Groundwater最新文献

A sophisticated understanding of the three-dimensional distribution of silt- and clay-rich bodies of strata (elements) in aquifers is critical given that they not only have the potential to act as aquitards or semi-confining units and vertically partition groundwater flow into separate aquifer zones, but also provide lateral barriers to groundwater flow, impacting contaminant distribution and groundwater flow dynamics. Additionally, when in prolonged contact with dense nonaqueous phase liquid (DNAPL) or contaminated groundwater, fine-grained elements may become storage zones for contaminant mass via matrix diffusion and thus serve as long-term secondary sources of contamination to groundwater that can confound remediation strategies and render remedy performance projections unreliable. The stratigraphic architecture of aquifer systems, including fine-grained facies architecture, is complex but is not random and can be effectively predicted through application of facies models. This paper reviews depositional models (“facies models”) for common depositional environments with a focus on alluvial end-members of braided fluvial, meandering fluvial, and alluvial fan facies models. We examine the facies models from the perspective of aquitards and present case studies to provide an overview of the expected aquitard dimensions and characteristics. The critical yet underappreciated role of the paleosol as a potential aquitard is also examined, and basic criteria for differentiating ancient floodplain clay units with high lateral continuity from other laterally discontinuous clay units are provided.

Leaching of marine salinity in the porewater of glaciomarine muds is one precursor to landslide hazard. In this study, groundwater modeling is used to quantify vertical groundwater flow, constrain paleosalinity, and characterize past and future progression of leaching with depth in Champlain Sea sediments. The Breckenridge Creek site, ~15 km northwest of Ottawa, Canada, was cored within a thick sequence (up to 98 m) of Champlain Sea muds that form a regional aquitard in the St. Lawrence Lowlands and Ottawa Valley. Porewater chloride concentrations ([Cl]), up to 12,250 mg/kg, and δ¹⁸O as high as −7.18‰, indicate remnant seawater. One-dimensional groundwater transport modeling simulates porewater [Cl] and δ¹⁸O with depth simultaneously and constrains specific discharge, q, from 2.40 to 2.51 mm/a. Groundwater transport modeling and three-component mixing of seawater, glacial meltwater and meteoric water constrain the range of initial [Cl] between 14,000 and 15,700 mg/kg (72–80% seawater) and initial δ¹⁸O between −5.99 and −5.61‰. The glacial meltwater component of Champlain Sea bottom waters at the Breckenridge site has a maximum δ¹⁸O value of −22.4‰. Downward leaching to the salinity threshold of <2 g/L for geotechnical sensitivity development reached a depth of 20.6 m. Modeling indicates the leaching front currently progresses at a rate of 2.5 m/1000 years, slower than advection of freshwater infiltration because of upward diffusion and dispersion of marine solutes. Notably for landslide hazard, the highest measurements of geotechnical sensitivity coincide with the leached zone.

The compaction simulation of compressible delay interbed is an important part of land subsidence simulation. Currently, the most widely used MODFLOW software has two modules, SUB and CSUB, both of which can simulate compressible delay interbed. The difference lies in that the head diffusion equation of the SUB module is based on the principle of head change, while CSUB can use either head change or geological stress variation principles. When based on the principle of geostress variation, the CSUB method is more physically reasonable. However, its limitation lies in the fact that, when solving the diffusion equation for compressible delay interbeds, it does not account for the effects of variations in the discrete nodal cell thickness and hydraulic conductivity of the interbed. This study improves the solution method for the head diffusion equation of compressible delay interbeds based on the principle of geostress variation. The Kozeny–Carman equation was introduced to establish a relationship between the hydraulic conductivity and porosity of the interbeds, while variations in the thickness of discrete nodal cells were also incorporated into the solution process. Collectively, these improvements lead to a more rigorous approach. To verify the effectiveness of the proposed simulation method, three representative test cases were developed and comprehensively compared with the CSUB results. The results indicate that notable discrepancies emerge between the two approaches when the interbed undergoes substantial compression, whereas the method proposed in this study effectively prevents the occurrence of “overcompaction” within the interbed.

Aquitards play critical roles in a variety of hydrogeologic processes. Despite their importance, coverage of aquitards in introductory hydrogeology textbooks is generally limited. This paper provides examples of classroom and field activities designed with an aquitard focus for instructors wishing to supplement textbook content. These activities emphasize high-resolution head profiles. Examining head profiles prompts students to think about how aquitards influence head with depth and conversely how these plots can be used to delineate and characterize aquitards. During a classroom activity, students explore the connection between changes in vertical gradient and changes in hydraulic conductivity by sketching conceptual head profiles based on given boundary conditions and several aquifer/aquitard scenarios. In a companion field exercise, students measure high-resolution head profiles using CMT multilevel systems at an outdoor learning laboratory. Students compare the high-resolution head profiles to lower resolution profiles they obtain from clusters of conventional wells. The field exercise provides students with a tactile experience that can help build intuition for vertical head changes, practice interpreting aquitards from head profiles, and an example of how lower resolution head profiles may create uncertainty in aquitard delineation and vertical gradient estimates. A paleosol at the site forms a prominent aquitard providing a unique basis for discussions about the geology of aquitards and characteristics influencing aquitard integrity. Regardless of the approach used, incorporating more aquitard content into hydrogeology courses at all levels will be beneficial for future hydrogeologists tackling a range of issues from sustainable water supplies to waste disposal.

Many public supply wells (PSWs) fail because their water chemistry does not meet regulatory standards, despite pilot hole water quality testing suggesting compliance. This is partly attributed to conventional testing that focuses on the mid-section of permeable zones, excluding low-permeability units or aquitards. The main goal is to prove yield, so groundwater is sampled within the tested interval for efficiency. Clay boundaries are typically excluded from zone testing because of low expected yields, but they may harbor elevated concentrations of constituents of concern. Well discharge concentrations may thus be non-compliant due to the blend of groundwater from permeable, high-yield zones and less permeable, elevated concentration zones. We evaluated flow and chemistry across the screens of 143 wells in California and Nevada, identifying the screen intervals with maximum arsenic, iron, manganese, and nitrate concentrations. We examined the relationship between sediment type, flow contribution, and maximum concentrations, focusing on the influence of aquitard boundaries and interbedded sequences on geochemical outcomes. Maximum concentrations occurred mostly (73–84%) in well screens associated with interbedded or coarse sediments with an aquitard boundary. Intervals with aquitard boundaries had higher arsenic concentrations (p = 0.02). In non-compliant wells, 64–69% of the maximum metal concentrations were sourced from fine-grained and interbedded sediments, warranting their inclusion in water quality zone testing. Approaches that may provide the geochemical resolution to determine the distance between aquitard boundaries and well screens are suggested to minimize the risk of constructing non-compliant PSWs that then require treatment.

The transboundary Hueco Bolson aquifer is the primary water supply for El Paso, Texas, and Ciudad Juárez, Mexico as well as U.S. Army Fort Bliss and smaller cities in Texas and Mexico. Binational groundwater data exchanges between the United States and Mexico that have focused on the aquifer have evolved over more than a century, shaped by scientific, political, and social dynamics. The history can be viewed through distinct periods. Efforts to understand the groundwater resource began with early reconnaissance surveys and have evolved through successive efforts to refine the characterization of groundwater flowpaths, residence times, surface water–groundwater interactions, and aquifer salinization. Lessons from these efforts highlight the importance of perseverance, mutual respect, and formal agreements, such as the 1999 memorandum between El Paso Water Utilities and the Junta Municipal de Agua y Saneamiento de Juárez, in sustaining long-term cooperation. The accumulated datasets chronicle the evolution of hydrogeologic conceptual and mathematical models while providing a foundation for ongoing research, sustainable water-use strategies, and the long-term stewardship of the Hueco Bolson aquifer shared by El Paso and Ciudad Juárez. The Hueco Bolson case demonstrates how long-term, cooperative data collection can improve scientific understanding and management of complex transboundary aquifer systems.