Experimental and modeling insights into mixing-limited reactive transport in heterogeneous porous media: Role of stagnant zones.

The understanding of mixing-controlled reactive dynamics in heterogeneous porous media remains limited, presenting significant challenges for modeling subsurface contaminant transport processes and for designing cost-effective environmental remedial efforts. The complexity of accurately observing, measuring, and modeling mixing-limited reactive transport has led to inadequate exploration of these critical processes. This study investigates the mixing and reaction kinetics affected by stagnant zones, which are commonly found in alluvial aquifers-aquitards and fracture-matrix systems. By conducting experiments involving conservative and bimolecular reactive transport through porous media within translucent chambers filled with two sizes of glass beads and under varying flow rates, we explored the effects of grain size and hydrodynamic conditions. Using a high-resolution camera, we monitored the concentration changes of conservative and reactive tracers, with subsequent interpretation through three-dimensional numerical simulations. The outcomes revealed the emergence of distinct mixing interfaces within both mobile and stagnant zones, culminating in a bi-peaked plume formation. Notably, the mixing and reaction times in media containing stagnant zones were found to be approximately 10 times longer than in homogeneous media. These findings, through experimental and modeling efforts, advance our understanding of mixing-limited reactive transport phenomena within heterogeneous media, underscoring the significant role of stagnant zones-a topic previously underexplored.

Limits, challenges, and opportunities of sampling groundwater wells with plastic casings for microplastic investigations.

Investigating microplastics (MPs) in groundwater suffers from problems already faced by surface water research, such as the absence of common protocols for sampling and analysis. While the use of plastic instruments during the collection, processing, and analysis of water samples is usually avoided in order to minimize unintentional contamination, groundwater research encompassing MPs faces unique challenges. Groundwater sampling typically relies on pre-existing monitoring wells (MWs) and water wells (WWs) that are often constructed with polyvinyl chloride (PVC) casings or pipes due to their favorable price-performance ratio. Despite the convenience, however, the suitability of PVC casings for MP research is questionable. Unfortunately, the specifics of these wells are often not detailed in published studies. Current literature does not indicate significant pollution risks from PVC casings, suggesting these wells might still be viable for MP studies. Our preliminary analysis of the existing literature indicates that if PVC exceeds 6 % of the total MP concentration, it is likely that casings and pipes made of PVC are a source of pollution. Above this threshold, additional investigations in MWs and WWs with PVC casings and pipes are suggested.

The changing nature of groundwater in the global water cycle.

In recent decades, climate change and other anthropogenic activities have substantially affected groundwater systems worldwide. These impacts include changes in groundwater recharge, discharge, flow, storage, and distribution. Climate-induced shifts are evident in altered recharge rates, greater groundwater contribution to streamflow in glacierized catchments, and enhanced groundwater flow in permafrost areas. Direct anthropogenic changes include groundwater withdrawal and injection, regional flow regime modification, water table and storage alterations, and redistribution of embedded groundwater in foods globally. Notably, groundwater extraction contributes to sea level rise, increasing the risk of groundwater inundation in coastal areas. The role of groundwater in the global water cycle is becoming more dynamic and complex. Quantifying these changes is essential to ensure sustainable supply of fresh groundwater resources for people and ecosystems.

Microplastics contamination of groundwater: Current evidence and future perspectives. A review.

Groundwater is a primary water source which supplies more than 2 billion people. The increasing population and urbanization of rural areas stresses and depletes the groundwater systems, reducing the groundwater quality. Among the emerging contaminants, microplastics (MPs) are becoming an important issue due to their persistency in the environment. Seepage through the pores and fractures as well as the interaction with colloidal aggregates can partially affect the MPs dynamics in the subsoil, making the detection of the MPs in the groundwater systems challenging. Based on literature, a critical analysis of MPs in groundwater is presented from a hydrogeological point of view. In addition, a review of the MPs data potentially affecting the groundwater systems are included. MPs in groundwater may have several sources, including the atmosphere, the interaction with surface water bodies, urban infrastructures, or agricultural soils. The characterization of both the groundwater dynamics and the heterogeneity of MPs is suggested, proposing a new framework named "Hydrogeoplastic Model". MPs detection methods aimed at characterizing the smaller fragments are necessary to clarify the fate of these contaminants in the aquifers. This review also aims to support future research on MP contamination in groundwater, pointing out the current knowledge and the future risks which could affect groundwater resources worldwide.

Groundwater Complexity in Urban Catchments: Shenzhen, Southern China.

Groundwater interactions with surface water and sewers in an urban setting are complex, and classic hydrogeological approaches must be combined with anthropogenic elements to characterize them. The level of detail needed to understand these interactions is illustrated by the analysis of an urban subcatchment in the megacity of Shenzhen in southern China that has had a drastic urban expansion in the last 40 years. The study area is characterized by the Yanshanian granite that is widespread across southern-eastern China. The urban setting is studied using multitemporal analysis of satellite images, borehole investigations and field surveys. Given the local hydrostratigraphy, a conceptual model was developed to identify the physical and anthropogenic factors that regulate the urban groundwater system. Based on the conceptual model and the data collected from the field or compiled from the literature, the average annual effective recharge is estimated to be 290 mm/year, after the urbanization process. From rural to urban conditions, it is estimated that the effective recharge increased by 170% and sewers intercept at least 23% of the effective recharge. Groundwater captured by sewers reduces river flows and increases the required capacity and costs for waste water treatment plants.