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Understanding the extent and quantity of groundwater drawdown is critical for developing a mitigation strategy for water management. This study illustrates that the data‐driven model can be used for the spatial estimation of groundwater drawdown using interferometric synthetic aperture radar (InSAR)‐based deformation data. Here, InSAR derived from Sentinel‐1 imagery is used to estimate surface deformations in the Choshui river alluvial fan, Taiwan, between 2016 and 2018. Spatial regression (SR) is applied to estimate the annual groundwater drawdown with a calculated R‐square of 0.96, which is shown to be superior to a nonspatial model. This study demonstrates the potential of the satellite‐based groundwater drawdown map prediction using InSAR‐derived land deformation. In predication, the SR model can reliably catch the patterns of annual predicted drawdown without requiring detailed groundwater observations. [ABSTRACT FROM AUTHOR]

The Blue Ridge Province contains ubiquitous northeast-southwest-trending thrust faults or smaller thrust 'slivers' that greatly impact the nature and character of ground water flow in this region. Detailed investigations at a field site in Floyd County, Virginia, indicate that high-permeability zones occur in the brittle crystalline rocks above these thrust faults. Surface and borehole geophysics, aquifer tests, and chlorofluorocarbon and geochemical data reveal that the shallow saprolite aquifer is separated from the deeper fault-zone aquifer by a low-fracture permeability bedrock confining unit, the hydraulic conductivity of which has been estimated to be six orders of magnitude less than the conductivity of the fault zones at the test site. Within the Blue Ridge Province, these fault zones can occur at depths of 300 m or more, can contain a significant amount of storage, and yield significant quantities of water to wells. Furthermore, it is expected that these faults may compartmentalize the deep aquifer system. Recharge to and discharge from the deep aquifer occurs by slow leakage through the confining unit or through localized breach zones that occur where quartz accumulated in high concentrations during metamorphism and later became extensively fractured during episodes of deformation. The results of this investigation stress the importance of thrust faults in this region and suggest that hydrogeologic models for the Blue Ridge Province include these ancient structural features. Faults in crystalline-rock environments may not only influence the hydrology, they may dominate the flow characteristics of a region., Background The Blue Ridge Physiographic Province in the eastern United States represents one of the most geologically complex regions in the world. Repeated episodes of compressional and extensional tectonism from [...]

Measurement and analysis of aquifer-system compaction have been used to characterize aquifer and confining unit properties when other techniques such as flow modeling have been ineffective at adequately quantifying storage properties or matching historical water levels in environments experiencing land subsidence. In the southeastern coastal plain of Virginia, high-sensitivity borehole pipe extensometers were used to measure 24.2 mm of total compaction at Franklin from 1979 through 1995 (1.5 mm/year) and 50.2 mm of total compaction at Suffolk from 1982 through 1995 (3.7 mm/year). Analysis of the extensometer data reveals that the small rates of aquifer-system compaction appear to be correlated with withdrawals of water from confined aquifers. One-dimensional vertical compaction modeling indicates measured compaction is the result of nonrecoverable hydrodynamic consolidation of the fine-grained confining units and interbeds, as well as recoverable compaction and expansion of coarse-grained aquifer units. The calibrated modeling results indicate that nonrecoverable specific storage values decrease with depth and range from 1.5 X [10.sup.-5]/m for aquifer units to 1.5 X [10.sup.-4]/m for confining units and interbeds. The aquifer and Potomac system recoverable specific storage values were all estimated to be 4.5 X [10.sup.-6]/m, while the confining units and interbeds had values of 6.0 X [10.sup.-6]/m. The calibrated vertical hydraulic conductivity values of the confining units and interbeds ranged from 6.6 X [10.sup.-4] m/year to 2.0 X [10.sup.-3] m/year. These parameter values will be useful in future management and modeling of ground water in the Virginia Coastal Plain., Introduction Withdrawals of ground water from the confined aquifers of the ~20,000 [km.sup.2] Virginia Coastal Plain, which includes an area of ~10,000 [km.sup.2] in southeastern Virginia (Figure 1), have increased [...]

The storage coefficient that is used ubiquitously today was first defined by the analytical work of Theis and Jacob over a half-century ago. Inherent within this definition is the restriction of purely vertical compression of the aquifer during a reduction in pressure. The assumption is revisited and quantitatively evaluated by comparing numerical results using both one- and three-dimensional strain models in the presence of three-dimensional flow. Results indicate that (1) calculated hydraulic head values are nearly identical for both models; (2) the release of water from storage in terms of volume strain is nearly identical for both models and that the location of maximum production moves outward from the well as a function of time; (3) the vertical strain components are markedly different with at least 50% of the total volume of water pumped originating from horizontal strain (and increasing to as much as 70%); and (4) for the one-dimensional strain model to yield the necessary quantity of water to the pu mped well, the resulting vertical compaction (land subsidence) is as much as four times greater and vertical strain is as much as 60% greater than the three-dimensional strain model. Results indicate that small changes in porosity resulting from horizontal strain can yield extremely large quantities of water to the pumping well. This study suggests that the assumption of purely vertical strain used in the definition of the storage coefficient is not valid., Thomas J. Burbey (1) Introduction O.E. Meinzer (Meinzer 1928) was one of the first hydrologists to recognize from field observations that confined aquifers were compressible and that they exhibited elastic [...]

Land subsidence caused by groundwater overexploitation is a serious global problem. The acquisition of spatiotemporal pumping rates and volumes is a first step for water managers to develop a strategic plan for mitigating land subsidence. This study investigates an empirical formulation to estimate the monthly maximum pumped volume over a 10‐year period based on electric power consumption data. A spatiotemporal variability analysis of monthly pumped volume is developed to provide an improved understanding of seasonal pumping patterns and the role of irrigation type. The analysis of regional pumped volume provides an approximation of the spatiotemporal patterns of the variations in pumped volume. Results show the effects of climate, seasonal changes in pumping from irrigation, and the local differences in pumping caused to crop types. A seasonal pumped volume peak occurs annually, with the highest and least pumped volumes occurring in March (highest peak) and September (lowest peak), respectively. However, the majority of the historical maximum pumped volumes have occurred during the last few years. Extracted volumes continue to increase in some locations. The analysis reveals increasing trends in pumping, thereby possibly providing the locations where increased effective stresses may lead to land subsidence. Article impact statement: The study provides the spatiotemporal patterns in pumped volume estimated from electricity consumption data, especially maximum pumped volumes. [ABSTRACT FROM AUTHOR]

Thomas J. Burbey [1] Abstract Subsidence data historically have been collected using costly extensometers. Recent advances in InSAR and GPS technology may provide noninvasive methods for collecting these data, which [...]

Abstract: Flow through reservoirs such as fractured media is powered by head gradients which also generate measurable poroelastic deformation of the rock body. The combined analysis of surface deformation and subsurface pressure provides valuable insights of a reservoir's structure and hydromechanical properties, which are of interest for deep‐seated CO2 or nuclear waste storage for instance. Among all surveying tools, surface tiltmeters offer the possibility to grasp hydraulically induced deformations over a broad range of time scales with a remarkable precision. Here we investigate the information content of transient surface tilt generated by the pressurization a kilometer scale subvertical fault zone. Our approach involves the combination of field data and results of a fully coupled poromechanical model. The signature of pressure changes in the fault zone due to pumping cycles is clearly recognizable in field tilt data and we aim to explain the peculiar features that appear in (1) tilt time series alone from a set of four instruments and 2) the ratio of tilt over pressure. We evidence that the shape of tilt measurements on both sides of a fault zone is sensitive to its diffusivity and its elastic modulus. The ratio of tilt over pressure predominantly encompasses information about the system's dynamic behavior and extent of the fault zone and allows separating contributions of flow in the different compartments. Hence, tiltmeters are well suited to characterize hydromechanical processes associated with fault zone hydrogeology at short time scales, where spaceborne surveying methods fail to recognize any deformation signal. [ABSTRACT FROM AUTHOR]

Fractured bedrock reservoirs are of socio-economical importance, as they may be used for storage or retrieval of fluids and energy. In particular, the hydromechanical behavior of fractures needs to be understood as it has implications on flow and governs stability issues (e.g., microseismicity). Laboratory, numerical, or field experiments have brought considerable insights to this topic. Nevertheless, in situ hydromechanical experiments are relatively uncommon, mainly because of technical and instrumental limitations. Here we present the early stage development and validation of a novel approach aiming at capturing the integrated hydromechanical behavior of natural fractures. It combines the use of surface tiltmeters to monitor the deformation associated with the periodic pressurization of fractures at depth in crystalline rocks. Periodic injection and withdrawal advantageously avoids mobilizing or extracting significant amounts of fluid, and it hinders any risk of reservoir failure. The oscillatory perturbation is intended to (1) facilitate the recognition of its signature in tilt measurements and (2) vary the hydraulic penetration depth in order to sample different volumes of the fractured bedrock around the inlet and thereby assess scale effects typical of fractured systems. By stacking tilt signals, we managed to recover small tilt amplitudes associated with pressure-derived fracture deformation. Therewith, we distinguish differences in mechanical properties between the three tested fractures, but we show that tilt amplitudes are weakly dependent on pressure penetration depth. Using an elastic model, we obtain fracture stiffness estimates that are consistent with published data. Our results should encourage further improvement of the method. [ABSTRACT FROM AUTHOR]

Surface coal mining has altered land cover, near-surface geologic structure, and hydrologic processes of large areas in central Appalachia, USA. These alterations are associated with changes in water quality such as elevated total-dissolved solids, which is usually measured via its surrogate, specific conductance (SC). The SC of valley fill effluent streams is a function of fill construction methods, materials, and age; yet hydrologic studies that relate these variables to water quality are sparse due to the difficulty of conducting traditional hydrologic studies in mined landscapes. We used electrical resistivity imaging (ERI) to visualize the subsurface geologic structure and hydrologic flow paths within a valley fill. ERI is a noninvasive geophysical technique that maps spatiotemporal changes in resistivity of the subsurface. We paired ERI with artificial rainfall experiments to track infiltrated water as it moved through the valley fill. Results indicate that ERI can be used to identify subsurface geologic structure and track advancing wetting fronts or preferential flow paths. Our results suggest that the upper portion of the fill contains significant fines, whereas the deeper profile is primarily large rocks and void spaces. Water tended to pond on the surface of compacted areas until it reached preferential flow paths, where it appeared to infiltrate quickly down to >15 m depth in 75 min. ERI applications can improve understanding of how fill construction techniques influence subsurface water movement, and in turn may aid in the development of valley fill construction methods to reduce water quality effects. [ABSTRACT FROM AUTHOR]

Las Vegas Valley has had a long history of groundwater development and subsequent surface deformation. InSAR interferograms have revealed detailed and complex spatial patterns of subsidence in the Las Vegas Valley area that do not coincide with major pumping regions. This research represents the first effort to use high spatial and temporal resolution subsidence observations from InSAR and hydraulic head data to inversely calibrate transmissivities ( T), elastic and inelastic skeletal storage coefficients ( Ske and Skv) of the developed-zone aquifer and conductance ( CR) of the basin-fill faults for the entire Las Vegas basin. The results indicate that the subsidence observations from InSAR are extremely beneficial for accurately quantifying hydraulic parameters, and the model calibration results are far more accurate than when using only groundwater levels as observations, and just a limited number of subsidence observations. The discrepancy between distributions of pumping and greatest levels of subsidence is found to be attributed to spatial variations in clay thickness. The Eglington fault separates thicker interbeds to the northwest from thinner interbeds to the southeast and the fault may act as a groundwater-flow barrier and/or subsidence boundary, although the influence of the groundwater barrier to this area is found to be insignificant. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing has become an important technique for enhancing the permeability of hydrocarbon source rocks and increasing aquifer transmissivity in many hard rock environments where natural fractures are limited, yet little is known about the nature or behaviour of these hydraulically induced fractures as conduits to flow and transport. We propose that these fractures tend to be smooth based on observed hydraulic and transport behaviour. In this investigation a multi-faceted approach was used to quantify the properties and characteristics of an isolated hydraulically induced fracture in crystalline rocks. Packers were used to isolate the fracture that is penetrated by two separate observation wells located approximately 33 m apart. A series of aquifer tests and an induced gradient tracer test were performed to better understand the nature of this fracture. Aquifer test results indicate that full recovery is slow because of the overall low permeability of the crystalline rocks. Drawdown tests indicate that the fracture has a transmissivity of 1-2 m2/day and a specific storage on the order of 2-9 × 10−7/m. Analysis of a potassium-bromide tracer test break through curve shows classic Fickian behaviour with minimal tailing analogous to parallel plate flow. Virtually all of the tracer was recovered, and the breakthrough curve dilution indicates that the swept area is only about 11% of a radial flow field and the estimated aperture is ≤0.5 mm, which implies a narrow linear flow region. These outcomes suggest that transport within these hydraulically induced 'smooth' fractures in crystalline rocks is rapid with minimal mixing, small local velocity fluctuations and no apparent diffusion into the host rock or secondary fractures. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Las Vegas Valley has had a long history of groundwater development and subsequent surface deformation. Much research has been done to estimate parameters within the Las Vegas Basin, but this research represents the first effort to use parameter estimation techniques to inversely calibrate hydrogeologic aquifer parameters for the principal aquifer of the entire basin. Three different inversion strategies are invoked to determine the most accurate and computationally efficient method for estimating transmissivities T and elastic and inelastic skeletal storage coefficients (Ske and Skr) at the basin scale: the zonation method (ZM), the adaptive multi-scale method, and the differential evolution adaptive metropolis Markov chain Monte Carlo scheme (DREAM MCMC). The three inversion methods are compared and contrasted based on quantitative measurements of model fit, computational efficiency, and user flexibility. The results indicate that overall, the adaptive multi-scale method, which is able to efficiently reconstruct the T, Ske, and Skv zones while providing more flexibility and accuracy than the other two methods, is the best strategy for calibrating optimal model parameters and providing a framework for developing an accurate hydrogeologic model for Las Vegas Valley. [ABSTRACT FROM AUTHOR]

Spatial distribution of sand bodies in caprock is important to its sealing potential in the evaluation of a potential waste disposal site. A numerical investigation was conducted to investigate the roles of sand bodies in caprock integrity, and a new leaking mechanism associated with the embedded sand bodies has been identified for the first time. This numerical model is characterized by a low proportion of sand bodies in caprock formation and stochastic permeability assignments to the shale facies with a consideration of stress-sensitive permeability. The research revealed that sand bodies in caprock, even initially unconnected due to their low concentration, represent the foundation for creating a percolating flow path if certain conditions are met. These conditions include a relatively high pore pressure gradient across the caprock, which may occur during fluid injection and a favorable geometric configuration of these sand bodies. Sand bodies of different shapes play different roles in forming a percolating flow path. Sand bars/dykes can lead pore pressure front to cover a considerable vertical thickness quickly, while thin sand sheets tend to transfer the pore pressure front across a large horizontal range and thus increase the possibility of involving other local sand bodies in the evolution of percolating flow paths. On the other hand, the presence of percolating flow path in caprock can limit the pore pressure increase in the aquifer formation during fluid injection. The more active the flow paths, the less increase the pore pressure that occurs in the aquifer. Stress-dependent permeability can facilitate the formation of percolating flow path, but evaluating how much impact it may have remains a question for future research. The modeling results indicate that the underlying geologically based discrete structure of permeable domains, even when masked by a lowpermeable rock matrix, exerts a significant influence on the location and magnitude of fluid flow within the system during fluid injection. [ABSTRACT FROM AUTHOR]

Ground deformation (both deformation patterns and deformation magnitudes) due to porous medium pressurization and fracture pressurization exhibits different characteristics. However, whether these differences can be detected during fluid storage is a complicated issue. Two analytical solutions have been previously developed to correlate ground deformation with underground fluid injection/extraction. One is Geertsma's (1973) solution on ground deformation associated with a disc-shaped reservoir (porous medium), and another is Sun's (1969) solution on ground deformation associated with a circular fracture. In this technique note, we compare these two solutions through a finite element modeling approach based on comparable boundary conditions, i.e., the similar areal extent, burial depth and fluid injection volume. We found that the deformation patterns are similar for these two cases on the ground surface, but very different underneath, especially in the horizontal direction. The radial extent of horizontal deformation is more restricted for the fracture pressurization case and more importantly, much higher magnitudes of ground deformation occur for the fracture pressurization case, which can provide new understanding of how to distinguish a fracture pressurization effect from porous medium pressurization. [ABSTRACT FROM AUTHOR]

Parameter estimation codes such as UCODE_2005 are becoming well-known tools in groundwater modeling investigations. These programs estimate important parameter values such as transmissivity ( T) and aquifer storage values ( Sa) from known observations of hydraulic head, flow, or other physical quantities. One drawback inherent in these codes is that the parameter zones must be specified by the user. However, such knowledge is often unknown even if a detailed hydrogeological description is available. To overcome this deficiency, we present a discrete adjoint algorithm for identifying suitable zonations from hydraulic head and subsidence measurements, which are highly sensitive to both elastic ( Sske) and inelastic ( Sskv) skeletal specific storage coefficients. With the advent of interferometric synthetic aperture radar ( InSAR), distributed spatial and temporal subsidence measurements can be obtained. A synthetic conceptual model containing seven transmissivity zones, one aquifer storage zone and three interbed zones for elastic and inelastic storage coefficients were developed to simulate drawdown and subsidence in an aquifer interbedded with clay that exhibits delayed drainage. Simulated delayed land subsidence and groundwater head data are assumed to be the observed measurements, to which the discrete adjoint algorithm is called to create approximate spatial zonations of T, Sske, and Sskv. UCODE-2005 is then used to obtain the final optimal parameter values. Calibration results indicate that the estimated zonations calculated from the discrete adjoint algorithm closely approximate the true parameter zonations. This automation algorithm reduces the bias established by the initial distribution of zones and provides a robust parameter zonation distribution. [ABSTRACT FROM AUTHOR]

The hydrological effects of earthquakes are determined by the style of fault displacement, rather than simply by the magnitude of the earthquake. In the past, many researchers used the analytical solution of Okada (1992) to estimate the pore-pressure field, which is derived through the stress field by Skempton's B coefficient, after the stress field is calculated from a given forced fault slip in Okada's method (Okada, 1992). This approach is a one-way coupling approach, as fluid has no effect on rock behavior, and Skempton's B coefficient must be given separately to know the pore-pressure field. We create a fault-slip model in conjunction with a poroelastic model and present a new approach in which fault movement is a consequence of re-duced friction occurring in a saturated porous medium. This model represents a numerical parallel to the traditional Okada's approach, but with two advantages: (1) it is well suited for more complex geometries and heterogeneous formations for which analytical methods fail; (2) it is fully coupled so that fluid effects are also taken into account. This model shows that all three different faulting regimes exhibit completely different pore-pressure change distributions in 3D space. The transient pore-pressure change after a strike-slip faulting event is axis symmetrical with respect to the imaginary vertical line passing through the epicenter and hypocenter, with in-creased and decreased areas evenly distributed in a juxtaposed pattern. The transient pore-pressure changes associated with normal and thrust faulting all display a circular pattern on the model surface, with the largest pore-pressure increase occurring at the epicenter for normal faulting and the largest decrease for thrust faulting. Many ob-served field phenomena have been explained in surprising detail by this model. [ABSTRACT FROM AUTHOR]

Reverse water-level fluctuations have been widely observed in aquitards or aquifers separated from a pumped confined aquifer (Noordbergum effect) immediately after the initiation of pumping. This same reverse fluctuation has been observed in a fractured crystalline-rock aquifer at the Coles Hill uranium site in Virginia in which the reverse water-level response occurs within a pumped fracture and results from an instantaneous strain response to pumping that precedes the pore-pressure response in observation wells of sufficient distance from the pumped well. This response is referred to as the Mandel-Cryer effect. The unique aspect of this water level rise during a controlled 24 h pumping test was that the reverse water levels lasted for approximately 100 min and reached a magnitude of nearly 1 cm prior to a typical drawdown response. The duration and magnitude of the response reflects the poromechanical properties of the fractured host rock and hydraulic properties of the pumped fracture. An axisymmetric flow and deformation model were developed using Biot2 in an effort to simulate the observed water-level response along an assumed 0.5 to 1.0 cm aperture horizontal fracture 176 m from the pumping well and to identify the importance of the poroelastic effect. Results indicate that traditional aquifer-testing methods that ignore the poromechanical response are not significantly different than results that include the response. However, the poroelastic effect allows for more accurate and efficient parameter calibration. [ABSTRACT FROM AUTHOR]

The threat of damage to buildings and other infrastructures resulting from land subsidence associated with groundwater pumping in urbanized areas is an ongoing problem requiring assessment. An important goal of subsidence vulnerability assessment is to construct a composite subsidence vulnerability index (SVI) that is represented by a set of indicators that focuses on four different thematic factors: physical, social, economic, and environmental vulnerability. These indicators are evaluated on the basis of indicator selection principles and then weighted by their contribution rate to the overall index. The weights reflect different measures assigned to the township-specific conditions. A complete and composite subsidence vulnerability assessment is developed in which future vulnerability management decision-making processes can be readily made. The vulnerability assessment includes not only the construction of the SVI, which involves selecting, assigning value to, weighting, and aggregating the vulnerability indicators, but also the presentation of the SVI decomposition. Research results demonstrate that a composite subsidence vulnerability assessment method can be made by first constructing and then decomposition-presenting the overall SVI. This allows for the relative comparison of subsidence vulnerability and the identification of the main vulnerable indicators; thus providing subsidence risk, which represents an important step toward vulnerability management of water resources. [ABSTRACT FROM PUBLISHER]

Identifying fracture pathways and connectivity between adjacent wells is vital for understanding flow characteristics, transport properties, and fracture characteristics. In this investigation, a simple, straightforward methodology is presented for assessing hydrofracing success and identifying possible fracture connectivity between neighboring boreholes, using water-level barometric response and tide signatures of individual fractures in a crystalline-rock setting. Water levels and barometric pressure heads were collected at two wells 27 m apart both prior to, and after, hydrofracing one of the wells at the fractured-rock research site in Floyd County, Virginia. Vastly different barometric and tidal signatures existed at the two wells prior to hydrofracing as well EX-1 had no discernable fractures, while W-03 was connected to an identified fault-zone aquifer and produced a notable water-level earth tide and barometric signatures. After hydrofracing EX-1, new fractures were induced and the resulting water-level tidal signature and barometric efficiencies were nearly identical to the W-03 well. Aquifer testing conducted from both wells verified this connectivity along the fault-zone aquifer. The small phase difference between the tidal responses in the two wells can be accounted for by the calculated differences in transmissivity and casing diameter. [ABSTRACT FROM AUTHOR]

The accurate estimation of aquifer parameters such as transmissivity and specific storage is often an important objective during a ground water modeling investigation or aquifer resource evaluation. Parameter estimation is often accomplished with changes in hydraulic head data as the key and most abundant type of observation. The availability and accessibility of global positioning system and interferometric synthetic aperture radar data in heavily pumped alluvial basins can provide important subsidence observations that can greatly aid parameter estimation. The aim of this investigation is to evaluate the value of spatial and temporal subsidence data for automatically estimating parameters with and without observation error using UCODE-2005 and MODFLOW-2000. A synthetic conceptual model (24 separate cases) containing seven transmissivity zones and three zones each for elastic and inelastic skeletal specific storage was used to simulate subsidence and drawdown in an aquifer with variably thick interbeds with delayed drainage. Five pumping wells of variable rates were used to stress the system for up to 15 years. Calibration results indicate that (1) the inverse of the square of the observation values is a reasonable way to weight the observations, (2) spatially abundant subsidence data typically produce superior parameter estimates under constant pumping even with observation error, (3) only a small number of subsidence observations are required to achieve accurate parameter estimates, and (4) for seasonal pumping, accurate parameter estimates for elastic skeletal specific storage values are largely dependent on the quantity of temporal observational data and less on the quantity of available spatial data. [ABSTRACT FROM AUTHOR]

A 62 day controlled aquifer test was conducted in thick alluvial deposits at Mesquite, Nevada, for the purpose of monitoring horizontal and vertical surface deformations using a high-precision global positioning system (GPS) network. Initial analysis of the data indicated an anisotropic aquifer system on the basis of the observed radial and tangential deformations. However, new InSAR data seem to indicate that the site may be bounded by an oblique normal fault as the subsidence bowl is both truncated to the northwest and offset from the pumping well to the south. A finite-element numerical model was developed using ABAQUS to evaluate the potential location and hydromechanical properties of the fault based on the observed horizontal deformations. Simulation results indicate that for the magnitude and direction of motion at the pumping well and at other GPS stations, which is toward the southeast (away from the inferred fault), the fault zone (5 m wide) must possess a very high permeability and storage coefficient and cross the study area in a northeast-southwest direction. Simulated horizontal and vertical displacements that include the fault zone closely match observed displacements and indicate the likelihood of the presence of the inferred fault. This analysis shows how monitoring horizontal displacements can provide valuable information about faults, and boundary conditions in general, in evaluating aquifer systems during an aquifer test. [ABSTRACT FROM AUTHOR]

Recent investigations describing the hydrogeology of the Blue Ridge Province of Virginia suggest the occurrence of multiple aquifers and flow paths that may be responsible for the variable flow behavior of springs and seeps appearing throughout the region. Deep, confined aquifers associated with ubiquitous faults and shallow, variably confined saprolite aquifers may contribute water to spring outlets resulting in significantly different quantities of discharge and water quality. Multiple analyses are required to adequately identify the flow paths to springs. In this investigation, hydrograph analyses, surface electrical resistivity surveys, aquifer tests, and nitrate concentrations are used in conjunction with previously reported analyses from borehole logs and age dating of ground water to identify two distinct flow paths. Results indicate that base flow occurs from a deep fault zone aquifer and such discharge can be maintained even during prolonged periods of drought, while increased discharge identified on hydrograph peaks suggests the occurrence of rapid flow through the saprolite aquifer within a radius of about 25 meters of the spring orifice. Springflow hydrograph analysis is suitable for rapid characterization of flow paths leading to spring outlets. Rapid characterization is important for evaluation of potential water quality problems arising from contamination of shallow and deep aquifers and for evaluation of water resource susceptibility to drought. The techniques evaluated here are suitable for use in other locations in fractured crystalline rock environments. [ABSTRACT FROM AUTHOR]

BSTRACT: Discharge from flooded abandoned subsurface coal mines is considered a potential source for water supplies where other acceptable water sources are not available. The objective of this study was to develop procedures for determining sustainability of mine-water discharge using rainfall and discharge data for a case study site. The study site is located in southwest Virginia where Late Paleozoic sequences of sandstone, coal, and shale predominate. A rain gauge and a flow rate monitoring system were installed at the site and data were collected for a period of 100 days. The recording period corresponded with one of the driest periods in recent years and, therefore, provided valuable information regarding the flow sustainability during baseflow conditions. From available data on underground mining patterns, geology, and ground water flow regimes, it was determined that a coal mine aquifer exhibits hydraulic characteristics very similar to the extremely heterogeneous systems observed in karst aquifers, and the mine discharge is analogous to springflow. Thus, techniques commonly used in karst-water systems and springflow analysis were used to develop rainfall/mine-discharge relationships. Springflow recession analysis was performed on five rainfall recessions and the coefficient for each recession was compared and interpreted in light of known geologic information. It was found that the recession coefficients described the mine discharge adequately and the mine aquifer response to a rainfall pulse was very similar to the response from certain types of karst aquifers. A cross-correlation analysis was performed to verify the results of the recession analysis and to develop a 'black box' statistical model for discharge data. The correlation analysis proved the validity of springflow recession analysis for mine discharge. The recorded data length was not adequate to create a statistical model, however, but a procedure was proposed for a statistical model that could be used with large flow records. For the study site, the mine discharge was found to be sustainable for a prolonged period of time. [ABSTRACT FROM AUTHOR]