Leaky Aquitard Research Articles

Saturated–unsaturated flow in a compressible leaky-unconfined aquifer

An analytical solution is developed for three-dimensional flow towards a partially penetrating large-diameter well in an unconfined aquifer bounded below by a leaky aquitard of finite or semi-infinite extent. The analytical solution is derived using Laplace and Hankel transforms, then inverted numerically. Existing solutions for flow in leaky unconfined aquifers neglect the unsaturated zone following an assumption of instantaneous drainage due to Neuman. We extend the theory of leakage in unconfined aquifers by (1) including water flow and storage in the unsaturated zone above the water table, and (2) allowing the finite-diameter pumping well to partially penetrate the aquifer. The investigation of model-predicted results shows that aquitard leakage leads to significant departure from the unconfined solution without leakage. The investigation of dimensionless time-drawdown relationships shows that the aquitard drawdown also depends on unsaturated zone properties and the pumping-well wellbore storage effects.

Analytical solutions for pressure perturbation and fluid leakage through aquitards and wells in multilayered‐aquifer systems

Large‐scale groundwater pumping or deep fluid injection in a multilayered subsurface system may generate pressure perturbation not only in the target formation(s), but also in over‐ and underlying units. Hydraulic communication in the vertical direction may occur via diffuse leakage through aquitards and/or via focused leakage through leaky wells. Existing analytical solutions for pressure perturbation and fluid flow in such systems consider either diffuse leakage or focused leakage, but never in combination with each other. In this study, we developed generalized analytical solutions that account for the combined effect of diffuse and focused leakage. The new solutions solve for pressure changes in a system of N aquifers with alternating leaky aquitards in response to fluid injection/extraction with any number, NI, of injection/pumping (active) wells, and passive leakage/recharge in any number, NL, of leaky wells. The equations of horizontal groundwater flow in the aquifers are coupled by the vertical flow equations in the aquitards and by the flow continuity equations in the leaky wells. The solution methodology, described in detail in this paper, involves transforming the transient flow equations into the Laplace domain; decoupling the resulting ordinary differential equations (ODEs) coupled by diffuse leakage via eigenvalue analysis; solving a system of NL × N linear algebraic equations for the unknown rates of flow through leakage wells; and superposing the solution of pressure buildup/drawdown in aquifers and aquitards resulting from flow in the NI active and NL leaky wells. Verification of the new methodology was achieved by comparison with existing analytical solutions for diffuse leakage and for focused leakage, and against a numerical solution for combined diffuse and focused leakage. Application to an eight‐aquifer system with leaky aquitards and one leaky well demonstrates the usefulness and efficiency of the approach, and illustrates the pressure behavior over a spectrum of leakage scenarios and parameters.

Quantitative assessment of the hydraulic role of subglaciofluvial interbeds in promoting deposition of deformation till (Northern Till, Ontario)

The Northern Till is a thick (>65 m) deformation till underlying some 7500 km 2 of Southern Ontario, Canada including the Peterborough Drumlin Field. It was deposited below the Lake Ontario ice stream of the Laurentide Ice Sheet. The till rests on glaciotectonized aquifer sediments and consists of multiple beds of till up to 6 m thick. These are separated by boulder lags, sometimes in the form of striated pavements, with thin (<30 cm) interbeds of poorly sorted waterlaid sand. The composite till stratigraphy indicates ‘punctuated aggradation’ where the subglacial bed was built up incrementally by the repeated ‘immobilization’ of deforming overpressured till layers. Boulders and sands indicate pauses in subglacial aggradation marked by sluggish sheet flows of water that reworked the top of the underlying till. Interbeds are laterally extensive and correlated using downhole electrical conductivity, core recovery and natural gamma data. A 3-D finite element model (FEFLOW) using data from 200 cored and geophysically logged boreholes, and a large digital water well dataset of 3400 individual records shows that the till functions as a ‘leaky aquitard’ as a consequence of water flow through interbeds. It is proposed that interbeds played a similar role in the subglacial hydraulic system below the Laurentide Ice Sheet by allowing drainage of excess porewater pressures in deforming sediment and promoting deposition of till. This is in agreement with theoretical studies of deforming bed dynamics and observations at modern glaciers where porewater in the deforming layer is discharged into underlying aquifers. In this way, the presence of interbeds may be fundamental in retarding downglacier transport of deforming bed material thereby promoting the build-up of thick subglacial till successions.

The hydrogeology of the Ballimore region, central New South Wales, Australia: an integrated study

The integrated use of geophysical, geological, hydrogeochemical and hydrogeological data has allowed the development of a plausible conceptual model for groundwater flow in the Ballimore region. A realistic model for this under-explored system could not be derived solely by the use of hydrogeological data. Interpretation of the available datasets indicates that two groundwater systems are active: a regional and a local system. These are separated by a regionally extensive aquiclude. Groundwater flow in the regional groundwater system is controlled by the structural fabric of the Palaeozoic basement rocks. The local groundwater system is restricted to the Permian to Recent sequence of cover rocks. The local groundwater system is subdivided into three cells: the deep, intermediate and shallow cells. Groundwater flow within the deep cell of the local groundwater system is controlled by fracture flow. Groundwaters from this aquifer are under artesian pressure and are effervescent (CO2-gas). The intermediate cell is a leaky aquitard that acts as a mixing zone between the deep and shallow cells. Groundwater flow within the shallow cell is controlled by the influx of surface waters which migrate laterally through permeable beds.

A numerical model and spreadsheet interface for pumping test analysis.

Curve-matching techniques have been the standard method of aquifer test analysis for several decades. A variety of techniques provide the capability of evaluating test data from confined, unconfined, leaky aquitard, and other conditions. Each technique, however, is accompanied by a set of assumptions, and evaluation of a combination of conditions can be complicated or impossible due to intractable mathematics or nonuniqueness of the solution. Numerical modeling of pumping tests provides two major advantages: (1) the user can choose which properties to calibrate and what assumptions to make; and (2) in the calibration process the user is gaining insights into the conceptual model of the flow system and uncertainties in the analysis. Routine numerical modeling of pumping tests is now practical due to computer hardware and software advances of the last decade. The RADFLOW model and spreadsheet interface presented in this paper is an easy-to-use numerical model for estimation of aquifer properties from pumping test data. Layered conceptual models and their properties are evaluated in a trial-and-error estimation procedure. The RADFLOW model can treat most combinations of confined, unconfined, leaky aquitard, partial penetration, and borehole storage conditions. RADFLOW is especially useful in stratified aquifer systems with no identifiable lateral boundaries. It has been verified to several analytical solutions and has been applied in the Snake River Plain Aquifer to develop and test conceptual models and provide estimates of aquifer properties. Because the model assumes axially symmetrical flow, it is limited to representing multiple aquifer layers that are laterally continuous.

Abstract: Fluvial Deposits and Leaky Aquitards of the Late Pleistocene Shallow Subsurface in Southwest Louisiana&amp;nbsp;

ABSTRACT Shallow subsurface Pleistocene deposits underlying southwest Louisiana exhibit multiple cycles of fluvial deposition, each herein referred to as a fluvial system. The fluvial systems are discernible in a geologic model that reflects correlations and facies analysis on a scale of approximately 200 to 300 feet. The fluvial systems consist of point bar, channel-fill, levee, crevasse splay and flood plain facies. Channel sequences up to 60 feet thick are composed of several fining-up sandy sequences each on the scale of 10 feet thick. Lateral to the upper portions of the channel sequence are sediments interpreted as crevasse splay deposits. Marsh and estuarine deposits representing flooding surfaces cap these fluvial cycles. Relatively thin, clays deposited on aggraded floodplains overlie some fluvial cycles. Laterally persistent flooding surfaces are locally eroded by successive channels. Fluvial systems exhibit distinctive hydrologic characteristics with similar water levels in each system. These systems form a leaky aquitard overlying the Chicot Aquifer. Factors responsible for the leaky nature of the clay-rich section are: 1) fractures in the clay-rich flood plain deposits that increase the vertical permeability, 2) scour of fluvial systems that erodes thin, relatively impermeable, fine-grained organic-rich sections capping the sequences causing vertical connections between different systems and 3) stress caused by water withdrawal from the underlying aquifer. Well-developed, highly permeable point bar sequences act as reservoirs and connect thin permeable, silty layers ubiquitous in these fluvial and interfluvial deposits. This discussion will illustrate the model with data acquired from a roughly one square mile study area. End_of_Record - Last_Page 1-------

Inverse modeling of groundwater flow in the semiarid evaporitic closed basin of Los Monegros, Spain

Only minor attention has been given in the past to the study of closed-basin hydrogeology in evaporitic environments, because these basins usually contain poor-quality groundwater. The motivation for hydrogeological research in the Los Monegros area in northeastern Spain was the approval in 1986 of a large irrigation project in the Ebre River basin. The irrigation of 60,000 ha is planned, partly in an evaporitic closed basin containing playa lakes. The project has given rise to environmental concerns. The evaluation of the hydrologic impacts of irrigation requires quantifying properly the hydrogeology of the area. With the available information, a conceptual hydrogeological model was formulated that identifies two main aquifers connected through a leaky aquitard. On the basis of the conceptual model, a numerical model was calibrated under steady-state conditions using the method of maximum-likelihood automatic parameter estimation (Carrera and Neuman, 1986a). The calibrated model reproduces the measured hydraulic heads fairly well and is consistent with independent information on groundwater discharge. By the solution of the inverse problem, reliable parameter estimates were obtained. It is concluded that anisotropy plays a major role in some parts of the lower aquifer. The geometric average of model conductivity is almost two orders of magnitude larger than the average conductivity derived from small-scale field tests. This scale effect in hydraulic conductivity is consistent with the findings of Neuman (1994) and Sanchez-Vila et al. (1996).

The relationship between the water table and the surface flow of a losing stream, lower Medano Creek, Great Sand Dunes National Monument, Colorado

Proposed groundwater withdrawals in the San Luis Valley of Colorado may lower the water table in Great Sand Dunes National Monument. In response, the National Park Service initiated a study that has produced a generalized conceptual model of the hydrologic system in order to assess whether a lowering of the water table might decrease the surface flow of lower Medano Creek. Based upon information obtained during the drilling of several boreholes, there appear to be five important hydrostratigraphic units underlying lower Medano Creek within the upper 30 m of the ground surface: 1. a perched aquifer overlying an aquitard located between about 5 and 6 m below the ground surface; 2. the aquitard itself; 3. an unconfined aquifer located between the upper and lower aquitards; 4. an aquitard located between about 27 and 29 m below the ground surface; and 5. a confined underlying the lower aquitard. Because the areal extent of the aquitards cannot be determined from the borehole data, a detailed conceptual model of the hydrogeologic system underlying lower Medano Creek cannot be developed. However, a generalized conceptual model can be envisioned that consists of a complex system of interlayered aquifers and leaky aquitards, with each aquifer having a unique hydraulic head. Water levels in the perched aquifer rise rapidly to their annual maximum levels in response to the arrival of the flow terminus of Medano Creek during the spring runoff event, and the location of the flow terminus is directly dependent upon the discharge of the creek. Water levels in the deeper, non-perched aquifers do not appear to fluctuate significantly in response to the arrival of the flow terminus, demonstrating that it is unlikely that the proposed groundwater withdrawals will decrease the surface flow of lower Medano Creek.

Distribution of selected elements in the shale of the Davis Formation, Buick Mine area, Viburnum Trend, Southeast Missouri

Ninety-five shale samples from drill holes that intersected the base of the Davis shale as well as a drill hole that cored the entire section of the formation were collected and analyzed for 26 elements by neutron activation. It was found that ore-forming and related solutions had produced a broad zone of enrichment at the base of the Davis shale that, for some elements, extends up to 0.5 and 1.0 km west and east of the ore zones, respectively. The zone of enrichment is defined, in various degrees, by Mn, Fe, Zn, and to a lesser extent, by Na, K, and As. These elements, with the exception of As, have been depleted (to back-ground levels) from the base of the shale that immediately overlies the ore zone. Within the shale over the ore zone, the central third of the Davis Formation is enriched with Na, Fe, and Zn, whereas most of the section is enriched in K. The Davis shale is also enriched in As at the upper and lower contacts of the formation and sporadically through the section.These data suggest that the shale of the Davis Formation may have acted as a sink and subsequently a source for Na, K, Mn, Fe, and Zn, as well as a sink for As. Episodic mineralization and leaching are believed to be responsible for the initial enrichment and subsequent removal of these elements in the Davis shale. Arsenic was probably precipitated and/or absorbed in a form that was more resistant to leaching. On the basis of the distribution of these selected elements, it is apparent that the alteration of the Davis shale in the area of the Buick mine was controlled by fracture permeability. Ore-forming and related solutions migrated along faults and fractures formed as a result of the collapse of the underlying dolomite. Thus, in the Buick mine area, the Davis Formation probably acted as a leaky aquitard during and after ore mineralization thereby allowing ore-forming and related solutions to migrate through, and probably along the top of, the Davis Formation.The enrichment zone at the base of the Davis shale, best delineated by Fe and Zn, is laterally extensive and could prove to be a useful guide to ore mineralization in this and other Mississippi Valley-type districts. Arsenic may be of use as an indicator of pathways taken by ore-forming and/or related solutions.

Seawater intrusion in continuous coastal aquifer-aquitard systems

The problem of seawater intrusion in a confined coastal aquifer is investigated. The aquifer is overlain by a leaky aquitard and both units extend continuously out under the sea. The problem is formulated in terms of the two governing differential equations, the fluid continuity equation written conveniently in terms of equivalent freshwater head, and the mass continuity equation. Use of linear rectangular finite elements, with direct integration and an iterative solution technique lead to an efficient numerical scheme that is capable of handling long simulation periods. The results, for a 300 m thick aquifer overlain by a 100 m thick aquitard, show that the aquitard has a controlling influence on the salt distribution. A zone of mixing in the aquifer is found to extend for several kilometres in the seaward as well as the landward direction. The time period required by the system to attain a state of dynamic equilibrium after a perturbation is applied may be of the order of centuries. The aquitard is found to cause a downward and seaward displacement of the average salt front.