Nitrate as a tracer of groundwater flow in a fractured multilayered aquifer

Abstract

Knowledge of the processes that control nitrate migration and its geochemical evolution in the subsurface are fundamental for the regional management of polluted aquifers. In this paper, the spatial distribution and transient variations of nitrate concentrations, associated with manure fertilization, are used to depict hydrogeological dynamics within the sedimentary aquifer system of la Plana de Vic in the Osona region of Spain. Flow systems are identified from geological, hydraulic head, hydrochemical and isotopic data, and by considering nitrate itself as a tracer that indicates how flow paths are modified by human pressures. In this area, nitrates move through fractured aquitards in flows induced by groundwater pumping. Moreover, the lack of casing in the boreholes permits a mixing of groundwater from distinct layers inside the wells, which negates any benefits from the low-nitrate groundwater found in the deepest aquifer layers. Therefore, impacts on groundwater quality are related to intensive manure fertilization as well as to inadequate well construction and exploitation strategies.

Citation Menció, A., Mas-Pla, J., Otero, N. & Soler, A. (2011) Nitrate as a tracer of groundwater flow in a fractured multilayered aquifer. Hydrol. Sci. J. 56(1), 108–122.     

Identifying connections in a fractured rock aquifer using ADFTs

Fractured rock aquifers are difficult to characterize because of their extremely heterogeneous nature. Developing an understanding of fracture network hydraulic properties in these aquifers is difficult and time consuming, and field testing techniques for determining the location and connectivity of fractures in these aquifers are limited. In the Clare Valley, South Australia, well interference is an important issue for a major viticultural area that uses a fractured aquifer. Five fracture sets exist in the aquifer, all dipping > 25 degrees . In this setting, we evaluate the ability of steady-state asymmetric dipole-flow tests (ADFTs) to determine the connections between a test well and a set of piezometers. The procedure involves dividing a test well into two chambers using a single packer and pumping fluid from the upper chamber to the lower chamber. By conducting a series of tests at different packer elevations, an "input" signal is generated in fracture zones connected to the test well. By monitoring the "output" response of the hydraulic dipole field at piezometers, the connectivity of the fractures between the test well and piezometers can be determined. Results indicate the test well used in this study is connected in a complex three-dimensional geometry, with drawdown occurring above and below areas of potentiometric buildup. The ADFT method demonstrates that the aquifer evaluated in this study cannot be modeled effectively on the well scale using continuum flow models.     

Asymmetric dipole-flow test in a fractured carbonate aquifer

In this study, a new method-the asymmetric dipole-flow test-is proposed and tested for characterization of conductive properties and structure of fractured aquifers. Analytical solutions were developed and then used for interpretation of a modification of the dipole-flow test with a single packer at the Bissen Quarry test site (Wisconsin, USA). The asymmetric dipole-flow tests were conducted by packing a well at various elevations, and fluids were pumped from the upper section (chamber) of the well to the lower section (chamber). The head was then monitored at 11 observation points and in both sections of the well, and the conductivities of the well segments were determined. The tests at seven packer elevations in the well were rapid (less than one hour to reach steady state). The asymmetric dipole-flow test demonstrates the potential to quantify heterogeneities of a fractured aquifer and delineate the applicability of the continuum and discrete approaches for conceptualization of ground water flow.     

Direct interpretation of self-potential anomalies produced by a vertical dipole

In the present paper a new method is proposed for the quantitative interpretation of self-potential anomalies which are produced by a vertical dipole. First the mathematical expression of the wavenumber spectrum of the self-potential anomaly is deduced. It is pointed out that at relatively high wavenumbers the behavior of the amplitude spectrum is controlled by the closer to the surface pole at depth h. On the other hand, the “width” of the amplitude spectrum depends on the depth h and the dipole length L.Making a proper mathematical transformation of the amplitude spectrum, and applying the least squares method, it is possible to calculate the depth to the upper pole. The dipole length may then be calculated, by solving numerically a characteristic algebraic equation, as long as the “width” of the amplitude spectrum has been previously defined.The proposed method is applied on a well known self-potential profile from Greece. The calculated parameters of the polarized body are in good agreement with real data. Experimentation with synthetic models in which random noise was introduced, showed that this method gives reliable results if the noise amplitude is not more than 20% of the signal amplitude. It is clearly more efficient than the methods which are based on the model of the point pole or the dipole with a small length. It can also give good results if the horizontal extensions of the polarized body are not more than a few tenths of the depth of the upper pole. If the polarized body is tilted, the depth of the upper pole can be calculated with satisfactory accuracy.The direct interpretation method which is proposed in the present paper, may be useful in mineral exploration, and particularly if the target of interest is the detection of massive sulfide mineralization.     

Ground water/surface water interaction in a fractured rock aquifer

In a recent field study of ground water/surface water interaction between a bedrock stream and an underlying fractured rock aquifer, it was determined that the majority of ground water discharge occurred through sparsely located vertical fractures. In this paper, the dominant mechanisms governing ground water/surface water exchange in such an environment are investigated using a numerical model. The study was conducted using several conceptual models based on the field study results. Although the field results provided the motivation for the modeling study, it was not intended to match modeling and field results directly. In addition, the extent of capture zones for discharging or recharging fractures was explored. The results of this study are intended to provide a better understanding of contaminant migration in the vicinity of bedrock streams. Based on the numerical results, the rate of ground water discharge (or recharge) was found to depend on the aperture size of the discharging feature, and on the distribution of hydraulic head with depth within the fracture network. It was determined that the extent of both the capture zone and reverse capture zone for an individual fracture can be extremely large, and will be determined by the height of the stream stage, the fracture apertures of the network, and the hydraulic-head distribution within the network. Because both the stream stage and the hydraulic-head distribution are transient, the size of the capture zone and/or the reverse capture zone for an individual fracture may change significantly over time. As a result, the migration path for contaminants within the fracture network and between the surface and subsurface will also vary significantly with time.     

裂缝介质岩石物理模型研究综述

随着我国油气勘探开发事业的快速发展,裂缝型油气藏将成为本世纪的重点勘探对象之一.由于裂缝模型对于预测裂缝型油气藏非常重要,因此,裂缝模型的研究备受关注.本文简单概述裂缝模型的发展历程,重点介绍Schoenberg模型、Hudson模型、Kachanov模型、Eshelby-Cheng模型、Thomsen模型、DEM模型...     

Introduction to hydromechanical well tests in fractured rock aquifers

This article introduces hydromechanical well tests as a viable field method for characterizing fractured rock aquifers. These tests involve measuring and analyzing small displacements along with pressure transients. Recent developments in equipment and analyses have simplified hydromechanical well tests, and this article describes initial field results and interpretations during slug and constant-rate pumping tests conducted at a site underlain by fractured biotite gneiss in South Carolina. The field data are characterized by displacements of 0.3 μm to more than 10 μm during head changes up to 10 m. Displacements are a hysteretic function of hydraulic head in the wellbore, with displacements late in a well test always exceeding those at similar wellbore pressures early in the test. Displacement measurements show that hydraulic aperture changes during well tests, and both scaling analyses and field data suggest that T changed by a few percent per meter of drawdown during slug and pumping tests at our field site. Preliminary analyses suggest that displacement data can be used to improve estimates of storativity and to reduce nonuniqueness during hydraulic well tests involving single wells.     

Analysis of solute transport in a divergent flow tracer test with scale‐dependent dispersion

It has been known for many years that dispersivities increase with solute displacement distance in a subsurface. The increase of dispersivities with solute travel distance results from significant variation in hydraulic properties of porous media and was identified in the literature as scale‐dependent dispersion. In this study, Laplace‐transformed analytical solutions to advection‐dispersion equations in cylindrical coordinates are derived for interpreting a divergent flow tracer test with a constant dispersivity and with a linear scale‐dependent dispersivity. Breakthrough curves obtained using the scale‐dependent dispersivity model are compared to breakthrough curves obtained from the constant dispersivity model to illustrate the salient features of scale‐dependent dispersion in a divergent flow tracer test. The analytical results reveal that the breakthrough curves at the specific location for the constant dispersivity model can produce the same shape as those from the scale‐dependent dispersivity model. This correspondence in curve shape between these two models occurs when the local dispersivity at an observation well in the scale‐dependent dispersivity model is 1·3 times greater than the constant dispersivity in the constant dispersivity model. To confirm this finding, a set of previously reported data is interpreted using both the scale‐dependent dispersivity model and the constant dispersivity model to distinguish the differences in scale dependence of estimated dispersivity from these two models. The analytical result reveals that previously reported dispersivity/distance ratios from the constant dispersivity model should be revised by multiplying these values by a factor of 1·3 for the scale‐dependent dispersion model if the dispersion process is more accurately characterized by scale‐dependent dispersion. Copyright © 2007 John Wiley & Sons, Ltd.     

Static and dynamic conceptual model of a complexly fractured crystalline rock aquifer

A conceptual model of anisotropic and dynamic permeability is developed from hydrogeologic and hydromechanical characterization of a foliated, complexly fractured, crystalline rock aquifer at Gates Pond, Berlin, Massachusetts. Methods of investigation include aquifer‐pumping tests, long‐term hydrologic monitoring, fracture characterization, downhole heat‐pulse flow meter measurements, in situ extensometer testing, and earth tide analysis. A static conceptual model is developed from observations of depth‐dependent and anisotropic permeability that effectively compartmentalizes the aquifer as a function of foliation intensity. Superimposed on the static model is dynamic permeability as a function of hydraulic head in which transient bulk aquifer transmissivity is proportional to changes in hydraulic head due to hydromechanical coupling. The dynamic permeability concept is built on observations that fracture aperture changes as a function of hydraulic head, as measured during in situ extensometer testing of individual fractures, and observed changes in bulk aquifer transmissivity as determined from earth tides during seasonal changes in hydraulic head, with higher transmissivity during periods of high hydraulic head, and lower transmissivity during periods of relatively lower hydraulic head. A final conceptual model is presented that captures both the static and dynamic properties of the aquifer. The workflow presented here demonstrates development of a conceptual framework for building numerical models of complexly fractured, foliated, crystalline rock aquifers that includes both a static model to describe the spatial distribution of permeability as a function of fracture type and foliation intensity and a dynamic model that describes how hydromechanical coupling impacts permeability magnitude as a function of hydraulic head fluctuation. This model captures important geologic controls on permeability magnitude, anisotropy, and transience and therefor offers potentially more reliable history matching and forecasts of different water management strategies, such as resource evaluation, well placement, permeability prediction, and evaluating remediation strategies.     

Influence of piezometer construction on groundwater sampling in fractured rock

A numerical model for groundwater flow and solute transport was employed to examine the influence of the screen and sandpack on the collection of a representative geochemical sample from a piezometer monitoring well installation in a discretely fractured bedrock aquifer. The optimization of screen and sandpack combinations was explored for the potential to reduce purging times and volumes in practice. Simulations accounted for the location of the fractures along the well screen, fracture aperture, screen length, and the pumping rate. The variability in the required purging times (t(99)-the time required to achieve 99% fractional contribution from the formation to pump discharge) can be explained by: (1) the relative hydraulic conductivities of the components of the system (fracture, sandpack, and screen), (2) the truncation of the flow field from the fracture to the screen by the upper and/or lower boundary of the sandpack of the flow field from another fracture, and (3) time-dependent drawdown. During pumping, only a portion of the sandpack may actually become hydraulically active. The optimal configuration (shortest purging time) is achieved when the ratios of the screen, sandpack, and fracture hydraulic conductivities are close to 1:1:1. More importantly, the role of the fracture hydraulic conductivity in the ratios is not as crucial to reducing t(99) as having the hydraulic conductivities of the screen and sandpack as similar as possible. This study provides a better understanding of well dynamics during pumping for the purpose of obtaining representative groundwater samples.     

High-resolution Electrical Resistivity Tomography monitoring of a tracer test in a confined aquifer

A permanent geoelectrical subsurface imaging system has been installed at a contaminated land site to monitor changes in groundwater quality after the completion of a remediation programme. Since the resistivities of earth materials are sensitive to the presence of contaminants and their break-down products, 4-dimensional resistivity imaging can act as a surrogate monitoring technology for tracking and visualising changes in contaminant concentrations at much higher spatial and temporal resolution than manual intrusive investigations. The test site, a municipal car park built on a former gasworks, had been polluted by a range of polycyclic aromatic hydrocarbons and dissolved phase contaminants. It was designated statutory contaminated land under Part IIA of the UK Environmental Protection Act due to the risk of polluting an underlying minor aquifer. Resistivity monitoring zones were established on the boundaries of the site by installing vertical electrode arrays in purpose-drilled boreholes. After a year of monitoring data had been collected, a tracer test was performed to investigate groundwater flow velocity and to demonstrate rapid volumetric monitoring of natural attenuation processes. A saline tracer was injected into the confined aquifer, and its motion and evolution were visualised directly in high-resolution tomographic images in near real-time. Breakthrough curves were calculated from independent resistivity measurements, and the estimated seepage velocities from the monitoring images and the breakthrough curves were found to be in good agreement with each other and with estimates based on the piezometric gradient and assumed material parameters.     

双组份裂缝岩石非线性有效应力模型研究

为认识黏土矿物和裂缝对岩石有效应力的影响,提出了双组份裂缝岩石椭圆模型.同时,将双组份裂缝岩石椭圆模型等效为共聚焦的椭圆岩石环与椭圆黏土环的叠加;基于复变函数和保角变换,分别得到了黏土环内椭圆长半轴和短半轴与压力间的关系式.进而结合椭圆孔渗透率计算式和岩石有效应力表达式,计算与分析了岩石孔隙度、黏土矿物含量、硬度比以及椭圆孔截面纵横比等参数对有效应力系数的影响.结果表明,当岩石中不含黏土矿物时,有效应力系数随纵横比的减小而增大,且小于1.0,这与Bernabé的观点一致.当岩石中含有易于压缩的黏土矿物,且纵横比为1.0（孔隙截面为圆形）时,有效应力系数随硬度比和黏土矿物含量的增大而增大,甚至远大于1.0,这与Al-Wardy等的研究结论一致;当纵横比小于1.0时,有效应力系数表现出了明显的非线性特征,随硬度比的增大而增大——甚至远大于1.0,而随黏土矿物含量的增大而减小,纵横比对有效应力系数的影响受黏土矿物含量大小的影响.易于压缩组分和裂缝的存在使得岩石有效应力系数变化特征更为复杂.

     

Numerical experiments for 3-dimensional flow analysis in a fractured rock with porous matrix

Groundwater flow in a 3-D domain with fracture planes is numerically investigated using the finite element method. A flexible mesh generation method for discretization is proposed in this paper. The method, based on Delaunay triangulation, divides the whole domain into subdomains separated by fracture planes. It then triangulates each subdomain independently into tetrahedra which are further subdivided into hexahedra. By putting together all of the meshes of the subdomain, a finite element mesh of the whole domain is obtained. The appropriateness of the mesh generation method is topologically proved. Several applications of the proposed method are given, and the numerical solutions are in good agreement with those obtained with a structured grid. It is concluded that the proposed mesh generation method can replace the structured grid.     

Influence of small-scale heterogeneities on contaminant transport in fractured crystalline rock

We present a sequence of purely advective transport models that demonstrate the influence of small-scale geometric inhomogeneities on contaminant transport in fractured crystalline rock. Special weight is placed on the role of statistically generated variable fracture apertures. The fracture network geometry and the aperture distribution are based on information from an in situ radionuclide retardation experiment performed at Grimsel test site (Swiss Alps). The obtained breakthrough curves are fitted with the advection dispersion equation and continuous-time random walks (CTRW). CTRW is found to provide superior fits to the late-arrival tailing and is also found to show a good correlation with the velocity distributions obtained from the hydraulic models. The impact of small-scale heterogeneities, both in fracture geometry and aperture, on transport is shown to be considerable.     

A comparison of cross-hole electrical and seismic data in fractured rock

Cross‐hole anisotropic electrical and seismic tomograms of fractured metamorphic rock have been obtained at a test site where extensive hydrological data were available. A strong correlation between electrical resistivity anisotropy and seismic compressional‐wave velocity anisotropy has been observed. Analysis of core samples from the site reveal that the shale‐rich rocks have fabric‐related average velocity anisotropy of between 10% and 30%. The cross‐hole seismic data are consistent with these values, indicating that observed anisotropy might be principally due to the inherent rock fabric rather than to the aligned sets of open fractures. One region with velocity anisotropy greater than 30% has been modelled as aligned open fractures within an anisotropic rock matrix and this model is consistent with available fracture density and hydraulic transmissivity data from the boreholes and the cross‐hole resistivity tomography data. However, in general the study highlights the uncertainties that can arise, due to the relative influence of rock fabric and fluid‐filled fractures, when using geophysical techniques for hydrological investigations.     

Electromagnetic fields of a vertical magnetic dipole above a laterally inhomogeneous thin layer of conductive overburden

An analytical formulation is developed for the resultant electromagnetic field of an oscillating vertical magnetic dipole located over a thin conductive sheet of infinite extent. The sheet is characterized by a conductivity-thickness product or conductance d that may be a function of the horizontal coordinates. The system of integral equations arising in the general formulation is simplified greatly when azimuthal symmetry prevails. Numerical results for a Gaussian variation of d in the radial direction are presented for the case of a symmetrically located source. These results are for the fields at the level of the source dipole over the conductive sheet. It is shown that the quadrature response of the sheet is enhanced when there is rapid variation of the conductance. The null in the resultant wave tilt is also found to be shifted toward the direction of increasing conductance.     

The field of the vertical electric dipole immersed in the heterogeneous half-space

The field of the vertical electric dipole (VED) immersed in the heterogeneous conductive halfspace (sea) is analyzed in time domain. In the near field of the source, the amplitudes of the electric and magnetic components of the field are proportional to power 3/2 and power 5/2 of the conductivity of the medium, respectively. After termination of the transmitter pulse, all the VED components decay with time as ～1/t ^5/2. The possibility of applying the VED field for estimating the electrical properties of the offshore geological sections is demonstrated.     

Seismic stability analysis of fractured rock slopes by yield design theory

This paper deals with the problem of stability of fractured rock slope located in seismic area. The rock mass is crossed by two sets of fractures which are considered to be planar, parallel and persistent. The effects of both horizontal seismic coefficient and strength characteristics of fractures are addressed. The analysis is based upon the kinematic approach of the yield design theory and the pseudo-static method as well. The fundamental inequality of the kinematic approach is invoked and the failure of the fractured rock slope is considered through simple translational mechanism involving a one-partsliding block. Rigorous upper bounds of the so-called stability factor for the structure under study of given slope angle, strength properties of constituent materials and seismic loads are obtained. The results are presented in the form of stability charts relating the estimated upper bound solutions to the friction angle of fractures.The used procedure highlights the destabilizing effects of the seismic loadings and is capable of conducting parametric studies. Furthermore, the results obtained far from various sets of data show good performance and further research work is planed to extend the analysis to include a large number of sub-problems.