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1.
Applicability of spectral analysis to determine hydraulic diffusivity   总被引：1，自引：1，他引：0  
This study is to evaluate the applicability of estimating the one-dimensional horizontal hydraulic diffusivity of an unconfined aquifer with time-dependent fluctuation of lateral head and vertical recharge boundaries using observed water level spectra. Different models of boundary condition are imposed to evaluate the statistical significance between the calculated hydraulic diffusivity (ξ′) with the given hydraulic diffusivity (ξ). The auto-spectra of the water level in observation wells tapping the same aquifer are closely related to those at the disturbed boundaries. For an aquifer with a constant hydraulic diffusivity, the water level fluctuation in the monitoring wells is linearly related to the water level spectra observed at the boundaries. The spectral density function of aquifer hydraulic head varies inversely with specific yield (S y) and directly with recharge. Given small variation in water level spectra at the disturbed boundaries, the water level fluctuation in the aquifer is affected by the recharge condition and the aquifer spectral density function is sensitive to S y. Using an iterative technique to estimate ξ from 1400 sets of given parameters, 99% of the ξ′/ξ values deviated within only one order of magnitude with the model length (L) being equal to 1 km and 10 km. For L equal to 100 m, approximately 82% of the ξ′/ξ population falls within two orders of magnitude. Therefore, spectral analysis of aquifer hydraulic head response can be used to estimate the hydraulic diffusivity of an unconfined aquifer which is affected by periodic variations in recharge and head at boundaries.  相似文献   

2.
The hydraulic diffusivity gives a measure of diffusion speed of pressure disturbances in groundwater system; large values of hydraulic diffusivity lead to fast propagation of signals in aquifer. This research provides a novel design and derives spectral representation to determine hydraulic diffusivity using spectral analysis of groundwater levels coupled with time-dependent boundary adjacent to marine system and no flow boundary in aquifer system. To validate the proposed method, water levels of fluctuated boundary and groundwater well in a sandy confined aquifer were collected. The hydraulic diffusivity is then obtained by an inverse process in the non-linear complex form of spectral relationship. The method essentially is constructed on the conceptual design of natural forcing transmitted in large aquifer. It is unlike the conventional field pumping test which is only used to determine hydraulic properties of groundwater in small range around the well. Hydraulic diffusivity of the confined aquifer is determined using real observation and then checked by comparing to the published range. It suggests that without local aquifer test to estimate hydraulic diffusivity in a coastal aquifer using spectral representation with its relevant flow system and boundary has become feasible.  相似文献   

3.
The study demonstrates spectral relationships in the time–frequency domain for one‐dimensional groundwater flow in aquifers bounded by fluctuating boundaries. By nature, the solutions of spectral equations are non‐linear complex functions. To determine hydraulic diffusivity in the governing equations, it is required that the data are collected from the spectra of water levels at the fluctuating boundaries and observation wells. Hydraulic diffusivity thus can be obtained by an iterative inverse approach. This paper presents an application in Pingtung County of Taiwan to determine the hydraulic diffusivity of a sandy aquifer under confined conditions. Spectral density function of water level obtained from tidal boundaries and observation wells are used to approximate hydraulic diffusivity, which yields an averaged value of 1·26 × 106 m2/h. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

4.
In this study, we attempted to analyse a drawdown pattern around a pumping well in an unconfined sandy gravelly aquifer constructed in a laboratory tank by means of both experimental and numerical modelling of groundwater flow. The physical model consisted of recharge, aquifer and discharge zones. Permeability and specific yield of the aquifer material were determined by Dupuit approximation under steady‐state flow and stepwise gravitational drainage of groundwater, respectively. The drawdown of water table in pumping and neighbouring observation wells was monitored to investigate the effect of no‐flow boundary on the drawdown pattern during pumping for three different boundary conditions: (i) no recharge and no discharge with four no‐flow boundaries (Case 1); (ii) no recharge and reservoir with three no‐flow boundaries (Case 2); (iii) recharge and discharge with two no‐flow boundaries (Case 3). Based on the aquifer parameters, numerical modelling was also performed to compare the simulated drawdown with that observed. Results showed that a large difference existed between the simulated drawdown and that observed in wells for all cases. The reason for the difference could be explained by the formation of a curvilinear type water table between wells rather than a linear one due to a delayed response of water table in the capillary fringe. This phenomenon was also investigated from a mass balance study on the pumping volume. The curvilinear type of water table was further evidenced by measurement of water contents at several positions in the aquifer between wells using time domain reflectometry (TDR). This indicates that the existing groundwater flow model applicable to an unconfined aquifer lacks the capacity to describe a slow response of water table in the aquifer and care should be taken in the interpretation of water table formation in the aquifer during pumping. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

5.
Groundwater in coastal areas is commonly disturbed by tidal fluctuations. A two‐dimensional analytical solution is derived to describe the groundwater fluctuation in a leaky confined aquifer system near open tidal water under the assumption that the groundwater head in the confined aquifer fluctuates in response to sea tide whereas that of the overlying unconfined aquifer remains constant. The analytical solution presented here is an extension of the solution by Sun for two‐dimensional groundwater flow in a confined aquifer and the solution by Jiao and Tang for one‐dimensional groundwater flow in a leaky confined aquifer. The analytical solution is compared with a two‐dimensional finite difference solution. On the basis of the analytical solution, the groundwater head distribution in a leaky confined aquifer in response to tidal boundaries is examined and the influence of leakage on groundwater fluctuation is discussed. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

6.
The effect of pressure-driven groundwater recharge signal propagation in the Andean-Atacamenian environment is investigated by assessing a record of 15 years of water table fluctuations of an unconfined–confined aquifer system. Based on a singular spectrum analysis of water table time series, it is shown that, in the given case, groundwater levels in the central Atacama Desert are hydraulically controlled by two distant recharge areas associated with the Andes. The maximum observed range of the pressure signal propagation is ~50 km over an elevation difference of more than 3000 m at a lag of ~25 months. Several findings indicate that an often-cited study misinterpreted a water level rise at the same site as an in-situ alluvial fan recharge. Thus, the effect’s impact on groundwater dynamics in complex aquifer systems can easily be overlooked. Singular spectrum analysis could be of use to investigate pressure effects at hydrologically comparable sites.  相似文献   

7.
Analytical solutions for the water table and lateral discharge in a heterogeneous unconfined aquifer with time-dependent source and fluctuating river stage were derived and compared with those in an equivalent homogeneous aquifer. The heterogeneous aquifer considered consists of a number of sections of different hydraulic conductivity values. The source term and river stage were assumed to be time-dependent but spatially uniform. The solutions derived is useful in studying various groundwater flow problems in a horizontally heterogeneous aquifer since the spatially piecewise-constant hydraulic conductivity and temporally piecewise-constant recharge and lateral discharge can be used to quantify variations in these processes commonly observed in reality. Applying the solutions derived to an aquifer of three sections of different hydraulic conductivity values shown that (1) the aquifer heterogeneity significantly increases the spatial variation of the water table and thus its gradient but it has little effect on lateral discharge in the case of temporally and spatially uniform recharge, (2) the time-dependent but spatially uniform recharge increases the temporal variation of groundwater table over the entire aquifer but its effect on lateral discharge is limited in the zone near the river, and (3) the effect of river stage fluctuation on the water table and lateral discharge is limited in the zone near the river and the effect of the heterogeneity is to increase lateral discharge to or recharge from the river.  相似文献   

8.
This paper focuses upon south-eastern Lithuanian Quaternary aquifer system groundwater resources formation modelling. Groundwater model calibration has been performed for a pre-development and transient flow conditions. The results demonstrate that there is an intense interaction between groundwater and surface water bodies which form groundwater resources and runoff. For Quaternary cover the majority of unconfined groundwater outflows to surface water streams, the remaining part discharges through the confined interglacial/interstadial aquifers and lateral outflows across the boundaries. Groundwater prognostic exploitable resources can be obtained without a significant negative impact on the geoenvironment. The main sources of exploitable resources formation are increase in groundwater recharge and lateral inflow (40.7%), decreased outflow via streams and lateral boundaries (41.2%) and drawing up additional flow from the hydraulically connected streams (14.5%).  相似文献   

9.
Geographically isolated wetlands (GIWs) are commonly reported as having hardpan or low hydraulic conductivity units underneath that produce perched groundwater, which can sustain surface water levels independently of regional aquifer fluctuations. Despite the potential of GIW-perched aquifer systems to provide important hydrological and ecological functions such as groundwater storage and native amphibian habitat, little research has studied the hydrologic controls and dynamics of these systems. We compared several ridge-top depressional GIW-perched groundwater systems to investigate the role of watershed morphology on hydroregime and groundwater-surface water interaction. Ridge-top depressional wetlands in the Daniel Boone National Forest, Kentucky were chosen because they offer natural controls such as lack of apparent connection to surface water bodies, similar climate, and similar soils. Three wetlands with different topographic slopes and hillslope structures were mapped to distinguish key geomorphic parameters and monitored to characterize groundwater-surface water interaction. Wetlands with soil hummocks and low upland slopes transitioned from infiltration to groundwater discharge conditions in the spring and during storm events. The magnitude and duration of this transition fell along a continuum, where higher topographic slopes and steeper uplands produced comparably smaller and shorter head reversals. This demonstrates that ridge-top GIW-perched groundwater systems are largely sensitive to the runoff-recharge relationship in the upland area which can produce significant groundwater storage on a small-scale.  相似文献   

10.
Principal component analysis (PCA) was applied to hydrochemical and isotopic data of 34 groundwater samples. This allowed the reduction of 20 variables to four significant PCs that explain 81.9% of the total variance; F1 (47.1%) explains the groundwater mineralization, whereas F2 (17%) shows isotopic enrichment and nitrate pollution. Based on an iso-factor scores map of F1, three water zones were delineated: Zone A (F1 < ?1), with fresh groundwater from the unconfined aquifer; Zone B (1 > F1 > ?1), with moderate mineralization from the confined–unconfined aquifer boundary; and Zone C (F1 > 1), with the most mineralized hot water from the confined aquifer. The iso-factor scores map of F2 delineates positive values representing samples from the unconfined aquifer, with freshwater and nitrate contamination associated with stable isotope enrichment, whereas negative values represent samples from the confined aquifer. The results clearly demonstrate the usefulness of PCA in groundwater hydrochemistry investigations.  相似文献   

11.
We consider the response of a deep unconfined horizontal aquifer to steady, annual, and monthly recharge. A groundwater divide and a zero head reservoir constrain the aquifer, so that sinusoidal monthly and aperiodic annual recharge fluctuations create transient specific discharge near the reservoir and an unsteady water table elevation inland. One existing and two new long-term data sets from the Plymouth-Carver Aquifer in southeastern Massachusetts calibrate and confirm hydraulic properties in a set of analytical models. [Geohydrology and simulated groundwater flow, 1992] data and a new power law for tritiugenic helium to tritium ratios calibrate the steady recharge that drives the classical parabolic model of steady hydraulics [Applied Hydrogeology, 2001]. Observed water table and gradient fluctuations calibrate the transient recharge models. In the latter regard, monitoring wells within 1 km of Buttermilk Bay exhibit appreciable specific discharge and reduced water table fluctuations. We apply [Trans Am Geophys Union 32(1951)238] periodic model to the monthly hydraulics and a recharge convolution integral [J Hydrol 126(1991)315] to annual flow. An infiltration fraction of 0.79 and a consumptive use coefficient of 1.08×10−8 m/s °C relate recharge to precipitation and daylight weighted temperature across all three time scales. Errors associated with this recharge relation decrease with increasing time scale.  相似文献   

12.
In eastern England the Chalk aquifer is covered by extensive Pleistocene deposits which influence the hydraulic conditions and hydrochemical nature of the underlying aquifer. In this study, the results of geophysical borehole logging of groundwater temperature and electrical conductivity and depth sampling for major ion concentrations and stable isotope compositions (δ18O and δ2H) are interpreted to reveal the extent and nature of the effective Chalk aquifer of north Norfolk. It is found that the Chalk aquifer can be divided into an upper region of fresh groundwater, with a Cl concentration of typically less than 100 mg l−1, and a lower region of increasingly saline water. The transition between the two regions is approximately 50 m below sea-level, and results in an effective aquifer thickness of 50–60 m in the west of the area, but less than 25 m where the Eocene London Clay boundary is met in the east of the area. Hydrochemical variations in the effective aquifer are related to different hydraulic conditions developed in the Chalk. Where the Chalk is confined by low-permeability Chalky Boulder Clay, isotopically depleted groundwater (δ18O less than −7.5‰) is present, in contrast to those areas of unconfined Chalk where glacial deposits are thin or absent (δ18O about −7.0‰). The isotopically depleted groundwater is evidence for groundwater recharge during the late Pleistocene under conditions when mean surface air temperatures are estimated to have been 4.5°C cooler than at the present day, and suggests long groundwater residence times in the confined aquifer. Elevated molar Mg:Ca ratios of more than 0.2 resulting from progressive rock-water interaction in the confined aquifer also indicate long residence times. A conceptual hydrochemical model for the present situation proposes that isotopically depleted groundwater, occupying areas where confined groundwater dates from the late Pleistocene, is being slowly modified by both diffusion and downward infiltration of modem meteoric water and diffusive mixing from below with an old saline water body.  相似文献   

13.
It was found in previous studies that groundwater levels may fluctuate as a temporal fractal. In this study numerical simulations of groundwater level fluctuations in an unconfined aquifer near a river were conducted to investigate the effects of aquifer heterogeneity and river stage variations on the fractal behavior of the water levels, h(t). Groundwater recharge was taken to be a white-noise process. The aquifer heterogeneity was simulated with a second-order stationary field of hydraulic conductivity (K) with an exponential variogram model. The results showed that groundwater levels fluctuate as a temporal fractal in both homogeneous and heterogeneous aquifers as long as K is less than 10 m/d. Most aquifers may indeed act as a fractal filter which takes a random non-fractal recharge inputs and produces a fractal responses of groundwater level fluctuations. A crossover in temporal scaling of h(t) may appear in more permeable aquifers. Fluctuations of the groundwater level in a homogeneous aquifer are dominated by the recharge process when the river stage is constant or by the river stage variations when the river stage varies in highly permeable aquifers. Heterogeneity plays an important role in the temporal scaling of h(t) in more permeable aquifers: the stronger the heterogeneity, the stronger the temporal scaling of h(t).  相似文献   

14.
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.  相似文献   

15.
In this second of two papers, analytical step-response functions, developed in the companion paper for several cases of transient hydraulic interaction between a fully penetrating stream and a confined, leaky, or water-table aquifer, are used in the convolution integral to calculate aquifer heads, streambank seepage rates, and bank storage that occur in response to stream-stage fluctuations and basinwide recharge or evapotranspiration. Two computer programs developed on the basis of these step-response functions and the convolution integral are applied to the analysis of hydraulic interaction of two alluvial stream–aquifer systems in the northeastern and central United States. These applications demonstrate the utility of the analytical functions and computer programs for estimating aquifer and streambank hydraulic properties, recharge rates, streambank seepage rates, and bank storage. Analysis of the water-table aquifer adjacent to the Blackstone River in Massachusetts suggests that the very shallow depth of water table and associated thin unsaturated zone at the site cause the aquifer to behave like a confined aquifer (negligible specific yield). This finding is consistent with previous studies that have shown that the effective specific yield of an unconfined aquifer approaches zero when the capillary fringe, where sediment pores are saturated by tension, extends to land surface. Under this condition, the aquifer's response is determined by elastic storage only. Estimates of horizontal and vertical hydraulic conductivity, specific yield, specific storage, and recharge for a water-table aquifer adjacent to the Cedar River in eastern Iowa, determined by the use of analytical methods, are in close agreement with those estimated by use of a more complex, multilayer numerical model of the aquifer. Streambank leakance of the semipervious streambank materials also was estimated for the site. The streambank-leakance parameter may be considered to be a general (or lumped) parameter that accounts not only for the resistance of flow at the river–aquifer boundary, but also for the effects of partial penetration of the river and other near-stream flow phenomena not included in the theoretical development of the step-response functions.  相似文献   

16.
Spatiotemporal variations of groundwater level due to a white noise recharge time series and a random transmissivity field in a bounded unconfined aquifer was studied. The analytical solutions for the variance and covariance of groundwater level were derived with non-stationary spectral analyses and superposition principle. It was found that the fluctuations of groundwater level are spatially non-stationary due to a fixed head boundary condition and temporal non-stationary at early time but gradually became stationary as time progresses due to effect of the initial condition. The variation in groundwater level is mainly caused by the random source/sink in the case of temporally random recharge and spatially random transmissivity. The effect of heterogeneity is to increase the variation of groundwater level and the maximum effect occurs close to the constant head boundary because of the linear mean hydraulic gradient. The heterogeneity also enhances the correlation of groundwater level, especially at large time intervals and small spatial distances.  相似文献   

17.
Much of what is known about groundwater circulation and geochemical evolution in carbonate platforms is based on platforms that are fully confined or unconfined. Much less is known about groundwater flow paths and geochemical evolution in partially confined platforms, particularly those supporting surface water. In north‐central Florida, sea level rise and a transition to a wetter climate during the Holocene formed rivers in unconfined portions of the Florida carbonate platform. Focusing on data from the Santa Fe River basin, we show river formation has led to important differences in the hydrological and geochemical evolution of the Santa Fe River basin relative to fully confined or unconfined platforms. Runoff from the siliciclastic confining layer drove river incision and created topographic relief, reorienting the termination of local and regional groundwater flow paths from the coast to the rivers in unconfined portions of the platform. The most chemically evolved groundwater occurs at the end of the longest and deepest flow paths, which discharge near the center of the platform because of incision of the Santa Fe River at the edge of the confining unit. This pattern of discharge of mineralized water differs from fully confined or unconfined platforms where discharge of the most mineralized water occurs at the coast. Mineralized water flowing into the Santa Fe River is diluted by less evolved water derived from shorter, shallower flow paths that discharge to the river downstream. Formation of rivers shortens flow path lengths, thereby decreasing groundwater residence times and allowing freshwater to discharge more quickly to the oceans in the newly formed rivers than in platforms that lack rivers. Similar dynamic changes to groundwater systems should be expected to occur in the future as climate change and sea level rise develop surface water on other carbonate platforms and low lying coastal aquifer systems. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

18.
Vulnerability indicators of sea water intrusion   总被引：5，自引：0，他引：5  
In this paper, simple indicators of the propensity for sea water intrusion (SWI) to occur (referred to as "SWI vulnerability indicators") are devised. The analysis is based on an existing analytical solution for the steady-state position of a sharp fresh water-salt water interface. Interface characteristics, that is, the wedge toe location and sea water volume, are used in quantifying SWI in both confined and unconfined aquifers. Rates-of-change (partial derivatives of the analytical solution) in the wedge toe or sea water volume are used to quantify the aquifer vulnerability to various stress situations, including (1) sea-level rise; (2) change in recharge (e.g., due to climate change); and (3) change in seaward discharge. A selection of coastal aquifer cases is used to apply the SWI vulnerability indicators, and the proposed methodology produces interpretations of SWI vulnerability that are broadly consistent with more comprehensive investigations. Several inferences regarding SWI vulnerability arise from the analysis, including: (1) sea-level rise impacts are more extensive in aquifers with head-controlled rather than flux-controlled inland boundaries, whereas the opposite is true for recharge change impacts; (2) sea-level rise does not induce SWI in constant-discharge confined aquifers; (3) SWI vulnerability varies depending on the causal factor, and therefore vulnerability composites are needed that differentiate vulnerability to such threats as sea-level rise, climate change, and changes in seaward groundwater discharge. We contend that the approach is an improvement over existing methods for characterizing SWI vulnerability, because the method has theoretical underpinnings and yet calculations are simple, although the coastal aquifer conceptualization is highly idealized.  相似文献   

19.
Groundwater recharge and mounding of water‐table is a complex phenomenon involving time‐ and space‐dependent hydrologic processes. The effect of long‐term groundwater mounding in the aquifer depends on soil, aquifer geometry and the area contributing to recharge. In this paper, a GIS‐based spatio‐temporal algorithm has been developed for the groundwater mound dynamics to estimate the potential rise in the water‐table and groundwater volume balance residual in an unconfined aquifer. The recharge and mound dynamics as predicted using the methodology recommended here were compared with those using the Hantush equation, and the differences were quite significant. The significance of the study is to assess the effectiveness of the basin in terms of its hydrologic and hydraulic properties for sustainable management of groundwater recharge. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

20.
Groundwater recharge studies are essential for investigating the feasibility of using the reclaimed lands in Singapore for subsurface storage and recovery of water. Through time‐series and spectral analyses, net recharge percentages and stress‐filtering characteristics at the reclaimed land were found to depend strongly on the stress transfer velocity, which was a combined function of rainfall pattern and vadose zone thickness. Based on stress transfer velocity, a theory was established to provide logical explanations for the rainfall–recharge relationship, the observed stress‐filtering characteristics and the recharge percentage characteristics at the unconfined sandy aquifer. Although the reclaimed land site has a lithollogically homogeneous soil profile, a non‐uniform recharge pattern was observed to be influenced pronouncedly by the uneven density distribution of bush grasses. Under a bare soil condition, significantly lower recharge percentages were observed for areas under the influence of offshore tides. The unconfined sandy aquifer appears to dampen out the wave propagation of offshore tides rapidly within a short distance from the tidal source, though it has a fairly straight shoreline and is created from highly compacted sand fills. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

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