Fractured Hard Rock Research Articles

An accessible hydrogeological tool to monitor critical groundwater resources in hard-rock aquifers

Groundwater resources are increasingly being relied on in rural areas for income generation and food security. However, there is currently a lack of simple, yet accessible hydrogeological tools to monitor critical groundwater resources, both for quantity and quality. This is particularly true in developing countries underlain by fractured hard rock aquifers, with low productivities. Electrical conductivity (EC) meters are presented here as an easy-to-use tool that can provide real-time data collection to enhance routine groundwater monitoring in rural areas. A program was established within a fractured hard rock watershed for over a year in Rajasthan, India to determine the effectiveness and controls over EC as a monitoring tool. The initial groundwater quality in this region was largely influenced by rainfall, modified by evapotranspiration with recognizable input from water-rock interaction in the later months following the monsoon season. Chloride concentrations were linearly correlated with EC in all of the sampled groundwater, but the strength of the correlation attenuated in the months following the monsoon. Recharge rates were estimated using the chloride mass balance (CMB) approach, and then compared to the recharge rates derived from using EC as a surrogate for in what is referred to here as the CMB-EC approach. Recharge rates estimated from the CMB and CMB-EC methods were statistically similar (p=0.44).

Application of Vertical Electrical Soundings (VES) for Delineating Sub Surface Lithology for Foundations. (CVG Link Canal A Case Study)

In today’s world, field investigation is a necessity to get a detailed overview of any area for civil engineering construction purpose. Precise determination of engineering geological properties is essential to plan for a proper design and successful construction for any civil engineering structure. The traditionally practiced conventional methods for the same are invasive, costly and time consuming. Electrical Resistivity Survey is an attractive tool for delineating subsurface geology without soil disturbance. Reliable correlation between electrical resistivity values and other field geological parameters can help in successful interpretation of the engineering properties and behavior of soil in evaluation of difficult terrains e.g. for obtaining hard rock position, obtaining continuity of rock strata and for knowing the position of various sub – stratifications. This has led to develop and put in practice the geophysical method of subsurface investigation for a more precise, economical and fast assessment of large areas like the present study area of Cauvery-Vaigai link canal project. The present paper presents the results of the use of Vertical Electrical Sounding coupled with Hydrogeological studies of the Cauvery-Vaigai-Gundar (CVG) link canal area. On the basis of generalized Vertical Electrical Soundings (VES) log it is seen that the aquifers in tertiary rocks are deep – seated, the depth varying from 80 to 300 m from the ground level. It was observed that the open wells are tapping unconfined aquifer system is highly weathered rock, moderately weathered/ fractured hard rock and highly fractured hard rock. The bore wells are tapping the confined aquifer system 40 m Below Ground Level (BGL) in highly fractured hard rock. The aquifers in the bore wells were reported to be in between 40 to 120 m BGL. Most of the bore wells are dry at present due to depletion of aquifer owing to very little precipitation and over exploitation. From resistivity survey it is inferred that where ever highly fractured hard rock are expected along the link, abundant quantity of ground water may be present there in that fractured zones. The result of resistivity values of different rock types of the area show resistivity of less than 20 ohm-m in the weathered soil, while highly fractured rocks show its range between 100 to 250 ohm-m, moderately fractured rocks shows range between 250 and 500 ohm-m and the massive crystalline rocks exhibit the range of more than 500 ohm-m. The water table shows periodical variations. During the excavation of the canal, the study of water table condition will give an idea about the chances of striking ground water to cut and cross drainage portions as ground water poses a serious hazard during the excavation works.

Hard rock aquifers and free‐living nematodes – an interdisciplinary approach based on two widely neglected components in groundwater research

AbstractThe subterranean part of the water cycle takes place in three different types of groundwater bodies: (1) unconsolidated sediments, (2) karstic aquifers and (3) fractured hard rock aquifers. Basically, different aquifer types require different investigation methods. However, the physico‐chemical properties of groundwater can aid in the understanding of flow history and aquifer dynamics of each aquifer type. Although groundwater biology is sensitive to hydrochemistry, water temperature and flow velocity, the possible benefits of groundwater animals as ‘hydrogeological agents’ is still rarely considered by hydrogeologists. The main research focus of hydrogeologists and groundwater biologists is shallow groundwater bodies in unconsolidated sediments and karst aquifers. Fractured hard rock aquifers are still poorly considered by both disciplines. This commentary has three purposes: (1) to show the special characteristics of fractured hard rock aquifers in comparison to porous and karst groundwater bodies, and to describe the resulting methological gap hydrogeologists face when investigating such aquifer types; (2) to give an overview of current groundwater biological research with its focuses and shortcomings, including an introduction of rarely known but common groundwater inhabitants, namely free‐living nematodes; and (3) to suggest a combination of methods, supported by a hypothetical example, and the possible benefits this combined knowledge from hydrogeologists and groundwater biologists can bring to both disciplines. Copyright © 2014 John Wiley & Sons, Ltd.

Time Lapse Tracer and SP Measurements to Characterize the Hydrodynamics of Fractured Granite Aquifer: A Case Study

Abstract: An attempt is made to identify and delineate the groundwater flow direction and rate in a fractured hard rock aquifer in Maheshwaram granite watershed in Andhra Pradesh using multiple tracers such as bromide, iodide and rhodamine-B under both natural and induced conditions. A main well in the center and three observation wells at 25 m in north-northwest, southeast and southwest directions respectively were constructed based on 222Rn anomalies and 4He measurements. Streaming potential survey was also carried out in the area before, during and after pumping test. The tracer tests indicated preferential groundwater flow in N-S direction. The groundwater flow velocity estimated based on arrival of tracer under natural and induced conditions are 0.52 m/d and 375 m/d respectively.

Evaluation of regional fracture properties for groundwater development using hydrolithostructural domain approach in variably fractured hard rocks of Purulia district, West Bengal, India

Estimation of geohydrologic properties of fractured aquifers in hard crystalline and/or metamorphosed country rocks is a challenge due to the complex nature of secondary porosity that is caused by differential fracturing. Hydrologic potentiality of such aquifers may be assessed if the geological controls governing the spatial distribution of these fracture systems are computed using a software-based model. As an exemplar, the Precambrian metamorphics exposed in and around the Balarampur town of Purulia district, West Bengal (India) were studied to find out the spatial pattern and consistency of such fracture systems. Surfer and Statistica softwares were used to characterize these rock masses in terms of hydrological, structural and lithological domains. The technique is based on the use of hydraulically significant fracture properties to generate representative modal and coefficient of variance (Cν) of fracture datasets of each domain. The Cν is interpreted to obtain the spatial variability of hydraulically significant fracture properties that, in turn, define and identify the corresponding hydrolithostructural domains. The groundwater flow estimated from such a technique is verified with the routine hydrological studies to validate the procedure. It is suggested that the hydrolithostructural domain approach is a useful alternative for evaluation of fracture properties and aquifer potentiality, and development of a regional groundwater model thereof.

Forward modeling using finite difference method for fractured hard rocks tracing via three-electrode array

Forward modeling using finite difference method for fractured hard rocks tracing via three-electrode array

Characteristics of local groundwater recharge cycles in South African semi-arid hard rock terrains: Rainfall–groundwater interaction

The semi-arid and arid regions occupy almost two-thirds of South Africa and fall in the winter and summer rainfall zones of the sub-continent. The annual rainfall patterns can be regarded as intermittent with a significant spatial variability due to the unique winter/summer synoptic systems manifesting over Southern Africa. Summer rainfall events indicate that episodic wet periods, consisting of up to 8 consecutive days, may contain falls that contribute to almost 45% to 60% of the total annual rainfall of an area; associated with relatively higher rain rates (1.5 to 10 mm·h-1). Hyetograph-hydrograph time-series data sets, however, indicate that episodic rainfall events are responsible for rapid, but sustainable groundwater recharge events. The recurrence rate of these events in the semi-arid and arid regions is still low and aquifer storage-recharge is therefore not an annual event. The recharge flow path through the unsaturated zone reservoir plays an important role in the underdrainage flow pattern, and subsequently the saturated flow regime. It resembles a simple L-shaped flow path driven by direct recharge mounding in the fractured hard rock terrain and indirect recharge to the surrounding sub-reservoirs. The lag-times between rainfall events and water table responses were found to be significantly short; a matter of a few hours to less than a few days. Evidence of multi-modal, time-related infiltration phases during the summer early and peak rainfall intervals, governed by the field capacity status of the unsaturated zone reservoir and the growth status of the local vegetation, were observed. A unique rainfall pattern in semi-arid and arid regions is required before a sustainable aquifer storage-recharge condition develops and such incidences could be a one-in-several-year (5 to 9) event.

Some key issues in the design of water wells in unconsolidated and fractured rock aquifers

The key well design issues considered in this paper are the length of the intake section; the hydraulic efficiency of the well; the length of grouted upper casing for wells in fractured rock aquifers and the potential trade-off between well yield and security against pollution; and the economics of well design. For wells in thick, relatively uniform unconsolidated aquifers, the well depth and screen length can be estimated using a simple discharge-drawdown relationship. This approach can help avoid constructing unnecessarily deep wells if alternative guidance to screen the bottom third of the aquifer was followed in such situations. Hydraulic efficiency is an important consideration in well design: the paper highlights that whereas screen entrance velocity has been a topic of much discussion in the literature, well upflow velocity has received less attention, but can be an important contributor to well losses in small diameter screens. In fractured hard rock aquifers, there may be a compromise required in well design between maximising well yield by exploiting shallow fracture zones whilst also providing adequate sanitary protection to the well by installing an upper grouted casing. Recent data from Ireland on the distribution of hydraulic conductivity with depth in poorly productive fractured rock aquifers are used to calculate the reduction in well yield that would result from increasing the length of the grouted upper well casing. Economic aspects of well design are especially important where there are a large number of wells to be drilled and/or where wells are required in poor rural communities in developing countries. The principles of cost-effective boreholes for developing countries are summarised, noting the opportunities for small-diameter shallow wells constructed with inexpensive manual or lightweight mechanical drilling rigs.

Genesis and geochemistry of high fluoride bearing groundwater from a semi-arid terrain of NW India

Occurrence of fluoride in natural water resources and the associated health hazards have been reported from many parts of India during the last decade. With the objective of organizing a systematic scientific programme to understand the geochemical behaviour of fluoride in natural water resources in relation to the local hydrogeological, climatic conditions and agricultural use, a typical semi-arid terrain in the northwestern part of India was chosen for detailed study. The present work is an attempt to study the fluoride contamination in groundwater in parts of Jaipur district, Rajasthan state, and to assess the origin and genesis of fluoride in groundwater. The area, falling under the semi-arid tract of central Rajasthan, is geologically occupied by alluvial formations and Precambrian rocks (gneisses and schists) where groundwater occurs under unconfined to semi-confined conditions. A total of 66 groundwater samples was collected during the pre-monsoon period and analysed, which gave a concentration of fluoride in water in the range of 0.35 to 9.3 mg/l with 62 % of the samples in excess of permissible limit of 1.5 mg/l. As compared to alluvial formations, fluoride enrichment in groundwater is higher in hard rock areas. The samples collected from the granite gneissic terrain show higher values when compared to those in the schist. The groundwater of the shallow aquifers (soil–clay complex weathered zone) has a higher concentration of fluoride than the deep (fractured hard rock) aquifer does. The possible sources of fluoride are chemical weathering and leaching of fluorine-bearing minerals present in hard rocks and alluvial formations under the alkaline environment in arid to semi-arid conditions. High pH as well as alkalinity and low levels of Ca, Mg and total hardness suggest favourable chemical conditions for the fluoride dissolution process. A higher rate of evapotranspiration, intensive and long-term irrigation and heavy use of fertilizers are the supplementary factors that further increase the fluoride content in the groundwater. A groundwater management programme is suggested.

Thermal response testing of a fractured hard rock aquifer with and without induced groundwater flow

This study quantifies experimentally the influence of groundwater on the thermal conductivity measurements via thermal response tests (TRT) in a fractured hard rock with low matrix permeability. An artificial groundwater flow towards a nearby well was induced by groundwater extraction and a TRT performed simultaneously. The results were compared with a TRT performed 24 days later in the same well without groundwater extraction. The effect of the groundwater flow is shown indirectly by the enhanced effective thermal conductivity and directly through the comparison of temperature profiles before and 4 h after both TRTs. Simulations in FEFLOW show that a groundwater flow velocity of 130–1,300 m d−1 through one open horizontal fracture of small volume increases the effective thermal conductivity by 11 % in the studied system.

Geogas transport in fractured hard rock – Correlations with mining seismicity at 3.54km depth, TauTona gold mine, South Africa

Abstract An on-site gas monitoring study has been conducted in the framework of an earthquake laboratory (The International NELSAM–DAFGAS projects) at the TauTona gold mine, South Africa. Five boreholes up to 60 m long were drilled at 3.54 km depth into the highly fractured Pretorius Fault Zone and instruments for chemical and seismic monitoring installed therein. Over the span of 4 years sensitive gas monitoring devices were continuously improved to enable the direct observation of geogas concentration variations in the DAFGAS borehole. The major gas concentrations are constant and air-like with about 78% N 2 , 21% O 2 , 1% Ar. The geogas components CO 2 , CH 4 , He and H 2 show the most interesting trends and variations on the minute-by-minute basis and significantly correlate with seismic data, while the 222 Rn activity remains constant. Time series and cross correlation analysis allow the identification of different gas components (geogas and tunnel air) and the identification of two processes influencing the borehole gas composition: (1) pumping-induced tunnel air breakthrough through networks of initially water-saturated fault fractures; and (2) seismicity induced permeability enhancement of fault fractures to above ∼5 × 10 -10 m 2 . The current set-up of the gas monitoring system is sensitive enough to quantify the resulting geogas transport during periods of intense blasting activities (including recorded blasts with seismic moment ⩽1 × 10 9 Nm, located within 1000 m of the cubby) and, it is suggested, also during induced earthquakes, a final goal of the project.

The underground application of Magnetic Resonance Soundings

Abstract The potential application of MRS technology in locating waterbearing fractures in underground mines is studied. The determination of the presence of water ahead of mining is important to prevent accidents and to ensure higher efficiency in mining operations. In the usual surface based measurements, with horizontal loop and water layer, the geometry of the problem can be summarized by the value of the inclination of the Earth magnetic field. For MRS measurements under the geometric conditions associated with underground mining, where the loop is non-horizontal, the geometry can be described in an effective inclination that can be expressed in terms of the Earth magnetic inclination and declination, together with two further parameters that characterize the orientation of the mine wall. We examine the consequences of the different geometries on the MRS signal. Since the loop size is severely restricted in underground conditions, the feasible target depth is also severely limited. The consequences of the fractured hard rock aquifer conditions, typical of deep mining or tunneling environments, are also examined. The overall conclusion is that in underground MRS applications the signal strength is too small to enable the practical identification of fractures containing large volumes of water ahead of the mining face.

Characterization of transmissivity in the fractured reservoirs in the Oumé area (Centre of Côte d’Ivoire)

The aims of this work were a better knowledge of hydraulic properties of fractured reservoirs in the Oume area and to estimate the transmissivity of highly fractured hard rock using geostatistical approaches. The boreholes available tapping the fractured reservoirs and have a total depth oscillate between 39 to 93m. The inflows are met between 1 to 63m from the top of crystalline basement. Aquifers of crystalline rocks are more productive that those in the schists. Geostatistical approach is used to perform an assessment of transmissivity on Oume area. The variographic analysis of logT showed a spatial structuring with a small nugget effect witch represented 17 % of the total dispersion. Cross-validation results showed that conditions of unbiasedness and optimality are met. The actual and estimated transmissivity values spread respectively on four and two orders of magnitude. This result was explained by missing of data and the kriging smoothing. Actual transmissivity values and kriging estimated values were comparables in statistical viewpoint. Estimated transmissivity values can be accepted and transmissivity map obtained will be used for the aquifer modelling. Geostatistical approach used in this study has supplied satisfactory results in the knowledge of this important parameter on the Oume area.

Evaluation des propriétés hydrauliques des aquifères fracturés des formations cristalline et métamorphique dans la région des Lacs (centre de la Côte d’Ivoire)

Groundwater is the main source of water supply for rural populations in the Region des Lacs area in central Cote d’Ivoire (West Africa). The area is underlain by the metamorphic and crystalline fractured hard rock aquifers. This paper focuses on the assessment of their hydraulic properties. To this end, a data base comprising pumping tests data and the technical reports were gathered. 105 values of transmissivity (T ) and specific capacity (Q/s) have been deduced after pumping tests interpretation by the Jacob recovery method. Statistical analyses of all these data have been done. Depth of wells range from 49.50 to 99 m with an average of 69 m. The thickness of the weathered zone has averaged 16.52 m and lies between 1.90 and 63.10 m. Wells average rate is 2.32 m3/s. The depths of open fractures are significantly in the first 60 meters of hard rock drilling and averaged 42.94 m. The transmissivity and specific capacity of each aquifer span over several orders of magnitude revealing the strong heterogeneity of the aquifer. Both variables are lognormal variables A significant empirical relationship between T and Q/s was found T = 0,937(Q/s)^1.118 with a coefficient of determination (R^2 = 0.82). This relationship enabled the transmissivity data to be supplemented with the 95% prediction interval in order to assess the uncertainty associated with the estimates of transmissivity in the area of the study. These results are significant and can be used as an input in forthcoming modeling these aquifers and facilitate groundwater management policy.

A constitutive model for water flow in unsaturated fractured rocks

AbstractA conceptual model for describing effective saturation in fractured hard rock is presented. The fracture network and the rock matrix are considered as an equivalent continuum medium where each fracture is conceptualized as a porous medium of granular structure and the rock matrix is assumed to be impermeable. The proposed model is based on the representation of a rough‐walled fracture by an equivalent porous medium, which is described using classical constitutive models. A simple closed‐form equation for the effective saturation is obtained when the van Genuchten model is used to describe saturation inside fractures and fractal laws are assumed for both aperture and number of fractures. The relative hydraulic conductivity for the fractured rock is predicted from a simple relation derived by Liu and Bodvarsson. The proposed constitutive model contains three independent parameters, which may be obtained by fitting the proposed effective saturation curve to experimental data. Two of the model parameters have physical meaning and can be identified with the reciprocal of the air entry pressure values in the fractures of minimum and maximum apertures. Effective saturation and relative hydraulic conductivity curves match fairly well the simulated constitutive relations obtained by Liu and Bodvarsson. Copyright © 2008 John Wiley & Sons, Ltd.

Percolation conditions in fractured hard rocks: A numerical approach using the three‐dimensional binary fractal fracture network (3D‐BFFN) model

We numerically investigate fracture connectivity and percolation conditions in fractured hard rocks using a three‐dimensional binary fractal fracture network (3D‐BFFN) model based on three fractal geometric parameters: the fractal dimensions (D2) of the spatial distribution of fractures, the exponent of the power‐law cumulative fracture length distribution (a), and the maximum fracture length (lmax) normalized by the domain length (L), lmax/L. Numerical results clarify that the percolation threshold in 3D‐BFFN models is strongly controlled by fractal geometric parameters and is independent of any anisotropy in the orientations Θ. In addition, when a < 1.8 and lmax/L < 1.0, percolation seldom occurs independently of D2 and Θ. In the current study the analytical solution of percolation probability (P) is presented as a function of the three fractal parameters within the 3D‐BFFN model. Application of the 3D‐BFFN model to seismogenic fractures determined from the earthquake catalogue in an offshore volcanic region between Miyake‐jima Island (MI) and Kozu‐shima Island (KI) off the Izu Peninsula, Japan, suggests that P is mainly affected by the error involved in determining a during actual surveys. Otherwise, P provides a useful index for determining whether a three‐dimensional domain percolates in fracture networks in fractured hard rocks. The basis of this approach is the observation from fracture network connections that domains with P > 0.55 are percolated domains. The zone of percolation within seismogenic fracture networks between MI and KI reveals that the networks formed from seismic‐swarm‐related seismogenic fractures over a 7‐week period related to the intrusion of a dyke, inferred previously from seismicity and deformation data.

A numerical approach for groundwater flow in unsaturated porous media

AbstractIn this article, a computational tool to simulate groundwater flow in variably saturated non‐deformable fractured porous media is presented, which includes a conceptual model to obtain analytical expressions of water retention and hydraulic conductivity curves for fractured hard rocks and a numerical algorithm to solve the Richards equation. To calculate effective saturation and relative hydraulic conductivity curves we adopt the Brooks–Corey model assuming fractal laws for both aperture and number of fractures. A standard Galerkin formulation was employed to solve the Richards' equation together with a Crank–Nicholson scheme with Richardson extrapolation for the time discretization.The main contribution of this paper is to group an analytical model of the authors with a robust numerical algorithm designed to solve adequately the highly non‐linear Richards' equation generating a tool for porous media engineering. Copyright © 2006 John Wiley & Sons, Ltd.

Geostatistical estimation of the transmissivity in a highly fractured metamorphic and crystalline aquifer (Man-Danane Region, Western Ivory Coast)

This work is aimed at estimating the transmissivity of highly fractured hard rock aquifers using a geostatistical approach. The studied aquifer is formed by the crystalline and metamorphic rocks of the Western Ivory Coast (West Africa), in the Man Danané area. The study area covers 7290 km 2 (90 km×81 km). The fracturing network is dense and well connected, without a marked fracture direction. A data base comprising 118 transmissivity ( T) values and 154 specific capacity ( Q/ s) values was compiled. A significant empirical relationship between T and Q/ s was found, which enabled the transmissivity data to be supplemented. The variographic analysis of the two variables showed that the variograms of T and Q/ s (which are lognormal variables) are much more structured than those of log T and log Q/ s (which are normal variables). This result is contrary to what was previously published and raises the question whether normality is necessary in geostatistical analysis. Several input and geostatistical estimations of the transmissivity were tested using the cross validation procedure: measured transmissivity data; supplemented transmissivity data; kriging; cokriging. The cross validation results showed that the best estimation is provided using the kriging procedure, the transmissivity field represented by the whole data sample (measured+estimated using specific capacity) and the structural model evaluated solely on the measured transmissivity. The geostatistical approach provided in fine a reliable estimation of the transmissivity of the Man Danané aquifer, which will be used as an input in forthcoming modelling.

A fractal constitutive model for unsaturated flow in fractured hard rocks

A physical conceptual model for water retention in self-similar fractured hard rocks is presented. The geometric pattern of the fracture network is described using a classical fractal object known as the Sierpinski carpet. Assuming this type of geometry, a relatively simple closed-form expression for the water content is obtained. All model parameters can be calculated from the density of the main fractures, the maximum and minimum values of the fracture aperture, and the residual water content. The resulting water content expression is then used to estimate the unsaturated hydraulic conductivity of the fractured medium based upon the well-known model of Burdine. It is found that for large enough ranges of fracture apertures the new constitutive model converges to the empirical Brooks–Corey model.

Characterisation of recharge processes and groundwater flow mechanisms in weathered-fractured granites of Hyderabad (India) using isotopes

In order to address the problem of realistic assessment of groundwater potential and its sustainability, it is vital to study the recharge processes and mechanism of groundwater flow in fractured hard rocks, where inhomogeneties and discontinuities have a dominant role to play. Wide variations in chloride, δ18O and 14C concentrations of the studied groundwaters observed in space and time could only reflect the heterogeneous hydrogeological setting in the fractured granites of Hyderabad (India). This paper, based on the observed isotopic and environmental chloride variations of the groundwater system, puts forth two broad types of groundwaters involving various recharge processes and flow mechanisms in the studied granitic hard rock aquifers. Relatively high 14C ages (1300 to ∼6000 yr B.P.), δ18O content (−3.2 to −1.5‰) and chloride concentration (<100 mg/l) are the signatures that identified one broad set of groundwaters resulting from recharge through weathered zone and subsequent movement through extensive sheet joints. The second set of groundwaters possessed an age range Modern to ∼1000 yr B.P., chloride in the range 100 to ∼350 mg/l and δ18O from −3.2 to +1.7‰. The δ18O enrichment and chloride concentration, further helped in the segregation of the second set of groundwaters into three sub-sets characterized by different recharge processes and sources. Based on these processes and mechanisms, a conceptual hydrogeologic model has evolved suggesting that the fracture network is connected either to a distant recharge source or to a surface reservoir (evaporating water bodies) apart from overlying weathered zone, explaining various resultant groundwaters having varying 14C ages, chloride and δ18O concentrations. The surface reservoir contribution to groundwater is evaluated to be significant (40 to 70%) in one subset of groundwaters. The conceptual hydrogeologic model, thus evolved, can aid in understanding the mechanism of groundwater flow as well as migration of contaminants to deep groundwater in other fractured granitic areas.