Aquifer Parameter Values Research Articles

Can Storage Coefficient Be Neglected in the Estimation of Unconfined Aquifer Parameters?

Unlike the Neuman solution (1974), the Dagan solution (1967) neglects elastic storage of unconfined aquifers. Whereas theformer has been commonly used in the analysis of aquifer pumpingtests, application of the latter to determination of aquiferparameters are rare. This article evaluates the reliabilityof the Dagan solution (1967) for the determination of the horizontaland vertical hydraulic conductivity Kr and Kz and specific yieldSy by neglecting the role of aquifer storage coefficient S.To be consistent in the analysis of aquifer test data, the Dagansolution was combined with an inverse computational procedureso that Kr, Kz, and Sy can be automatically calculated. Computationsconverge relatively fast because evaluation of the Dagan solutionis mathematically simpler than is the Neuman solution. Datafrom three pumping tests conducted in alluvial aquifers in thePlatte and Republican River valleys, Nebraska, were analyzed.Earlytime drawdown data from the site in the Platte River valleyare scarce, but plenty of early-time drawdowns were collectedfrom the two sites in the Republican River valley. Comparisonof the results from the Dagan and the Neuman solution indicatesthat the Dagan solution provides aquifer parameter values veryclose to those of the Neuman solution in most situations. Formost practical purposes, the Dagan solution is as good as theNeuman solution for aquifer test analysis.

Determination of aquifer properties in northern Qatar for application to artificial recharge

Abstract The aquifer system in Qatar has been heavily exploited for agricultural purposes for over 20 years, leading to a decline in groundwater quality. In 1992 the Ministry of Municipal Affairs and Agriculture appointed Entec to carry out a two-year study into the feasibility of using artificial recharge to restore the aquifer system. This study involved field testing and modelling of the Rus and Umm Er Radhuma (UER) aquifers at four sites. Step, constant rate and tracer injection/reabstraction tests were carried out on each aquifer at each site. The step tests on boreholes in the UER indicated that non-Darcian behaviour occurred in the aquifer and extended a significant distance from the pumping boreholes. This was not observed during the step tests on boreholes in the Rus. A variety of techniques were used to analyse data from the constant rate tests and these provided a range of values for standard aquifer parameters as well as allowing a conceptual model of flow processes in the aquifer to be developed. This understanding was used in developing finite difference solute transport models to replicate the results from the tracer tests. Values of dispersivity of 0.5–5 m were derived for the Rus aquifer whilst values in the range 10–150 m were derived for the UER. Effective porosities were high at around 0.2 for both aquifers.

Uncertainty in wellhead protection area delineation due to uncertainty in aquifer parameter values

The importance of modeling in hydrogeologic investigations has been stressed in a number of studies. Since the applicability of any model is dependent on the type of need and availability of hydrogeological information, a parametric analysis is essential to determine its applicability. This paper presents the results of a parametric analysis conducted to evaluate the effect of data uncertainty on wellhead protection area (WHPA) delineation. The precision of aquifer parameter values is the most important factor in a WHPA delineation process in relation to the model itself. To demonstrate the effect of variable values in a wellfield, we tested modified version of time-related analytical groundwater flow concept using a RESSQC model, which is used for capture zone delineation and to delineate contaminant front for injection wells. The RESSQC model is a modified version of the RESSQ code which was initially designed to only delineate contaminant fronts for injection wells. Field measured aquifer parameters for a shallow aquifer were used in the analysis. The results indicate that great caution must be taken when aquifer parameters such as hydraulic gradient, thickness, transmissivity and porosity are being used for WHPA delineation, since over or under-protection of WHPA can jeopardize the public health. An extended aquifer test and installation of various observation wells in or near different lithologic and depth zones may increase reliability of parameter values.

Determination of Unconfined Aquifer Parameters Using Boulton, Neuman and Streltsova Methods

Analysis of pumping test data to estimate aquifer parameters is normally carried out using standard procedures such as the Theis or Jacob method. These methods, however, were derived with assumptions regarding the type of aquifer and wells that may not be satisfied under some conditions. In unconfined aquifers the effects of delayed yield, partial penetration of wells and other effects make the use of Theis or Jacob methods impractical in many cases. The objective of this work is to analyze pumping test data from seven different well sites of five wadis taking into consideration the effects of delayed yield, partial penetration of wells and the decreased saturated thickness. The methods of Boulton, Neuman and Streltsova were applied on pumping test data from the five unconfined aquifers in the south-western part of Saudi Arabia. The aquifer parameters resulting from the three methods were evaluated and compared to each other. Using the resulting values of aquifer parameters, the drawdowns were recalculated and compared to the original data in order to define best values of aquifer parameters for the seven test sites.

A Neural‐Network Approach to the Determination of Aquifer Parameters

AbstractA new approach to determine aquifer parameter values from aquifer‐test data has been developed that uses the pattern‐matching capability of a neural network. The network is trained to recognize patterns of normalized drawdown data as input and the corresponding aquifer parameters as output. The Theis and Hantush‐Jacob solutions for confined and leaky‐confined aquifer conditions are used to derive the input patterns based on the parameter values selected from predetermined ranges. The trained network produces output of aquifer parameter values when it receives the aquifer‐test data as the input patterns. The results obtained from this new approach are in good agreement with published results using other techniques. The advantages of the present approach are the automated process of obtaining aquifer parameter values and the ability of the network to associate drawdown to the corresponding Theis and Hantush‐Jacob solutions.

Sample Functions as Indicators of Aquifer Heterogeneities

An adaptive slope matching method is used to evalute the aquifer heterogeneity from constant rate aquifer test data. The basic aquifer parameters such as the storativity and transmissivity are considered to vary spatially and the effects of such changes appear as temporal variations during a pumping test period. It is stressed that this is due to the depression cone expansion with time around the pumping well. The plots of individual parameters versus time are referred to as the sample functions. These functions have erratic variations indicating that the aquifers considered in this study have local heterogeneities. Furthermore, a moving average technique is applied to each parameter sample function for the purpose of finding regional trends in the parameter variations. It is interesting to notice that the arithmetic average values of sample functions yield aquifer parameter values as obtained by the classical type curve matching techniques which assume that the aquifer concerned is homogeneous. Hence the use of the sample function concept is recommended for the detailed interpretation of the aquifer heterogeneity.

Finite-element simulation of low-temperature, heat-pump-coupled, aquifer thermal energy storage

A two-dimensional, Galerkin finite-element, transport model was applied to a feasibility study of low temperature, heat pump coupled, aquifer thermal energy storage in a confined sandstone aquifer in northeastern Ohio. Simulations were run to determine the relative sensitivity of the model to transient versus steady-state radial flow, grid size, time-step lengths, and injection temperatures. In order to determine the effects of uncertainties in the values of aquifer and aquitard parameters, energy recovery factors and recovery temperatures were calculated for worst-case, best-case, and best-estimate simulations. Aquifer and aquitard parameters were either measured directly or were estimated based on previous hydrogeologic investigations. All simulations were performed assuming a 150 d injection phase with a pumping rate of 15 m 3h −1, a 35 d storage phase, and a 150 d production phase, also with a pumping rate of 15 m 3h −1. The worst-case simulation for an injection temperature of 23.9°C produced an energy recovery factor of 0.43 and a final production temperature of 14.6°C. The best-case simulation resulted in an energy recovery factor of 0.69 and a final production temperature of 16.2°C. The best-estimate simulation predicted an energy recovery factor of 0.50 and a final production temperature of 15.0°C. Both the energy recovery factor and the recovery temperature were most affected by the uncertainty in the storage formation longitudinal dispersivity. Using the best-estimate parameter values, an injection temperature of 18.0°C resulted in a final production temperature of 13.2°C and an energy recovery factor of 0.50. The best-estimate parameter values used with an injection temperature of 27.0°C yielded a final production temperature of 15.9°C and an energy recovery factor of 0.50.

Determination of Aquifer Parameters by the Slope‐Matching Method

ABSTRACTA method has been developed by which the slope between any two successive data points on a time‐drawdown plot can be used for determining aquifer parameters in nonleaky and leaky aquifers. The method yields values of transmissivity and storage coefficient which are in good agreement with the results of the classically known techniques. The method of slope analysis offers several advantages in its use. These advantages include the following: Changes in values for the aquifer parameters that might occur during a pumping test can be identified. Confidence limits can be calculated for average values of aquifer parameters. The method yields meaningful aquifer parameter estimates even for short duration pumping tests. Subjective interpretation of the data is minimized.

Comparison of Aquifer Characteristics Derived from Local and Regional Aquifer Testsa

ABSTRACTA comparison of the aquifer parameter values obtained through the analysis of a local and a regional aquifer test involving the same area in southeast Georgia is made in order to evaluate the validity of extrapolating local aquifer‐test results for use in large‐scale flow simulations. The case study involves an increase in pumpage of 6.3 Mgal/d (million gallons per day) for the local test and a reduction in pumpage of 66 Mgal/d for the regional test, each involving the same well field. The test involved the Floridan aquifer system, a limestone sequence characterized predominantly by secondary permeability in this area. Time‐drawdown and time‐recovery data were analyzed by using both graphical and least‐squares fitting of the data to the Theis curve. Additionally, directional transmissivity, transmissivity tensor, and angle of anisotropy were computed for both tests. The major and minor axes of the transmissivity ellipse determined from the regional test show general agreement with those determined from the local test. A slightly higher ratio of anisotropy was observed on the regional scale: 2.1:1, than on the local scale: 1.5:1. Regional storage coefficients ranged from 3.4 × 10−4 to 7.2 × 10−4 as compared with local storage coefficients ranging from 1.8 × 10−4to 5.3 × 10−4.

Migration of contaminants in groundwater at a landfill: A case study: 1. Groundwater flow and plume delineation

A landfill-derived contaminant plume with a maximum width of ∼600 m, a length of ∼700 m and a maximum depth of 20 m in an unconfined sand aquifer was delineated by means of a monitoring network that includes standpipe piezometers, multilevel point-samplers and bundle-piezometers. The extent of detectable contamination caused by the landfill, which began operation in 1940 and which became inactive in 1976, was determined from the distributions of chloride, sulfate and electrical conductance in the sand aquifer, all of which have levels in the leachate that are greatly above those in uncontaminated groundwater. The maximum temperature of groundwater in the zone of contamination beneath the landfill is 12°C, which is 4–5°C above background. The thermal plume in the aquifer extends ∼150 m downgradient from the centre of the landfill. A slight transient water-table mound exists beneath the landfill in the late spring and summer in response to snowmelt and heavy rainfall. Beneath the landfill, the zone of leachate contamination extends to the bottom of the aquifer, apparently because of transient downward components of hydraulic gradient caused by the water-table mound and possibly because of the higher density and lower viscosity of the contaminated water. Values of hydraulic conductivity, which show variations due to local heterogeneity, were obtained from slug tests of piezometers, from pumping tests and from laboratory tests. Because of the inherent uncertainty in the aquifer parameter values, the 38-yr. frontal position of the plume calculated using the Darcy equation with the assumption of plug flow can differ from the observed frontal position by many hundreds of metres, although the use of mean parameter values produces a close agreement. The width of the plume is large relative to the width of the landfill and can be accounted for primarily by variable periods of lateral east- and westward flow caused by changes in water-table configuration due to the variable nature of recharge. Northward from the landfill, the vertical thickness of the plume decreases and the top of the plume is farther below the water table. The thickness of the zone of uncontaminated groundwater above the plume increases northward as the area of recharge of uncontaminated water downflow from the landfill increases. Because dispersion in the vertical direction is weak, there is very little mixing between the overlying zone of recharge water and the contaminant plume. Concentration profiles are irregular beneath and near the landfill and become smooth downgradient where the maximum concentrations are much less than those beneath landfill. These features are attributed to a strong influence of longitudinal dispersion. The plume passes beneath a small shallow stream near the landfill without significant influence on the stream.

Normal-mode analysis of the structure of baseflow-recession curves

It has been observed for several years that semi-logarithmic plots of the baseflow recession for many streams which partially penetrate aquifers in the United Kingdom are not single straight-line plots. Simple modelling studies indicate that this behaviour, for typical values of aquifer parameters, occurs for even the simplest of groundwater systems. In general such studies show that the baseflow component arising from even a structurally uncomplicated aquifer should be viewed as a superposition of many, distinct exponential terms. The origin of these terms does not necessarily lie in a varied hydrogeological structure, but can arise purely from the dynamics of groundwater flow. Such a form for the baseflow recession can be used to perform empirical fits to observed data. Moreover, where more than one exponential term is contributing to the total baseflow, great care must be taken in the interpretation of associated data.

Modification of routed streamflow by channel loss and base flow

The convolution integral is used to compute continuous variations in channel loss and base flow that result from a reservoir release on the North Canadian River in central Oklahoma. The open channel flow hydrograph is routed by using the unit response method and then modified for interaction with the aquifer. Stream losses and gains are evaluated from the arbitrary fluctuations in stream stage by using average values of aquifer parameters. In spite of gross simplification pertaining to the nature of the groundwater system, good agreement with the actual flow hydrograph is obtained.

Use of Harmonic Analysis To Study Tidal Fluctuations in Aquifers near the Sea

Sixteen days of water level records were collected from a tidal gage and from four wells situated in a line perpendicular to the shore at Borden, Prince Edward Island. These two sets of records are separated by harmonic analysis into three tidal components. In a confined aquifer, groundwater flow obeys linear potential theory so that each component represents a distinct test on the aquifer, which is analyzed for hydraulic conductivity and specific storage. The use of a line of wells permits the determination of the effective distance and an accurate determination of true tidal efficiency by two different plots. The values of hydraulic conductivity determined this way agree very closely with the values determined by packer tests on nearby wells. These results permitted examination of the composite wave and the single well technique. The unseparated composite wave was analyzed by using the maxima and minima from four days of records and by assuming a periodicity of 745 minutes. The calculated values of aquifer parameters differed very little from those determined by the individual components. In addition, each well was analyzed individually and the results were compared with those determined from the line of wells. The individual analyses can produce large errors because they do not permit an accurate determination of the effective distance.