Aquifer overexploitation: what does it mean?

Groundwater overexploitation and aquifer overexploitation are terms that are becoming common in water-resources management. Hydrologists, managers and journalists use them when talking about stressed aquifers or some groundwater conflict. Overexploitation may be defined as the situation in which, for some years, average aquifer abstraction rate is greater than, or close to the average recharge rate. But rate and extent of recharge areas are often very uncertain. Besides, they may be modified by human activities and aquifer development. In practice, however, an aquifer is often considered as overexploited when some persistent negative results of aquifer development are felt or perceived, such as a continuous water-level drawdown, progressive water-quality deterioration, increase of abstraction cost, or ecological damage. But negative results do not necessarily imply that abstraction is greater than recharge. They may be simply due to well interferences and the long transient period that follow changes in the aquifer water balance. Groundwater storage is depleted to some extent during the transient period after abstraction is increased. Its duration depends on aquifer size, specific storage and permeability. Which level of "aquifer overexploitation" is advisable or bearable, depends on the detailed and updated consideration of aquifer-development effects and the measures implemented for correction. This should not be the result of applying general rules based on some indirect data. Monitoring, sound aquifer knowledge, and calculation or modelling of behaviour are needed in the framework of a set of objectives and policies. They should be established by a management institution, with the involvement of groundwater stakeholders, and take into account the environmental and social constraints. Aquifer overexploitation, which often is perceived to be associated with something ethically bad, is not necessarily detrimental if it is not permanent. It may be a step towards sustainable development. Actually, the term aquifer overexploitation is mostly a qualifier that intends to point to a concern about the evolution of the aquifer-flow system in some specific, restricted points of view, but without a precise hydrodynamic meaning. Implementing groundwater management and protection measures needs quantitative appraisal of aquifer evolution and effects based on detailed multidisciplinary studies, which have to be supported by reliable data. Electronic Publication     

The effect of aquifer heterogeneity on natural attenuation rate of BTEX

Monitored natural attenuation can be a viable option for remediation of groundwater contamination by BTEX compounds. Under the field conditions, the rate of contaminant mass attenuation through natural processes, such as biodegradation, to a large extent affected by the groundwater flow regime, which is primarily controlled by the aquifer heterogeneity. Numerical simulation techniques were used to describe quantitatively the relationship between biodegradation rate of BTEX and aquifer heterogeneity. Different levels of aquifer heterogeneity were described by random hydraulic conductivity fields (K) having different statistical parameters, the coefficient of variation (CV) and the correlation length (h). The Turning Bands Algorithm was used to generate such K fields. Visual MODFLOW/RT3D was used to simulate the fate and transport of dissolved BTEX plume within heterogeneous aquifers. The multispecies reactive transport approach described BTEX degradation using multiple terminal electron-accepting processes. First-order biodegradation rate constants were calculated from simulated BTEX plumes in heterogeneous flow fields. The results showed that aquifer heterogeneity significantly affected biodegradation rate; it decreased with increasing CV when h was in the range of up to 12 m, whereas it increased with increasing CV when h was greater than about 12 m. For well characterized aquifers, this finding could be of great value in assessing the effectiveness of natural attenuation during feasibility studies at BTEX contaminated sites.     

Transmissivity estimation for highly heterogeneous aquifers: comparison of three methods applied to the Edwards Aquifer,Texas, USA

Obtaining reliable hydrological input parameters is a key challenge in groundwater modeling. Although many quantitative characterization techniques exist, experience applying these techniques to highly heterogeneous real-world aquifers is limited. Three geostatistical characterization techniques are applied to the Edwards Aquifer, a limestone aquifer in south-central Texas, USA, for the purposes of quantifying the performance in an 88,000-cell groundwater model. The first method is a simple kriging of existing hydraulic conductivity data developed primarily from single-well tests. The second method involves numerical upscaling to the grid-block scale, followed by cokriging the grid-block conductivity. In the third method, the results of the second method are used to establish the prior distribution for a Bayesian updating calculation. Results of kriging alone are biased towards low values and fail to reproduce hydraulic heads or spring flows. The upscaling/cokriging approach removes most of the systematic bias. The Bayesian update reduced the mean residual by more than a factor of 10, to 6 m, approximately 2.5% of the total head variation in the aquifer. This agreement demonstrates the utility of automatic calibration techniques based on formal statistical approaches and lends further support for using the Bayesian updating approach for highly heterogeneous aquifers.     

Intrinsic aquifer vulnerability assessment: validation by environmental tracers in San Miguel de Allende, Mexico

Vulnerability maps are important tools for water decision makers and land-use planners for protection of aquifers against contamination. The vulnerability map, according to the parametric method SINTACX for assessing intrinsic aquifer vulnerability, was validated in a case study with chlorofluorocarbon tracer technologies (CFC-11, CFC-12, and CFC-113) of groundwater. The tested area was the 1,295 km² volcano-sedimentary area of San Miguel de Allende (SMA), Mexico. From the results of this area, it appears that the vulnerability map is in parts inconsistent with the underlying groundwater flow system. Thus, the vulnerability map was corrected with tracer information. The validated vulnerability map indicates that the degree of vulnerability varies from low to moderate-high. Low vulnerability values are found in the graben extending from north to south along the SMA fault system and high values in recharge areas southeast and northwest of the study area. The investigation is a demonstration that the scientific reliability of results of the parametric method can be improved by validation with tracer techniques representing the groundwater dynamics. The flexible structure of SINTACX allows revising and adjusting scores and weights of the parameter maps to rebuild a new vulnerability map consistent with the hydrological system.     

The sources of geogenic arsenic in aquifers at Datong basin,northern China: Constraints from isotopic and geochemical data

Mineralogical, geochemical and zircon U–Pb dating studies were carried out to identify the sources of arsenic in the shallow aquifers of Datong Basin in northern China. A sediment sample from 18 m depth containing 10.3 mg/kg arsenic showed a Zircon U–Pb concordant age of 2528 ± 20 to 271 ± 4 Ma that can be divided into two groups (2528 ± 20 to 1628 ± 21 Ma and 327 ± 4 to 271 ± 4 Ma) and is comparable to that of the sedimentary rocks of Taiyuan (upper Carboniferous) and Shanxi Formation (lower Permian) outcropping to the west of Datong Basin. In contrast, a sediment sample from 22.5 m depth containing 5.7 mg/kg arsenic displayed a Zircon U–Pb concordant age ranging from 2561 ± 21 to 1824 ± 26 Ma that is comparable to that of the Hengshan Complex (Ne-Archaean Precambrian) outcropping to the east of .     

深基坑历史抽排水估算方法及应用

城市对地下水的开采主要用于城市供水,但近年来大规模城市建设伴随的基坑降水也在大量开采地下水。基坑降水不但大量浪费地下水资源,也是引发城市工程性地面沉降的重要因素,研究基坑降水与地面沉降关系具有重要意义。但历史上大部分基坑降水未系统开展排水量监测,亟待发展新的技术和方法,估算历史基坑抽水量。本文针对仅具有面积、挖深和粗略位置信息的基坑,提出了估算基坑抽水量的解析法和数值法。以上海为例,利用地质分区确定降水目标含水层,利用三维地质信息估算基坑含水系统安全降深,采用虚拟帷幕的大井法和数值法估算基坑排水量。2020年,估算上海市基坑总体排水量达到971万立方米,单个基坑排水量最少为28.8立方米,最大达到77.8万立方米。本文提出的估算方法可以为城市地下水资源历史分析与地面沉降相关性分析提供参考依据。     

Geoelectrical survey over perched aquifers in the northern part of Upper Sakarya River Basin,Türkiye

In this study, a groundwater exploration survey was conducted using the DC Resistivity (DCR) method in a hydrogeological setting containing a perched aquifer. DCR data were gathered and an electrical tomography section was recovered using conventional four-electrode instruments with a Schlumberger array and a two-dimensional (2D) inversion scheme. The proposed scheme was tested over a synthetic three-dimensional (3D) subsurface model before deploying it in a field situation. The proposed method indicated that gathering data with simple four-electrode instruments at stations along a line and 2D inversion of datasets at multiple stations can recover depth intervals of the studied aquifer in the hydrogeological setting even if it has a 3D structure. In this study, 2D inversion of parallel profiles formed a pseudo-3D volume of the subsurface resistivity structures and mapped out multiple resistive (>25 ohm·m) bodies at shallow (between 50–100 m) and deep sections (>150 m). In general, the proposed method is convenient to encounter geological units that have limited vertical and spatial extensions in any direction and presents resistivity contrast from groundwater-bearing geologic materials.     

On the relation between excess hydraulic head and flow in a conduit imbedded within a porous matrix

This article provides a partial answer to the question “What is the relation between excess hydraulic head and volume flux of water in a conduit within a porous matrix?”, focusing on the case that the forcing is steady. The conduit is modelled as a horizontal circular cylinder, imbedded within a porous matrix of rectangular cross section, having constant head prescribed on the sidewalls and being confined top and bottom. Laminar flow in the matrix is assumed to obey Darcy's law, while turbulent flow in the conduit is quantified using the Darcy–Weisbach equation. Analysis of the latter equation shows that the length scale of variations in the direction of the conduit is large compared with the scale of lateral and vertical variations. This permits separation of the full three-dimensional non-linear problem into a two-dimensional linear problem for head within the matrix and a one-dimensional non-linear problem for head within the conduit. Analytic solutions are obtained for the distribution of head in the matrix and in a conduit of either infinite or finite length. In both cases, the volume flux of water is proportional to the excess head to the 2/3 power, the conduit radius to the 5/3 power, the matrix permeability to the 1/3 power and gravity to the 1/3 power. The scale of variation of head along the conduit is proportional to the excess head to the ?1/3 power, the conduit radius to the 5/3 power, the matrix permeability to the ?2/3 power and gravity to the 1/3 power.     

Quantification of aquatic sediment prokaryotes—A multiple-steps optimization testing sands from pristine and contaminated aquifers

The total number of prokaryotic cells in aquatic sediments is a crucial measure tightly linked to energetic constraints, microbial productivity, and ecosystem status. While counting cells in water samples is routine, reliable procedures for the determination of prokaryotic cells associated to aquatic sediments are especially needed. A protocol for the direct quantification of prokaryotic cell abundance in aquatic sediments was developed and tested with sandy material from pristine and contaminated aquifers. Individual steps required in sample preparation, including preservation, cell dislodgement, density gradient centrifugation providing sample purification, and staining, were critically evaluated and optimized. Quantification of cells is conducted by flow cytometry promoting a high-throughput sample processing. Although tested for only two different types of aquifer sediments, researchers may easily adapt the protocol for their individual samples and purposes, thus conducting a brief efficiency re-evaluation of individual steps highlighted in this study.     

Geophysical characterization of the León-Chinandega aquifer,Nicaragua

Electrical geophysical surveys in the mode of vertical electrical soundings (VES) and continuous vertical electrical soundings (CVES) were conducted in the León-Chinandega plains, northwestern Nicaragua, in order to obtain detailed information about the geometry of the different hydrogeological layers in the aquifer and depth to the basement. A total of 51 VES were carried out within the plains. The results show a complex structure towards the north east of the area, and the southwestern part of the plains presents a smoother stratification. The geoelectrical measurements and borehole information indicate that the basement topography is characterized by hills and deep valleys with highly variable basement depths. Fifty CVES where done in a smaller area in the center of the plain. The resistivity data yielded considerable information revealing the existence of two main geo-electrical units. The combined interpretation of geological and geophysical data shows an environment typical of sedimentary volcanic coastal plains. The information collected during this investigation provides valuable data for estimating the fresh-water resources of the León-Chinandega aquifer system and for development of a groundwater management plan.