水位波动下包气带透镜体影响LNAPL迁移的数值模拟研究

轻质非水相液体（LNAPLs）在土壤包气带中具有多相态特征,非均质性、地下水位波动等因素将显著增加包气带内LNAPLs污染的复杂程度。已有研究多关注于揭示包气带内自由相LNAPLs的污染过程,少有更为深入地探究水位波动时非均质结构对LNAPLs迁移及各相态分布规律的影响。基于TOUGH2程序构建包气带多相流数值模型,以揭示透镜体结构与地下水位波动共同作用下LNAPLs迁移过程及相态分布。研究结果表明:（1）含水率变化是LNAPLs迁移分布的主要控制因素,包气带内受透镜体介质岩性、水位波动影响所呈现的含水率变化直接控制LNAPLs迁移规律及分布特征;（2）水位恒定时,细砂透镜体使LNAPL呈“蓄积穿透-横向扩展-绕流”迁移,粗砂透镜体则是LNAPL垂向迁移的“优势通道”,水位波动引起的细砂透镜体含水率变化使“绕流”显著增强,粗砂透镜体则进一步呈现“优势空间”作用;（3）水位恒定时,细砂透镜体模型中LNAPL滞留于透镜体内部,粗砂模型中LNAPL则集中于透镜体下方,水位波动下透镜体附近LNAPL分布范围扩展,两模型LNAPL分布面积较水位恒定时分别增大51%、63%;（4）两模型中LNAPL挥发通量均呈“先减小-后增大”规律,并受LNAPL-气体接触条件及LNAPL分布状况共同作用,水位波动打破三相平衡状态,主要表现为水位抬升阶段LNAPL挥发增强,此时两模型中平均挥发量较水位恒定时增大124%～126%。研究为非均质石油污染场地中的LNAPL污染过程认识提供了科学的理论依据。     

Field validation of radon monitoring as a screening methodology for NAPL-contaminated sites

Screening methodologies aim at improving knowledge about subsurface contamination processes before expensive intrusive operations, i.e. drilling and core-sampling, well installation and development, sampling of groundwater and free-phase product, are implemented. Blind field tests carried out at a hydrocarbon storage and distribution center in NE Spain suggest that Rn monitoring can be effectively used to locate the boundaries of subsurface accumulations of NAPLs. Sixty seven measurements of Rn in soil air were performed with a SARAD RTM 2100 current-ionization alpha-particle spectrometer following a 10 m square grid. Reductions of ²²²Rn concentration above a pool of LNAPL due to the preferential partition of Rn into the organic phase were spatially analyzed and resolved to yield the surface contour of the NAPL source zone. This surface trace of the source zone agreed well with the extent and situation inferred from measurements of free-phase thickness taken at eight monitoring wells at the site. Moreover, the good repeatability (as measured by replicate measurements at the same sampling point) and spatial resolution of the technique suggest that the boundaries of the plume can be delineated at the sub-decametre level.     

Analysis of subsurface temperature data to quantify groundwater recharge rates in a closed Altiplano basin,northern Chile

Quantifying groundwater recharge is a fundamental part of groundwater resource assessment and management, and is requisite to determining the safe yield of an aquifer. Natural groundwater recharge in arid and semi-arid regions comprises several mechanisms: in-place, mountain-front, and mountain-block recharge. A field study was undertaken in a high-plain basin in the Altiplano region of northern Chile to quantify the magnitude of in-place and mountain-front recharge. Water fluxes corresponding to both recharge mechanisms were calculated using heat as a natural tracer. To quantify in-place recharge, time-series temperature data in cased boreholes were collected, and the annual fluctuation at multiple depths analyzed to infer the water flux through the unsaturated zone. To quantify mountain-front recharge, time-series temperature data were collected in perennial and ephemeral stream channels. Streambed thermographs were analyzed to determine the onset and duration of flow in ephemeral channels, and the vertical water fluxes into both perennial and ephemeral channels. The point flux estimates in streambeds and the unsaturated zone were upscaled to channel and basin-floor areas to provide comparative estimates of the range of volumetric recharge rates corresponding to each recharge mechanism. The results of this study show that mountain-front recharge is substantially more important than in-place recharge in this basin. The results further demonstrate the worth of time-series subsurface temperature data to characterize both in-place and mountain-front recharge processes.     

轻非水相液体在地下环境中的运移特征与模拟预测研究

通过自行设计制备的二维砂箱模拟轻非水相液体(LNAPL)在湿润多孔介质中的渗漏,得出LNAPL在地下环境中的入渗、迁移及分布规律,并利用多相流体理论对其污染机理进行分析.在实验基础上建立模型来预测渗漏的基本特征,即渗流带中污染锋面扩展速度和透镜体的最终形状,预测结果与实测结果较为接近.这种预测模型对控制和治理地下水中LNAPL污染是至关重要的.     

Multiphase Transport of Tritium in Unsaturated Porous Media—Bare and Vegetated Soils

Tritium is a short-lived radioactive isotope (T _1/2=12.33 yr) produced naturally in the atmosphere by cosmic radiation but also released into the atmosphere and hydrosphere by nuclear activities (nuclear power stations, radioactive waste disposal). Tritium of natural or anthropogenic origin may end up in soils through tritiated rain, and may eventually appear in groundwater. Tritium in groundwater can be re-emitted to the atmosphere through the vadose zone. The tritium concentration in soil varies sharply close to the ground surface and is very sensitive to many interrelated factors like rainfall amount, evapotranspiration rate, rooting depth and water table position, rendering the modeling a rather complex task. Among many existing codes, SOLVEG is a one-dimensional numerical model to simulate multiphase transport through the unsaturated zone. Processes include tritium diffusion in both, gas and liquid phase, advection and dispersion for tritium in liquid phase, radioactive decay and equilibrium partitioning between liquid and gas phase. For its application with bare or vegetated (perennial vegetation or crops) soil surfaces and shallow or deep groundwater levels (contaminated or non-contaminated aquifer) the model has been adapted in order to include ground cover, root growth and root water uptake. The current work describes the approach and results of the modeling of a tracer test with tritiated water (7.3×10⁸ Bq m⁻³) in a cultivated soil with an underlying 14 m deep unsaturated zone (non-contaminated). According to the simulation results, the soil’s natural attenuation process is governed by evapotranspiration and tritium re-emission. The latter process is due to a tritium concentration gradient between soil air and an atmospheric boundary layer at the soil surface. Re-emission generally occurs during night time, since at day time it is coupled with the evaporation process. Evapotranspiration and re-emission removed considerable quantities of tritium and limited penetration of surface-applied tritiated water in the vadose zone to no more than ∼1–2 m. After a period of 15 months tritium background concentration in soil was attained.     

太行山山前平原地下水补给规律分析:以河北栾城为例

采用水均衡法与通量法对太行山山前平原(河北栾城)进行了地下水补给评价,结果表明:近20 a地下水入渗补给量为8～172mm/a(水均衡法),平均值49.3 mm/a,变化幅度大.其中2004年入渗补给量(63mm/a)比采用通量法计算结果(28.2mm/a)大一倍,表明土壤优先流对入渗补给作用显著.年入渗补给量总体上随有效降雨量增加、地下水位下降速率减少、作物实际蒸发蒸腾量减少而增加,但与有效灌溉量和灌溉量之和的相关性不显著.通过长序列地下水位与灌溉量等数据估算得到研究区侧向净补给量为125.2 mm/a,约占地下水总补给量的71.7％(175.4 mm/a),说明地下水主要以侧向补给为主.同时计算了入渗补给与侧向补给地下水的延迟时间,分别为60 a和1个月.     

零通量面法在沈阳地区地下水资源评价中的应用

降水对地下水的入渗补给和大气蒸发作用下地下水的消耗是地下水资源的重要组成部分。降水入渗和地下水蒸发这两种作用均是通过包气带进行的。从包气带着手，运用零通量法确定这些量已在实际中得到应用。以沈阳地区为例，针对该区集中频繁降水与蒸发排泄交替进行导致零通量紊乱易变的特点，在对有关问题深入探讨的基础上，对浅层潜水入渗补给量（包括蒸发量）和非饱和渗透系数进行了计算，取得了较好的效果。     

浅层地下水补给对人类活动影响的响应特征研究

本文通过数据整理分析, 查明滹沱河流域平原区1976-2005年以开采量、灌区引水量和河道过水量为代表的人类活动逐渐增强, 分析了浅层地下水补给在大气降水减少和人类活动逐渐增强背景下大幅减少的响应特征: 综合补给量所占比率由1976-1980年的21.6%下降到2000-2005年的11.3%。随后, 从开采量、河道过水量和灌区引水量三个方面分析了浅层地下水补给响应人类活动的变化特征: 地下水补给量与开采量呈y=65.412x-0.2576模式随降水量增减而负相关变化; 河道渗漏补给量和渠水入渗量在地下水位不同埋深条件下表现出随来水量、引水量增加而增大的态势, 但在不同埋深条件下, 河道渗漏补给量与来水量之间、渠水入渗量和引水量之间关系不同。     

Estimation of groundwater recharge of shallow aquifer on humid environment in Yaounde,Cameroon using hybrid water-fluctuation and hydrochemistry methods

A study of environmental chloride and groundwater balance has been carried out in order to estimate their relative value for measuring average groundwater recharge under a humid climatic environment with a relatively shallow water table. The hybrid water fluctuation method allowed the split of the hydrologic year into two seasons of recharge (wet season) and no recharge (dry season) to appraise specific yield during the dry season and, second, to estimate recharge from the water table rise during the wet season. This well elaborated and suitable method has then been used as a standard to assess the effectiveness of the chloride method under forest humid climatic environment. Effective specific yield of 0.08 was obtained for the study area. It reflects an effective basin-wide process and is insensitive to local heterogeneities in the aquifer system. The hybrid water fluctuation method gives an average recharge value of 87.14 mm/year at the basin scale, which represents 5.7% of the annual rainfall. Recharge value estimated based on the chloride method varies between 16.24 and 236.95 mm/year with an average value of 108.45 mm/year. It represents 7% of the mean annual precipitation. The discrepancy observed between recharge value estimated by the hybrid water fluctuation and the chloride mass balance methods appears to be very important, which could imply the ineffectiveness of the chloride mass balance method for this present humid environment.     

Sources of oxygen flux in groundwater during induced bank filtration at a site in Berlin, Germany

The microbial degradation of pharmaceuticals found in surface water used for artificial recharge is strongly dependent on redox conditions of the subsurface. Furthermore the durability of production wells may decrease considerably with the presence of oxygen and ferrous iron due to the precipitation of trivalent iron oxides and subsequent clogging. Field measurements are presented for oxygen at a bank filtration site in Berlin, Germany, along with simplified calculations of different oxygen pathways into the groundwater. For a two-dimensional vertical cross-section, oxygen input has been calculated for six scenarios related to different water management strategies. Calculations were carried out in order to assess the amount of oxygen input due to (1) the infiltration of oxic lake water, (2) air entrapment as a result of water table oscillations, (3) diffusive oxygen flux from soil air and (4) infiltrating rainwater. The results show that air entrapment and infiltrating lake water during winter constitute by far the most important mechanism of oxygen input. Oxygen input by percolating rainwater and by diffusive delivery of oxygen in the gas phase is negligible. The results exemplify the importance of well management as a determining factor for water oscillations and redox conditions during artificial recharge.     

Chloride and the environmental isotopes as the indicators of the groundwater recharge in the Gobi Desert,northwest China

The long term recharge in Gobi Desert from Hexi Corridor to Inner Mongolia Plateau was estimated to be 1 mm year⁻¹ by using the chloride mass balance method from one unsaturated zone profile, which shows that no effective modern recharge is taking place. A good rainfall database from Zhangye provides definition of the stable isotopic composition of modern rainfall. The signature of groundwater from the late Pleistocene differs markedly from that of the Holocene, shown clearly by the compositions of −10.5‰ δ¹⁸O as compared with values of −7‰ at the present day. It is apparent that the groundwaters in the Minqin Basin, Ejina Basin and feeding the lake system of the Badain Jaran are part of a regional flow network related to a wetter past climate as source of recharge. The recharge source in the past and to a limited extent in the more arid conditions of the present day included the foothills of the mountains of the Tibetan Plateau. The tritium age determinations accurate to the year are impossible and of no meaning to groundwater studies. A tritium value in the groundwater means multiple recharge ages in this region.     

Conceptual model of recharge to southeastern Badain Jaran Desert groundwater and lakes from environmental tracers

Sources of groundwater recharge to the Badain Jaran Desert in China have been investigated using geochemical and isotopic techniques. Stable isotope compositions (δ¹⁸O and δ²H) of shallow groundwater and surface water from oasis lakes evolve from a starting composition considerably depleted compared to local unsaturated zone moisture, confirming inferences from chloride mass balance that direct infiltration of precipitation is not a volumetrically important source of recharge to the shallow aquifer in the study area. Shallow phreatic and deeper confined groundwater bodies appear unconnected based on chemical composition and radiocarbon activities. Hydrogeologic evidence points toward a bordering mountain range (Yabulai) as a likely recharge zone, which is consistent with tracer results. A mean residence time in the range of 1–2 ka for the desert’s southeastern margin is inferred from radiocarbon. These results reveal that some replenishment to the desert aquifer is occurring but at a rate much lower than previously suggested, which is relevant for water resources planning in this ecologically sensitive area.     

Geochemistry of sediment moisture in the Badain Jaran desert: Implications of recent environmental changes and water-rock interaction

Unsaturated zone pore water has the potential to record history of recharge, palaeoenvironment, pollution movement and water-rock interaction as it percolates through and moves towards the water table. In this study, two 6-m cores from the Badain Jaran desert (NW China) were collected to explore this potential using directly extracted moisture. Pore waters in these unsaturated zone sediments (1–5% moisture by wet weight) were directly extracted using immiscible liquid displacement and then analysed for major anions, cations and trace elements. Results show enrichment in pore water chemistry in the top 1–2 m where strong temperature and moisture fluxes occur. The enrichment in cations relative to chloride is primarily due to silicate mineral dissolution during infiltration. High nitrate and low iron concentrations indicate the overall oxidizing environment, which allows the mobility of oxyanions, such as uranium, arsenic and chromium. The trace elements show enrichment in the upper zone of fluctuation where chemical gradients are strong, but with lesser reaction lower in the profile. The calculated groundwater recharge rates using the chloride mass balance are negligible in this arid region between 1.5 and 3.0 mm/year. The modern rainfall infiltration signature contrasts with that of the underlying groundwater body, which has a distant, regional recharge signature.This reconnaissance study demonstrates the potential for a new geochemical approach to studying geochemical processes in the unsaturated sediments in semi-arid environments due to both natural and human influences. The use of directly extracted water, rather than extraction by dilution (elutriation), facilitates an improved understanding of hydrological and geochemical processes in the unsaturated zone and into the capillary fringe at the water table, because it avoids potential chemical changes induced during elutriation.     

Noble gas and isotope geochemistry in western Canadian Arctic watersheds: tracing groundwater recharge in permafrost terrain

In Canada’s western Arctic, perennial discharge from permafrost watersheds is the surface manifestation of active groundwater flow systems with features including the occurrence of year-round open water and the formation of icings, yet understanding the mechanisms of groundwater recharge and flow in periglacial environments remains enigmatic. Stable isotopes (δ¹⁸O, δD, δ¹³C_DIC), and noble gases have proved useful to study groundwater recharge and flow of groundwater which discharges along rivers in Canada’s western Arctic. In these studies of six catchments, groundwater recharge was determined to be a mix of snowmelt and precipitation. All systems investigated show that groundwater has recharged through organic soils with elevated P_CO2, which suggests that recharge occurs largely during summer when biological activity is high. Noble gas concentrations show that the recharge temperature was between 0 and 5 °C, which when considered in the context of discharge temperatures, suggests that there is no significant imbalance of energy flux into the subsurface. Groundwater circulation times were found to be up to 31 years for non-thermal waters using the ³?H-³He method.     

Groundwater flow modeling of Quaternary aquifer Ras Sudr,Egypt

Recently, Ras Sudr (the delta of Wadi Sudr) area received a great amount of attention due to different development expansion activities directed towards this area. Although Quaternary aquifer is the most prospective aquifer in Ras Sudr area, it has not yet completely evaluated. The present work deals with the simulation of the Quaternary groundwater system using a three-dimensional groundwater flow model. MODFLOW code was applied for designing the model of the Ras Sudr area. This is to recognize the groundwater potential as well as exploitation plan of the most prospective aquifer in the area. The objectives were to determine the hydraulic parameters of the Quaternary aquifer, to estimate the recharge amount to the aquifer, and to determine the hydrochemistry of groundwater in the aquifer. During this work, available data has been collected and some field investigation has been carried out. Groundwater flow model has been simulated using pilot points’ method. SEAWAT has been also applied to simulate the variable-density flow and sea water intrusion from the west. It can be concluded that: (1) the direction of groundwater flow is from the east to the west, (2) the aquifer system attains a small range of log-transformed hydraulic conductivity. It ranges between 3.05 and 3.35 m/day, (3) groundwater would be exploited by about 6.4 × 10⁶ m³/year, (4) the estimated recharge accounts for 3 × 10⁶ m³/year, (5) an estimated subsurface flow from the east accounts for 2.7 × 10⁶ m³/year, (6) the increase of total dissolved solids (TDS) most likely due to dilution of salts along the movement way of groundwater from recharge area to discharge area in addition to a contribution of sea water intrusion from the west. Moreover, it is worth to note that a part of TDS increase might be through up coning from underlying more saline Miocene sediments. It is recommended that: (1) any plan for increasing groundwater abstraction is unaffordable, (2) reliable estimates of groundwater abstraction should be done and (3) automatic well control system should be made.     

A GIS-based DRASTIC model for assessing shallow groundwater vulnerability in the Zhangye Basin,northwestern China

Groundwater vulnerability is a cornerstone in evaluating the risk of groundwater contamination and developing management options to preserve the quality of groundwater. Based on the professional model (DRASTIC model) and geographical information system (GIS) techniques, this paper carries out the shallow groundwater vulnerability assessment in the Zhangye Basin. The DRASTIC model uses seven environmental parameters (depth to water, net recharge, aquifer media, soil media, topography, impact of vadose zone, and hydraulic conductivity) to characterize the hydrogeological setting and evaluate aquifer vulnerability. According to the results of the shallow groundwater vulnerability assessment, the Zhangye Basin can be divided into three zones: low groundwater vulnerability risk zone (risk index <120); middle groundwater vulnerability risk zone (risk indexes 120–140) and high risk zone (risk index >140). Under the natural conditions, the middle and high groundwater vulnerability risk zones of the Zhangye Basin are mainly located in the groundwater recharge zones and the important cities. The high, middle and low groundwater vulnerability risk zones of the Zhangye Basin cover around 17, 21 and 62% of study area, respectively.     

岩溶含水系统降水入渗补给研究进展

岩溶含水系统中赋存着丰富的优质地下水,而大气降水是浅部可供开采的岩溶地下水的最主要补给来源。受岩溶含水系统各向异性、不均一性和直接观测难度大等因素的影响,降水入渗补给量的计算是一个非常复杂的过程。确定岩溶含水系统的汇水范围是降水入渗补给计算的首要问题,示踪法与经验公式法被证明是最有效的两种方法。降水入渗补给量的计算方法主要包括水文过程线法分析法、氧同位素法、氯质量平衡法、基于GIS的多变量综合分析法和模型法。本文对目前岩溶含水系统降水入渗补给计算方法的关键点和适用条件进行了总结和对比,同时指出大气降水物理化学性质的时空特征以及水-岩反应可以作为未来研究岩溶含水系统降水补给的研究方向之一。     

种植条件下降雨灌溉入渗试验研究

基于清水河平原头营和黑城试验场降雨(灌溉)入渗过程土壤水分运移观测试验数据的分析研究,笔者应用能量观点描述了包气带水分运移的分带性、降雨(灌溉)入渗补给地下水的水分条件和地下水入渗补给过程的基本特征。应用蒸散量模型、土壤水分通量模型,计算了作物生长期的蒸发蒸腾量、土壤贮水量的变化量、400cm深度处的土壤水分渗漏量及渗漏系数。从多年的角度分析了深层土壤水分渗漏量、渗漏系数与地下水入渗补给量和补给系数的关系。它对分析降雨(灌溉)入渗对地下水的补给过程和定量分析地下水入渗补给量、入渗补给系数具有重要价值。     

含单裂隙非饱和带中轻非水相流体修复的数值模拟

针对非饱和带中油类污染物时空分布的研究,室内实验很难定量分析运移机理,野外检测成本高且破坏地层。数值模拟法作为一种应用广且成熟的方法,可以用来分析油类污染物在非饱和带中的运移规律。为了研究单井抽提及原位冲洗修复时,含单裂隙非饱和带中轻非水相流体（Light non-aqueous phase liquids,LNAPL）的时空变化规律,建立了数值模型,分析不同条件下LNAPL的修复效果及时空变化规律。模拟结果发现,LNAPL注入时优先流入裂隙,停止注入时优先流出裂隙。单井抽提修复模拟表明,抽提流量越大,修复效率越高。原位冲洗技术能有效补充地下水,防止产生新的环境问题;注水井起到“冲洗”及稀释污染物的作用,模拟最优方案修复面积达到96%,修复率达到75%,LNAPL饱和度控制在约0.05;对比分析发现,注水井布设在污染物范围的上边界时修复效果最好,能有效“冲洗”污染物并携带至抽提井中抽出地表。该研究为受轻油污染的土壤及地下水修复提供了科学的理论依据及有效的评估方法。     

Alluvial aquifer recharge enhanced by a natural dam: feasibility assessment based on multidisciplinary characterization (Khuzestan,Southwest Iran)

In the Kushkak Valley (Khuzestan, Southwest Iran) an anticlinal structure has partially impounded an ephemeral stream. This natural impounded area has been chosen for an artificial recharge site due to its current rate of recharge, capability to store water and favorable situation for the construction of man-made barriers to stream flow. The aquifer to be recharged is the Kushkak unconfined aquifer which consists of medium to coarse-grained alluvial deposits that overlie consolidated conglomerate rock. In this semi-arid area with infrequent relatively heavy falls of rain, alluvial aquifer recharge can be an important process that sustains shallow, over-exploited groundwater bodies. In this investigation a multidisciplinary approach including: hydrometerological studies, and a detailed hydrogeochemical survey, have been carried out. Other essential prerequisite parameters for the scheme were also taken into account to determine the suitability of this location for groundwater artificial recharge. The assessment has brought out that (1) the proposed reservoir will conserve a major part of the water being lost, (2) annual runoff of about 0.27 MCM can be injected into the aquifer through recharge from impounded water, (3) hydrochemical data from surface water and from the Kushkak aquifer water demonstrates that dilution and change in compositional trend in the groundwater proximal to the impounded alluvial bed areas would be expected based on the infiltration capacity of this site, and (4) cost–benefit ratio of the project is 1:2 and it is assumed to recover the investment within six years.