Semi-confined Aquifer Research Articles

Groundwater as a feedwater source for membrane treatment plants: Hydrogeologic controls on water quality variation with time

A major misconception in the design of membrane water treatment facilities is that if a groundwater source is used to feed the plant, water quality will be relatively stable with time. There are wide variations in the rate of groundwater quality changes caused by pumping aquifers based on the local geology and hydrology. Brackish groundwater sources tend to show increases in the total dissolved solids concentration with time, while both brackish and seawater groundwater sources can have changes in the chemistry of the feedwater from basic dilute or standard seawater to water enriched in specific cation concentrations, particularly with iron or manganese. Modeling of these possible water quality changes should be accomplished prior to the final design of any membrane treatment facility.The primary geologic controls on groundwater quality within a subsurface aquifer system are the natural barriers which control the vertical inflow of water from adjacent aquifers containing water with different chemistries. Since the pumping rate of production wells is greater than the natural horizontal recharge rate in virtually all semi-confined aquifer systems, aquifer flow under an equilibrium condition is predominantly vertical through the confining beds. Therefore, the confining bed leakance coefficient along with other aquifer hydraulic characteristics, and the pattern and rate of pumping tend to control the rate of water quality change within any production aquifer. This water quality change is caused by mixing of the leaked water with the natural water contained in the production aquifer.Aquifer and confming bed materials tend to control the type of chemical composition changes in the water yielded to the wells with time. In limestone aquifers separated by compacted lime muds, such as in Florida, there are few changes in water chemistry outside of dilute seawater type compositions with the possible exception of sulfate and hardness. In clastic aquifers separated by clay mineral confining beds, there is a tendency for enrichment of the feedwater in dissolved iron or manganese. In fractured rock aquifers within igneous or metamorphic rocks, a wide range in water chemistry changes can occur.Most significant water quality changes can be predicted to some degree of accuracy based on the hydrogeologic test program conducted prior to wellfield design. If the geology is known, the relevant aquifer characteristics are measured, and the initial water quality in the aquifer above and below the production aquifer are measured, then changes in water quality with time can be modeled. Since the long-term water quality changes can be controlled to a large degree by the design of the feedwater wellfield, the economics of both water treatment and plant construction can also be significantly altered by well spacing and pumping rates. The accuracy of predictive solute transport modeling is directly related to the availability of data on the aquifer system modeled. Large errors in the modeled water quality changes can occur when data are insufficient. When significant restrictions on data occur, a very conservative wellfield design approach is prudent.

Stochastic analysis of ground-water flow in semiconfined aquifer : M. G. Satish & Jianting, Zhu, Journal of Hydraulic Engineering — ASCE, 120(2), 1994, pp 147–168

Stochastic analysis of ground-water flow in semiconfined aquifer : M. G. Satish & Jianting, Zhu, Journal of Hydraulic Engineering — ASCE, 120(2), 1994, pp 147–168

Exact solutions for two-dimensional groundwater flow in a semiconfined aquifer

Two-dimensional groundwater flow problems involving a semipervious boundary can be solved using an analytical function ( V) of the complex potential (ω) and its derivative with respect to the complex coordinate ( z). So far boundary conditions however have been restricted to a semipervious layer of constant resistance, vertical equipotential lines and vertical stream lines. This paper gives solutions for problems in a semiconfined aquifer of finite depth, so involving a semipervious boundary as well as a horizontal stream line boundary. Exact solutions are given for flow with an arbitrary number of polder levels above the semipervious boundary; also sinks and sources may be present.

Stochastic Analysis of Ground‐Water Flow in Semiconfined Aquifer

A stochastic analysis for two‐dimensional steady‐state ground‐water flow through a homogeneous random shallow semiconfined aquifer is presented. The flow problems through a semiconfined aquifer with a random leakage factor but subject to deterministic boundary conditions and domain recharge are investigated by perturbation techniques associated with the boundary element method. The leakage factor is related to the physical properties of both the main aquifer and the leaky layer. The hierarchical equations based on the expansion of the potential into the perturbation series are derived and the boundary element method is applied for obtaining the solutions of these equations. The results from the present study are compared with those obtained using the Monte Carlo simulation method as well as the analytical solutions. The perturbation‐based boundary element method does not require the specification of the probability density function of the leakage factor, but only its mean, variance (or standard deviation), and/or higher‐order moments. Therefore, the perturbation‐based boundary element method is particularly useful when the statistical information is incomplete. The results of the present study are quite comparable with those of Monte Carlo simulations, but perturbation‐based boundary element method needs much less computational effort.

Unsteady free flow to an observation well from a semi-confined leaky aquifer

This paper presents an analytical solution to the problem of unsteady free flow of water from a leaky confined aquifer, fed by an external supply source and draining into a ditch. Both the non-homogeneous governing equation and the boundary conditions are converted to Sturm-Liouville problems. The closed form solution so obtained is significant because it not only illustrates the process of groundwater recharge and depletion, but can constitute a bench-test for numerical models.

Management Of Saline Groundwater Discharge By Long-term Windmill Pumping In The Wheatbelt, Western Australia

Land and stream salinization in the Western Australian wheatbelt developed as a result of clearing of native vegetation. This caused a shift in the hydrological balance and a 10- to 30-fold increase in recharge. Rising water levels rejuvenated groundwater flow through paleao channels and increased baseflow to drainage lines. An increase in saline groundwater ascension (TDS 5000–30 000 mgl−1) to the near-surface, induced by rising pressure heads in the deeper semi-confined aquifer system, advanced land and stream salinization. Areas in a catchment where groundwater flow is channelled upwards by subsurface geological strctures are especially susceptible to salinization and water logging.

Modeling Ground‐Water Remediation in the High Plains Aquifer

AbstractThe Princeton Transport Code (PTC), a three‐dimensional finite‐element flow and transport model, was used to evaluate contaminant movement and recovery in the vicinity of a manufacturing facility in western Nebraska. The modeling was complicated by seasonal recharge from a nearby irrigation canal, a drainage ditch flowing across the site, municipal wells pumping at variable rates and screened at different levels in the underlying semiconfined aquifer, and the pumping of the three high‐capacity wells being used to recover contaminated ground water. The flow model was calibrated in several steps: steady‐state calibration using the unstressed (no pumping) conditions; steady‐state calibration using the prerecovery pumping conditions; and finally, transient calibration using results obtained for the early recovery pumping period–first for the nonirrigation season and then for the irrigation season. The calibrated flow model was further tested by simulating transport during the early stages of the recovery pumping.

Large‐Scale Aquitard Consolidation Near Mexico City

AbstractThe extensive lacustrine Chalco Plain in the southeastern part of Mexico City is underlain by an aquitard up to 300 m thick composed of a layered sequence of very porous (80‐90%) fine‐grained, organic‐rich Quaternary deposits, with thin horizontal interbeds of volcanic sand (“Capas Duras”). The aquitard overlies a thick sequence of alluvial‐pyroclastic sediments which form a highly productive regional aquifer. The Chalco Plain was a shallow lake until the 1940s when it was drained for agricultural use and human habitation. Historic information indicates that the Chalco Plain was an area of ground‐water discharge prior to the onset of heavy ground‐water extraction from the semiconfined aquifer. Due to aquifer pumping, however, the hydraulic gradient has reversed throughout the full thickness of the aquitard in areas where the aquitard is thin (< 100m), and recharging conditions now prevail. Where the aquitard is thick, the hydraulic head data show a progressive decline with time even though the hydraulic gradient still indicates upward flow in at least the upper part of the lacustrine sequence. Between the early 1960s, when major ground‐water extraction began beyond the periphery of the aquitard, and the onset in 1982 of heavy pumping from aquifers beneath the aquitard, the land surface subsided approximately 3 m. An additional subsidence of 2 m occurred between 1984 and 1989, causing a shallow lake to form and gradually expand. If the present rate of ground‐water withdrawal from the Chalco Basin continues, total land subsidence in the middle of the plain will probably continue to a rate of about 0.4 m per year for many years, and could eventually reach a total subsidence of tens of meters in the thickest part of the Chalco Plain. Pore water in much of the aquitard is saline; however, release of salts and other chemical constituents to the underlying aquifer has not yet significantly impaired the aquifer water quality. A better understanding of the behavior of the aquitard under the influence of aquifer pumping is needed to assist in long‐term management of ground water and land use in the Chalco Plain.

Geophysical Surveys of Leaking Tailings Impoundments

Alcoa of Australia's bauxite treatment plant at Kwinana discharges alkaline liquor to impoundments constructed on Quaternary formations of the Perth Basin. Contaminated groundwater has been detected outside some of these impoundments. The impoundments are constructed with an impermeable basal clay layer overlain by a permeable, thin sand blanket. Wet tailings slurry is poured in, the fine residue fraction is dried and stacked higher than the dyke walls allowing access of tracked vehicles and geophysical equipment to the impoundments. Leakage is initially detected from monitoring bores placed outside the impoundments. Approximate leak locations can then be detected from water level lows in piezometers placed within the semi-confined aquifer of the sand blanket. The mise-a-la-masse technique uses current electrodes placed within the impoundment and within conductive, leaking water at some distance away. A current pathway is provided between the impoundment and the conductive groundwater via the leakage zone and this can be mapped as voltage highs using potential electrodes within piezometer holes near the base of the impoundment. In some cases the detection of these leakage points is complicated, but not impossible, by multiple leaks, PVC liners placed above the basal clay layer and the strongly varying conductivity within the impoundment. The mise-a-la-masse anomalies only give leak location with differences in intensity between anomalies not being diagnostic of leak properties.

Distribution of recharge and discharge areas in a first-order catchment as interpreted from water level patterns

A major problem in managing and reclaiming saline land is identifying areas of recharge and discharge in a catchment. In a first-order catchment in the wheatbelt of Western Australia, four trends of water level changes have been observed: monotonically rising water levels; continuously rising water levels with seasonal fluctuations; continuously falling water levels; seasonally fluctuating water levels. Each pattern is associated with a specific hydrologic mechanism — recharge, recharge-discharge and discharge. The spatial distribution of the aquifer systems in the catchment follows a configuration which is controlled by the basin morphology. Recharge takes place in confined aquifers at the watershed and in the uplands; recharge-discharge occurs in the unconfined to semiconfined aquifer of the midslopes, and discharge becomes dominant along the unconfined aquifers of the drainage lines. The main areas of discharge of higher-salinity groundwater occur just upstream of geological structures along the drainage line. Surface water dams were found to contribute to the establishment of new discharge areas. They cause increased pressure in the deep aquifers, resulting in a rise in water levels and groundwater discharge below the dams.

Analytical solutions of unsteady drainage problems for soils subjected to variable recharge from a semi-confined aquifer

In drainage of agricultural lands, the upward vertical recharge from a semi-confined aquifer depends on the difference of the piezometric heads on the two sides of the semi-impermeable layer through which this recharge takes place. This means that the recharge through the semi-impermeable base depends on the unknown height of the unsteady water table. In the nonhomogeneous Boussinesq equation, which describes the drainage problems, the downward uniform rate of the recharge from rain or irrigation and the recharge from the semiconfined aquifer are expressed by two terms. By solving the Boussinesq equation expressions for the nondimensional height of the water table and the nondimensional discharge of the drains per unit drained area are obtained for three different initial conditions. Some known solutions are shown as special cases of the present solutions. Variation of nondimensional water table heights at half distance of the drain spacing and the nondimensional discharge of the drains with nondimensional time have been graphically illustrated with the help of synthetic examples.

Desalting and ground water management in the San Joaquin Valley, California

Desalting brackish ground water can be combined with ground water management in the San Joaquin Valley to partially alleviate environmental and economic problems caused by agricultural drainage water. Desalting subsurface drainage water produced from on—farm shallow drainage systems is costly due to high salinity and the potential for scaling and fouling of reverse osmosis (RO) membranes. When subsurface drainage water is transported in open canals, algal growth and accompanying organic transformations present pretreatment problems that have yet to be solved. Similar problems have been experienced at desalting facilities outside the San Joaquin Valley using comparable feed waters. The proposed pumping of usable ground water from the semiconfined aquifer on the Valley's west side to lower the shallow water table and thus eliminate waterlogging of crop root zones may result in degradation and mining of usable ground water and contamination of the confined aquifer below the Corcoran clay, the principal confining layer in the Valley. A combination of brackish ground water desalting and ground water management by pumping unusable shallow ground water from an interval between the crop root zones and the usable water body at the bottom of the semiconfined aquifer would result in (1) a lower salinity feed water that will reduce the cost of desalting, (2) a feed water that requires little, if any, filtration to protect RO membranes, and (3) the removal of the body of water containing high levels of salt and selenium that endanger usable water in both aquifers. The combination of brackish ground water desalting and ground water management would result in the production of usable quantities of fresh water for California's ever-increasing population and a reduction in the cost of reclaiming and managing drainage water by desalination.

Boundary element method for analyzing flow through a random semiconfined aquifer

With the aid of perturbation technique a boundary element procedure is developed for solution of two-dimensional Helmholtz equation with a random parameter of small variability. The perturbation boundary element method does not require specification of probability density function of the parameter but only its mean and standard deviation. The method is used to analyze steady-state groundwater flows in a horizontal shallow semiconfined aquifer (homogeneous and isotropic) of probabilistic properties. To illustrate the applicability of the method, a simple numerical example is presented. As the nature of perturbation method suggests, the developed method is valid only when the perturbed value of the parameter is of a small order.

Modelling of flow pattern in a shallow aquifer affected by reservoirs

A process for identifying and quantifying a spatial subsurface flow pattern in a shallow semi-confined aquifer is presented. The conceptual model is aimed at describing the hydraulic effects of surface reservoirs on a shallow semi-confined conducting layer that results in salinization of the top soil. A qualitative study of the relationship between water load in reservoirs, the artesian conducting layer, and water in the top soil was based on chemical and isotopic characteristics. Results indicate limited infiltration from reservoirs into the top soil layers and probably no penetration of water into the conducting layer. Results were then used to establish the regional flow pattern emphasizing the role of surface reservoirs on the build up of artesian pressure in the shallow conducting layer. The flow system was formulated using a compartmental approach in which the unknown parameters are the various sources of recharge and the physical parameters of the aquifer. A complete mathematical setup and suggested solution for the mathematical algorithm are presented in a second paper.

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.

Stochastic approach for groundwater flow in a semiconfined aquifer subject to random boundary conditions

A methodology is presented for analyzing the groundwater flow through a shallow semiconfined aquifer with random parameters, governed by the Helmholtz equation and subject to stochastic boundary conditions. Stochasticity in an aquifer parameter is considered along with the stochasticity in the Dirichlet and Neumann boundary conditions. The hierarchical equations based on the expansion of the potential into the perturbation series are derived and the regular boundary element procedure is applied for obtaining the solutions of these equations. The standard deviation in the potential is observed to increase with an increase in either the standard deviation in the random quantities or the strength of the correlation between the random quantities. The boundary element method based on perturbation does not require specification of the probability density function of the parameters but only their expectations, variances and the covariance of the random quantities. The applicability of the method is demonstrated by presenting a groundwater flow example as well as by comparing the numerical results with analytical solutions for a special case without the Neumann boundary condition uncertainty.

Automated analysis of pump test data for leaky confined aquifers

Automated type-curve matching can be used to analyze pump test data from semi-confined (leaky) aquifers. The computer algorithm evaluates aquifer and aquitard transmissivity, storage coefficient, and leakage coefficient, where the leakage is assumed to occur vertically from the confining semipervious layer (aquitard). Initial estimates for the type-curve matching algorithm are automatically obtained from the data. The method is completely automated, requiring only the pump test data; it converges rapidly, and identifies and ignores spurious data in the analysis.

Annual fluctuations of groundwater levels as a result of loading by surface moisture

Long-term hydrographs for deep confined aquifers in southern Saskatchewan, Canada, show annual fluctuations characterized by a gradual rise in head from October to May/June and a rapid drop from May/June to October. These fluctuations are distinctly different from the seasonal fluctuations observed in surficial and shallow semi-confined aquifers, which reflect the response of these aquifers to recharge derived from snow melt and early spring rains. On the basis of field data and theoretical considerations, it is demonstrated that seasonal fluctuations of the water table are not transmitted down to the deep aquifers, provided the confining layers are thick and have sufficiently low hydraulic diffusivity. The annual fluctuations in the deep aquifers thus do not reflect seasonal recharge to these aquifers. Plots of moisture accumulation (i.e. precipitation minus calculated areal evapotranspiration) are strikingly similar to the observed hydrographs for the deep aquifers, in timing and magnitude of the annual fluctuations. It is postulated that the annual head fluctuations in the deep confined aquifers reflect changes of the mechanical load on the formations, caused by seasonal changes of the soil moisture, snow, and storage at the water table. The deep confined aquifers thus act as large-scale, inexact, weighing lysimeters.

Groundwater contamination at the Kesterson Reservoir, California: 1. Hydrogeologic setting and conservative solute transport

Shallow groundwater under Kesterson Reservoir, Merced County, California, was contaminated over a period of 6 years while the reservoir was used for disposal of brackish subsurface agricultural drainage water from farmlands in the San Joaquin Valley. Drainage water contained an average of 10,000 mg/L total dissolved solids (TDS), high concentrations of selenium (300 μg/L), nitrate (37 mg/L as nitrogen), boron (15 mg/L), and lesser amounts of mercury, chromium, and molybdenum. Infiltration and lateral migration of this water created a 20‐m‐deep plume of high‐TDS, boron‐rich water covering an area of about 5 km2. Transport of selenium and nitrate was inhibited by biogeochemical reactions in the pond bottom sediments, so that migration of these two constituents into the aquifer was limited. This paper focuses on physical parameters affecting seepage and lateral migration of nonreactive solutes and on hydrogeological factors associated with selenium break‐through into the shallow aquifer. An accompanying paper describes biogeochemical processes that limit nitrate and selenium migration (White et al., this issue). The regional hydrogeologic system at Kesterson Reservoir is dominated by the presence of four lithostratigraphic units. These are, in order of increasing depth, a fine‐grained surfical layer extending to depths of as much as 8 m, a semiconfined sandy aquifer extending to 60 m, a widespread lacustrine clay confining layer, and a confined sandy aquifer. Within each of these units, order of magnitude variations in the hydrologic properties of the sediments occur ubiquitously over lateral distances of tens of meters and vertical distances of a fraction of a meter. Seepage rates from the ponds into the underlying aquifer are dominated by local hydrogeologic properties and thickness of the fine‐grained surface layer. Selenium migrated into the aquifer in an area where the fine‐grained surface layer was thin or absent. Once in the aquifer, local rates of migration are controlled by the small‐scale structure and hydrogeologic properties of the sediments. The aquifer is composed of a heterogeneous mixture of sands, silts, and clays. Individual sand beds, which average about 0.3 m thick, do not appear to extend over lateral distances of more than 30 m. The large range of velocities combined with a limited lateral extent of individual layers are expected to create a broadly dispersed plume of solutes migrating downgradient from Kesterson. A noninvasive electromagnetic survey of electrical conductivity distributions in the shallow aquifer appears to confirm this conceptual model for transport in the shallow aquifer.

Comparison Between Model Simulations and Field Results for In‐Situ Biorestoration of Chlorinated Aliphatics: Part 1. Biostimulation of Methanotrophic Bacteria

AbstractA nonsteady‐state model is presented for the in‐situ biostimulation of a microbial population in saturated porous media. The model includes basic processes of microbial growth, utilization of electron donor and acceptor, advective transport, dispersion, and sorption in porous media. Model simulations are compared with results from a series of controlled field experiments at the Moffett Naval Air Station, where the growth of an indigenous population of methane‐utilizing bacteria (methanotrophs) was stimulated by the controlled addition of dissolved methane and oxygen (DO) into a semiconfined aquifer. Simulations provide good matches to the observed transient uptake of methane and DO, demonstrating that the observed response resulted from the growth of methanotrophs in the test zone. Simulations duplicate results from alternate pulsed addition of methane (electron donor) and oxygen (electron acceptor), used as a means for distributing microbial growth throughout the test zone. The model permits estimation of changes in microbial population distribution at various stages of the two‐year experiment. Temporal changes in model‐fitted kinetic parameters indicate that the microbial population evolved to one that more effectively utilized the methane at lower concentrations. These analyses demonstrate that a relatively simple model, which includes basic microbial and transport processes, can be of use in the design and evaluation of in‐situ biotreatment processes. The model user, however, must provide judgment in the selection of appropriate input parameters, as well as being aware of model limitations.