Aquitard Thickness Research Articles

A new analytical method for estimating hydraulic parameters of a nonlinear consolidated aquitard with variable drawdown in adjacent aquifers

Estimation of hydraulic parameters and groundwater depletion of aquitards is necessary to evaluate the groundwater resources and land subsidence. In this study, an analytical solution for water release from a nonlinear consolidated aquitard was derived by approximating the drawdown history as a continuous piecewise linear drawdown function (PLD) with several stages, while the water level in adjacent aquifers varies arbitrarily over time. According to dimensionless and parameter sensitivity analyses, groundwater depletion of an aquitard increases with an increase in compression index, aquitard thickness, and water drawdown rate in adjacent aquifers, decreases with an increase in initial void ratio and effective stress, and is not affected by the consolidation coefficient. In addition, we proposed a type-curve fitting method to calculate the hydraulic parameters of the aquitard under the condition of PLD in adjacent aquifers. The proposed method was applied to estimate the hydraulic parameters of aquitards at two field sites in the Yangtze River Delta in China, i.e., Changzhou and Shanghai. Results show that the compression index of the aquitards in Changzhou and Shanghai are 0.074 and 0.24, respectively, while the consolidation coefficients are 7.62 × 10-7 m2/s and 2.16 × 10-6 m2/s, respectively. The hydraulic conductivity and specific storage estimated by the proposed method are in good agreement with previous studies and the results of the geological method of Konikow and Neuzil (2007). The proposed analytical method for estimating hydraulic parameters and groundwater depletion of aquitards will contribute to the sustainable management of groundwater resources in multi-layer aquifer systems.

Aquifer depressurization and water table lowering induces landscape scale subsidence and hydrophysical change in peatlands of the Hudson Bay Lowlands

The depositional history of the Hudson Bay Lowlands (HBL) in Ontario, Canada has created a low relief, poorly drained landscape, favouring the formation of one of the largest peatland complexes in the world. High volume dewatering associated with resource extraction in this area, such as the De Beers Victor Diamond Mine, tests the ability of the underlying confining layer to limit water losses in the peatlands above. This research quantifies the deepening of water tables and increase in effective stress related to mine dewatering and the resulting changes to bog and fen peatland hydrophysical structure and function. Long-term implications of these impacts are discussed. One impacted and two unimpacted transects were instrumented for meteorological (precipitation and evapotranspiration) and hydrophysical (hydraulic head, hydraulic conductivity (Ksat), and surface elevation) monitoring over a 12-year period in the vicinity of the Victor Mine. Over this study period, the unimpacted peatlands operated within relative hydrological equilibrium, demonstrated through shallow water tables, negligible subsidence, and stable Ksat. Contrastingly, all impacted peatlands experienced deeper watertables, larger downwards gradients, and measurable long-term subsidence (4–15 cm). Hydrological impacts were highest in bogs with a thin underlying confining layer even if they were farther from the point of dewatering, highlighting the need for environmental monitoring programs which incorporate an assessment of aquitard thickness. Where subsidence occurred, associated decreases in Ksat deflected bog-fen-tributary flow-paths deeper, reducing the upwards transport of solute rich water to downgradient fens. The long-term effects of these landscape scale changes should be studied further, particularly since climate change in this region will potentially increase water deficits and further alter peatland connectivity. Peatland studies should be conducted in different landscapes experiencing water table lowering due to drought or depressurization in order to better understand the associated subsidence patterns and hydrophysical changes in varying geological and morphological regimes.

Influence of inter-aquifer leakage on well-injection capacity: Theory and aquifer-scale mapping for artificial recharge

Aquifer storage and recovery (ASR) is an important water resources management technique that involves the injection of a large volume of water underground. For the successful implementation of an ASR project, a target aquifer should have a sufficient injection capacity, which is the maximum volume of water that can be safely injected. In nature, no aquitard is perfectly impermeable, and inter-aquifer leakage may have a major impact on injection capacity. Despite the importance of determining the injection capacity for ASR planning, there is no quantitative methodology that estimates the injection capacity of leaky aquifers. In this study, we first develop a solution for injection capacity with inter-aquifer leakage based on the Hantush - Jacob solution, and conduct a comprehensive sensitivity analysis to elucidate the influence of inter-aquifer leakage on injection capacity. From the sensitivity analysis, we show that inter-aquifer leakage can impact injection capacity by more than one order of magnitude, depending on the hydrogeological and operational parameters. We then develop a practical mapping methodology that estimates the injection capacity of leaky aquifers. We demonstrate the proposed methodology by applying it to a potential ASR site in Minnesota, USA, where ASR is considered as a solution to alleviate groundwater contamination by PFAS chemicals. The case study results reveal significant spatial variability in injection capacity over the study area and show an average increase in the injection capacity of about 26% compared to that in the nonleaky scenario. We also analyze the uncertainty in the estimated injection capacity due to the variability of aquitard properties and show that the variability of aquitard vertical hydraulic conductivity leads to a larger uncertainty in the estimated injection capacity than does the variability of aquitard thickness. This study elucidates the effects of inter-aquifer leakage on injection capacity and provides a practical methodology for injection capacity mapping.

Seepage and heat transfer characteristics of sandstone thermal reservoir under aquitard damage

Flow and thermal breakthroughs are the main reasons for the reduction in the operational efficiency of a single-well groundwater heat pump (SWGWHP) system. Furthermore, a large initial pumping flow rate, attributed to the large initial heating/cooling load of the building, can consolidate and damage the aquifer. In this study, an aquitard damage test bench was constructed to investigate the formation and development of flow and thermal breakthroughs during the operation of the SWGWHP system by varying the initial recharge rate (Q), aquifer thickness (H), and the time difference between pumping and recharge (Δt); and to investigate the aquifer damage, seepage, and heat transfer patterns. The results indicated that pressure was the main factor that damaged the aquitard. Aquitard damage can cause or aggravate the flow breakthrough phenomenon. The flow breakthrough strength is the main factor affecting the thermal breakthrough strength. The results of the orthogonal test analysis indicate that the order of the primary and secondary influencing factors of the flow breakthrough strength and operating time efficiency is aquitard thickness (H) > initial recharge rate (Q) > pumping and recharging time difference (Δt). The existence of the aquitard reduced the flow breakthrough strength by 15.2–33.2% and prolonged the efficient operation time of the system by 2.5–5 times. Based on quantitative analyses of the flow and thermal breakthrough strengths, and the variation in the system coefficient of performance, this study reveals the reasons for the formation of thermal breakthrough and provides theoretical suggestions to ensure a safe and stable operation of the SWGWHP system. The results of the study can guide practical installations as well as future research on SWGWHP systems.

Subsidence monitoring by integration of time series analysis from different SAR images and impact assessment of stress and aquitard thickness on subsidence in Tehran, Iran

Subsidence monitoring by integration of time series analysis from different SAR images and impact assessment of stress and aquitard thickness on subsidence in Tehran, Iran

Delineation of groundwater potential zones using a parsimonious concept based on catastrophe theory and analytical hierarchy process

A cost-effective “parsimonious” approach to delineating groundwater potential zones is proposed, based on catastrophe theory (CT) and analytical hierarchy process (AHP) in a geographic information system (GIS). Eleven indicators that influence groundwater storage (slope, drainage density, surface-water body (proximity), soil permeability, aquitard thickness, aquifer thickness, hydraulic conductivity, specific yield, recharge, aquitard resistivity and aquifer resistivity) were prepared. The suitable weights of the factors and the index values of the features of the factors were normalized using AHP and CT multicriteria decision analysis (MCDA) techniques for the development of a groundwater potential index (GWPI) map. Finally, the relative sensitivity of the factors was evaluated to develop a parsimonious groundwater potential index (P-GWPI) map using the most sensitive themes. GWPI and P-GWPI maps were validated using 14-year average annual post-monsoon depth to groundwater level data of 36 monitoring wells in a study area in Bangladesh. The generated GWPI map classified the study area as moderately good, good and very good groundwater potential covering an area of 19.5, 40.3 and 40.2% respectively. Subsequently, a modified GWPI map was developed using effective weights derived from single-parameter sensitivity analysis. The P-GWPI map developed using the most sensitive factors categorized the groundwater potential zones as moderately good (13.0%), good (38.2%) and very good (48.8%). The results of this study can serve as guidelines for future groundwater exploration, planning and management of the area, and the methodology used can also be easily adopted in other similar and data-scarce areas.

Modeling Saline Mudflat and Aquifer Deformation Synthesizing Environmental and Hydrogeological Factors Using Time-Series InSAR

Clarifying the surface and subsurface deformation is important for protecting the infrastructures attached to the ground and reasonably exploiting the underground resources. This article presents an improved deformation model combining environmental factors (i.e., precipitation and temperature) with hydrogeological parameters to separate the saline-soil deformation from the aquifer deformation over Qarhan Salt Lake, China. First, the vertical ground deformation was derived by 119 ascending and 113 descending synthetic aperture radar images of Sentinel-1A collected from July 2015 to May 2020. Subsequently, we estimated the deformation components derived from temperature, precipitation, seasonal oscillation, magnitude, and decay coefficient of the hydrodynamic function by the proposed deformation model. The estimated deformation and decay coefficient maps pinpoint the existing and previously unknown faults. Next, we compared our proposed prototype with the sinusoidal and exponentially decaying model to present the reliability and efficiency in separating the deformation components and estimating the decay coefficient. Finally, we collected the stratigraphic data from 50 drilling wells to validate our model results by simulating the cumulative silt and clay layers (aquitard) thickness utilizing the Ordinary Kriging interpolation method. Findings show that by quantifying the aquitard thickness in the drilling wells, there is a strong relationship in the spatial distribution between cumulative thickness and the decay coefficient. The results suggested that both the decay coefficient and the aquitard-layer thickness can be used as the vital parameters to partition hydrogeological units, which provides an opportunity to detect and mitigate the potential geological hazards in the saline mudflat.

An Analytical Method to Calculate Groundwater Released From an Aquitard Undergoing Nonlinear Consolidation

AbstractThe estimation of groundwater released from aquitards is important for the groundwater resource management. In this study, a governing equation of drawdown variation within the aquitard, which undergoes nonlinear consolidation, was obtained based on the consolidation theory of Gibson et al. (1967, https://doi.org/10.1680/geot.1967.17.3.261). Then, analytical solutions of drawdown variations within an aquitard were derived for a single aquifer underlying an aquitard, as well as for the case when there are aquifers above and below an aquitard. Drawdown solutions were obtained for both the cases when there was a sudden hydraulic head decline in the confined aquifer(s). The characteristics of the groundwater dynamics and water balance in the aquitard have been analyzed with the analytical solution. A laboratory experiment was conducted to verify the analytical solution, and a type‐curve fitting method was presented to calculate the aquitard's hydraulic parameters. Our study revealed that the cumulative water released from the aquitard predicted by Gibson's theory (1967) is less than that obtained through Terzaghi's theory (1943). Groundwater released from the aquitard increases with increasing compression index, aquitard thickness, and drawdown of the adjacent aquifer, and with decreasing initial void ratio and initial effective stress. The time for the drawdown to reach steady state increases with increasing compression index, initial void ratio, and aquitard thickness, and with decreasing initial effective stress and hydraulic conductivity.

Application of a new evaluation method for floor water inrush risk from the Ordovician fissure confined aquifer in Xiayukou coal mine, Shanxi, China

As the mining depth of North China-type coal fields increases year by year, the threat of Ordovician limestone karst water damage to coal seam mining is increasing, and the risk of floor water inrush increases gradually. To ensure safe production in coal mines and avoid casualties and economic losses, it is essential to accurately predict the risk of coal floor water inrush. The traditional approach to determine whether there is risk during the mining process is the water inrush coefficient method, which has certain limitations because it does not fully consider the water yield of the aquifer and the actual hydraulic resistance of the aquitard. Therefore, in this paper, a new method was put forward and applied for predicting the floor water inrush risk attributed to the Ordovician karst fissure confined aquifer from the Permian-age coal deposits in the Xiayukou coal field, Hancheng mining area, Shanxi, China. The influence of floor water inrush is comprehensively considered from the three aspects of the aquifer’s water yield, the hydraulic resistance characteristics of the aquitard and the tectonic development of the study area. Based on the new analytic hierarchy process-linear weighted average (AHP-LWA) method, a water abundance structure index (WASI) was created using the burial depth of the Ordovician aquifer (Dp), water abundance layer (Ly) and morphological features of the aquifer (Mf). Similarly, a hydraulic resistance structure index (HRSI) model was established using aquitard thickness (Tk), number of aquifers and aquitards (Num), sand–mud ratio (SMr) and core recovery rate (CRr); additionally, fractal dimension values were used to characterize the degree of structural development. Finally, a water inrush risk evaluation model was constructed, and a contour map was drawn using the risk index method of floor water inrush (RIWI) values and revealed three water inrush risk zones: normal mining area (0.18–0.32), medium-risk area (0.32–0.38) and high-risk area (0.38–0.52). Model verification was accomplished by the actual exposed water inrush points during the No. 3 coal seam mining work, which are all located in the high-risk area zoned by the RIWI method, and the water sources are all from the third section of the Upper Majiagou strong aquifer. The results show that the new evaluation model has high prediction accuracy, small error and wide applicability, and the model meets the basic requirements of mine production safety, provides favourable technical support for floor water inrush prediction during the Carboniferous–Permian lower coal group mining above the confined aquifer and is of great significance to ensure safe and efficient production of the coal mine.

Modeling regional-scale groundwater arsenic hazard in the transboundary Ganges River Delta, India and Bangladesh: Infusing physically-based model with machine learning

For the last few decades, toxic levels of arsenic (As) in groundwater from the aquifers of the Ganges River delta, India and Bangladesh, have been known to cause serious public health concerns. Innumerable studies have advocated the control of geomorphologic, geologic, hydrogeologic, biogeochemical, and anthropogenic factors on arsenic mobilization, flow, and distribution patterns within the Ganges River delta. We have developed transboundary regional-scale models for computing the probability of groundwater As concentrations to exceed the WHO permissible thresholds for drinking water of 10 μg/L within the Ganges River delta as a function of the various geomorphologic-(hydro)geologic-hydrostratigraphic-anthropogenic controlling factors, using statistical methods and artificial intelligence (AI) [i.e., machine learning] techniques namely, Random Forest (RF), Boosted Regression Trees (BRT) and Logistic Regression (LR) algorithms, followed by probabilistic delineation the high As-hazard zones within the delta. A “hybrid multi-modeling approach” was adapted for this study, which involved the introduction of hydrostratigraphic parameters (aquifer connectivity and surficial aquitard thickness) derived from a high-resolution transboundary hydrostratigraphic model developed for the Ganges River delta aquifer system, as predictors for modeling groundwater As probabilities within the delta.The RF model outperforms the BRT and LR model in terms of model performance. Model outputs suggest the dominant influence of surficial aquitard thickness and groundwater-fed irrigated area (%) on groundwater As. While, the north-central and southern regions of the Ganges River delta show low As-hazard (<10 μg/L), the western and north-eastern regions demonstrate elevated hazard level (>10 μg/L). An estimated 30.3 million people are found to be exposed to elevated groundwater As within the study area. Thus, our study demonstrates that such hybrid, predictive models are not only helpful in delineating the regional-scale distribution of groundwater As-hazard zones in the areas with limited As data but is also useful in identifying the possible exogenous forcing that may have led to the worst, natural pollution in human history.

Impacts of Aquitard Properties on an Overlying Unconsolidated Aquifer in a Mining Area of the Loess Plateau: Case Study of the Changcun Colliery, Shanxi

Quaternary unconsolidated porous aquifers are crucial to industrial, agricultural, and domestic water as well as ecological requirements in semiarid regions. Underground mining can influence aquifers even when they are located out of the fractured zone. Slight but continuous leakage should not be ignored. This study focused on the flow from a leaky aquifer. The deformation and fissure distribution characteristics and changes in the water resistance of a clay aquitard underlying an unconsolidated aquifer were studied by physical modeling, and the impacts on the unconsolidated aquifer were analyzed. Considering the mining-induced changes in the overburden hydraulic conductivity, we used numerical groundwater flow simulations of an unconsolidated aquifer to predict the flow dynamics of the aquifer for different scenarios. The influences of aquitard thickness and permeability on the groundwater flow regime of the regional unconsolidated aquifer under the mining area were quantitatively analyzed. This case study shows that the degree of coal extraction influence on the leaky confined aquifer depends on the properties of the aquitard beneath the aquifer. When the thickness was as high as 40 m and the hydraulic conductivity was as low as 10−6 cm/s, the aquitard could effectively prevent a water level depression. The results have practical implications for coal mining with water resource conservation.

Assessment and validation of confined aquifer vulnerability based on the VEBHAT method: a case study in Heilongjiang Province, northeastern China

Groundwater vulnerability assessment is an effective tool for groundwater pollution prevention and control. However, very few methods are available to address the vulnerability assessment of confined aquifers, though the risk of confined aquifer pollution has been increasing. This paper describes a method that integrates the vulnerability of the unconfined aquifer (V), extraction rate (E), aquitard thickness (Bt), head difference between unconfined and confined aquifers (H), aquitard media (At), and transmissivity of the confined aquifer (Tc), i.e., VEBHAT, to assess the vulnerability of confined aquifers. Hydraulic connections between the unconfined and confined aquifers, the vulnerability of unconfined aquifers, and the properties of confined aquifers were explicitly considered in this method. Moreover, the FO-TOPSIS method was used, which integrates fuzzy set theory (FST), along with an optimization model of multi-objective programming (OMP) and the classical TOPSIS (Technique for Order Preference by Similarity to Ideal Solution) method, to determine the weights of assessment factors, and to reduce subjectivity of the weight assignment. To illustrate the applicability and reliability of the VEBHAT and FO-TOPSIS methods, they were applied to assess the vulnerability of confined aquifers in Zhaoyuan County, Heilongjiang Province, northeastern China. The Pearson correlation coefficient between the ammonium concentration data and the vulnerability assessment values was used to verify the validity of the methods. The results showed that the VEBHAT method and FO-TOPSIS method could be used to effectively assess the vulnerability of confined aquifers.

Do CSIA data from aquifers inform on natural degradation of chlorinated ethenes in aquitards?

Back-diffusion of chlorinated ethenes (CEs) from low-permeability layers (LPLs) causes contaminant persistence long after the primary spill zones have disappeared. Naturally occurring degradation in LPLs lowers remediation time frames, but its assessment through sediment sampling is prohibitive in conventional remediation projects. Scenario simulations were performed with a reactive transport model (PHT3D in FloPy) accounting for isotope effects associated with degradation, sorption, and diffusion, to evaluate the potential of CSIA data from aquifers in assessing degradation in aquitards. The model simulated a trichloroethylene (TCE) DNAPL and its pollution plume within an aquifer-aquitard-aquifer system. Sequential reductive dechlorination to ethene and sorption were uniform in the aquitard and did not occur in the aquifer. After 10 years of loading the aquitard through diffusion from the plume, subsequent source removal triggered release of TCE by back-diffusion. In the upper aquifer, during the loading phase, δ13C-TCE was slightly enriched (up to 2‰) due to diffusion effects stimulated by degradation in the aquitard. In the upper aquifer, during the release phase, (i) source removal triggered a huge δ13C increase especially for higher CEs, (ii) moreover, downstream decreasing isotope ratios (caused by downgradient later onset of the release phase) with temporal increasing isotope ratios reflect aquitard degradation (as opposed to downstream increasing and temporally constant isotope ratios in reactive aquifers), and (iii) the carbon isotope mass balance (CIMB) enriched up to 4‰ as lower CEs (more depleted, less sorbing) have been transported deeper into the aquitard. Thus, enriched CIMB does not indicate oxidative transformation in this system. The CIMB enrichment enhanced with more sorption and lower aquitard thickness. Thin aquitards are quicker flushed from lower CEs leading to faster CIMB enrichment over time. CIMB enrichment is smaller or nearly absent when daughter products accumulate. Aquifer CSIA patterns indicative of aquitard degradation were similar in case of linear decreasing rate constants but contrasted with previous simulations assuming a thin bioactive zone. The Rayleigh equation systematically underestimates the extent of TCE degradation in aquifer samples especially during the loading phase and for conditions leading to long remediation time frames (low groundwater flow velocity, thicker aquitards, strong sorption in the aquitard). The Rayleigh equation provides a good and useful picture on aquitard degradation during the release phase throughout the sensitivity analysis. This modelling study provides a framework on how aquifer CSIA data can inform on the occurrence of aquitard degradation and its pitfalls.

Step-drawdown test as a tool for the assessment of the Nubia sandstone aquifer in East El-Oweinat Area, Egypt

The Cretaceous Nubia sandstone aquifer is the main aquifer in East El-Oweinat area along the Egyptian–Sudanese borders and in NE Africa. East El-Oweinat project has been recently considered one of the main agricultural projects in the southwest of the Egyptian Western Desert. So, the hydrogeological assessment for the Nubia aquifer in East El-Oweinat area has been done through studying and characterising the hydraulic parameters and step-drawdown test. The well and aquifer losses coefficients (C and B, respectively), the specific capacity, and the well efficiency as well as the aquifer and aquitard thickness have been estimated and traced through a total of 46 wells that have been distributed through the study area in two sectors, southern and northern. For the purpose of further exploration, the effective porosity (∅), hydraulic conductivity (K), and transmissivity (T) have been estimated for the wells in the northern sector. In addition, an aquifer assessment quality index has been introduced to assess and to rank the studied aquifer. Therefore, it is concluded that the hydraulic and the step-draw down parameters as well as the aquifer and aquitard thicknesses are prospective for further exploration to the north and the east of the northern sector in the study area. The well efficiency is mostly affected by the studied parameters, particularly the well loss coefficient, the hydraulic parameters and the well design as well. GIS approach and ArcGIS software have been applied to delineate the spatial distribution of the well performance characteristics, in order to compare between the studied groundwater wells and to integrate the different-acquired data. A set of isocontour maps have been introduced to match the lateral variation of the studied parameters and the aquifer potential to refer to the best direction for further underground water exploration.

Evaluation of water inrush from underlying aquifers by using a modified water-inrush coefficient model and water-inrush index model: a case study in Feicheng coalfield, China

Water inrush from underlying aquifers seriously threatens mining of Permo-Carboniferous coal seams in many North China coalfields. To evaluate the risk of water inrush from underlying aquifers, a modified water-inrush coefficient method—using the water inrush coefficient (T) and geological structure index (G)—and a water-inrush index model (WII model) were proposed. The T_G model improved the traditional water-inrush coefficient method by quantifying the degree of geological structure development, considering three main controlling factors: G, aquifer water pressure (P) and aquitard thickness (M). The WII model was constructed to assess the risk of floor water inrush by the entropy weight method, which integrated six factors: G, P, M, the depth at which ground pressure creates a broken-rock zone (Cp), aquifer water yield property (Y), and percentage of brittle rock within the aquitard (B). Results from the engineering practice data analysis validated the T_G and WII models as operational tools to evaluate the risk of water inrush from an underlying aquifer. The comparative analysis of the predictions by these two models shows that the prediction accuracy of the WII model is 13% higher than that of the T_G model, and approximately 21% of the two model predictions are not in agreement. A more reasonable prediction was obtained with application of the T_G and WII models to Feicheng coalfield in Shandong Province to evaluate water inrush risk from the underlying aquifer, and the prediction offers guidance on different preventive measures against water hazards in the underlying Ordovician limestone in the different zones.

Contaminant transport in a largely-deformed aquitard affected by delayed drainage

This paper employed a one-dimensional large-deformation model in consideration of the coupling of mechanical consolidation and solute transport, to study the transport of contaminants in a largely-deformed aquitard. An analytical solution has been derived to describe the drawdown variation in a largely-deformed aquitard which is subjected to abrupt hydraulic head decline in adjacent confined aquifers. The pore water flux and void ratio variation were obtained on the basis of the analytical solution. The equation for transient contaminant flux was solved by the finite difference method. A hypothetical case study was done to explore the effect of consolidation on the contaminant transport in a largely-deformed aquitard. The transit time of contaminant transport in the aquitard is mainly determined by the hydraulic conductivity, thickness, partitioning coefficient, void ratio and effective diffusion coefficients of aquitard, as well as the drawdown in the adjacent confined aquifer. The impact of delayed drainage on the contaminant transport in the largely-deformed aquitard is mainly controlled by two factors: the transient water flow and the decrease of aquitard thickness, in the process of aquitard consolidation. The former increases the breakthrough time of contaminant transport in the aquitard, and the latter gives rise to an opposite case with its effect decreasing with increasing contaminant partitioning coefficient for soil particles sorption. A larger deformation, which may be induced by a larger thickness, higher specific storativity of aquitard or a larger drawdown of the adjacent confined aquifer, and a lower hydraulic conductivity of aquitard cause a more significant impact of delayed drainage on the contaminant transport in an aquitard.

Quartz-Helium Method to Estimate Fluid Flow in Thick Aquitards, Gunnedah Basin, Australia.

The hydraulic integrity of aquitards is generally assumed and relies on a few core‐scale permeability measurements, drill‐stem tests, or textbook values. This approach is because hydraulic data across the full aquitard thickness is generally lacking. Proper assessment of aquitard integrity should be studied at the formation (spanning its entire thickness at a single point) or regional (formation properties at multiple locations throughout the basin) scale. One formation‐scale approach uses environmental tracers and advection‐dispersion modeling to constrain fluid flow rates. This study demonstrates the use of helium concentrations in quartz as a method of constraining the rate of fluid flow in a 520‐m thick aquitard in the Gunnedah Basin, NSW, Australia. Quartz was separated from existing core samples in the Watermark and Porcupine Formations at depths from 750 to 1200 m. The helium was released from these samples by heating and select samples were impregnated with helium to determine the rate of helium diffusion through the quartz. One‐dimensional advection‐dispersion modeling of the helium profile accounting for diffusive helium exchange between quartz and pore water revealed, that (1) vertical fluid velocity has been on the order of 0.02 mm/year or less for tens to thousands of years, (2) helium is in equilibrium between quartz and pore water, and (3) the helium profile is transient indicating that helium concentrations in the underlying Maules Creek Formation has varied over geological time. Further modeling identified aquitard conditions (thickness and temperature) for which equilibrium exists, a precondition for deriving formation‐scale permeability.

GIS integration of hydrogeological and geoelectrical data for groundwater potential modeling in the western part of greater Kushtia district of Bangladesh

Geographical Information System (GIS) has been used in this paper to delineate groundwater resources potential in the western part of greater Kushtia district of Bangladesh, where urgent attention for augmentation of irrigation water supply is required. Thematic maps of transmissivity, hydraulic conductivity, specific yield, net recharge, aquifer thickness, surface water bodies, aquifer resistivity, overburden aquitard thickness and its resistivity have been prepared and assigned weight according to their relative importance using Analytical Hierarchical Process for the preparation of groundwater potential model. Since the values within each thematic map vary significantly, they are classified into various ranges or types and assigned ratings. Finally, the thematic maps are integrated using GIS to prepare the groundwater potential map for the study area in terms of Ground Water Potential Index (GWPI). The evolved map indicates that 22.51% of the study area have GWPI more than 0.70 and therefore, have excellent prospective for exploitation. About 69.12% of the area with GWPI ranging from 0.50 to 0.70 is also quite promising for groundwater abstraction, while the rest 8.37% area having GWPI below 0.50 indicates moderate potential. The obtained map of groundwater potential is found in good agreement with the yields of available pumping test data.

A Simple Analytical Formula for the Leakage Flux Through a Perforated Aquitard.

In this methods note, we present a simple analytical formula to quantify the steady-state leakage flux (Q) over a perforated aquitard. The flux depends on the aquitard thickness (D), the radius of the perforation (R), the hydraulic conductivity of the material inside the perforation (kfill ), the conductivities of the overlying and underlying aquifers (k1 and k2 , respectively), and the head difference between the two aquifers (ΔH): [Formula: see text]. This equation assumes an aquitard separating two homogeneous and infinite aquifers (R ≪ aquifer thickness) in which radial flow to and from the perforation occurs, with no other recharge or discharge boundaries near the perforation. The flux through a perforation in a hypothetical case study with D = 10 m, k1 = 10 m/d, k2 = 20 m/d, R = 0.072 m, and ΔH = 1 m ranges between less than 1 mL/d if the hole is backfilled with bentonite (k(fill ) = 10(-4) m/d), to several liters per day if the perforation is backfilled with sand from the overlying aquifer (k(fill) = 10 m/d), to several m(3) /d if the perforation forms an open conduit (k(fill) = 10(5) m/d). The leakage fluxes calculated with this model agree well with those calculated using a numerical model (MODFLOW).

Numerical investigation of leakage in sloping aquifers

A sloping aquifer resting on impermeable bedrock is a common paradigm in hydrological modeling. The underlying assumption of this paradigm is examined in this study of leaky hillslope systems. Leakage is simulated with a three-dimensional finite element Richards equation model for a 100 m synthetic hillslope composed of an unconfined and a confined aquifer separated by an aquitard. The simulations examine different configurations of aquifer and aquitard properties (hydraulic conductivity, aquitard thickness), hillslope geometry (uniform, convergent, divergent), hillslope inclination (0.2, 5, and 30%), and boundary conditions (Dirichlet, seepage face), as well as the interplay between leakage, water levels, and outflow. The results show that leakage generally percolates in both directions, with downward (positive) leakage in upslope portions of the aquifer and upward (reverse or negative) leakage in downslope regions. Geometry is found to be a main determinant of the partitioning of leakage along a hillslope, with for instance upward leakage in large portions of convergent slopes but only in a restricted downslope region for divergent slopes. In steep hillslopes, the reverse leakage that occurs downslope as a result of quick upslope drying represents a major component of the water budget. Outflow boundary conditions also exert a major control on the volume and direction of leakage, with the placement and extent of Dirichlet or seepage face nodes along the outflow face being particularly important factors. A dimensional analysis is used to synthesize the main findings and to highlight the differences in response between leaky and non-leaky hillslope conceptualizations. Leakage is also examined for a larger scale aquifer system, in a preliminary assessment of the importance of this exchange process for river basin models that are based on extensions of simple hillslope conceptualizations.