基于解析法和数值法的非稳定流抽水试验参数反演

含水层的水文地质参数是进行地下水资源计算、地下水污染防控等所必需的基础数据,结合数值模拟技术进行含水层参数反演很有必要.按照1:5万水文地质调查规范在江汉平原仙桃市杨林尾镇复兴水厂不同含水层位开展抽水试验,包括深层含水层单孔抽水试验以及浅层含水层中群孔(2孔)抽水试验.对于单孔抽水试验,应用第1类越流系统井流理论进行参数反演;对于群孔抽水试验,推导了特定综合井函数,并利用特定标准曲线匹配法和直线图解法求解了含水层参数.随后利用FEFLOW软件建立了相应数值模型,拟合了含水层参数.结果表明:浅层含水层的渗透系数变化范围为21.66~54.00 m/d,贮水率变化范围为1.28×10-5~8.00×10-4 m-1;深层含水层渗透系数变化范围为1.27~7.00 m/d,贮水率变化范围为3.90×10-6~5.00×10-6 m-1.对于深层承压含水层而言,越流补给量较大.采用数值模拟方法结合抽水试验数据求参,综合考虑了含水层结构,拟合效果好,所得结果更加可靠.      

Numerical groundwater-flow modeling to evaluate potential effects of pumping and recharge: implications for sustainable groundwater management in the Mahanadi delta region,India

Process-based groundwater models are useful to understand complex aquifer systems and make predictions about their response to hydrological changes. A conceptual model for evaluating responses to environmental changes is presented, considering the hydrogeologic framework, flow processes, aquifer hydraulic properties, boundary conditions, and sources and sinks of the groundwater system. Based on this conceptual model, a quasi-three-dimensional transient groundwater flow model was designed using MODFLOW to simulate the groundwater system of Mahanadi River delta, eastern India. The model was constructed in the context of an upper unconfined aquifer and lower confined aquifer, separated by an aquitard. Hydraulic heads of 13 shallow wells and 11 deep wells were used to calibrate transient groundwater conditions during 1997–2006, followed by validation (2007–2011). The aquifer and aquitard hydraulic properties were obtained by pumping tests and were calibrated along with the rainfall recharge. The statistical and graphical performance indicators suggested a reasonably good simulation of groundwater flow over the study area. Sensitivity analysis revealed that groundwater level is most sensitive to the hydraulic conductivities of both the aquifers, followed by vertical hydraulic conductivity of the confining layer. The calibrated model was then employed to explore groundwater-flow dynamics in response to changes in pumping and recharge conditions. The simulation results indicate that pumping has a substantial effect on the confined aquifer flow regime as compared to the unconfined aquifer. The results and insights from this study have important implications for other regional groundwater modeling studies, especially in multi-layered aquifer systems.     

Modeling approaches and strategies for data-scarce aquifers: example of the Dar es Salaam aquifer in Tanzania

Management of groundwater resources can be improved by using groundwater models to perform risk analyses and to improve development strategies, but a lack of extensive basic data often limits the implementation of sophisticated models. Dar es Salaam in Tanzania is an example of a city where increasing groundwater use in a Pleistocene aquifer is causing groundwater-related problems such as saline intrusion along the coastline, lowering of water-table levels, and contamination of pumping wells. The lack of a water-level monitoring network introduces a problem for basic data collection and model calibration and validation. As a replacement, local water-supply wells were used for measuring groundwater depth, and well-top heights were estimated from a regional digital elevation model to recalculate water depths to hydraulic heads. These were used to draw a regional piezometric map. Hydraulic parameters were estimated from short-time pumping tests in the local wells, but variation in hydraulic conductivity was attributed to uncertainty in well characteristics (information often unavailable) and not to aquifer heterogeneity. A MODFLOW model was calibrated with a homogeneous hydraulic conductivity field and a sensitivity analysis between the conductivity and aquifer recharge showed that average annual recharge will likely be in the range 80–100 mm/year.     

Analytical solutions for constant-rate test in bounded confined aquifers with non-Darcian effect

This paper proposes a simplified analytical solution considering non-Darcian and wellbore storage effect to investigate the pumping flow in a confined aquifer with barrier and recharge boundaries. The mathematical modelling for the pumping-induced flow in aquifers with different boundaries is developed by employing image-well theory with the superposition principle, of which the non-Darcian effect is characterized by Izbash’s equation. The solutions are derived by Boltzmann and dimensionless transformations. Then, the non-Darcian effect and wellbore storage are especially investigated according to the proposed solution. The results show that the aquifer boundaries have non-negligible effects on pumping, and ignoring the wellbore storage can lead to an over-estimation of the drawdown in the first 10 minutes of pumping. The higher the degree of non-Darcian, the smaller the drawdown.     

Oscillatory Pumping Test to Estimate Aquifer Hydraulic Parameters in a Bayesian Geostatistical Framework

Comprehensive information about the spatial distribution of the subsurface hydraulic properties is crucial to model groundwater flow, to predict solute transport in aquifers and to design remediation actions. In this work, a Bayesian Geostatistical approach, as implemented in bgaPEST, was adopted to estimate the hydraulic properties of a well field located at the Campus of Science and Technology of the University of Parma (Northern Italy), in a contest of a highly parameterized inversion. Head data, collected by means of multi frequency oscillatory pumping tests, were used to both estimate the hydraulic parameters and validate the results. The groundwater flow processes were modelled by means of MODFLOW 2005 and an adjoint-state formulation of the same software was used to efficiently calculate the sensitivity matrix, required by the inverse procedure. The Bayesian Geostatistical approach estimated the hydraulic conductivity and specific storage fields, handling a large number of parameters. The results of the inversion are consistent with the alluvial nature of the investigated aquifer and the preliminary traditional pumping tests carried out at the site.     

论“单位涌水量就是导水系数”成立的特殊条件

针对截取侧向径流的抽水井,兰太权提出一个解释单位涌水量的公式,认为“单位涌水量就是导水系数”(q=T),并以砂箱试验的结果作为证据。然而,这个等式的推导过程缺少严谨的水动力学基础。砂箱试验的边界条件等价于含有1个或2个定水头边界的矩形承压含水层,单井抽水的稳定流场可以用镜像井点叠加法求解,从理论上表明单位涌水量与导水系数成正比,而比例系数取决于含水层的形状和抽水井的半径。在特定条件下,含水层和抽水井的尺寸可以造成单位涌水量与导水系数相等的情况,兰太权发表于文献中的2组砂箱试验恰好符合这种条件,但并不能证明q=T具有普适性。新的砂箱试验避免了几何尺寸的这种特殊组合,得到了显著偏离q=T的结果。     

The dependence of stream depletion by seasonal pumping on various hydraulic characteristics and engineering factors

Compensation pumping is used to alleviate deficiencies in streamflow discharge during dry seasons. Short-term groundwater pumping can use aquifer storage instead of catchment-zone water until the drawdown reaches the edge of the stream. The capacitance is a complex, dimensionless parameter of an aquifer system that defines the delayed effect on streamflow when there is groundwater pumping. This parameter is a function of aquifer hydraulic characteristics, pumping time, and distance between the well and stream edge; the latter can involve stream leakance and vertical leakance of an associated aquitard. Three typical hydraulic cases of combined water systems (major catchment-zone wells close to the stream and compensation pumping wells) were classified depending on their capacitance structure (i.e. the relationship between surface water and groundwater): (1) perfect hydraulic connection between the stream and aquifer; (2) imperfect hydraulic connection between the stream and aquifer; and (3) essentially imperfect hydraulic connection between the stream and the underlying confined aquifer. The impact of various hydraulic characteristics and engineering factors on stream depletion was examined by conceptual and numerical modeling. To predict the suitability and efficiency of a combined water system application, regression tests were undertaken on unit stream depletion and capacitance, and power dependencies were defined.     

地面沉降对含水层参数及给水能力的影响研究

地面沉降主要由深层地下水开采造成,含水层的压缩释水是深层地下水开采量主要构成来源,同时随其压缩变形孔隙比减小造成储水系数、渗透系数的减小,对弱透水层非弹性释水量、越流补给量造成影响。地面沉降过程中的水文地质参数非线性变化及对承压含水层系统的反馈作用也成为水文地质领域的前沿问题。为对地下水开采量、沉降量、地质参数变化以及给水能力变化之间的关系做一个较为定量定性的探究,以含水层压缩过程中的物理机制为依据,并基于沧州地区深层承压含水层的地下水位变化过程和水文地质参数,采用数学手段构建储水系数和渗透系数变化的一维非线性沉降模型。模拟结果显示随承压水头下降,储水系数最高可减小77%,含水层系统的给水能力和储水能力会随压缩变形减小50%甚至更多。研究成果为深入认识水文地质参数与应力变化相关关系、科学评价承压含水层地下水储水调节能力有重要借鉴意义。     

Hydrogeologic control of shear-related fracturing in Blue Ridge basement rocks: a case study from production wells in the Marshall Metagranite,Fauquier County,Virginia (USA)

Borehole geophysical logging of four production wells completed in Precambrian metagranite in Fauquier County, Virginia (USA), was conducted to characterize stratigraphy, collect water-bearing fracture orientations and describe vertical hydraulic gradients in the vicinity of each borehole. Long-term (48–90 h) single-well pump test data were reevaluated for each well to better characterize this locally important aquifer system. Single-well aquifer test analyses indicate mid-to-late-time infinite acting radial flow conditions within the fractured rock aquifer, followed by increasing late-time contribution of stored groundwater from recharge boundaries. Later-time pump test results are believed to indicate that water-bearing fractures within the metagranite are ultimately recharged by groundwater stored within the regolith. Assumptions about the presence of a recharge boundary sourced by the regolith were tested with a simple groundwater flow model that was calibrated to observed drawdown data associated with one of the long-term pump tests. This study identifies fracturing associated with shear-related metamorphic fabrics in the wellbore and demonstrates the significance of these fractures as mechanisms for accessing groundwater. Results from this investigation indicate that shear-related metamorphic fabrics can be important structures for integrating transmissive fracture networks within the Marshall Metagranite and possibly within other Blue Ridge basement rocks possessing similar metamorphic history.

     

Assessment of artificial aquifer recharge potential in the Kucuk Menderes River Basin, Turkey

The Kucuk Menderes River Basin in western Turkey has been facing continuous groundwater-level decline for decades. Previous studies have suggested that, to avoid aquifer depletion in the basin, artificial recharge structures should be constructed. To assess artificial aquifer recharge potential in one of the subbasins, a two-dimensional (2-D) groundwater model was set up using SEEP/W software. The material functions and parameters used in the model for both saturated and unsaturated conditions were taken from previous studies. The model has been calibrated under transient conditions. The excess runoff volume that could be collected in the recharge basins was estimated from flood frequency analysis. Various scenarios were simulated to observe the change in groundwater level and storage with respect to different exceedance probabilities. Simulation results suggest that a significant increase in groundwater storage is achieved by applying surface artificial-recharge methods. In addition to the recharge basins, to reinforce the effect of artificial recharge, simulations are repeated with underground dam construction at the downstream side of the basin. Although groundwater storage is increased with the addition of the dam, the increase in groundwater storage was not sufficient to warrant the construction.     

A case history of field pumping tests in a deep gravel formation in the Taipei Basin,Taiwan

33 large-diameter wells embedded in 2-m thick, 63-m deep diaphragm walls were constructed to reduce both the uplift pressures and the groundwater inflow during the excavations. As the actual thickness of the pumped aquifer is unknown, the installed wells are regarded as partial penetration wells. Single-well and multi-well pumping tests were conducted in the deep gravel formation of Taipei Basin to derive the hydraulic parameters and to investigate the drawdown characteristics at both the construction and remote sites. However, the tidal effect on the drawdown of both the pumping well and nearby observation wells was found significant. Additionally, wellbore storage, skin, and leakage need to be taken into account for deriving the hydraulic parameters. Hence, a method to remove these five factors influencing the drawdown curve is developed, which takes advantage from the late-time characteristics of drawdown data and the early-time behavior of drawdown. Some currently available semi-log graphic techniques are therefore proven applicable for parameter determination. Validity of the proposed method is verified by the good agreement between the calculated and the measured drawdown of both the pumping well and observation well.     

Quantitative analysis of springs

Growing demand for groundwater resources and stringent environmental concerns has led to large groundwater investigations, including characterization of aquifer systems that are hydraulically connected to springs. A pumping test is one of the most reliable means of quantifying hydraulic characteristics and the response of natural springs to pumping because it yields results that, in general, are representative of a larger area than are results from a single point observation. Recharge to the aquifer sustaining discharge from springs must be evaluated prior to the utilization of springs. The spring hydrograph is analyzed, as the shape of a hydrograph is a reflection of the response of the aquifer to recharge. The form and rate of recession provide significant information on the storage, lithological composition, and structural characteristics of the aquifer system sustaining the spring. Water tracing techniques have been developed and used over a period of centuries to delineate catchment boundaries, estimate groundwater flow velocities, determine areas of recharge, and identify sources of pollution of spring water.     

Application of advanced borehole geophysical logging to managed aquifer recharge investigations

Communities and water utilities are increasingly being forced to implement more hydrogeologically complex alternative water supply and storage options to meet increasing freshwater demands. The performance of managed aquifer recharge projects, including aquifer storage and recovery, is controlled by the movement and mixing of stored freshwater and native groundwater, and fluid–rock interactions, which, in turn, are strongly influenced by aquifer heterogeneity. Advanced borehole geophysical logging techniques developed for the oil and gas industry such as neutron-gamma ray spectroscopy, microresistivity imaging, and nuclear magnetic resonance, can provide hitherto unavailable fine-scale data on porosity (total and effective), hydraulic conductivity, salinity, and the mineralogical composition of aquifers. Data on aquifer heterogeneity obtained from advanced borehole geophysics logs, combined with information on larger-scale aquifer hydraulics obtained from pumping tests, have the potential for improving aquifer characterization and modeling needed for feasibility assessments and the design and optimization of the operation of managed aquifer recharge systems.     

Land subsidence caused by groundwater exploitation in Suzhou City,China

Suzhou City, located at the lower reaches of the Yangtze River in southeastern Jiangsu Province, is one of the few cities in China which suffer from severe ground settlement. A research project was carried out to investigate this problem. Geological and hydrogeological studies show that there is a multi-layered aquifer system with three distinct, soft mud layers of marine and lagoonal origins. An examination of historical records of groundwater extraction, water levels, and ground settlement shows that the ground subsidence is associated with the continuously increasing groundwater extraction in the deep, confined aquifer. It is believed that the consolidation of the soft mud layers, especially the third layer which is thick and close to the main pumped aquifer, contributes to the ground settlement. A three-dimensional finite difference numerical model representing the multi-layered aquifer system was developed to study the ground settlement in response to groundwater extraction. By calibrating the model with both the measured groundwater level and ground settlement, the aquifer parameters were estimated. The model outputs fit reasonably well with the observed results, which indicates that the numerical model can reproduce the dynamic processes of both groundwater flow and soil consolidation. The hydraulic conductivity of the third mud layer near the center of the ground settlement has been reduced by over 30% in the last 14 years. The gradual deterioration in the hydraulic conductivity of the mud may have significant adverse effect on the sustainable groundwater resource of the deep confined aquifer, since the recharge from the shallow aquifers through the mud layer is the only source of water to the deep aquifer. An analysis of the spatial distributions of groundwater drawdown and ground settlement shows that the area with maximum drawdown is not necessarily the area with maximum ground settlement due to the occurrence of the soft mud layer. A simple reallocation in pumping rates on the basis of the spatial distribution of the thick mud layer could significantly reduce the ground settlement. Electronic Publication     

The impact of groundwater recharge on land subsidence: a case study from the Cangzhou test area,Hebei Province,China

Groundwater recharge has become a method to prevent the expansion of land subsidence. A groundwater pumping–recharge test in Cangzhou city, Hebei Province (China), is taken as a representative example in order to monitor the changes in borehole water levels and land subsidence. The aquifer system consists of three main confined aquifers interspersed with silt and clays. Based on the lithology, the test and the Biot consolidation theory, a three-dimensional fully coupled numerical model is established to analyse the influence of groundwater recharge on the seepage field and displacement field. The results show that, due to recharge of the deeper confined aquifers (III1 and III2), the water levels rise rapidly and remain stable for a long time. By 163 days, the water-level rise is approximately 0.5 m, and the land also shows an obvious rebound. Therefore, groundwater recharge takes a long time to control land subsidence. Groundwater recharge controls the occurrence of land subsidence by increasing pore-water pressure and changing the characteristics of the aquifer system. However, the limitation is that the scope of impact of single-well recharge is limited, which is still a challenge to the governance of large areas.

     

Evaluation of groundwater withdrawal from a mountain watershed, Colorado, USA

The purpose of this study is to evaluate the groundwater-withdrawal potential of the Fraser River watershed, a mountainous drainage system in north-central Colorado. Laboratory tests, field investigations, and numerical modeling are conducted to present a quantitative understanding of the watershed’s groundwater-flow system. Aquifer hydraulic conductivity values obtained from aquifer tests range from 1E?5 to 1E?3 m/s. Groundwater withdrawal is concentrated in channel-fill deposits of the Troublesome Formation within the Fraser basin. A steady state groundwater-flow model of the Fraser River watershed is developed and calibrated using 24 observation wells in the Fraser River valley and estimated baseflow of the Fraser River. Modeling results suggest that surface recharge is the major source of groundwater in the watershed. Groundwater exits the watershed through evapotranspiration and discharge to rivers. Transient groundwater-flow modeling evaluates future withdrawal scenarios using the hydraulic head distribution from the steady state model as the initial condition. Drawdown within Troublesome Formation aquifers from the current pumping schedule approaches 2 m. When the daily pumping rate is doubled, drawdown approaches 4 m. The radius of influence is hundreds of meters to 1 km. Pumping wells withdraw approximately 2 and 15 % of groundwater flowing through the well field for hydraulic conductivity of 1E?3 and 1E?5 m/s, respectively. This study suggests that the groundwater system at the Fraser Valley could sustain current and future withdrawals, given that the current recharge condition is maintained.     

Analysis of transient groundwater flow through a stochastic modelling approach

This study demonstrates the application of a stochastic procedure to enhance confidence in the predictions of numerical groundwater flow models. The objective was to assess the hydrogeological conditions and groundwater potential of a semi-confined aquifer system in Legon, Accra. The study applied the parameter randomization procedure in the Groundwater Modelling System (GMS) to evaluate model uncertainties arising from uncertainties in the data of model input parameters. A total of 2500 equally likely solutions of the numerical model were simulated through the Latin hypercube stochastic option. Regarding the size of the terrain and the number of parameters used for calibrating the model, the 2500 model realizations were adjudged to be largely sufficient to represent the possible solutions that could arise from the various permutations of the parameter data. High variances would suggest significant differences in the realizations, indicating high levels of non-uniqueness in the solutions. The 2500 solutions were compared using standard deviations computed from the differences in the estimated horizontal hydraulic conductivity, specific yield and specific storage estimates. The transient model calibrated for this domain suggests very close similarities in the model-computed aquifer parameters for all the 2500 solutions. This suggests that any of the solutions could be used to represent the conditions prevailing in the domain. Risk analyses were then performed on abstractions through all the 2500 solutions. The study shows that the domain is sustained largely by lateral subsurface inflows from adjoining areas amounting to 399,278 m³/day, with direct vertical infiltration and percolation of precipitation accounting for an average of 2450 m³/day. The analysis suggests that at this rate of input, the domain can sustain groundwater abstraction to meet all water needs in the area. In addition, the domain has demonstrated resilience against annual increment in abstraction rates by 5 % over a 20-year period at the current recharge conditions. However, increasing abstraction rates by up to 200 % of the current abstraction rates over the 20-year period (2015–2035) but with a gradual decrease in groundwater recharge by up to 10 % over the period will result in drastic lowering of the hydraulic head especially in the south-central portions of the domain. This situation might induce reversal of the current flow and potentially lead to contamination and/or aquifer dewatering in the domain.     

Hydrogeological characterization of aquifer in palla flood plain of Delhi using integrated approach

The Yamuna flood plains spread across the northern part of Indian subcontinent is home to millions of people. The ever-growing population in these plains make it difficult to sustain the demand of freshwater resources. However, the productive aquifers of flood plains could provide solution for these issues. In this context, it is necessary to understand the aquifer characteristics. Thus, the paper attempts to characterize the aquifer in Palla area of the flood plain using integrated approach. Besides, grain size analysis and site-litholog study, the nature of aquifer material was also ascertained from bulk mineralogy of the sediments using X-Ray Diffraction. The aquifer parameters were estimated with help of long duration pumping test data. Moreover, the effect of pumping on salinity variation and hydrochemical facies evolution was also examined. The sand dominant, unconfined aquifer was estimated to have horizontal hydraulic conductivity in the range of 25 m/day and vertical hydraulic conductivity of 6–7 m /day. While the specific yield of the aquifer was estimated in the range of 0.07–0.08. It is observed that under conducive active flood plain environment, the given sand mineralogy at the site does not allow salinity increase in groundwater even after more than a decade of groundwater pumping. In fact, over years, hydrochemical facies have evolved towards Bicarbonate type. These things put together make the active flood plain aquifer a sustainable groundwater resource.     

水力层析法与克立金法估算非均质含水层渗透系数场比较

含水层非均质性空间分布特征的识别,是准确模拟地下水流和污染物运移的前提.基于室内非均质含水层砂箱实验,分别利用水力层析法和克立金插值法刻画了非均质含水层渗透系数场.研究结果表明:(1) 水力层析法与克立金法相比,不仅可以更好地刻画非均质含水层渗透系数场,还可以更高精度地预测地下水流过程;(2) 水力层析抽水实验中,通过增加抽水实验组数可以有效地提高水力层析参数反演的精度,但是抽水实验组数增加到一定程度以后,再增加抽水组数不会显著提升参数反演的效果.后续需要进一步研究水力层析抽水实验合适的组数,进一步对抽水井进行优化布设.