Storage in confined aquifer: Spectral analysis of groundwater in responses to Earth tides and barometric effect

Dilatation of aquifer and associated water level fluctuation in groundwater well is known to be driven periodically from lunar, solar, or other tidal forces. Time‐dependent variables in groundwater system, such as water level, can be converted to power spectra in the frequency domain using Fourier transform to evaluate significant fluctuation. The major innovation of this research is to develop spectral representation in frequency domain for the groundwater system that the storage in confined aquifer can be determined considering dilatation affected by Earth tides and barometric effect. In order to verify applicability of the evolved method, time series of Earth tides and barograph are collected; aquifer storage is then determined inversely by selecting significant semidiurnal and diurnal components in spectra computation. It suggests that to discover groundwater storage using groundwater level with barograph and tidal potential of Earth in frequency domain becomes accessible and feasible.     

New Insights from Well Responses to Fluctuations in Barometric Pressure

Hydrologists have long recognized that changes in barometric pressure can produce changes in water levels in wells. The barometric response function (BRF) has proven to be an effective means to characterize this relationship; we show here how it can also be utilized to glean valuable insights into semi‐confined aquifer systems. The form of the BRF indicates the degree of aquifer confinement, while a comparison of BRFs between wells sheds light on hydrostratigraphic continuity. A new approach for estimating hydraulic properties of aquitards from BRFs has been developed and verified. The BRF is not an invariant characteristic of a well; in unconfined or semi‐confined aquifers, it can change with conditions in the vadose zone. Field data from a long‐term research site demonstrate the hydrostratigraphic insights that can be gained from monitoring water levels and barometric pressure. Such insights should be of value for a wide range of practical applications.     

Identifying Aquifer Type in Fractured Rock Aquifers using Harmonic Analysis

Determining aquifer type, unconfined, semi‐confined, or confined, by drilling or performing pumping tests has inherent problems (i.e., cost and complex field issues) while sometimes yielding inconclusive results. An improved method to cost‐effectively determine aquifer type would be beneficial for hydraulic mapping of complex aquifer systems like fractured rock aquifers. Earth tides are known to influence water levels in wells penetrating confined aquifers or unconfined thick, low‐porosity aquifers. Water‐level fluctuations in wells tapping confined and unconfined aquifers are also influenced by changes in barometric pressure. Harmonic analyses of water‐level fluctuations of a thick (～1000 m) carbonate aquifer located in south‐central Oklahoma (Arbuckle‐Simpson aquifer) were utilized in nine wells to identify aquifer type by evaluating the influence of earth tides and barometric‐pressure variations using signal identification. On the basis of the results, portions of the aquifer responded hydraulically as each type of aquifer even though there was no significant variation in lithostratigraphy. The aquifer type was depth dependent with confined conditions becoming more prevalent with depth. The results demonstrate that harmonic analysis is an accurate and low‐cost method to determine aquifer type.     

Case 21 water level and strain changes preceding and following the August 4, 1985 Kettleman Hills, California, earthquake

Two of the four wells monitored near Parkfield, California, during 1985 showed water level rises beginning three days before theM _w 6.1 Kettleman Hills earthquake. In one of these wells, the 3.0 cm rise was nearly unique in five years of water level data. However, in the other well, which showed a 3.8 cm rise, many other changes of comparable size have been observed. Both wells that did not display pre-earthquake rises tap partially confined aquifers that cannot sustain pressure changes due to tectonic strain having periods longer than several days. We evaluate the effect of partial aquifer confinement on the ability of these four wells to display water level changes in response to aquifer strain. Although the vertical hydraulic diffusivities cannot be determined uniquely, we can find a value of diffusivity for each site that is consistent with the site's tidal and barometric responses as well as with the rate of partial recovery of the coseismic water level drops. Furthermore, the diffusivity for one well is high enough to explain why the preseismic rise could not have been detected there. For the fourth well, the diffusivity is high enough to have reduced the size of the preseismic signal as much as 50%, although it should still have been detectable. Imperfect confinement cannot explain the persistent water level changes in the two partially confined aquifers, but it does show that they were not due to volume strain. The pre-earthquake water level rises may have been precursors to the Kettleman Hills earthquake. If so, they probably were not caused by accelerating slip over the part of the fault plane that ruptured in that earthquake because they are of opposite sign to the observed coseismic water level drops.     

The unusual and large drawdown response of buried-valley aquifers to pumping

The buried-valley aquifers that are common in the glacial deposits of the northern hemisphere are a typical case of the strip aquifers that occur in many parts of the world. Pumping from a narrow strip aquifer leads to much greater drawdown and much more distant drawdown effects then would occur in a sheet aquifer with a similar transmissivity and storage coefficient. Widely used theories for radial flow to wells, such as the Theis equation, are not appropriate for narrow strip aquifers. Previously published theory for flow to wells in semiconfined strip aquifers is reviewed and a practical format of the type curves for pumping-test analysis is described. The drawdown response of strip aquifers to pumping tests is distinctive, especially for observation wells near the pumped well. A case study is presented, based on extensive pumping test experience for the Estevan Valley Aquifer in southern Saskatchewan, Canada. Evaluation of groundwater resources in such buried-valley aquifers needs to take into account the unusually large drawdowns in response to pumping.     

Field Testing the Hypothesis of Darcian Flow Through a Carbonate Aquifer

The acceptability of the hypothesis of Darcian flow through a semiconfined carbonate aquifer was tested prior to running a multiple-day aquifer test in Pinellas County, Florida. The approach used to test the hypothesis was to run a number of hour-long aquifer tests at different discharges with drawdown measured at the same time during each test in two observation wells, one at 35 feet and the other at 733 feet from the pumped well. If the hypothesis were acceptable, a plot of drawdown versus discharge should describe a straight line. A linear relation was described in 15-minute and 1-hour data plots of drawdown versus discharge for both observation wells. Thus, the hypothesis of Darcian flow through the semiconfined carbonate aquifer was deemed acceptable. Drawdown data from the distant observation well collected during the following multiple-day aquifer test were then analyzed for aquifer-property values of transmissivity, storage coefficient, and leakance coefficient using standard methods. Discharge for the multiple-day test was within the discharge range of the hour-long tests.     

Removal of barometric pressure effects and earth tides from observed water levels

The effects of barometric pressure and earth tide changes are often observed in ground water level measurements. These disturbances can make aquifer test interpretation difficult by masking the small changes induced by aquifer testing at late times and great distances. A computer utility is now available that automatically removes the effects of barometric pressure and earth tides from water level observations using regression deconvolution. This procedure has been shown to remove more noise then traditional constant barometric efficiency techniques in both confined and unconfined aquifers. Instead of a single, instantaneous barometric efficiency, the procedure more correctly accounts for the lagged responses caused by barometric pressure and earth tide changes. Simultaneous measurements of water levels (or total heads) and nearby barometric pressures are required. As an additional option, the effects of earth tides can also be removed using theoretical earth tides. The program is demonstrated for two data sets collected at the Waste Isolation Pilot Plant, Carlsbad, New Mexico. The program is available free by request at http://www.hydrology.uga.edu/tools.html.     

Hydraulic parameters of a multilayered aquifer system in Kuwait City

The Kuwait Group consists mainly of clastic sediments overlying unconformably the Dammam Formation of Tertiary age. The Kuwait Group is generally divided into three main hydrostratigraphic units: the upper and lower aquifers separated by an aquitard. The upper aquifer is further divided into the water table aquifer, an aquitard and a semiconfined aquifer. This semiconfined unit was pumped and the drawdowns were observed in piezometers screened in various subunits of the Kuwait Group. Some pumping tests of short duration were carried out in the top water table aquifer as well. These tests showed that the subunits of the Kuwait Group are hydraulically interconnected to a varying degree.

The pumping test data were analysed using conventional analytical solutions. The semiconfined pumping test was also simulated by a quasi-three-dimensional model using a leaky multiaquifer modelling technique. The initial hydraulic parameters were improved manually in the model till best fit drawdowns were obtained.

The final parameters obtained by simulation of the pumping tests were used in designing a pilot drainage system for the control of a rising groundwater table in parts of Kuwait City.     

A Leaky Aquifer below Champlain Sea Clay: Closed‐Form Solutions for Natural Seepage

Closed‐form solutions are proposed for natural seepage in semiconfined (leaky) aquifers such as those existing below the massive Champlain Sea clay layers in the Saint‐Lawrence River Valley. The solutions are for an ideal horizontal leaky aquifer below an ideal aquitard that may have either a constant thickness and a constant hydraulic head at its surface, or a variable thickness and a variable hydraulic head at its surface. A few simplifying assumptions were needed to obtain the closed‐form solutions. These have been verified using a finite element method, which did not make any of the assumptions but gave an excellent agreement for hydraulic heads and groundwater velocities. For example, the difference between the two solutions was smaller than 1 mm for variations in the 5 to 8 m range for the hydraulic head in the semiconfined aquifer. Note that fitting the hydraulic head data of monitoring wells to the theoretical solutions gives only the ratio of the aquifer and aquitard hydraulic conductivities, a clear case of multiple solutions for an inverse problem. Consequently, field permeability tests in the aquitard and the aquifer, and pumping tests in the aquifer, are still needed to determine the hydraulic conductivity values.     

A general solution for groundwater flow in an estuarine's leaky aquifer system after considering aquifer anisotropy

This paper presents an analytical model for describing the tidal effects in a two‐dimensional leaky confined aquifer system in an estuarine delta where ocean and river meet. This system has an unconfined aquifer on top and a confined aquifer on the bottom with an aquitard in between the two. The unconfined and confined aquifers interact with each other through leakage. It was assumed that the aquitard storage was negligible and that the leakage was linearly proportional to the head difference between the unconfined and confined aquifers. This model's solution was based on the separation of variables method. Two existing solutions that deal with the head fluctuation in one‐dimensional or two‐dimensional leaky confined aquifers are shown as special cases in the present solution. Based on this new solution, the dynamic effect of the water table's fluctuations can be clearly explored, as well as the influence of leakage on the behaviour of fluctuations in groundwater levels in the leaky aquifer system. Copyright © 2015 John Wiley & Sons, Ltd.     

承压井水位对气压动态过程的响应

本文从气压影响承压井水位的偏微分方程出发,考虑到井孔和含水层之间相互渗流的边界条件,得出了方程的解。给出了承压井水位对气庄动态过程响应的气压效率和位相滞后。讨论了气压效率和位相滞后与水井含水层系统各参数的关系。并指出气压效率越大的承压井,其含水层对应力应变的敏感程度越差。     

Dissolution in a variably confined carbonate platform: effects of allogenic runoff,hydraulic damming of groundwater inputs,and surface–groundwater exchange at the basin scale

In variably confined carbonate platforms, impermeable confining units collect rainfall over large areas and deliver runoff to rivers or conduits in unconfined portions of platforms. Runoff can increase river stage or conduit heads in unconfined portions of platforms faster than local infiltration of rainfall can increase groundwater heads, causing hydraulic gradients between rivers, conduits and the aquifer to reverse. Gradient reversals cause flood waters to flow from rivers and conduits into the aquifer where they can dissolve limestone. Previous work on impacts of gradient reversals on dissolution has primarily emphasized individual caves and little research has been conducted at basin scales. To address this gap in knowledge, we used legacy data to assess how a gradient of aquifer confinement across the Suwannee River Basin, north‐central Florida affected locations, magnitudes and processes of dissolution during 2005–2007, a period with extreme ranges of discharge. During intense rain events, runoff from the confining unit increased river stage above groundwater heads in unconfined portions of the platform, hydraulically damming inputs of groundwater along a 200 km reach of river. Hydraulic damming allowed allogenic runoff with SI_CAL < ?4 to fill the entire river channel and flow into the aquifer via reversing springs. Storage of runoff in the aquifer decreased peak river discharges downstream and contributed to dissolution within the aquifer. Temporary storage of allogenic runoff in karst aquifers represents hyporheic exchange at a scale that is larger than found in streams flowing over non‐karst aquifers because conduits in karst aquifers extend the area available for exchange beyond river beds deep into aquifers. Post‐depositional porosity in variably confined carbonate platforms should thus be enhanced along rivers that originate on confining units. This distribution should be considered in models of porosity distribution used to manage water and hydrocarbon resources in carbonate rocks. Copyright © 2013 John Wiley & Sons, Ltd.     

井水位的固体潮效应和气压效应与含水层参数间的定量关系

本文分析了水井水位的固体潮效应和气压效应与承压含水层参数之间的定量关系。结果表明,含水层的固体骨架体积压缩系数和孔隙度越小,水井水位的固体潮系数则越大;含水层的固体骨架的体积压缩系数越小,孔隙度越大,則水井水位的气压效率越大。本文还把理论结果和固体潮及气压的实际观测结果进行了对比,两者符合得较好。     

A method for evaluating horizontal well pumping tests

Predicting the future performance of horizontal wells under varying pumping conditions requires estimates of basic aquifer parameters, notably transmissivity and storativity. For vertical wells, there are well-established methods for estimating these parameters, typically based on either the recovery from induced head changes in a well or from the head response in observation wells to pumping in a test well. Comparable aquifer parameter estimation methods for horizontal wells have not been presented in the ground water literature. Formation parameter estimation methods based on measurements of pressure in horizontal wells have been presented in the petroleum industry literature, but these methods have limited applicability for ground water evaluation and are based on pressure measurements in only the horizontal well borehole, rather than in observation wells. This paper presents a simple and versatile method by which pumping test procedures developed for vertical wells can be applied to horizontal well pumping tests. The method presented here uses the principle of superposition to represent the horizontal well as a series of partially penetrating vertical wells. This concept is used to estimate a distance from an observation well at which a vertical well that has the same total pumping rate as the horizontal well will produce the same drawdown as the horizontal well. This equivalent distance may then be associated with an observation well for use in pumping test algorithms and type curves developed for vertical wells. The method is shown to produce good results for confined aquifers and unconfined aquifers in the absence of delayed yield response. For unconfined aquifers, the presence of delayed yield response increases the method error.     

Is the forward problem of ground water hydrology always well posed?

Complex aquifer systems are often modeled with quasi-three-dimensional models, which consider two-dimensional horizontal flow in the aquifers and one-dimensional vertical flow through aquitards. When the aquifer system consists of a phreatic aquifer and one or more semiconfined aquifers connected by aquitards, the discrete model consists of a nonlinear system of algebraic equations, because the transmissivity of the phreatic aquifer depends on the phreatic head. If the water extraction is very high, the phreatic aquifer can be depleted and the equations of the model must be modified accordingly. There are not simple and general criteria to state if the phreatic aquifer is depleted before solving the system of equations. Therefore, the iterative procedures (e.g., relaxation methods), used to find the solution to the forward problem, must handle these particular conditions and can suffer several problems of convergence. These problems can be caused by the choice of the initial head values or of the relaxation coefficient of the iterative algorithms; however, they can also be caused by the nonexistence or nonuniqueness of the solution to the system of nonlinear equations. The study of existence and uniqueness of the general problem is very difficult and, therefore, we consider a simplified problem, for which the discrete model can be handled analytically. The results of the numerical experiments show that the solution to the forward problem can be nonunique. Only for some cases it is possible to invoke physical arguments to eliminate tentative solutions.     

Using existing data to estimate aquifer properties, Great Lakes Region, USA

To determine specific storage and porosity, areally limited and time-consuming aquifer tests are frequently done. Hydrogeologic studies often do not have the resources to collect such data and rely on existing data sources for aquifer properties. An alternative tool for determining these aquifer properties is the analysis of earth tides. The objective of this study was to determine whether existing water-level and barometric-pressure data could be used to determine aquifer properties, such as porosity and specific storage, on a regional scale. In this study, national databases from the Great Lakes Region were queried for continuous records of groundwater-level and barometric-pressure data. Records from 37 selected wells were then analyzed for barometric efficiency and earth-tide responses. Specific-storage (S(s) ) and porosity values were determined, and the quality of the results were assessed with a measure of the "goodness of fit" (percent variance) of reconstruction of the response. Records from wells completed in several aquifer systems were analyzed with varying degrees of success. Aquifer S(s) values ranging from 5.9 × 10(-8) to 3.8 × 10(-6) /m were derived, with percent variance of reconstruction ranging from 1% to 78%. Comparisons with aquifer and laboratory testing of S(s) and porosity are favorable if the percent variance of reconstruction is above about 30%. Although the earth-tide-analysis method is not suitable for every situation, the S(s) and porosity of aquifers can, in many places, be estimated with existing water-level and barometric-pressure data or with data that are relatively inexpensive to collect.     

Modelling tidal signals enhanced by a submarine spring in a coastal confined aquifer extending under the sea

Submarine springs discharge offshore groundwater from confined aquifers extending under the sea. The effects of these springs on the propagation of tidal oscillations in coastal confined aquifers are not known. This paper presents an approximate analytical solution of tidal head fluctuations in a confined aquifer with one submarine spring. The aquifer is assumed to extend in all directions infinitely. The spring is represented by a permeable round column on the seabed, which penetrates completely the impermeable layer overlying the confined aquifer. The error of the approximate solution is negligible if the distance from the spring to the coastline is much greater than the radius of the permeable column representing the spring. Through a hypothetical example, we demonstrate that it is possible to identify the spring’s location using tidal signals observed from inland wells. Tidal groundwater head fluctuations from three inland observation wells at least are needed to determine the 5 model parameters, including the location (2 parameters), the radius of the permeable column representing the spring, the diffusivity of the aquifer, and the tidal loading efficiency of the system.     

Assessing radial transmissivity variation in heterogeneous aquifers using analytical techniques

Pumping tests are one of the most commonly used in situ testing techniques for assessing aquifer hydraulic properties. Numerous researches have been conducted to predict the effects of aquifer heterogeneity on the groundwater levels during pumping tests. The objectives of the present work were as follows: (1) to predict drawdown conditions and to estimate aquifer properties during pumping tests undertaken in radially symmetric heterogeneous aquifers, and (2) to identify a method for assessing the transmissivity field along the radial coordinate in radially symmetric and fully heterogeneous transmissivity fields. The first objective was achieved by expanding an existing analytical drawdown formulation that was valid for a radially symmetric confined aquifer with two concentric zones around the pumping well to an N concentric zone confined aquifer having a constant transmissivity value within each zone. The formulation was evaluated for aquifers with three and four concentric zones to assess the effects of the transmissivity field on the drawdown conditions. The specific conditions under which aquifer properties could be identified using traditional methods of analysis were also evaluated. The second objective was achieved by implementing the inverse solution algorithm (ISA), which was developed for petroleum reservoirs to groundwater aquifer settings. The results showed that the drawdown values are influenced by a volumetric integral of a weighting function and the transmissivity field within the cone of depression. The weighting function migrates in tandem with the expanding cone of depression. The ability of the ISA to predict radially symmetric and log‐normally distributed transmissivity fields was assessed against analytical and numerical benchmarks. The results of this investigation indicated that the ISA method is a viable technique for evaluating the radial transmissivity variations of heterogeneous aquifer settings. Copyright © 2013 John Wiley & Sons, Ltd.     

Tide‐induced groundwater head fluctuation in a coastal aquifer system with a submarine outcrop covered by a thin silt layer

We present an analytical solution of groundwater head response to tidal fluctuation in a coastal multilayered aquifer system consisting of an unconfined aquifer, a leaky confined aquifer and a semi‐permeable layer between them. The submarine outcrop of the confined aquifer is covered by a thin silt layer. A mathematical model and the analytical solution of this model are given. The silt layer reduces the amplitude of the hydraulic head fluctuation by a constant factor, and shifts the phase by a positive constant (time lag), both of which depend on the leakances of the silt layer and the semi‐permeable layer. The time lag is less than 1·5 h and 3·0 h for semi‐diurnal and diurnal sea tides respectively. When the leakance of the semi‐permeable layer or the silt layer assumes certain special values, the solution becomes the existing solutions derived by previous researchers. The amplitude of the hydraulic head fluctuation in the confined aquifer increases with the leakance of the silt layer and decreases with the leakance of the semi‐permeable layer, whereas the phase shift of the fluctuation decreases with both of them. A hypothetical example shows that neglecting the silt layer may result in significant parameter estimation discrepancy between the amplitude attenuation and the time‐lag fittings. Copyright © 2007 John Wiley & Sons, Ltd.     

Multiple-aquifer characterization from single borehole extensometer records

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(-5)/m for aquifer units to 1.5 x 10(-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(-6)/m, while the confining units and interbeds had values of 6.0 x 10(-6)/m. The calibrated vertical hydraulic conductivity values of the confining units and interbeds ranged from 6.6 x 10(-4) m/year to 2.0 x 10(-3) m/year. These parameter values will be useful in future management and modeling of ground water in the Virginia Coastal Plain.