Evaluation of a capture zone overlay method for designing groundwater remediation systems

 Simplified approaches are often used to model the removal of groundwater contamination. These approaches can yield poor remediation schemes because they incorrectly portray the effects of multiple pumping wells. In this study, a pumping configuration designed by graphically overlaying capture zones having an identical, quasi-elliptical shape was evaluated with a numerical mass transport model. After a 3-year period (within which the hypothetical aquifer was to be remediated) the contaminant mass had been reduced by 77%. Due to stagnation zones which developed between extraction wells, approximately 15 years of pumping was required to remediate the aquifer with the overlay configuration. An alternative design, consisting of an extraction well between two injection wells along the long axis of the plume, removed the contaminant within the 3-year design period. Received: 23 October 1995 · Accepted: 18 June 1996     

Mass transport modelling to assess contamination of a water supply well in Sabarmati river bed aquifer, Ahmedabad City, India

 Drinking water supply wells were constructed in the Sabarmati river bed aquifer of Ahmedabad city using radial pipes and are known as French Collector wells. Contamination of groundwater from one of the French wells near Sabarmati railway bridge was noticed in 1992. The suspected pollution sources are Duff-nala of Shahibaug and two other sources from slum dwellings on either side of Sabarmati river. A combined groundwater flow, pathlines and a mass transport model was constructed covering an area of 9 km² to analyse the capture zone of the French well under two different scenarios. Aquifer parameters of the river bed aquifer were available. Dry river bed condition was simulated under scenario I and controlled flow in the river bed was simulated under scenario II. The groundwater velocity and migration of contaminant particles from sources was analysed in the pathline model. Total dissolved solids (TDS) concentration contours originating from sources in the mass transport model (MT3D) were computed by solving an advection-dispersion equation. The computed pathlines and TDS concentration contours indicate likely migration of contaminant plume from pollutant sources to the French well during 365 days under two scenarios. The model results confirm the tracer injection studies carried out to know the likely migration of contaminants towards the French well. The modelling study emphasised the necessity of controlled release of surface water in Sabarmati river bed from Dharoi reservoir throughout the year. Received: 28 October 1998 · Accepted: 17 June 1999     

Groundwater flow and capture zone analysis of the Central Passaic River Basin, New Jersey

Delineating capture zones of pumping wells is an important part of safe drinking water and well protection programs. Capture zones or contributing areas of a groundwater extraction well are the parts of the aquifer recharge areas from which the wells draw their water. Their extent and location depend on the hydrogeologic conditions such as groundwater recharge, pumping scenario and the aquifer properties such as hydraulic conductivity, porosity, heterogeneity of the medium and hydraulic gradient. Different methods of delineation can be used depending on the complexity of the hydrogeologic conditions. In this study, a 3-dimensional transient numerical MODFLOW model was developed for the Central Passaic River Basin (CPRB), and used with a MODPATH particle tracking code to determine 3-dimensional transient capture zones. Analytically calculated capture zones from previous studies at the site were compared with the new numerically simulated capture zones. The study results revealed that the analytical solution was more conservative, estimating larger capture zones than the numerical models. Of all the parameters that can impact the size, shape and location of a capture zone, the hydraulic conductivity is one of the most critical. Capture zones tend to be smaller in lower hydraulic conductivity areas.     

地下水污染监测网的设计研究进展

地下水污染监测网的设计包括取样点在空间上的采样位置和时间上的取样频率这两方面的确定，其目的是为了准确刻画污染羽在含 水层中随时间的变化状况。概要地回顾了近20年来地下水污染监测网设计的研究成果。分别介绍了统计方法、模拟方法和模拟-优化模型 等监测网设计方法的研究进展。监测网设计方法的选择取决于最终的监测目的以及可供利用的基础资料。指出参数的不确定性是影响污染监 测设计结果的最重要因素。如何将地下水污染监测网设计的理论研究真正与实际应用相结合是今后的主要研究方向。     

基于不同地质统计方法的渗透系数场对污染物运移的影响

渗透系数场的空间变异性是影响污染物运移结果的决定因素,而地质统计方法是解决渗透系数空间变异性的主要技术手段。本文利用野外场地实测数据,采用普通克里格法和指示克里格法、顺序高斯模拟法和顺序指示模拟法四种地质统计方法,插值估测和模拟再现随机渗透系数场,进而对比研究四种渗透系数场对大尺度污染物运移的影响。研究结果表明,污染羽的质心位置（一阶矩）主要由渗透系数的平均值来决定;污染羽在空间上的展布范围（二阶矩）主要受渗透系数空间变异方差的影响;条件模拟克服了估计法的平滑效果,较好地再现真实曲线的波动性,渗透系数（ lnK）估计方差与污染羽空间二阶矩随着条件模拟次数的增加而减小,并且顺序指示模拟程度更加明显。     

Application of multiple-point geostatistics on modelling groundwater flow and transport in a cross-bedded aquifer (Belgium)

Sedimentological processes often result in complex three-dimensional subsurface heterogeneity of hydrogeological parameter values. Variogram-based stochastic approaches are often not able to describe heterogeneity in such complex geological environments. This work shows how multiple-point geostatistics can be applied in a realistic hydrogeological application to determine the impact of complex geological heterogeneity on groundwater flow and transport. The approach is applied to a real aquifer in Belgium that exhibits a complex sedimentary heterogeneity and anisotropy. A training image is constructed based on geological and hydrogeological field data. Multiple-point statistics are borrowed from this training image to simulate hydrofacies occurrence, while intrafacies permeability variability is simulated using conventional variogram-based geostatistical methods. The simulated hydraulic conductivity realizations are used as input to a groundwater flow and transport model to investigate the effect of small-scale sedimentary heterogeneity on contaminant plume migration. Results show that small-scale sedimentary heterogeneity has a significant effect on contaminant transport in the studied aquifer. The uncertainty on the spatial facies distribution and intrafacies hydraulic conductivity distribution results in a significant uncertainty on the calculated concentration distribution. Comparison with standard variogram-based techniques shows that multiple-point geostatistics allow better reproduction of irregularly shaped low-permeability clay drapes that influence solute transport.     

Numerical modeling of contaminant transport in a stratified heterogeneous aquifer with dipping anisotropy

The combined influence of dip angle and adsorption heterogeneity on solute transport mechanisms in heterogeneous media can be understood by performing simulations of steady-state flow and transient transport in a heterogeneous aquifer with dipping anisotropy. Reactive and non-reactive contaminant transport in various types of heterogeneous aquifer is studied by simulations. The hydraulic conductivity (K) of the heterogeneous aquifer is generated by HYDRO_GEN with a Gaussian correlation spectrum. By considering the heterogeneity of the adsorption distribution coefficient (K _d), a perfect negative correlation between lnK and lnK _d is obtained by using the spherical grains model. The generated K and K _d are used as input to groundwater flow and transport models to investigate the effects of dipping sedimentary heterogeneity on contaminant plume evolution. Simulation results showed that the magnitude of the dip angle strongly controls the plume evolution in the studied anisotropic and heterogeneous aquifer. The retarded average pore-water velocity (v/R) of the adsorption model significantly controls the horizontal spreading of the plume. The bottom plume is intensively retarded in the zones between the dipping lenses of lower hydraulic conductivity and the no-flow bottom boundary. The implications of these findings are very important for the management of contaminated heterogeneous aquifers.     

Modeling in situ benzene bioremediation in the contaminated Liwa aquifer (UAE) using the slow-release oxygen source technique

Dissolved benzene was detected in the shallow unconfined Liwa aquifer, UAE, which represents the main freshwater source for the nearby residence Bu Hasa camp area. The main source of this contamination is believed to be the rejected water released from Bu Hasa liquid recovery plant. In this paper, a finite element model (METABIOTRANS) is used to simulate the fate and transport of the dissolved benzene plume in Liwa aquifer. Different remediation scenarios were simulated in which the slow-release oxygen source (SOS) technique is utilized to minimize benzene concentrations at the nearest camp supply wells downstream of the contamination zone. Results of the remediation scenarios show that the highest biodegradation rates occur when the oxygen source is placed near the plume center; where higher benzene concentrations exist. The nearest oxygen release source to the contamination zone caused higher stimulation to bacterial growth than further down-gradient oxygen sources. It also exhibited longer resident time of oxygen in the aquifer; and therefore, yielded higher reductions in benzene concentrations. However, using one central SOS proved to be insufficient as contaminant escaped laterally. Additional four transverse oxygen sources were necessary to capture benzene that laterally spread away from the contamination zone. These lateral SOSs were essential to reduce benzene concentrations at the supply wells that are located at the plume fringes. Finally, it was found that increasing oxygen release from one source did not always improve remediation; and that using several SOSs with lower release rates could be a more practical approach to enhance benzene biodegradation in the aquifer.     

Testing of stack-unit/aquifer sensitivity analysis using contaminant plume distribution in the subsurface of Savannah River Site, South Carolina, USA

Published information on the correlation and field-testing of the technique of stack-unit/aquifer sensitivity mapping with documented subsurface contaminant plumes is rare. The inherent characteristic of stack-unit mapping, which makes it a superior technique to other analyses that amalgamate data, is the ability to deconstruct the sensitivity analysis on a unit-by-unit basis. An aquifer sensitivity map, delineating the relative sensitivity of the Crouch Branch aquifer of the Administrative/Manufacturing Area (A/M) at the Savannah River Site (SRS) in South Carolina, USA, incorporates six hydrostratigraphic units, surface soil units, and relevant hydrologic data. When this sensitivity map is compared with the distribution of the contaminant tetrachloroethylene (PCE), PCE is present within the Crouch Branch aquifer within an area classified as highly sensitive, even though the PCE was primarily released on the ground surface within areas classified with low aquifer sensitivity. This phenomenon is explained through analysis of the aquifer sensitivity map, the groundwater potentiometric surface maps, and the plume distributions within the area on a unit-by- unit basis. The results of this correlation show how the paths of the PCE plume are influenced by both the geology and the groundwater flow.     

Detection of contaminant plumes released from landfills: numerical versus analytical solutions

Contaminant leaks released from landfills are a threat to groundwater quality. The groundwater monitoring systems installed in the vicinity of such facilities are vital. In this study the detection probability of a contaminant plume released from a landfill has been investigated by means of both a simulation and an analytical model for both homogeneous and heterogeneous aquifer conditions. Since the detection probability is a sensitive quantity, we first compare the two methods for homogeneous aquifer conditions to assess the errors that are encountered by performing simulations. The analysis shows that the simulation model yields the detection probabilities of a contaminant plume at a given monitoring well quite well in the homogeneous case. For heterogeneous aquifers we apply the approximated analytical model based on macro-dispersivities. Here we find that this model is insufficient in monitoring system design, since the obtained analytical values of the detection probabilities at a given monitoring well differ significantly from those computed by simulation.     

Tritium�Chelium groundwater age used to constrain a groundwater flow model of a valley-fill aquifer contaminated with trichloroethylene (Quebec, Canada)

Tritium?Chelium groundwater dating was carried out in a trichloroethylene (TCE)-contaminated valley-fill aquifer system in Quebec, Canada, where a numerical groundwater flow model was developed. Forty seven discrete groundwater and dissolved gas samples were obtained along two flow paths originating from known TCE source zones whose related plumes converge down gradient to form a single plume. Sampling points in monitoring wells were projected onto vertical sections showing particle tracks along the two flow paths. At these points, simulated advective ages obtained from particle tracking were matched to tritium?Chelium ages using different porosity values; the best match was 0.35. Ages were also obtained above and below a prodeltaic silty aquitard in a portion of the aquifer where some source zones are located, which provide groundwater and TCE transit times through the aquitard as well as a mean vertical hydraulic conductivity that agrees with previous estimates used in the model. In certain locations, anomalously old ages associated with high terrigenic ⁴He indicate areas where groundwater from the underlying proglacial unit flows upward into the deltaic sand aquifer through aquitard windows. Upflow locations correspond with increased TCE concentrations, suggesting significant TCE provenance through the proglacial unit originating from a previously unrecognized TCE source zone.     

Site characterization and monitoring of natural attenuation indicator parameters in a fuel contaminated coastal aquifer: Karaduvar (Mersin,SE Turkey)

This paper presents results from a site characterization and monitoring study at Karaduvar area (Mersin, SE Turkey), where high concentrations of refined petroleum products have been detected in domestic and irrigation water wells. The saturated and unsaturated zones in the deltaic aquifer are contaminated by large quantities of gasoline and diesel range fuel hydrocarbons (GRHs and DRHs) released from diverse sources that include accidental spills, storage tank fires, pipeline breaks, deliberate discharge of waste petroleum products from slop tanks and illegal tanker truck washing facilities. At the site, due to the complex nature of the pollution sources, overlapping contaminant plumes exist and cover an area of about 0.5 km². In both polluted and unpolluted parts of the aquifer, monitoring of groundwater physicochemical parameters in a total of 55 sampling points was carried out between 2006 and 2007. The results show that the terminal electron acceptors (e.g. dissolved oxygen, nitrate, Mn(IV), Fe(III), sulfate) were reduced near the source area(s) indicating presence of actively operating biodegradation processes at the site. Close to the contaminant source area(s), conditions in the plume are highly anoxic and reducing; where high amounts of transformation products (e.g. bicarbonate, dissolved iron, and manganese) are present in solution. Additionally, at the site, excessive pumping, careless land use, and deliberate wastewater discharges significantly deteriorated the quality and quantity of groundwater. Excessive groundwater pumping for industrial and agricultural uses has resulted in substantial water level declines (2–3 m) near the coastal part where seawater intrusion threatens the groundwater resources.     

Probability of plume interception using conditional simulation of hydraulic head and inverse modeling

A procedure to estimate the probability of intercepting a contaminant groundwater plume for monitoring network design has been developed and demonstrated. The objective of the procedure is to use all available information in a method that accounts for the heterogeneity of the aquifer and the paucity of data. The major components of the procedure are geostatistical conditional simulation and parameter estimation that are used sequentially to generate flow paths from a suspected contaminant source location to a designated monitoring transect. From the flow paths, a histogram is constructed that represents the spatial probability distribution of plume centerlines. With an independent estimate of the plume width, a relationship between the total cost and the probability of detecting a plume can be made. The method uses geostatistical information from hydraulic head measurements and is conditioned by the data and the physics of groundwater flow. This procedure was developed specifically for the design of monitoring systems at sites where very few, if any, hydraulic conductivity data are available.     

Modeling of transient groundwater flow,pollutant transport,and biodegradation in an aquifer with large hydraulic head variations

Industrially sourced dense non-aqueous phase liquids (DNAPLs) contaminated an alluvial aquifer in France decades ago. The location(s) and nature of the pollution source zone(s) were unknown, and the dissolved concentrations of volatile organic compounds in the monitoring wells varied greatly with time. The aquifer was in hydraulic equilibrium with an artificial canal whose water level was highly variable (up to 5 m). These variations propagated into the aquifer, causing changes in the groundwater flow direction; a transient numerical model of flow and solute transport showed that they correlate with the concentration variations because the changes in the flow direction resulted in the contaminant plume shifting. The transient hydrogeological numerical model was built, taking into account solvent biodegradation with first-order chain, since biodegradation has a significant influence on the pollutant concentration evolution. The model parameterization confirms the position of the source zones among the potential troughs in the bedrock where DNAPLs could have accumulated. The groundwater model was successfully calibrated to reproduce the observed concentration variations over several years and allowed a rapid validation of the hypotheses on the functioning of the polluted system.     

Delaunay方法的改进及其在地下水污染监测网设计中的应用

地下水污染监测网的设计包括取样点在空间上的采样位置和时间上的取样频率这两方面的确定,其目的是为了准确刻画污染羽在含水层中随时间的变化状况。介绍了一种新的基于Delaunay三角形剖分方法用于地下水污染监测网在空间上的去冗余设计。同时,针对Delaunay方法没有考虑含水层参数变化和不能新增必要监测点的缺陷,提出了改进措施。改进的Delaunay方法可以直接与污染物的运移模型耦合,通过获取的含水层参数能够比较好地检验监测网在空间上的去冗余设计效果。实例研究表明,该方法在保证监测精度的基础上,能够找到最优的空间取样位置点,同时与改进前相比,利用该方法得到的地下水污染监测网能更加客观地反映污染物在含水层中的空间分布特征。     

Analysis of the distribution of inorganic constituents in a landfill leachate-contaminated aquifer: Astrolabe Park,Sydney, Australia

An investigation was conducted at Astrolabe Park landfill, a decommissioned municipal landfill in Sydney, Australia, to assess the physical and chemical processes affecting the distribution of inorganic constituents in the leachate plume. The plume is migrating from the landfill towards a groundwater-fed pond into which leachate-impacted groundwater discharges. Borehole geophysical logging and depth-discrete groundwater sampling were used to delineate the distribution of the leachate plume along two groundwater flow paths between the landfill and the shore of the pond. Borehole geophysical logs indicate a strong correlation between bulk and fluid electrical conductivity (EC) values, and help to identify small-scale heterogeneities that comprise a major constraint on contaminant transport within the aquifer. Variations in the distribution of several indicator parameters (EC, HCO₃^–, pH, Eh, NH₄⁺/NO₃^–, S^2–/SO₄^2–) are used to assess the dominant processes affecting contaminant distribution along the flow path, including mixing of fresh and contaminated groundwater, oxidation/reduction reactions and ion exchange.     

基于过程模拟的李官堡水源地地下水污染预警

基于过程模拟的地下水污染预警是基于包气带和饱和带过程模拟的耦合实现的,且融合了涵盖整个地下水系统的四个预警指标。选取浑河冲洪积扇的李官堡水源地为例,基于Hydrus-1D和Visual Modflow分别进行包气带过程模拟和饱和带过程模拟,针对潜层和承压层的复杂程度分别制定了各自的预警临界值;经分析可知,随着时间的推移,地表污染物持续进入到地下水中,污染晕范围不断扩大,地下水中最大浓度值和水源井浓度值也持续增加,且承压含水层的预警级别要高于潜水含水层;同时基于潜层和承压层中污染物最大浓度值与时间分别近似呈线性关系和指数关系可进行长时间尺度的预警;并提出了零级预警区和一级预警区管理措施以监测和预防为主,二级预警区、三级预警区和四级预警区管理措施以控制和监测为主。     

昆明地热田岩溶地下热水系统中溶质运移的模拟

通过对昆明地热田深层基岩地下热水系统的地质、水文地质条件和开采现状的分析,建立了考虑温压变化和越流条件的岩溶热储层中地下热水的水流和溶质(污染物)运移的准三维非稳定流数学模型。对开采条件下地热田Ⅱ 块段地下热水系统中水位及F- 、Cl- 、NH+4 、SO2-4 浓度的模拟结果表明,所建立的模型合理、可靠,具有较高的仿真性。模型可预测不同条件下地热田地下热水的流场和溶质浓度的动态变化趋势,为防治地下热水环境的进一步恶化提供参考。      

某氨氮污染地下水体抽出 -处理系统优化模拟研究

抽出 -处理系统设计多侧重于考虑修复初期的效率,在修复后期通常效率低下,产生拖尾现象,其优化的关键在于布设的井群系统能否高效抽出受污染的地下水体。利用溶质运移数值模拟可为井群布设和抽水方案优化提供依据。本研究旨在优化我国北方某化肥厂高浓度氨氮污染的地下水体的抽出 -处理修复系统,节约时间和成本。在水文地质调查及氨氮浓度监测的基础上,综合考虑井数、抽水天数和总抽水量三个变量,采用中轴线法与三角形法结合的布井方法,利用GMS软件反复试算,筛选出三种较优抽水方案并进一步模拟优化,最终从中选出最优抽水方案。结果,相比最初方案（方案1）,最优方案（方案3）将修复周期缩短了23个月,抽水总量减少了约31.9×104 m3,而抽水井数量仅增加了1口。该模型进行了稳定流水位拟合验证和4期非稳定流实测溶质浓度验证,较符合实际。结果表明,针对抽水井数量不足引起的拖尾问题,关键因素在于合理的井位布设与分阶段的抽水模式。在修复过程中,及时对地下水中污染物进行监测,并随着污染羽变化过程及时调整抽水方案,保证高浓度区一直有抽水井进行较大流量抽水,可有效提高修复效率并缩短修复周期。     

Hydrogeology and historical assessment of a classic sequential-land use landfill site,Illinois, U.S.A.

The Blackwell site in northeastern Illinois was a classic sequential-use project combining land reclamation, a sanitary landfill, and a recreational park. This paper adds a recent assessment of leachate generation and groundwater contamination to the site's unfinished record. Hydrogeological studies show that (1) the landfill sits astride an outwash aquifer and a till mound, which are separated from an underlying dolomite aquifer by a thin, silty till; (2) leachate leaks from the landfill at an estimated average rate between 48 and 78 m³/d; (3) the resultant contaminant plume is virtually stagnant in the till but rapidly diluted in the outwash aquifer, so that no off-site contamination is detected; (4) trace VOC levels in the dolomite probably indicate that contaminants have migrated there from the landfill-derived plume in the outwash. Deviations from the original landfill concepts included elimination of a leachate collection system, increased landfill size, local absence of a clay liner, and partial use of nonclay cover. The hydrogeological setting was unsuitable for the landfill as constructed, indicating the importance of detailed geological consideration in landfill and land-use planning.