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Understanding how groundwater level changes affect the permeability of bedrock aquifer‐aquitard systems is important for groundwater management, yet this relationship remains poorly understood. This study focuses on Tangshan in the northeastern North China Plain, utilizing tidal response analysis to investigate the dynamic interplay between groundwater level trends and permeability variations in bedrock aquifer‐aquitard systems. High‐frequency groundwater level data from two monitoring wells were employed to reveal a significant positive correlation: rising groundwater head leads to increased permeability of the bedrock aquifer‐aquitard system, primarily due to adjustments in groundwater head. This research provides direct evidence that both climate variability and human activities can influence bedrock aquifer‐aquitard permeability through changes in the groundwater head. The findings highlight the importance of integrating models of dynamic permeability induced by hydrological processes into groundwater resource management frameworks and hazard assessments, particularly in regions experiencing groundwater level recovery, such as the North China Plain.

Fractured aquifers are heterogeneous due to the variable frequency, orientation, and intersections of rock discontinuities. A ~100-m-thick Silurian dolostone sequence provides a bedrock aquifer supplying the city of Guelph, Canada. Here, fracture network characteristics and associated influences on hydraulic head were examined using several data types obtained from 24 cored holes in a study that is novel for the quantity and quality of data. High (50–90°) angle joint orientations, heights, and terminations relative to bedding features were determined from acoustic televiewer logs and outcrop scanlines. These data were compared to high-resolution hydraulic head profiles showing head loss over depth-discrete intervals identifying zones with lower vertical hydraulic conductivity. This study reveals that the marl-rich Vinemount Member, traditionally considered the principal aquitard, corresponds to head loss in only 62% of the 24 boreholes. The vertical position of head loss varies across the 90-km2 study area and occurs in any of the lithostratigraphic units of the Lockport Group. Within this sedimentary sequence, aquitards are laterally discontinuous or “patchy” at variable depths and relate to: (1) the frequency of the high-angle joints; (2) shorter joint height; and (3) the type of joint terminations. The head loss occurs in thin (2–2.5 m) intervals where the frequency of the high-angle joints is low. Where a large proportion of small joints cross-cut marl bedding planes, head loss is negligible, suggesting that the vertical hydraulic conductivity is not reduced. Overall, these findings are potentially applicable to assessing aquitard and cap rock integrity in carbonate sedimentary sequences worldwide.

Coastal groundwater dynamics and solute transport were influenced by multiple factors including aquitards, tides, evaporation, and slope breaks in coastal aquifers. However, quantification of the impacts of these factors on groundwater flow and salinity distribution remains a challenge. In this study, both field observations and numerical modeling were applied to investigate hydraulic heads and groundwater salinity in a tidal flat with large‐scale seepage faces at Laizhou Bay, China. Results showed that seepage‐face evaporation increased groundwater salinity landward and promoted groundwater and salt exchange within the intertidal zone significantly in comparison to the case without evaporation. Seawater infiltrated the aquifer on the left of the slope break and discharged on the right, forming a groundwater circulation cell, which notably influenced leakage flow between unconfined and confined aquifers through the aquitard. The aquitard prevented approximately 85% of inland freshwater discharge near the slope break, resulting in the formation of two atypical freshwater discharge tubes in the upper and middle intertidal zones. Two additional groundwater circulation cells developed in the lower intertidal zone due to the spring‐neap tidal cycle. The outflow and inflow fluxes over a spring‐neap tidal cycle were numerically estimated to be 1.46 and 1.27 m2/d, respectively, with evaporation accounting for 45% of the outflow flux. These findings provide significant insights for further investigations on groundwater dynamics and solute transport in multi‐layered coastal aquifers, and have strong implications for biogeochemical processes within the intertidal zone. Plain Language Summary The coastal aquifer serves as a crucial connection between terrestrial and marine systems, with groundwater flow and salt transport in coastal regions influenced by factors such as topographic variations (e.g., slope break), tides, aquitards (low‐permeability layers among permeable layers), and evaporation. Quantification of these complex processes is a challenge. Here, we combined field observations and numerical simulations to quantify the effects of slope break, tides, aquitard, and evaporation on groundwater flow paths and salinity distribution beneath a tidal flat. It was found that evaporation may significantly increase groundwater salinity landward, and promoted the mass exchange between groundwater and seawater on the tidal flat surface. The combined effects of slope break, spring‐neap tidal cycle, and aquitard notably altered the pathways of groundwater flow and solute transport in coastal aquifers. These may profoundly influence the biogeochemical conditions in multi‐layered coastal aquifers, with important implications for coastal management and environmental protection. Key Points Seepage‐face evaporation significantly increases groundwater salinity and promotes groundwater/salt exchange within the intertidal zone Two new freshwater discharge tubes and three groundwater circulation cells develop due to the aquitard, spring‐neap tidal cycle, slope break Significant exchanges of groundwater and solutes occur between unconfined and confined aquifers via leakage under the slope break

Aquitards are an important part of multi-layer aquifer systems. The hydraulic properties of each aquitard are significant to the management of the groundwater resources in an aquifer system. Specific storage is one of the fundamental hydraulic properties, which indicates the aquitard’s capacity to release water from storage, or take water into storage, when the hydraulic head changes. However, there is a limited number of studies that systematically summarize the methods used to estimate aquitard specific storage and present the representative specific storage values of different types of aquitard materials. This review synthesizes the existing methods published in the literature used to estimate the specific storage of aquitards. Specific storage data from 99 aquitards were collected, covering 11 types of aquitard material, including both unconsolidated deposits and rocks. The specific storage values of unconsolidated deposits mostly lie in the range 10–7–10–2 m–1, while those of rocks lie in the range 10–8–10–4 m–1. The corresponding range of hydraulic conductivity values is larger than that of specific storage. A positive correlation was found between specific storage and porosity. A positive relationship also exists between specific storage and hydraulic conductivity. The relationship between specific storage and aquitard thickness is not obvious. Specific storage of aquitards is found to decrease with depth. Different methods to estimate specific storage are compared and future research directions are recommended. The findings of this study will help achieve a better understanding of the specific storage of different aquitard materials.

The behavior of iron (Fe) in clayey aquitards has a significant effect on the groundwater environment. However, the release processes and impact of Fe within clayey sediments during compaction remain unknown. Two groups of simulation experiments were carried out to demonstrate the migration and transformation mechanisms of Fe during clayey sediment compaction. Experiment A, which simulated a natural deposition condition, revealed that pressurization changed the reaction environment from oxidative to reductive by isolating oxygen. Oxidation of ferrous ions was followed by reduction dissolution of poorly crystalline Fe (III) and crystalline Fe (III) oxides. Under the microbial utilization of organic matter, the main transformation process of sediment Fe was the dissimilatory reduction of poorly crystalline Fe (III) oxides. The total Fe concentration in pore water was 0.09–11.61 mg/L, with ferrous ions predominating among the Fe species. The lower moisture content (<~36%) in the later stage of compaction inhibited the dissimilatory reduction of Fe (III), and the formation of Fe (II) minerals resulted in a decrease in Fe concentration. Experiment B, which simulated an artificial compaction state, revealed that the sediment Fe was primarily released by physical dissolution because of changes in pore structure and solubility. The concentration of total Fe in pore water was 0.02–1.96 mg/L, with a significant increase in response to a rapid increase in pressure. According to the estimates in the Chen Lake wetland (eastern China), the contribution of clay pore water release accounted for 19.9–31.9% of the average Fe concentration in groundwater during natural deposition.

Research on the migration behaviors of contaminants in the aquitard has been deficient for an extended period. Clay is commonly employed as an impermeable layer or barrier to stop the migration of contaminants. However, under certain conditions, the clay layer may exhibit permeability to water, thereby allowing contaminants to infiltrate and potentially contaminate adjacent aquifers. Consequently, it holds immense importance to scrutinize and investigate the migration characteristics of light non-aqueous phase liquid (LNAPL) within the aquitard for the purposes of groundwater pollution control and remediation. To evaluate the environmental risk posed by organic contaminants in the aquitard, an experimental model was formulated and devised to monitor the LNAPL concentration in the aquitard under pumping conditions. The correlation between pumping rate and LNAPL concentration was investigated. A self-developed plexiglass sandbox model was used to simulate the migration characteristics of LNAPL in the aquitard under pumping conditions. Four experimental scenarios were designed, varying pumping rates, aquitard thicknesses, and groundwater level changes. The LNAPL concentration curve was derived by systematically tracking and analyzing LNAPL levels at various locations within the aquitard. The results indicated that higher pumping rates corresponded to increased migration of LNAPL, resulting in greater LNAPL ingress into the pumping well during extraction. A thicker aquitard demonstrated a more pronounced inhibitory effect on LNAPL, leading to an extended penetration time of LNAPL within the aquitard. The drawdown within the aquitard exerted a discernible influence on LNAPL migration, with the LNAPL concentration continuing to decrease in tandem with declining water levels during pumping. These research findings can establish a scientific foundation for the control and remediation of contaminants within aquitards.

Seasonal climate variations (SCV) have a significant impact on the exchange of surface and groundwater in the reservoir area. As a crucial climatic element, temperature also has an important effect on the hydrological cycle. In the plain reservoir area, the hysteresis response of groundwater in the aquitard to reservoir level (RL) fluctuations is ever-present. To study the impacts of SCV on the hysteresis response of groundwater in the aquitard, the response of groundwater level (GL) of Jiangxiang Reservoir to the variation of the RL was examined. Considering the seasonal temperature variations (STV), the hydraulic diffusivity of the aquitard was calculated. Relying on the STV, the formula for calculating the variation of the GL with the RL was derived. The results showed that the hydraulic diffusivity is sensitive to temperature changes. The higher the temperature, the greater the hydraulic diffusivity, which results in a faster response of groundwater to changes in reservoir level. Additionally, the lag of groundwater level change in the aquitard became smaller. Considering STV, the calculated immersion scope with variable parameters was smaller than that with constant parameters.

Many hydrogeological processes are intricately linked to both the spatial heterogeneity and temporal dynamics of the involved parameters. The variability in hydrogeological parameters across both space and time domains is defined as the spatiotemporal heterogeneity. This study introduces a partitioning geostatistical inversion approach to characterize the spatiotemporal heterogeneity, and further applies this methodology to the long‐term multi‐extensometer data from the second aquitard beneath Qingliang Primary School, Changzhou City, China. The Principal Component Geostatistical Approach (PCGA) is used to capture the temporal variability in the hydraulic conductivity (K) and specific storage (Ss) across the three sub‐layers of the aquitard of interest. Comparative analysis reveals that models accounting for spatiotemporal heterogeneity achieve significantly higher accuracy than those considering only spatial heterogeneity. PCGA effectively captures the temporal variations in aquitard K and Ss, with sub‐layer 3 dominating the temporal dynamics. Additionally, neglecting the initial delayed drainage within the aquitard leads to a notable overestimation of K. Moreover, utilizing a stage‐wise distribution of parameters as an initial guess significantly enhances inversion accuracy. The proposed methodology not only deepens the understanding of aquitard deformation but also holds broad potential for advancing the characterization of spatiotemporal heterogeneity in 4D hydrogeological modeling.

The distribution of saline water in the upper aquifer and freshwater in the lower aquifer is a characteristic of groundwater resources in the North China Plain (NCP). The phenomenon of groundwater depression cones in confined aquifers, primarily caused by excessive groundwater extraction, has been extensively documented. In line with Darcy’s law, it is noteworthy that the migration of shallow groundwater into confined aquifers can occur due to a substantial difference in hydraulic head between the unconfined and confined aquifer systems. However, based on the monitoring data, the quality of deep groundwater generally remains good. This paper attempts to explain this phenomenon from the perspective of non-Darcian flow in aquitards. A finite difference method is used to solve low-velocity non-Darcian flow to a well in the NCP. The mathematical model considers the threshold pressure gradient to describe non-Darcian flow in the aquitard and assumes Darcian and horizontal flows for both shallow and confined aquifers. The comparison with traditional Darcian flow indicates that the leaky area decreases rapidly when considering the threshold pressure gradient. The leaky area is negatively correlated with the aquitard thickness and the transmissivity of the confined aquifer, and positively correlated with the pumping rate. The non-Darcian vertical flow velocity is significantly lower than that obtained from Darcian theory. The vertical velocity difference between Darcian and non-Darcian flows is significant under the situation of a small aquitard thickness, large pumping rate, low transmissivity and large leakage coefficient when the threshold pressure gradient is large.

Seven high‐resolution (0.3–0.6 m depth intervals), 1‐D vertical profiles of the δ²H of pore water were collected across a 300 km2 study area in southern Saskatchewan, Canada, to define the vertical controls on solute transport in a >120 m thick, two‐layered aquitard system. The 1‐D profiles were augmented with an existing δ²H profile collected from a previous study. The surficial aquitard in the area consists of Quaternary deposits (either glacial till or lacustrine deposits; 13 to 128 m thick) underlain by an upper Cretaceous claystone aquitard (80–110 m thick). The shape of the individual δ²H profiles and associated 1‐D transport modeling suggest diffusion is the regionally dominant vertical transport mechanism across the aquitards. The profile shape is controlled by the thickness of the Quaternary deposit and the δ²H value at the upper boundary, which coincides with the depth of the water table. The upper boundary δ²H value varies considerably across the area (−149‰ to −101‰), perhaps due to differences in local hydrological conditions (e.g., slope, aspect, infiltration) across the landscape. Modeling of all profiles shows the timing for till deposition and the timing of climate change during the Holocene are consistent across the area (∼30 ka and 7–10 ka before the present, respectively), corroborating other studies. This study provides insights into the hydrogeologic controls on solute transport in an aquitard system and associated geologic and climatic changes for a prairie region over the past 30 ka, and improves our understanding of initial and time‐dependent transport boundary conditions for the study of aquitards. Key Points Results corroborate timing for geologic events Diffusion is the regionally‐dominant vertical transport mechanism The profile shape is controlled by the thickness of the Quaternary deposit