Aquifer Hydraulic Properties Research Articles

Examining individual recession events instead of a data cloud: Using a modified interpretation of dQ/dt–Q streamflow recession in glaciated watersheds to better inform models of low flow

Summary To examine stream recession rates, hydrologists have plotted the rate of change in discharge (dQ/dt) versus the mean discharge (Q). Such plots typically result in a large cloud of data points where the slope of the lower bound of the cloud is often used to infer aquifer hydraulic properties as informed by certain interpretations of the Boussinesq Equation. For seven watersheds in New York State, USA ranging from 100 km2 to 6415 km2, we distinguish data points in the dQ/dt–Q plot belonging to individual recession events instead of looking at the entire data cloud. The recession curve of individual events consistently shifts upward during the summer and late fall (relative to spring and late fall curves) and much of the scatter in the data cloud appears to be due to seasonal variations in recession. We speculate that these seasonal variations in recession rate may be due to variations in watershed evapotranspiration (ET). Additionally, most individual recession events have slopes of approximately two when plotted as log dQ/dt versus log Q. Application of the Boussinesq Equation to watershed-scale recession date predicts slopes shifting from 3 to 3/2 as recession progresses, thus the observed slope of two casts doubt on whether it is appropriate to apply hydraulic aquifer theory to these types of watersheds to infer aquifer properties or predict low flows. As a further validation of the hydrologic information content of individual recession curves, the individual recession curves from time periods with minimal ET (early spring and late fall) were used to establish storage–discharge functions for two of the smaller watersheds. With these storage–discharge functions, a simple hydrologic model could reasonably simulate time series with minimal calibration (Nash–Sutcliffe R2 of 0.71 and 0.67 on log transformed discharge). Overall, this paper suggests that the analysis of individual event recession curves (particularly when compared among watersheds and seasons) can be a valuable tool for gaining insights into hydrological processes at the scale of large watersheds and can justify alternatives to current low flow models.

Controlled CO2 injection into heterogeneous geologic formations for improved solubility and residual trapping

Geologic CO2 storage (GCS) has been proposed as a potentially viable climate change mitigation option. Among the trapping mechanisms known for permanent CO2 storage in saline aquifers, solubility and residual trapping are important for safe short‐term entrapment. These storage mechanisms are hampered by density‐driven upward CO2 movement that inhibits the lateral migration of the plume, leaving a large portion of the aquifer volume unexposed to CO2 and unavailable for storage. Heterogeneity of aquifer hydraulic properties and high horizontal/vertical hydraulic conductivity ratio of geologic formations are two competing mechanisms that can hinder upward CO2 migration. Postinjection displacement of free‐phase CO2 is somewhat controlled by the heterogeneity in rock permeability and porosity distributions. In particular, low permeability shale layers that act as vertical flow barrier and high‐permeability horizontal channels that form flow conduits can spread the CO2 plume laterally in the aquifer. In this paper, we consider CO2 storage in heterogeneous saline aquifers and propose controlled CO2 injection, based on existing knowledge of heterogeneity, to increase CO2 contact with brine and improve the solubility and residual trapping and the overall aquifer storage potential. We examine two optimization methods: directly maximizing the total stored gas in the aquifer, and maximizing the sweep efficiency of the CO2 flood to promote uniform displacement in all directions. We consider the effect of geologic uncertainty on the performance of the controlled injection schemes by using an ensemble of model realizations to represent the uncertainty in aquifer property distribution. We also show how a controlled injection can be used to mitigate the risk of leakage from potential pathways, such as an abandoned well, by restricting CO2 movement toward the leakage zone. Our results suggest that controlled injection can lead to substantial improvements in residual and dissolution trapping and can be used to divert a CO2 plume from approaching sensitive zones or leakage pathways.

Fluorescent dye imaging of the volume sampled by single well forced-gradient tracer tests evaluated in a laboratory-scale aquifer physical model

This study presents a new method to visualise forced-gradient tracer tests in 2-D using a laboratory-scale aquifer physical model. Experiments were designed to investigate the volume of aquifer sampled in vertical dipole flow tracer tests (DFTT) and push–pull tests (PPT), using a miniature monitoring well and straddle packer arrangement equipped with solute injection and recovery chambers. These tests have previously been used to estimate bulk aquifer hydraulic and transport properties for the evaluation of natural attenuation and other remediation approaches. Experiments were performed in a silica glass bead-filled box, using a fluorescent tracer (fluorescein) to deduce conservative solute transport paths. Digital images of fluorescein transport were captured under ultraviolet light and processed to analyse tracer plume geometry and obtain point-concentration breakthrough histories. Inorganic anion mixtures were also used to obtain conventional tracer breakthrough histories. Concentration data from the conservative tracer breakthrough curves was compared with the digital images and a well characterised numerical model. The results show that the peak tracer breakthrough response in dipole flow tracer tests samples a zone of aquifer close to the well screen, while the sampling volume of push–pull tests is limited by the length of the straddle packers used. The effective sampling volume of these single well forced-gradient tests in isotropic conditions can be estimated with simple equations. The experimental approach offers the opportunity to evaluate under controlled conditions the theoretical basis, design and performance of DFTTs and PPTs in porous media in relation to measured flow and transport properties. 2 2 DFTT = Dipole Flow Tracer Test, PPT = Push Pull Tracer Test.

Geoelectrical studies for estimating aquifer hydraulic properties in Enugu State, Nigeria

2over eight main geological formations. The thickness, lateral extent and resistivity of the aquiferous layers were determined by the electrical survey. Also, zones of high yield potentials were inferred from the resistivity information. Transmissivity values were inferred using the empirical relationship between hydraulic conductivity and formation factor. Results show highly variable thickness of the main aquifer in the study area. Aggregate longitudinal conductance indicates greater depth of the substratum in the central part of the area, underlain by the Ajali and Nsukka Formations. High values of transmissivity, specific yield and moderate porosity also predominate, thus suggesting thick and prolific aquiferous zone. Lower average values of these parameters were obtained in southwestern and eastern parts underlain by the Imo and Ezeaku/ AwguNdeaboh/ Nkporo Formations, with the eastern part recording the lowest values, except porosity with the highest estimated average value. Maps of various geoelectrical and hydraulic parameters have been produced and the overall results can serve as a useful guide in planning a drilling program in the study area.

Drought-driven transient aquifer compaction imaged using multitemporal satellite radar interferometry

In unconsolidated, heterogeneous aquifer systems, low rates of pore-pressure diffusion of applied effective stresses due to the drainage of thick, low-permeability, clay-rich layers with time constants of decades to centuries cause delayed, residual permanent compaction and land subsidence. Current application of satellite differential radar interferometry (DInSAR—differential interferometric synthetic aperture radar) to estimate aquifer hydraulic properties (compressibility and/or storage) in these systems is limited by the temporal availability of synthetic aperture radar data (1992–present). In this paper we study the degree of aquifer compaction due to water extraction using DInSAR through an example in southeast Spain. Ground deformation data indicate large-scale deformation and in particular the discovery of the highest rates of groundwater-related land subsidence recorded in Europe (>10 cm/yr), affecting the Guadalentin River basin (>200 km 2 ), the largest tributary of the Segura River. Modeling of the ground surface time series of the Guadalentin Basin indicates that deformation is mainly driven by nonlinear time-delayed flow processes in the underlying aquifer. After a drought period (1990–1995), the aquifer responded with an exponential decay of the land subsidence (lasting ∼8 yr), suggesting transient groundwater pore-pressure flow. We show that multitemporal satellite radar interferometry analysis and its modeling can be a stimulating way to study nonlinear soil mechanics and groundwater flows at aquifers. A deeper understanding of such processes could help the management of water resources and land subsidence of unconsolidated coastal and Quaternary alluvial aquifers in a highly evolving climate region (the Mediterranean Sea and elsewhere).

Estimating Aquifer Properties from the Water Level Response to Earth Tides

Water level fluctuations induced by tidal strains can be analyzed to estimate the elastic properties, porosity, and transmissivity of the surrounding aquifer material. We review underutilized methods for estimating aquifer properties from the confined response to earth tides. The earth tide analyses are applied to an open well penetrating a confined carbonate aquifer. The resulting range of elastic and hydraulic aquifer properties are in general agreement with that determined by other investigators for the area of the well. The analyses indicate that passive monitoring data from wells completed in sufficiently stiff, low porosity formations can provide useful information on the properties of the surrounding formation.

Effects of Karst and geological structure on groundwater flow: The case of Yarqon-Taninim Aquifer, Israel

This study demonstrates the significant influences of the geological structure (especially folding and lithology) and the karst system on groundwater flow regime. Folds divert groundwater flow from the general hydraulic gradient; marly layers sustain several perched sub-aquifers above the regional aquifer; and karstification increases the hydraulic conductivity by several orders of magnitude. These phenomena are quantitatively demonstrated within the Yarqon-Taninim (YT) basin, Israel, which is a complex groundwater system, combining several (extremely) opposite characteristics: humid and arid recharge zones, phreatic and confined parts, shallow and deep sub-aquifers, stratified and relatively-homogeneous sub-basins, saline and fresh water bodies, as well as stagnant and fast-flowing groundwater regions. We have introduced a 3D geological-based grid for the basin (for the first time). It was implemented into a numerical code (FEFLOW), which was used thereafter to analyze quantitatively the flow regime, the groundwater mass balance, and the aquifer hydraulic properties. We present up to date conceptual understanding and numerical modeling of the YT flow field, especially at its mountainous parts. Based on the calibration procedure and the sensitivity analyses, we obtained the best-fitted hydraulic conductivity values for the aquifer mesh. The general phenomenon observed is that as groundwater flow quantity increases, the hydraulic conductivity also increases. We interpret this result by the karstification mechanism (including paleo-karst). Thus, where groundwater flow-lines converge and where groundwater discharge amount increases, the karstification process intensifies and permeability increases. Consequently, at the mountainous region, along the syncline axes, where groundwater flow-lines converge, higher conductivities are found. Modeling results also exhibit that at the lowland confined area, the geological structure does not play a major role in directing groundwater flow. Rather, the flow field is controlled by the well-developed karst system and the relatively homogenous carbonate section. It is hypothesizes that the extensive karstification took place at the Messinian Salinity Crises, ∼5.5 Ma, during which groundwater heads as well as sea level were lowered by several 100 m.

Geological controls and effects of floodplain asymmetry on river–groundwater interactions in the southeastern Coastal Plain, USA

Channel sediment and alluvial aquifer hydraulic properties exert a major control on river–groundwater interactions. Channels and floodplains are often asymmetrical, resulting in differences in sediment hydraulic properties across the river. Floodplain asymmetry is common along Coastal Plain rivers in South Carolina and North Carolina, USA. The Tar River, North Carolina, has an asymmetrical valley. The study objective was to characterize the effects of floodplain asymmetry and geological controls on river–groundwater interactions. Floodplain and river channel sediments adjacent to the river were characterized with split spoon cores and hand auger samples along a 22-km reach. Hydrogeology was characterized with 38 piezometers and water level recorders in and adjacent to the river. Ground penetrating radar was used to define the shallow stratigraphy. Channel sediments were significantly different between the north and south sides of the river. Hydraulic conductivity and groundwater inputs were greater on the side of the river (north) that contained more permeable fluvial deposits. Groundwater chemistry (δ18O, specific conductance) data also suggested greater exchange between surface water and groundwater on the north side of the river channel. A conceptual hydrogeological model illustrates that groundwater movement and contaminant transport to the river differs across the channel due to asymmetrical geology.

Estimation of effective aquifer hydraulic properties from an aquifer test with multi-well observations (Taiwan)

The representative anisotropic parameters for an aquifer located at the campus of the National Yunlin University of Science and Technology in Taiwan have been acquired. A constant-rate pumping test was carried out, and drawdown-time data were collected from ten observation wells. Applications of the conventional aquifer test analysis, which assumes aquifer homogeneity for each observed well hydrograph, yielded spatially varying transmissivity and storage coefficient estimates, contradicting the homogeneous assumption. A direct approach and a nonlinear-least squares minimization of distance-drawdown data were then employed to analyze anisotropy of the transmissivity of an equivalent homogeneous aquifer. Results show that the direction and values of anisotropic transmissivities vary with time and the estimates depend on the number of observation wells used. These field results are consistent with results from recent theoretical investigations which questioned the suitability of the conventional aquifer test analysis.

Métodos estocásticos para modelagem de escoamento estacionário e transiente em meios porosos

Uma maneira de incorporar a incerteza das medidas de campo e a variabilidade espacial nas propriedades hidráulicas de aqüíferos é estabelecer distribuições de probabilidade para os parâmetros físicos do meio. As propriedades estatísticas do potencial hidráulico são então determinadas pela solução numérica das equações diferenciais estocásticas que regem o regime de escoamento no aqüífero. Neste trabalho descrevemos a utilização de dois métodos de elementos finitos estocásticos (o método de Monte Carlo e o método da Colocação) para estimar a média e a variância do potencial hidráulico para fluxo saturado em meio estatisticamente heterogêneo, supondo que o coeficiente de transmissividade hidráulica é descrito por um processo lognormal. Um dos fatores decisivos na precisão numérica dos métodos é o comprimento de correlação associado à transmissividade. Discutimos também algumas configurações de baterias de extração que foram recentemente propostas para uma exploração adequada do aqüífero Recôncavo na bacia do rio Capivara (Bahia, Brasil), comparando dois modelos clássicos e introduzindo a aleatoriedade da transmissividade em um dos arranjos de poços que foram propostos.

Groundwater modeling in semiarid Central Sudan: adequacy and long-term abstraction

The present study assesses groundwater resources in the semiarid central Sudan, where 20 deep productive wells were installed to supply a major city in the region, El Obeid. The wells, which has an average 20 L/s discharge each, are taping a deep semiconfined to confined aquifer of fluvial silisiclastics deposited in the Tertiary–Pleistocene. Groundwater modeling was used as a technique to interpret the hydrologic system in arid to semiarid central Sudan and to simulate the future influence of the project on the hydrogeologic system. The simulation confirmed that steady-state flow conditions have been currently reached as indicated by consistency of computed heads. It also calibrated the values of the conductivity and recharge and ensured the sustainability of the El Obeid water supply project. A total of 3.5 × 107 m3/year can be continually extracted from the deep aquifer to supply El Obeid city without endangering the groundwater resources in the region. The decline in water level will not exceed 25 m during the first 10 years, while indefinite continuous pumping will affect only the vicinity of the wells in a circle of 30 km diameter. Therefore, aquifer storage capacity and hydraulic properties encourage further groundwater exploitation. The present use of groundwater is extremely lower than the present demand, and it can potentially cover future demands without introducing significant changes to the system. The increase of pumping cost due to the decline in head subsequent to project operation was found to be minimal and of local effect.

Analysis of groundwater quality using water quality index and conventional graphical methods: the Volta region, Ghana

Conventional graphical and statistical methods were used with water quality indices to characterize the hydrochemistry of groundwater from the northern part of the Volta region of Ghana. The objective was to determine the processes that affect the hydrochemistry and the variation of these processes in space among the three main geological terrains: the Buem formation, Voltaian System and the Togo series that underlie the area, and to determine the suitability of groundwater from the area for drinking purposes. The Q-mode cluster analysis reveals three main water groups. The groups established from the Q-mode HCA appear to indicate different degrees of weathering which could further indicate varying levels of fracturing aquifer hydraulic properties. R-mode HCA and factor analysis (using varimax rotation and Kaiser Criterion) were then applied to determine the significant sources of variation in the hydrochemistry. This study finds that groundwater hydrochemistry in the area is controlled by the weathering of silicate and carbonate minerals, as well as the chemistry of infiltrating precipitation. Mineral activity diagrams for the CaO–Na2O–Al2O3–SiO2–H2O and CaO–MgO–Al2O3–SiO2–H2O systems plotted for the area indicate stability in the smectite field and attribute hydrochemistry to the weathering of silicate minerals. Silicate mineral weathering and the effects of precipitation appear to be pervasive among all the three main geological terrains, whereas carbonate weathering is localized among the Voltaian aquifers. Cation exchange does not appear to play a significant role in the hydrochemistry but mild Water quality indices (WQI) were calculated for the samples using the concentrations of Na+, Ca2+, Mg2+, Cl−, NO3 −, F−, and EC at the various sample locations. The WQI values indicate that groundwater from the study area is of excellent quality for drinking purposes. WQI values from groundwater samples are averagely higher than samples taken from surface water sources in the area. This implies that geology has had an impact on the WQI of groundwater in the area.

Site characterisation for improved assessment of contaminant fate in fractured aquifers

A methodology for the improved characterisation of organic contaminant fate in fractured aquifers is presented. Using rotary cored boreholes, the methodology combines borehole logging techniques (caliper, gamma, resistivity, digital televiewer logs) and hydraulic testing (1–2m straddle-packers) with real-time on-site measurement of aquifer lithology, fracture and contaminant distribution in rock core and groundwater samples. The ensemble data are integrated to interpret the fracture network properties, aquifer hydraulic properties and contaminant distribution in vertical profile, which identifies hydraulically significant fractures, vertical flow components and preferential transport pathways supporting the design of a network of level-specific multilevel groundwater sampler monitoring wells. A key feature is the use of discrete-zone monitoring for site characterisation and long-term groundwater quality monitoring, to provide representative data on the spatial and temporal variation in aquifer hydraulic properties and hydrochemistry. This methodology is a cost-effective improvement on current site characterisation practice for fractured aquifers, providing more reliable assessment of contaminant transport and biodegradation processes, peak contaminant concentrations, plume geometry and behaviour for the development and validation of robust conceptual site models. This investment ultimately reduces long-term site management costs owing to less uncertainty in risk assessment and greater confidence in the targeting of more effective remediation. The methodology is demonstrated for the assessment of monitored natural attenuation at a site on the Chalk aquifer contaminated by a release of unleaded petroleum fuel.

The effect of river bend morphology on flow and timescales of surface water–groundwater exchange across pointbars

Head gradients along meandering rivers induce surface water–groundwater exchange across pointbars. We analyze this exchange process by finite-element modeling of groundwater flow and transport through model-generated river planforms taken from previous studies. The river is considered as a constant-head boundary that decreases linearly along the direction of the channel. This leads to an irregular groundwater flowfield in the pointbar that is controlled by channel sinuosity. Simulations under homogeneous aquifer conditions shows that the variable flowpath lengths coupled with the heterogeneous flow velocities lead to non-Fickian transport characterized by power-law residence time distributions. This anomalous transport behavior is enhanced by increased aquifer dispersivity. The broad transit time distribution implies that time-dependent chemical transformation processes in near-stream zones may be equally variable as the flowfield, even in the absence of heterogeneity in aquifer hydraulic properties.

Porosity and Salt Load Prediction from Airborne EM and Borehole EC

Dissolved salt in aquifers is a potential threat to fresh water resources and the environment. Interpolated grid maps at aquifer depths, derived from borehole EC measurements on water samples, are combined with more detailed bulk electrical conductivities from airborne electromagnetics to provide detailed estimates of total dissolved salt load and subsurface porosity values. A resistive host matrix assumption implies that our calculated porosity and salt load values are maximum values. This mapping technique is a large area coverage remote senor method which has been tested in different areas of the salt-threatened Murray-Darling basin. This technique provides extensive information on the hydraulic properties of aquifers, important for quantitative hydrology.

Steady flows driven by sources of random strength in heterogeneous aquifers with application to partially penetrating wells

Average steady source flow in heterogeneous porous formations is modelled by regarding the hydraulic conductivity K(x) as a stationary random space function (RSF). As a consequence, the flow variables become RSFs as well, and we are interested into calculating their moments. This problem has been intensively studied in the case of a Neumann type boundary condition at the source. However, there are many applications (such as well-type flows) for which the required boundary condition is that of Dirichlet. In order to fulfill such a requirement the strength of the source must be proportional to K(x), and therefore the source itself results a RSF. To solve flows driven by sources whose strength is spatially variable, we have used a perturbation procedure similar to that developed by Indelman and Abramovich (Water Resour Res 30:3385–3393, 1994) to analyze flows generated by sources of deterministic strength. Due to the linearity of the mathematical problem, we have focused on the explicit derivation of the mean head distribution G d (x) generated by a unit pulse. Such a distribution represents the fundamental solution to the average flow equations, and it is termed as mean Green function. The function G d (x) is derived here at the second order of approximation in the variance σ2 of the fluctuation $$\varepsilon \left({{\mathbf{x}}}\right) = 1- \frac{K{\left({{\mathbf{x}}}\right)}}{K_{A}}$$ (where K A is the mean value of K(x)), for arbitrary correlation function ρ(x), and any dimensionality d of the flow domain. We represent G d (x) as product between the homogeneous Green function G (0) (x) valid in a domain with constant K A , and a distortion term Ψ d (x) = 1 + σ2ψ d (x) which modifies G (0) (x) to account for the medium heterogeneity. In the case of isotropic formations ψ d (x) is expressed via one quadrature. This quadrature can be analytically calculated after adopting specific (e.g.. exponential and Gaussian) shape for ρ(x). These general results are subsequently used to investigate flow toward a partially-penetrating well in a semi-infinite domain. Indeed, we construct a σ2-order approximation to the mean as well as variance of the head by replacing the well with a singular segment. It is shown how the well-length combined with the medium heterogeneity affects the head distribution. We have introduced the concept of equivalent conductivity K eq(r,z). The main result is the relationship $$\frac{K^{\rm eq}\left(r,z\right)} {K_{A}} = 1-\sigma^{2}\psi^{\left(w\right)}\left(r,z\right)$$ where the characteristic function ψ(w)(r,z) adjusts the homogeneous conductivity K A to account for the impact of the heterogeneity. In this way, a procedure can be developed to identify the aquifer hydraulic properties by means of field-scale head measurements. Finally, in the case of a fully penetrating well we have expressed the equivalent conductivity in analytical form, and we have shown that $$K^{({\rm efu})}\leq K^{\rm eq}\left(r\right) \leq K_{A}$$ (being $$K^{({\rm efu})}$$ the effective conductivity for mean uniform flow), in agreement with the numerical simulations of Firmani et al. (Water Resour Res 42:W03422, 2006).

Time to Change the Way We Collect and Analyze Data for Aquifer Characterization

Over the past few decades, enormous effort has been expended on conducting traditional aquifer tests and model calibration for characterizing aquifers. Traditional aquifer tests generally involve pumping at one well and using the observed drawdown data at another well to estimate aquifer hydraulic properties. Analysis of the test is built upon Theis’ or Jacob’s approximate solution. These solutions assume aquifer homogeneity and uniform thickness despite the fact that aquifers are inherently heterogeneous at multiple scales and their thicknesses vary significantly spatially. These solutions at best predict an averaged drawdown over some volume of the aquifer (apples), which is likely different from the actual drawdown observed at a well, i.e., a single point in space (oranges). An application of these solutions to estimation of averaged aquifer properties based on a hydrograph from only one well is tantamount to comparing apples and oranges. Despite this inconsistency and impropriety, traditional aquifer tests continue to be widely adopted because the methods yield results, however questionable. Unfortunately, these results can significantly affect water resources management. For example, a recent study of traditional aquifer tests in heterogeneous aquifers by Wu et al. (2005) shows that the transmissivity estimate is a nonunique average of heterogeneity over the cone of depression, which evolves with time and depends on the location of the observation well. On the other hand, the estimated storage coefficient is mainly influenced by the geology between the pumping and the observation well. These findings suggest that ground water availability may have been either underestimated or overestimated, and our current management strategies of ground water resources as such are subject to great uncertainty. Likewise, calibrating a ground water model with spatially distributed parameters has been known as solving an ill-posed problem, which is often attributed to a lack of data. Despite of this fact, modelers have continued to search for magic algorithms to solve these problems that inherently have nonunique solutions and seldom have emphasized the need for collecting more data. Moreover, both modelers and practitioners rarely attempt to recognize the kinds of data (the necessary and sufficient conditions, see Yeh and Sim _ unek [2002]) that make an inverse problem well posed. As such, designs of laboratory and field experiments or monitoring networks often collect information irrelevant to resolve the ill-posed issue. Instead of collecting data to meet the necessary and sufficient conditions, which can be expensive and prohibitive for many large-scale field problems, a new technology, known as hydraulic tomography (HT), which collects data intelligently and analyzes data smartly, has recently been developed to provide high-resolution aquifer characterization (Gottlieb and Dietrich 1995; Renshaw 1996; Vasco et al. 2000; Yeh and Liu 2000; Bohling et al. 2002; Liu et al. 2002; Brauchler et al. 2003; McDermott et al. 2003; Zhu and Yeh 2005, 2006). In simple terms, HT is a sequential cross-hole hydraulic test followed by inversion of all the data to map the spatial distribution of aquifer hydraulic properties. Specifically, HT involves installation of multiple wells in an aquifer, which are partitioned into several intervals along the depth using packers. A sequential aquifer test at selected intervals is then conducted. During the test, water is injected or withdrawn (i.e., a pressure excitation) at a selected interval in a given well. Pressure responses of the subsurface are then monitored at other intervals at this well and other wells. This test produces a set of pressure excitation/response data of the subsurface. Once a test is completed, the pump is moved to another interval and the test is repeated to collect another set of data. The same procedure is then 1Corresponding author: Department of Hydrology and Water Resources, The University of Arizona, Tucson, AZ 85721; (520) 621-5943; fax (520) 621-1422; yeh@hwr.arizona.edu 2Department of Resources Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China. 3Department of Resources Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China; leech@mail.ncku. edu.tw Received November 2006, accepted November 2006. Copyright a 2007 The Author(s) Journal compilationa2007National GroundWaterAssociation. doi: 10.1111/j.1745-6584.2006.00292.x

Groundwater flow along and across structural folding: an example from the Judean Desert, Israel

The considerable influence of the geological structure on groundwater flow regime is exhibited in the thick carbonate aquifer beneath the Judean Desert, Israel. Groundwater flow is diverted from the general steep hydraulic gradient, creating a subsurface ‘river-like’ meandering flow pattern. The structure of the extensive-folded anticlinorium forces groundwater flow through synclinal axes in the upper aquifer and in places it overflows from one to an adjacent syncline. Groundwater outflows are at Tsukim, Kane, Samar and En-Gedi springs near the Dead Sea shore and by sub-surface flow across the Graben faults towards the Dead Sea. In this study all available data are integrated and processed, first ever, to form a complete representation of the three-dimensional hydrostratigraphy and hydrogeology. Using numerical modeling (MODFLOW), we analyzed quantitatively the flow regime, leakage rates between upper and lower sub-aquifers and between adjacent sub-basins, the groundwater mass balance, and aquifer hydraulic properties. This study has practical implications regarding recent groundwater management, future possibilities of groundwater development for the benefit of both Israelis and Palestinians residing in the area, and conservation of nature reserves located along the Dead Sea.

Combining geology and geophysics to develop a hydrogeologic framework for salt interception in the Loxton Sands aquifer, central Murray Basin, Australia

Salt interception schemes (SIS) have been developed to manage high salt loads and to improve the health of the River Murray, both in the Riverland (South Australia) and Sunraysia (Victoria/NSW) areas. Currently SIS at Loxton and Bookpurnong, in the South Australian Riverland are being developed, both incorporating borefields in the Loxton-Parilla Sands aquifer to intercept saline groundwater flux from groundwater mounds that have formed beneath irrigated areas. A detailed interpretation of borehole geology, ground and airborne geophysics, combined with the analysis of sediments for the Loxton Sands and underlying Bookpurnong Beds has resulted in an improved hydrogeological model for the area. Lateral and vertical changes in sedimentary facies associated with the main aquifer systems relevant to the SIS are now better understood. This is an important precursor in the development of a predictive model for aquifer hydraulic properties using hydrogeological and geophysical data. Relatively thin zones of high hydraulic conductivity, associated with the lower Loxton sands sedimentary package, are characterized by slightly reduced electrical conductivity response at the watertable, and represent a target for ground and airborne EM systems. These zones have been identified as elements of a basin-wide beach-barrier strandline sequence that developed in the Pliocene. Results from the constrained inversion of helicopter EM data have helped to better define the geometry of this sedimentary system, the location of these zones and have provided elements of a predictive framework for a more informed approach to the design, development and potential performance of the Loxton Sands SIS borefields.

Reply to Comment on "Sensitivity analysis and determination of streambed leakance and aquifer hydraulic properties" by S. Christensen

The study of Christensen [Christensen, S., 2000. On the estimation of stream flow depletion parameters by drawdown analysis. Ground Water 38 (5), 726–734], based on the solution of Hunt [Hunt, B., 1999. Unsteady stream depletion from groundwater pumping. Ground Water 37 (1), 98–102], analyzed three streamflow depletion parameters T, S, and λ of a simplified stream-aquifer system. Although such a stream-aquifer hydrologic condition does not commonly exist in the real world, Christensen [Christensen, S., 2000. On the estimation of stream flow depletion prameters by drawdown analysis. Ground Water 38 (5), 726–734] concluded that his results can be used to analyze any hydrogeological setting and his proposed methodology can analyze any possible pumping test design. Stream-aquifer-well systems are far more complex in the real world: lack of a semipervious layer that covers the whole stream channel, anisotropic hydraulic conductivity of stream sediments and the surrounding aquifers, effects of multiple channels or stream width, etc. It is clear that the results of Christensen [Christensen, S., 2000. On the estimation of stream flow depletion prameters by drawdown analysis. Ground Water 38 (5), 726–734] have limitations in the analyses of streamflow depletion parameters. It is then very apparent and logical that further research on streamflow depletion parameters for a more complex stream-aquifer system and for a less ideal design of pumping and observation well network is needed. Chen and Chen [Chen, X.H., Chen, X., 2003a. Sensitivity analysis and determination of streambed leakance and aquifer hydraulic properties. Journal of Hydrology 284, 270–284] used numerical models to simulate stream-aquifer interactions and fullfilled at least part of this need. The study of realistic and complex stream-aquifer-well conditions on streamflow depletion parameters remains an important topic of research.