinetic deposition model was applied to describe the interactions between the GO and limestone media and model simulations fitted the observed experimental data very well. As limestone is an important component of aquiferous media in subsurface, findings from this study elucidated the key factors and processes controlling the fate of GO particles in limestone media, which can inform the prediction and assessment of the risks of GO in groundwater environment.

Experimental and numerical modeling of chemical osmosis in the clay samples of the aquitard in the North China Plain

This study focuses on addressing the existence of chemical osmosis in the clay samples of the aquitard in the North China Plain (NCP). For this purpose, chemical osmotic experiments in the clay samples collected from the aquitard in the NCP were implemented to estimate the reflection coefficient or chemico-osmotic efficiency in clayey sediments. More specifically, under a certain chemical potential gradient, the reflection coefficients were, respectively, achieved by measuring the osmotic flow and the induced pressure in terms of the two sets of undisturbed clay core samples and their remolded clay samples equipped in a rigid wall permeameter. The measured reflection coefficient is the value of 0.023 for in situ clay sample, much smaller than that of 0.098 for remolded sample. Also, the constant-head tests for determining the hydraulic conductivity of clay samples were conducted before and after the osmotic tests. It is indicated that the permeability of the remolded sample is smaller than the undisturbed sample in both periods. Moreover, a chemical osmosis continuum model was used to fit the evolution of the osmotically driven hydraulic pressure in the clay samples where more intrinsic parameters of the samples were calibrated. Additional analysis demonstrates the sensitivity of the osmosis pressure to the different parameters input to the model for the in situ clay sample. This study could provide a reliable basis for further evaluating the role of clay sediments in the desalination of shallow saline groundwater and salinization of deeper fresh groundwater in the NCP.

Delineation of contaminant plume for an inorganic contaminated site using electrical resistivity tomography: comparison with direct-push technique

Precise delineation of contaminant plume distribution is essential for effective remediation of contaminated sites. Traditional in situ investigation methods like direct-push (DP) sampling are accurate, but are usually intrusive and costly. Electrical resistivity tomography (ERT) method, as a non-invasive geophysical technique to map spatiotemporal changes in resistivity of the subsurface, is becoming increasingly popular in environmental science. However, the resolution of ERT for delineation of contaminant plumes still remains controversial. In this study, ERT and DP technique were both conducted at a real inorganic contaminated site. The reliability of the ERT method was validated by the direct comparisons of their investigation results that the resistivity acquired by ERT method is in accordance with the total dissolved solid concentration in groundwater and the overall variation of the total iron content in soil obtained by DP technique. After testifying the applicability of ERT method for contaminant identification, the extension of contaminant plume at the study site was revealed by supplementary ERT surveys conducted subsequently in the surrounding area of the contaminant source zone.

The Co-application of Willow and Earthworms/Horseradish for Removal of Pentachlorophenol from Contaminated Soils

Pentachlorophenol (PCP) was once commonly used as a pesticide worldwide, and is now a toxic and recalcitrant environmental pollutant. To explore a practical approach in the remediation of PCP-contaminated soils in China, we evaluated the efficacy of a local willow (Salix × aureo-pendula CL “J1011”) on removing PCP and assessed the potential of a native earthworm (Metaphire guillelmi) and horseradish (Armoracia rusticana Gaerth), alone and in combination with willow, to enhance the efficiency of removing PCP from spiked field soils. Willow, horseradish, and earthworms alone significantly increased PCP removal from soil. After 45 days, only 47.2% of the PCP remained in the presence of willow alone; 68.4, 51.4, and 46.3% of the PCP remained with 50, 100, and 200 g horseradish m−2, respectively; and 41.1% of the PCP remained in the presence of earthworms. The removal of PCP from soils significantly increased with co-application of willow and horseradish (23.1% remaining) and especially with the co-application of willow and earthworms (2.2% remaining). Our results indicate that in terms of remediation efficiency, economic value, ecological benefits, and ease of use, a system consisting of this autochthonous willow and indigenous earthworms has great potential for the remediation of PCP-polluted soil in China.

Retention and Transport of Bisphenol A and Bisphenol S in Saturated Limestone Porous Media

The release of bisphenols such as bisphenol A (BPA) and its alternative bisphenol S (BPS) into the subsurface environment may cause serious pollutions to soil and groundwater. However, only few works have examined their fate and transport in porous media. In this work, batch and column experiments and mathematical modeling were conducted to study the transport behaviors of BPA and BPS in water-saturated limestone porous media. The effects of contaminant input concentration, solution ion type, and solution ionic strength on the retention and transport of BPA and BPS in the columns were investigated. BPS had higher mobility in limestone porous media than that of BPA. With its input concentration decreased, BPA showed lower mobility, while the transport of BPS in the media was not affected by the input concentration perturbations. The retention of both BPA and BPS was higher in divalent calcium ion solution than that in monovalent sodium solution in limestone porous media. Ionic strength showed little effect on the retention and transport of BPA and BPS except that high concentration of Ca2+ inhibited the migration of BPS in the media. Because of its relatively high mobility and toxicity, BPS may present a great risk to groundwater quality and thus may not be an environmentally friendly bisphenol alternative.

Joint inversion of physical and geochemical parameters in groundwater models by sequential ensemble-based optimal design

Joint inversion of physical and geochemical parameters in groundwater reactive transport models is still a great challenge due to the intrinsic heterogeneities of natural porous media and the scarcity of observation data. In this study, we make use of a sequential ensemble-based optimal design (SEOD) method to jointly estimate physical and geochemical parameters of groundwater models. The effectiveness and efficiency of the SEOD method are illustrated by the comparison between the sequential optimization strategy and the conventional strategy (using fixed sampling locations) for two synthetic cases. Since the SEOD method is an optimization method based on the ensemble Kalman filter (EnKF), it invokes the time-consuming genetic algorithm at every assimilation step of the EnKF to obtain the optimal sampling locations. To enhance its computational efficiency, we improve the SEOD method by replacing the EnKF with the ensemble smoother with multiple data assimilation. Furthermore, the influence factors of the original and improved SEOD method are also discussed. Our results show that the SEOD method provides an effective designed sampling strategy to accurately estimate heterogeneous distribution of physical and geochemical parameters. Moreover, the improved SEOD method is more advantageous than the original one in computational efficiency, making this SEOD framework more promising for future application.

Retention and Transport of PAH-Degrading Bacterium Herbaspirillum chlorophenolicum FA1 in Saturated Porous Media Under Various Physicochemical Conditions

Dispersal of functional microorganisms is a rate-limiting process during in situ bioremediation of contaminated soil and groundwater. In this work, series of column experiments were conducted to investigate the retention and transport behaviors of Herbaspirillum chlorophenolicum FA1, a promising bacterial agent for bioremediation, in saturated porous media under conditions of different combinations of grain size, solution pH, solution ionic strength (IS), and humic acid (HA) concentration. Experimental data showed that the mobility of FA1 in saturated porous media was strongly dependent on the physicochemical conditions. The breakthrough curves (BTCs) indicated that the amounts of FA1 in the effluent increased with increasing in sand size, solution pH, and HA concentration, but decreased with increase of solution IS. The shape of retention profiles (RPs) was hyper-exponential. The amounts of retained bacteria in the media also varied with the experimental conditions with opposite trends to that of effluent. Both experimental BTCs and RPs were simulated by a mathematical model that accounted for deposition kinetics to better interpret the effects of physicochemical conditions on FA1 deposition dynamics. Findings from this study showed that fate and transport of the functional bacterium FA1 in porous media strongly relied on the environmental conditions. Both experimental and modeling results can provide guidelines for field application of functional bacteria for soil and groundwater remediation.

Comprehensive evaluation of shallow groundwater quality in Central and Southern Jiangsu Province, China

Evaluation of the shallow groundwater quality in Central and Southern Jiangsu Province (CSJ) in China is important not only to public health but also to sustainable development and utilization of groundwater resources. In this work, 968 groundwater samples were collected by field surveys during 2006 to 2010 to investigate spatial distribution and extent of pollution. The single factor pollution standard index method, considering the values of both the natural groundwater background and the standard of safe drinking water, was used to assess the groundwater quality in the study area. Results showed that the shallow groundwater was severely polluted with Grade V and VI (i.e., severe and extremely severe pollution) groundwater covered about 1/3 and 1/4 of the study area, respectively. The main groundwater contaminants that cause the severe and extremely severe pollutions were manganese, iron, arsenic, and nitrogen. Findings from this work showed that natural hydrogeochemical processes were the main causes of the iron and manganese pollution, however, human activities (e.g., industrial, agricultural, and domestic pollutions) mainly contributed to the nitrogen pollution in the study area.