Semiconfining Layer Research Articles

Model-based interpretation of hydrogeochemistry and arsenic mobility in a low-enthalpy hydrothermal system

Low-enthalpy geothermal systems are widely spread around the world. Their exploitation for geothermal energy requires an accurate knowledge of the system. Detailed hydrogeochemical characterization is of pivotal importance since geothermal systems are often associated to the occurrence of toxic gases and elements which may contaminate any connected water resources. In this study we demonstrate how increased knowledge of a low-enthalpy geothermal system (Cimino-Vico, Central Italy) can be acquired from the analysis and interpretation of major chemistry and 87Sr/86Sr of local spring waters and groundwaters. With a model-based approach, we assess the main processes governing the major ion composition and arsenic (As) mobility in the system. The occurrence of high concentration of arsenic in the groundwater of the study area is a severe problem for the inhabitants that use the resource for domestic purposes. The system's hydrogeology consists of a shallow highly permeable aquifer, composed of alkaline-potassic volcanic rocks and characterized by fresh waters, a semi-confining layer at the base of the fresh water aquifer, and a deeper thermal aquifer in Mesozoic carbonates and Triassic evaporites. Upwelling of hot waters (up to 63 °C) to the shallow aquifer is related to the presence of faults and fractures in the semi-confining layer. The major chemistry of the deep thermal waters was found to be controlled by dedolomitization, while the fresh waters chemistry is governed by the interaction with the volcanic rocks and the mixing with the upwelling CO2-rich thermal waters. The outcomes of geochemical modeling are consistent with a conceptual model positing that arsenic bound to iron-(hydr)oxides becomes mobilized in the shallow, volcanic aquifer when thermal waters ascend into such shallow aquifer, where they promote the desorption of arsenic in favor of bicarbonate.

Experimental investigation of assessment of the contribution of heterogeneous semi-confining shale layers on mixing and trapping of dissolved CO2 in deep geologic formations

In most conceptual models of dissolution trapping of CO2, it is assumed that mixing of dissolved supercritical CO2 and formation brine occurs through density-driven convection. In our previous modeling study, we showed that the presence of continuous low-permeability shale layers in the formations causes convective shutdown through disruption of fingers, which impacts the effectiveness of mixing and trapping processes. However, these layers are naturally heterogeneous due to variations in compositional and textural properties. In the present study, we investigate the potential effects of heterogeneity present within semi-confining low-permeability layers on the overall mixing and trapping of dissolved CO2. Since accurate field experimentation in deep geologic formations is difficult due to inability to adequately characterize geology and define boundary conditions of the formation, we designed well-controlled experiments using NaBr solution and water under laboratory conditions. We conducted intermediate-scale 3D laboratory experiments under two homogeneous and one heterogeneous multilayered sand packing configurations. The results of these experiments show that connectivity of relatively higher permeability material within the semi-confining low-permeability layers contributes to mixing through (1) brine leakage between upper and lower aquifers (reverse convection), and (2) trapping through diffusion and back diffusion of initially trapped mass due to reversed concentration gradients in the long term. These findings suggest that when estimating the effectiveness of dissolution trapping in CO2 sequestration one needs to consider possible deviations from the traditional convective mixing theory in homogeneous media. Under some conditions of natural formation heterogeneity, diffusion can contribute to trapping; in some others, it may not.

Using transient temperature–depth profiles to assess vertical groundwater flow across semi-confining layers in the Chianan coastal plain aquifer systeme, southern Taiwan

The quantification of vertical groundwater fluxes across semi-confining layers is fundamental to evaluate groundwater recharge and discharge rates to and from aquifer systems. Methods to estimate vertical groundwater fluxes from temperature–depth profiles have been available since the 1960s. While some methodologies assume steady-state conditions, changes in land-surface temperatures as well as hydrogeological conditions can lead to transient heat flow conditions. Indeed, many studies have indicated that transient temperatures in deeper confined aquifers are widespread. A study is presented that uses transient-temperature–depth curves obtained from groundwater observation wells in the Chianan coastal plain in southern Taiwan. In this area, sedimentary aquifer systems consist of a stack of alternating sand and mud layers, over several hundred meters in thickness. Groundwater has been abstracted from these aquifers for decades, resulting in large hydraulic gradients between the shallow and deeper aquifers. Hence, vertical groundwater flow is likely enhanced across finer-grained, semi-confining units. A set of temperature–depth profiles is available from this area. Constrained by these profiles, numerical models of one-dimensional transient heat transfer were used to infer vertical fluxes of 3.3 × 10−8 to 3.9 × 10−8 m/s using thermal data from 2013 to 2016. An analytical solution was also employed that assumes steady-state conditions. Calculated fluxes using the latter approach were lower, at approximately 1.1 × 10−8 to 1.6 × 10−8 m/s. The study suggests that vertical fluxes derived from using Bredehoeft and Papadopulos’s analytical solutions result in underestimates of actual vertical seepage rates across aquitards.

The effect of loading efficiency on the groundwater response to water level changes in shallow lakes and streams

AbstractThe loading efficiency (sometimes called the tidal efficiency) is often neglected when simulating the head response in an aquifer to water level changes in lakes and streams. This is not appropriate when the lake or stream only partially penetrates the aquifer. In such cases, the aquifer extends below the lake or stream and is hydraulically connected through a semiconfining layer of lower permeability. The loading efficiency is the ratio between the instantaneous head response below a lake or stream and the water level change in the lake or stream. In sand and clay, whose particles are not cemented together, the instantaneous head response below a stream or lake is nearly equal to the stage change, and the loading efficiency is close to 1. New semianalytic solutions are presented for the groundwater response to water level changes in shallow lakes and streams that account for the loading efficiency of the aquifer. It is shown that the loading efficiency may have a significant effect on the head response. The effect is larger for larger values of the vertical resistance of the semiconfining layer and larger width of the stream and is much more pronounced in confined aquifers than in unconfined aquifers. The importance of the loading efficiency declines with time and with distance from the lake or stream. Graphs are presented that may be used to determine whether a certain combination of parameters gives a significant difference in the head at the lake shore or river bank when the loading efficiency is taken into account.

Water Resources Evaluation in Ghadamis Basin, Libya

Ghadames basin is one of the important groundwater basins in Libya. This basin covers an area of about 215,000 km2 in northwestern Libya. This groundwater basin consists of two major aquifer systems. The upper aquifer system mostly formed of carbonate rocks (Mizdah and Nalut formations). The lower aquifer system formed mostly of sandstone known as Kiklah formation. The two aquifer systems are separated by semi-confining layers known as Yefren marl. The lower aquifer system is considered as the main aquifer for water supply. The water resource (mostly from the lower aquifer system) in these systems is used for domestic, irrigation and fodder drinking. The domestic use of water is to supply water to the cities and towns in western part of the country and in the northern flanks of the basin, and plans to transport water to the coastal cities in northwestern part of the country. The existing total amount of extracted water is 132 Mm3/year, and about 20% of it used for domestic uses. The planned water extraction is 223.8 Mm3/year about 55% of it will be used for domestic uses. The hydrogeological situation for this basin has been studied in this study using data from previous hydrogeological exploration programs, evaluation of the data from the existing extraction and observation water wells, pumping tests data used to evaluate aquifers hydraulic parameters, and construction of hydrogeological cross-sections to know the aquifer systems geometry. Hydogeological computer model was constructed to evaluate the aquifer hydraulic parameters using the historical water extraction, and water levels and drawdowns. The model is then used to predict the future drawdown which has been found to be in the range 55 to 60 m after 50 years of extraction.

Gas bubble transport and emissions for shallow peat from a northern peatland: The role of pressure changes and peat structure

Gas bubbles are an important pathway for methane release from peatlands. The mechanisms controlling gas bubble transport and emissions in peat remain uncertain. The effects of hydrostatic pressure and peat structure on the dynamics of gas bubbles in shallow peat were therefore tested in laboratory experiments. A peat monolith was retrieved from a raised bog and maintained in a saturated state. Three distinct layers were identified from noninvasive permittivity measurements supported by soil physical properties (porosity, bulk density). Phase I of the experiment involved monitoring for the accumulation of gas bubbles under steady pressure and temperature conditions. The data showed evidence for gas bubbles being impeded by a shallow semiconfining layer at depths between 10 and 15 cm. Visible gas bubbles observed on the side of the sample box were recorded over time to estimate changes in the vertical distribution of volumetric gas content. Porosity estimates derived using the Complex Refraction Index Model (CRIM) suggest that gas bubbles enlarge the pore space when the exerted pressure is high enough. Phase II involved triggering release of trapped bubbles by repeatedly increasing and decreasing hydrostatic pressure in an oversaturated condition. Comparison of changes in pressure head and methane density in the head space confirmed that the increasing buoyancy force during drops in pressure is more important for triggering ebullition than increasing mobility during increases in pressure. Our findings demonstrate the importance of changes in hydrostatic pressure on bubble size and variations in resistance of the peat fabric in regulating methane releases from peatlands.

Quantity and quality of groundwater discharge in a hypersaline lake environment

Geophysical and geochemical surveys were conducted to understand groundwater discharge to Great Salt Lake (GSL) and assess the potential significance of groundwater discharge as a source of selenium (Se). Continuous resistivity profiling (CRP) focusing below the sediment/water interface and fiber-optic distributed temperature sensing (FO-DTS) surveys were conducted along the south shore of GSL. FO-DTS surveys identified persistent cold-water temperature anomalies at 10 separate locations. Seepage measurements were conducted at 17 sites (mean seepage rate = 0.8 cm/day). High resistivity anomalies identified by the CRP survey were likely a mirabilite (Na2SO4·10H2O) salt layer acting as a semi-confining layer for the shallow groundwater below the south shore of the lake. Positive seepage rates measured along the near-shore areas of GSL indicate that a ∼1-m thick oolitic sand overlying the mirabilite layer is likely acting as a shallow, unconfined aquifer. Using the average seepage rate of 0.8 cm/day over an area of 1.6 km2, an annual Se mass loading to GSL of 23.5 kg was estimated. Determination of R/Ra values (calculated 3He/4He ratio over the present-day atmospheric 3He/4He ratio) <1 and tritium activities of 1.2–2.0 tritium units in groundwater within and below the mirabilite layer indicates a convergence of regional and local groundwater flow paths discharging into GSL. Groundwater within and below the mirabilite layer obtains its high sulfate salinity from the dissolution of mirabilite. The δ34S and δ18O isotopic values in samples of dissolved sulfate from the shallow groundwater below the mirabilite are almost identical to the isotopic signature of the mirabilite core material. The saturation index calculated for groundwater samples using PHREEQC indicates the water is at equilibrium with mirabilite. Water samples collected from GSL immediately off shore contained Se concentrations that were 3–4 times higher than other sampling sites >25 km offshore from the study site and may be originating from less saline groundwater seeps mixing with the more saline water from GSL. Additional evidence for mixing with near shore seeps is found in the δD and δ18O isotopic values and Br:Cl ratios. Geochemical modeling for a water sample collected in the vicinity of the study area indicates that under chemically reducing conditions, arsenic- (As) bearing minerals could dissolve while Se-bearing minerals will likely precipitate out of solution, possibly explaining why the shallow groundwater below and within the mirabilite salt layer contains low concentrations of Se (0.9–2.3 μg/L).

Three‐Dimensional Flow in the Storative Semiconfining Layers of a Leaky Aquifer

An analytical solution for three-dimensional (3D) flow in the storative semiconfining layers of a leaky aquifer fully penetrated by a production well is developed in this article to provide a method from which accurate hydraulic parameters in the semiconfining layers can be derived from aquifer test data. The analysis of synthetic aquifer test data with the 3D analytical solution in the semiconfining layers provided more accurate optimal hydraulic parameters than those derived using the available quasi-two-dimensional (2D) solution. Differences between the 3D and 2D flow solutions in the semiconfining layers become larger when a no flow boundary condition is imposed at either at the top of the upper semiconfining layer or at the bottom of the lower semiconfining layer or when the hydraulic conductivity ratio of the semiconfining layer to the aquifer is larger than 0.001. In addition, differences between the 3D and 2D flow solutions in the semiconfining layers are illustrated when the thickness ratio of the semiconfining layer to the aquifer is changed. Analysis of water level data from two hypothetical and one real aquifer test showed that the 3D solution in the semiconfining layers provides lower correlation coefficients among hydraulic parameters than the 2D solution.

Hydrogeology of thermal waters in Viterbo area, central Italy

A conceptual hydrogeological model of the Viterbo thermal area (central Italy) has been developed. Though numerous studies have been conducted on its geological, geochemical and geothermal features, there is no generalized picture defining the origin and yield of the hydrothermal system. These latter aspects have therefore become the objectives of this research, which is based on new hydrogeological and geochemical investigations. The geological setting results in the coexistence of overlapped interacting aquifers. The shallow volcanic aquifer, characterized by fresh waters, is fed from the area around the Cimini Mountains and is limited at its base by the semiconfining marly-calcareous-arenaceous complex and low-permeability clays. To the west of Viterbo, vertical upflows of thermal waters of the sulphate-chloride-alkaline-earth type with higher gas contents, are due to the locally uplifted carbonate reservoir, the reduced thickness of the semiconfining layer and the high local geothermal gradient. The hot waters (30–60°C) are the result of deep circulation within the carbonate rocks (0.5–1.8 km) and have the same recharge area as the volcanic aquifer. The upward flow in the Viterbo thermal area is at least 0.1 m3/s. This flow feeds springs and deep wells, also recharging the volcanic aquifer from below.

Permeability estimation from transient vadose zone pumping tests in shallow coastal-plain sediments

Short-term, field-scale transient vadose zone pumping tests were performed on several wells at the Savannah River Site, located near Aiken, South Carolina. A total of 18 vacuum extraction tests were conducted in shallow coastal-plain vadose zone sediments, which yielded 38 data sets for analysis. Data collected during these tests were analyzed using two different modified Hantush-Jacob models to estimate the physical properties of the unsaturated sediments. The first model used was a commercially available software package used to estimate aquifer properties. This model assumed the test fluid (air) to be incompressible. The second model used was provided by Clemson University and accounted for the compressibility of the test fluid. This model was used primarily to provide confirmation of results from the first model. Comparisons between the results of the two models showed that both produced similar estimates of permeability for a given data set. Results from the analysis of the 38 data sets showed that the radial air permeability values ranged from 4.7 to 84.0 d. Vertical air permeability values were found to range from 0.93 to 11.7 d. The permeability values for the overlying semiconfining layer ranged from 2.7 1010 to 2.01 d. The results from these tests can be used to establish a general range of air permeabilities for shallow coastal-plain vadose zone sediments.

Hydrogeologic Setting and Ground Water Flow Beneath a Section of Indian River Bay, Delaware

AbstractThe small bays along the Atlantic coast of the Delmarva Peninsula (Delaware, Maryland, and Virginia) are a valuable natural resource, and an asset for commerce and recreation. These coastal bays also are vulnerable to eutrophication from the input of excess nutrients derived from agriculture and other human activities in the watersheds. Ground water discharge may be an appreciable source of fresh water and a transport pathway for nutrients entering the bays. This paper presents results from an investigation of the physical properties of the surficial aquifer and the processes associated with ground water flow beneath Indian River Bay, Delaware. A key aspect of the project was the deployment of a new technology, streaming horizontal resistivity, to map the subsurface distribution of fresh and saline ground water beneath the bay. The resistivity profiles showed complex patterns of ground water flow, modes of mixing, and submarine ground water discharge. Cores, gamma and electromagnetic‐induction logs, and in situ ground water samples collected during a coring operation in Indian River Bay verified the interpretation of the resistivity profiles. The shore‐parallel resistivity lines show subsurface zones of fresh ground water alternating with zones dominated by the flow of salt water from the estuary down into the aquifer. Advective flow produces plumes of fresh ground water 400 to 600 m wide and 20 m thick that may extend more than 1 km beneath the estuary. Zones of dispersive mixing between fresh and saline ground water develop on the upper, lower, and lateral boundaries of the plume. The plumes generally underlie small incised valleys that can be traced landward to streams draining the upland. The incised valleys are filled with 1 to 2 m of silt and peat that act as a semiconfining layer to restrict the downward flow of salt water from the estuary. Active circulation of both the fresh and saline ground water masses beneath the bay is inferred from the geophysical results and supported by geochemical data.

Use of stable isotopes to quantify flows between the Everglades and urban areas in Miami-Dade County Florida

An isotopic study was performed to assess the movement of groundwater for a site located in Miami-Dade County, Florida. The site encompasses portions of a protected wetland environment (northeast Everglades National Park) and suburban residential Miami, incorporating municipal pumping wells and lakes formed by rock mining. Samples of ground, surface, and rainwater were analyzed for their isotopic composition (oxygen-18 and deuterium). Various analytical and graphical techniques were used to analyze this data and two conceptual box models were developed to quantify flows between different regions within the site. Results from this study indicate that the aquifer underlying the study site (the Biscayne aquifer) is highly transmissive with the exception of two semi-confining layers of reduced hydraulic conductivity. Everglades surface water infiltrates into the aquifer and migrates east toward residential areas. In these urban areas, ‘shallow’ groundwater (above the deeper semi-confining layer) is substantially affected by urban rainfall while ‘deep’ groundwater (below the deeper semi-confining layer) maintains a composition similar to that of Everglades water. Rock mining lakes in the area provide ‘breaks’ in the semi-confining layers that allow for mixing of shallow and deep groundwater. As water travels eastward, municipal well intakes, screened to a depth below the deeper semi-confining layer, draw upon not only shallow urban water (predominantly comprised of urban rainfall) and lake water (having influences from both urban rainfall and Everglades water) but also deep water that originated in the Everglades. Results from one of the box models estimate that over 60% of the water being removed by municipal pumping originated in the Everglades. These conclusions suggest that Everglades water, both directly through deep groundwater flow and indirectly through mixing with rock-mining lakes, is being drawn into the operating municipal wellfield.

The dynamic form of rainwater lenses in drained fens

Inflow of nutrient poor alkaline groundwater is generally seen as a prerequisite for the development of species rich fen vegetation. Drainage and groundwater abstraction may lead to the development of so-called rainwater lenses in the upper groundwater of fens, which prevent upward seeping alkaline groundwater from reaching the fen root zone. This threatens the conservation of the species rich vegetation. To provide insight in rainwater lens dynamics as a basis for conservation and restoration of fen biodiversity, numerical groundwater flow model simulations were performed for a hypothetical case area in the Netherlands. The simulations indicate that drainage canal surface water levels mainly affect the phreatic level in the fens, while groundwater inflow fluxes mainly affect the thickness of the rainwater lens. Increasing thickness of a semi-confining surface layer results in a rise in phreatic level. As the phreatic level reaches the soil surface, the rainwater lens may split in two as a result of groundwater discharge windows, which come into existence in the centre of the parcel. The effect of seasonal recharge fluctuations on the rainwater lens appears small. Spells of dry and wet years have a considerably larger effect especially on the lower boundary of the rainwater lens, resulting in thinner lenses during spells of dry years. Most simulations show rainwater lens thicknesses well exceeding rooting depths of fen plants. This is inconsistent with the fact that fen vegetation is found in nature reserves for which the simulations are considered representative. Chemical buffer processes in the soil may explain this inconsistency, masking an acidification process under way. As buffer capacities become exhausted over time, acidification and fen deterioration will occur at a certain future moment. To arrive at more definite conclusions about this hypothesis, our simulations of convective groundwater flow need to be supplemented by modelling of hydrochemical processes in rainwater lenses, followed by verification at actual field locations.

Use of stable isotopes to quantify flows between the Everglades and urban areas in Miami-Dade County Florida

An isotopic study was performed to assess the movement of groundwater for a site located in Miami-Dade County, Florida. The site encompasses portions of a protected wetland environment (northeast Everglades National Park) and suburban residential Miami, incorporating municipal pumping wells and lakes formed by rock mining. Samples of ground, surface, and rainwater were analyzed for their isotopic composition (oxygen-18 and deuterium). Various analytical and graphical techniques were used to analyze this data and two conceptual box models were developed to quantify flows between different regions within the site. Results from this study indicate that the aquifer underlying the study site (the Biscayne aquifer) is highly transmissive with the exception of two semi-confining layers of reduced hydraulic conductivity. Everglades surface water infiltrates into the aquifer and migrates east toward residential areas. In these urban areas, ‘shallow’ groundwater (above the deeper semi-confining layer) is substantially affected by urban rainfall while ‘deep’ groundwater (below the deeper semi-confining layer) maintains a composition similar to that of Everglades water. Rock mining lakes in the area provide ‘breaks’ in the semi-confining layers that allow for mixing of shallow and deep groundwater. As water travels eastward, municipal well intakes, screened to a depth below the deeper semi-confining layer, draw upon not only shallow urban water (predominantly comprised of urban rainfall) and lake water (having influences from both urban rainfall and Everglades water) but also deep water that originated in the Everglades. Results from one of the box models estimate that over 60% of the water being removed by municipal pumping originated in the Everglades. These conclusions suggest that Everglades water, both directly through deep groundwater flow and indirectly through mixing with rock-mining lakes, is being drawn into the operating municipal wellfield.

The dynamic form of rainwater lenses in drained fens

Inflow of nutrient poor alkaline groundwater is generally seen as a prerequisite for the development of species rich fen vegetation. Drainage and groundwater abstraction may lead to the development of so-called rainwater lenses in the upper groundwater of fens, which prevent upward seeping alkaline groundwater from reaching the fen root zone. This threatens the conservation of the species rich vegetation. To provide insight in rainwater lens dynamics as a basis for conservation and restoration of fen biodiversity, numerical groundwater flow model simulations were performed for a hypothetical case area in the Netherlands. The simulations indicate that drainage canal surface water levels mainly affect the phreatic level in the fens, while groundwater inflow fluxes mainly affect the thickness of the rainwater lens. Increasing thickness of a semi-confining surface layer results in a rise in phreatic level. As the phreatic level reaches the soil surface, the rainwater lens may split in two as a result of groundwater discharge windows, which come into existence in the centre of the parcel. The effect of seasonal recharge fluctuations on the rainwater lens appears small. Spells of dry and wet years have a considerably larger effect especially on the lower boundary of the rainwater lens, resulting in thinner lenses during spells of dry years. Most simulations show rainwater lens thicknesses well exceeding rooting depths of fen plants. This is inconsistent with the fact that fen vegetation is found in nature reserves for which the simulations are considered representative. Chemical buffer processes in the soil may explain this inconsistency, masking an acidification process under way. As buffer capacities become exhausted over time, acidification and fen deterioration will occur at a certain future moment. To arrive at more definite conclusions about this hypothesis, our simulations of convective groundwater flow need to be supplemented by modelling of hydrochemical processes in rainwater lenses, followed by verification at actual field locations.

Use of hydraulic head to estimate volumetric gas content and ebullition flux in northern peatlands

Hydraulic head was overpressured at middepth in a 4.2‐m thick raised bog in the Glacial Lake Agassiz peatlands of northern Minnesota, and fluctuated in response to atmospheric pressure. Barometric efficiency (BE), determined by calculating ratios of change in hydraulic head to change in atmospheric pressure, ranged from 0.05 to 0.15 during July through November of both 1997 and 1998. The overpressuring and a BE response were caused by free‐phase gas contained primarily in the center of the peat column between two or more semielastic, semiconfining layers of more competent peat. Two methods were used to determine the volume of gas bubbles contained in the peat, one using the degree of overpressuring in the middepth of the peat, and the other relating BE to specific yield of the shallow peat. The volume of gas calculated from the overpressuring method averaged 9%, assuming that the gas was distributed over a 2‐m thick overpressured interval. The volume of gas using the BE method averaged 13%. Temporal changes in overpressuring and in BE indicate that the volume of gaseous‐phase gas also changed with time, most likely because of rapid degassing (ebullition) that allowed sudden loss of gas to the atmosphere. Estimates of gas released during the largest ebullition events ranged from 0.3 to 0.7 mol m−2 d−1. These ebullition events may contribute a significant source of methane and carbon dioxide to the atmosphere that has so far largely gone unmeasured by gas‐flux chambers or tower‐mounted sensors.

Evaluation of a pumping test of the Snake River Plain aquifer using axial-flow numerical modeling

The Snake River Plain aquifer in southeast Idaho is hosted in a thick sequence of layered basalts and interbedded sediments. The degree to which the layering impedes vertical flow has not been well understood, yet is a feature that may exert a substantial control on the movement of contaminants. An axial-flow numerical model, RADFLOW, was calibrated to pumping test data collected by a straddle-packer system deployed at 23 depth intervals in four observation wells to evaluate conceptual models and estimate properties of the Snake River Plain aquifer at the Idaho National Engineering and Environmental Laboratory. A delayed water-table response observed in intervals beneath a sediment interbed was best reproduced with a three-layer simulation. The results demonstrate the hydraulic significance of this interbed as a semi-confining layer. Vertical hydraulic conductivity of the sediment interbed was estimated to be about three orders of magnitude less than vertical hydraulic conductivity of the lower basalt and upper basalt units. The numerical model was capable of representing aquifer conceptual models that could not be represented with any single analytical technique. The model proved to be a useful tool for evaluating alternative conceptual models and estimating aquifer properties in this application.

Modelling scenarios for the emplacement of palaeowaters in aquifer systems

Abstract The response of coastal aquifer systems to global sea-level rise, the presence of permafrost and glaciation has been analysed using analytical and numerical models. The hydraulic connectivity between confined coastal aquifers and the sea largely controls their response to global sea-level rise. Open aquifer systems have direct hydraulic contact with the sea where they sub crop along the continental slope and base-level history for the aquifer is defined by sea-level history. In these systems, hydraulic heads equilibrate quickly to sea-level change at rates controlled by the aquifer hydraulic diffusivity. Interface movement lags behind the equilibration of the hydraulic heads and is controlled largely by the rate at which freshwater can be flushed from the aquifer through overlying semi-confining units. Interface movement occurs over time periods of tens of thousands of years. In contrast, closed aquifer systems lack direct hydraulic connectivity with the sea, which is controlled by the thickness and permeability of overlying semi-confining layers. During the Late Pleistocene-Early Holocene the base-level history for closed aquifer systems underlying the North Sea Basin was defined by the location of rivers and lakes in areas that are now offshore. These aquifers became coastal aquifers during the Holocene when rivers and lakes were inundated by rising sea levels. Salinization of closed aquifer systems may occur due to the downward diffusion of salts through overlying semi-confining layers in the presence of upward freshwater seepage. Aquifers overridden by glaciers respond in a manner that is largely controlled by aquifer transmissivity and geometry. The presence of permafrost inhibits recharge resulting in lower hydraulic heads and a reduction in aquifer fluxes.

Application of the top specified boundary layer (TSBL) approximation to initial characterization of an inland aquifer mineralization:: 2. Seepage of saltwater through semi-confining layers

This paper presents a generalized basic study that addresses practical needs for an understanding of the major mechanisms involved in the mineralization of groundwater in the Great Bend Prairie aquifer in south-central Kansas. This Quaternary alluvial aquifer and associated surface waters are subject to contamination by saltwater, which in some areas seeps from the deeper Permian bedrock formation into the overlying freshwater aquifer through semiconfining layers. A simplified conceptual model is adopted. It incorporates the freshwater aquifer whose bottom is comprised of a semiconfining layer through which a hydrologically minor but geochemically important saline water discharge seeps into the aquifer. A hierarchy of approximate approaches is considered to analyze the mineralization processes taking place in the aquifer. The recently developed top specified boundary layer (TSBL) approach is very convenient to use for the initial characterization of these processes, and is further adapted to characterization of head-driven seepage through semi-confining layers. TSBL calculations indicate that the seeping saline water may create two distinct new zones in the aquifer: (1) a completely saline zone (CSZ) adjacent to the semiconfining bottom of the aquifer, and (2) a transition zone (TZ) which develops between the CSZ and the freshwater zone. Some possible scenarios associated with the various mineralization patterns are analyzed and discussed.

Effect of Horizontal Heat and Fluid Flow on the Vertical Temperature Distribution in a Semiconfining Layer

By including the constant flow of heat and fluid in the horizontal direction, we develop an analytical solution for the vertical temperature distribution within the semiconfining layer of a typical aquifer system. The solution is an extension of the previous one‐dimensional theory by Bredehoeft and Papadopulos [1965]. It provides a quantitative tool for analyzing the uncertainty of the horizontal heat and fluid flow. The analytical results demonstrate that horizontal flow of heat and fluid, if at values much smaller than those of the vertical, has a negligible effect on the vertical temperature distribution but becomes significant when it is comparable to the vertical.