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.

An extensive water level survey of the water-table aquifer (i.e., shallow aquifer) within Shelby County, Tennessee, was conducted in the dry (fall 2020) and wet (spring 2021) seasons. Water-table surfaces were generated using cokriging to observe seasonal differences to identify anomalous water-table depressions, indicative of an underlying aquitard breach. Seasonal differences were attributed to non-coincident control and timing between the surveys and when optimum dry (fall) and wet (spring) conditions existed, as observed through comparisons with continuous historical water levels from 12 shallow monitoring wells. Additionally, data from fall 2020 were compared to previous studies in 2005 and 2015 to determine decadal changes in levels and shape of the water-table surface which were mostly attributed to changes in data control and potential climate variations. A prediction error map was generated from the 2020 dataset to identify areas of the county with high-prediction error (>7.0 m) to offer guidance on where future well control would be optimal.

Low-permeability aquitards may serve as secondary sources of slow-releasing contaminants into the adjacent aquifer system, creating considerable obstacles to groundwater cleanup. Accurately capturing the exchange of contaminant mass between aquitards and aquifers can facilitate site management and remediation. Previous simulation studies were mainly limited to one-dimensional (1D) back diffusion from aquitards during the remediation of the source zone. In this study, a novel two-dimensional (2D) back-diffusion model is developed to investigate the storage and release of contaminants in aquitards after source isolation. This model coupled the dynamical decay of isolated sources and the diffusion-sorption process of contaminants in the layered aquitards. Exact analytical solutions for the present 2D multilayer model were derived using the finite cosine transform, Duhamel Theorem, separation of variables, and transfer matrix method. Results indicated that the previous 1D model would overestimate the contaminant concentration in the aquitard and the back-diffusion risk when the source zone was isolated. The proposed 2D back-diffusion model enables quantitative prediction of how source zone width, source concentration, and aquitard heterogeneity impact plume trailing time, thus aiding in understanding the mechanisms of aquifer contamination beyond barrier-controlled source zones.

Recalcitrant groundwater contamination is a common problem at hazardous waste sites worldwide. Groundwater contamination persists despite decades of remediation efforts at many sites because contaminants sorbed or dissolved within low-conductivity zones can back diffuse into high-conductivity zones, and therefore act as a continuing source of contamination to flowing groundwater. A review of the available literature on remediation of plume persistence due to back diffusion was conducted, and four sites were selected as case studies. Remediation at the sites included pump and treat, enhanced bioremediation, and thermal treatment. Our review highlights that a relatively small number of sites have been studied in sufficient detail to fully evaluate remediation of back diffusion; however, three general conclusions can be made based on the review. First, it is difficult to assess the significance of back diffusion without sufficient data to distinguish between multiple factors contributing to contaminant rebound and plume persistence. Second, high-resolution vertical samples are decidedly valuable for back diffusion assessment but are generally lacking in post-treatment assessments. Third, complete contaminant mass removal from back diffusion sources may not always be possible. Partial contaminant mass removal may nonetheless have potential benefits, similar to partial mass removal from primary DNAPL source zones.

The clayey aquitard has the potential to release geogenic poisonous chemicals such as arsenic (As) to the adjacent aquifer owing to complex hydrologic or biogeochemical processes. However, it remains unclear whether the aquitard has effect on As enrichment in the underlying aquifer in regions without extensive groundwater pumping, and the related processes have been poorly known. Based on piezometer water chemistry, stable water isotopes, sediment chemistry and reactive-transport model, this study aims to reveal the impact process of the aquitard to As accumulation of underlying aquifer from central Yangtze River Basin, a As-affected area without extensive groundwater pumping. On the whole, As migrated from top to bottom of the aquitard (especially the depth over 10 m) and significantly influenced the As accumulation in the underlying aquifer. Nonetheless, the results of three topical boreholes showed two different hydrogeological conditions affected As release in the aquitard and enrichment in the underlying aquifer. Different hydrogeological conditions could result in the input of different species organic carbon and then impact As concentrations in the aquifer. When the aquitard was near surface water bodies, the reductive dissolution of iron oxides was the main driver for As release and the aquitard had a significant influence on the enrichment of arsenic in the aquifer. At areas without surface water bodies nearby, the desorption of As(V) from minerals was the main source of As and the concentrations of As in pore water were quite low; this pattern had little effect on the enrichment of arsenic in the aquifer.

Nitrogen pollution of groundwater has created problems worldwide. Riparian zones form a connection hub for terrestrial and aquatic ecosystems. As a potential source of ammonium in groundwater, aquitards have an important effect on the environment of riparian zones. The spatial distribution and factors influencing the ammonium content in the riparian zone aquitard of a small watershed were analyzed through three geological boreholes with increasing distances from the river: boreholes A > B > C. The results show that the distribution of ammonium was closely related to the lithology of sediments. Under the influence of the river and floods, the average content of ion exchange form of ammonium of sediments in borehole A (stable sedimentary environment) was 94.31 mg kg-1, accounting for 21.2% of the transferable ammonium. The average proportions of ion exchange form of ammonium in the transferable ammonium of boreholes B and C (unstable sedimentary environment) were 19.1% and 17.4%, respectively. The carbonate and iron-manganese oxide forms of ammonium content of sediments in three boreholes were 0.96-15.28 mg kg-1 and 2.3-54.4 mg kg-1, respectively; this was mainly affected by the pH and Eh of the sedimentary environment. Organic sulfide, the form of transferable ammonium of sediments mainly exists in organic matter. The ammonium content in pore water generally increased with depth and was mainly derived from the mineralization of humic-like organic matter in borehole A. The ammonium in pore water in boreholes B and C mixed with ammonium from the mineralization of organic matter and the desorption of ion exchange form ammonium within sediments. The ammonium content in the pore water (up to 5.34 mg L-1) was much higher than the limit for drinking water of 0.5 mg L-1 in China. Therefore, the aquitard has a high risk of releasing ammonium and poses a certain threat to the quality of groundwater.

The contribution of aquitards to aquifer water quality can be pronounced but is rarely considered. The aims of this study were to delineate the spatial distribution of iron in a shallow aquitard-aquifer system within Jianghan Plain (JHP) of central China and to identify the origin of high iron within aquifers. Infiltration, hydraulic gradients and sediment chemistry influence the distribution of iron in the aquitard pore water which has a significant effect on the underlying aquifer. Chemical equilibrium modeling of pore water was used to simulate chemical processes influencing aquifer chemistry and determined the possible precipitation of FeCO3, FeS minerals (FeSx) and Fe-oxides (representing hydroxides, oxyhydroxides, and oxides of ferric iron). We presented a conceptual chemical-physical scenario to explain the observed Fe distributions: (1) Increasing iron concentrations with low-level sulfide in aquitard pore water. (2) Increasing iron concentrations with low-level sulfide in aquitard pore water underlying ponded water. (3) Decreasing iron concentrations with high-level sulfide in aquitard pore water. In combination, our findings illustrate the influence of aquitards on aquifer chemistry using Fe within the Jianghan Plain as an example.

As an important part of groundwater systems, aquitards may have a considerable impact on the quality of groundwater in an aquifer. Organic carbon (OC) serves as an important component in biogeochemical processes which affects the hydrochemical composition of pore water; the association of OC with (Iron) Fe-containing minerals is recognized as an important stabilization mechanism for OC. However, the characteristics of the process forming complexes between OC and Fe oxides in subsurface aquitards and the contribution of OC to aquifers has been poorly understood. In this study, the content, speciation and reaction types of OC and Fe oxides were investigated to reveal the characteristics and mechanisms of OC-Fe interaction in subsurface aquitards at different depths in two typical sedimentary facies of the Jianghan Plain in central China. The total organic carbon (TOC) contents of aquitard sediments in alluvial-lacustrine and residual slope facies were 7.25 ± 5.10 and 2.79 ± 0.86 mg g-1, respectively. In general, the proportion of the heavy fraction of OC (HFOC) to TOC gradually increased with increasing depth because consumption of the light fraction of OC (LFOC) that caused more HFOC to remain in sediment. On average, Fe bound-OC contributed 30.3% and 31.6% of TOC by adsorption and coprecipitation, respectively. The adsorption-stabilized OC has not changed obviously but the coprecipitation-stabilized OC increased gradually with depth. Coprecipitation stabilized more OC in a stable environment (residual slope facies) when compared with an unstable environment (alluvial-lacustrine facies), which can be supported by the greater average ratio of FePP bound-OC:TOC ratio and increased enrichment of carboxylates and aromatics in a stable environment. The transformation of OC-Fe complexes could affect the transport of As, Cr and ammonium which chemically bind to organic matter (OM) and Fe minerals from sediment to pore water by reductive dissolution. The findings of this study are helpful in understanding the interaction between OC and Fe oxides in subsurface aquitard environments, in which reaction products may affect adjacent aquifers.

Two important factors that affect groundwater contaminant persistence are the temporal pattern of contaminant source depletion and solute diffusion into and out of aquitards. This study provides a framework to evaluate the relative importance of these effects on contaminant persistence, with emphasis on the importance of thin aquitards. We developed one-dimensional (1D) analytical solutions for forward and back diffusion in a finite domain with a no flux boundary using the method of images and demonstrated their applicability to measured data from three well-controlled laboratory diffusion experiments with exponentially depleting sources. We used both in situ aquitard solute concentrations and aquifer breakthrough curves for sorbing and non-sorbing solutes. The finite-domain no flux boundary solutions showed better agreement with measured data than was available with semi-infinite approaches, with increasing discrepancy for dimensionless relative diffusion length scale beyond a critical threshold value (Zd > 0.7). We also used a mass balance to demonstrate that the temporal pattern of contaminant source depletion controls the duration of solute mass accumulation in the aquitard, as well as the total solute mass release back into the aquifer. Lower rates of source depletion result in a longer period of mass accumulation in the aquitard and later back diffusion initiation time. The amount of solute mass stored in the aquitard increases with longer loading duration, thereby contributing to overall longer contaminant persistence in aquifers. This study entails widespread implications for anthropogenic waste and contamination sites, which are all dependent on efficient and cost-effective contaminant management strategies.

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.