Rui Hu

Travel–time-based thermal tracer tomography (TTT) has emerged as a promising technique for characterizing aquifer heterogeneity. However, the influence of travel–time definitions and data density on inversion performance is not well understood. In this study, we present a controlled two-dimensional sandbox experiment designed to systematically investigate three travel–time definitions (early-time t₁₀, intermediate t₅₀, and peak-time t_peak) under data-rich (32 travel times) and data-sparse (10 travel times) conditions. The obtained hydraulic conductivity (K) fields are benchmarked against permeameter measurements and a geostatistical inversion that assimilates dense steady-state head observations. The results demonstrate that all three travel–time definitions satisfactorily reproduce the primary layered heterogeneity when abundant travel–time data are available, with t₅₀ and t_peak providing marginally better structural fidelity under data-rich conditions. However, only the early-time t₁₀ definition preserves the spatial continuity of dominant geological structures under data-sparse conditions, exhibiting superior robustness. All TTT inversions systematically underestimate the K ranges and exhibit pronounced range compression, whereas the geostatistical inversion overestimates K and introduces spurious high-value extremes. Forward thermal transport simulations reveal that TTT-derived K fields yield systematically delayed thermal breakthroughs, while the geostatistical inversion yields more accurate predictions. These findings highlight the critical interplay between travel–time diagnostics and observation density. They also underscore the necessity of jointly inverting hydraulic and thermal data to overcome the limitations of single-dataset approaches for reliable aquifer characterization and transport prediction.

Hydraulic tomography (HT) based on the successive linear estimator (SLE) offers the high-resolution characterization of aquifer heterogeneity but conventionally requires prolonged pumping to achieve steady-state conditions, limiting its applicability in contamination-sensitive or low-permeability settings. This study bridged theoretical and practical gaps (1) by identifying spatial periodicity (hole effect) as the mechanism underlying divergences in steady-state cross-correlation patterns between random finite element method (RFEM) and first-order analysis, modeled via an oscillatory covariance function, and (2) by validating a novel short-term sampling strategy for SLE-based HT using field experiments at the University of Göttingen test site. Utilizing early-time drawdown data, we reconstructed spatially congruent distributions of hydraulic conductivity, specific storage, and hydraulic diffusivity after rigorous wavelet denoising. The results demonstrate that the short-term sampling strategy achieves accuracy comparable to that of long-term sampling strategy in characterizing aquifer heterogeneity. Critically, by decoupling SLE from steady-state requirements, this approach minimizes groundwater disturbance and time costs, expanding HT’s feasibility to challenging environments.

In the realm of groundwater science, characterization of heterogeneous aquifers is pivotal for resolving diverse groundwater resource and engineering-related problems that require the detailed spatial distribution of hydraulic parameters. As research progresses, one hydraulic tomographical method, which is based on hydraulic travel time inversion, emerges as a promising and rapid method due to its robust and efficient calculation. In the field, the acquisition of hydraulic excitation and head observation data required for inversion is less time-consuming. Data collection from a single hydraulic test (such as a pumping test) typically takes only a few minutes or even a few tens of seconds. However, the field application of this method faces challenges. Hydraulic travel time is typically generated in the early stages of hydrogeological tests (e.g., early drawdown of a pumping test), yet accurate data may not be readily available because of the noise signals from test equipment, which can contaminate travel time signals, leading to inaccurate inversion results. A potential solution lies in utilizing the smooth head observation during the recovery period after the pump is turned off, which yields more accurate travel times for inversion calculations. In this paper, the mathematical development suggests that the travel time of the recovery phase aligns with that of the pumping phase when pumping reaches a steady or quasi-steady state. Subsequently, by employing Monte-Carlo simulations, 1200 realizations of two-dimensional heterogeneous confined aquifer models were generated for simulating pumping tests with different pumping durations. The calculated head data were then utilized to compute the travel time derived from drawdown data (t) and recovery data (t′), respectively. Comparisons showed that t is equal to t′ when drawdown reaches a steady or quasi-steady state. Conversely, when the pump is turned off before reaching a quasi-steady state, t differs from t′. However, results also indicate the fact that a decent hydraulic travel time diagnosis can be obtained, especially for the cases when travel times are smaller than 15 s. Given the statistical results of Monte-Carlo simulations, as well as experience during pumping tests in the field with different scenarios, using the recovery data from 60 s of pumping duration, or extended pumping durations of 100 s or 200 s as a more conservative alternative, can replace the aquifer characterization based on drawdown data. The new inversion strategy not only has less data uncertainty and equivalent inversion accuracy, but also can greatly enhance the repeatability of field tests and reduce the environmental impact of long-term pumping tests.

Water pollution is calling for a sustainable remediation method such as the use of metallic iron (Fe⁰) to reduce and filter some pollutants, yet the reactivity and hydraulic conductivity of iron filters decline over time under field conditions. Here we review iron filters with focus on metallic corrosion in porous media, flaws in designing iron filters, next-generation filters and perspectives such as safe drinking water supply, iron for anaemia control and coping with a reactive material. We argue that assumptions sustaining the design of current Fe⁰filters are not valid because proposed solutions address the issues of declining iron reactivity and hydraulic conductivity separately. Alternatively, a recent approach suggest that each individual Fe⁰atom corroding within a filter contributes to both reactivity and permeability loss. This approach applies well to alternative iron materials such as bimetallics, composites, hybrid aggregates, e.g. Fe⁰/sand, and nano-Fe⁰. Characterizing the intrinsic reactivity of individual Fe⁰materials is a prerequisite to designing sustainable filters. Indeed, Fe⁰ratio, Fe⁰type, Fe⁰shape, initial porosity, e.g. pore size and pore size distribution, and nature and size of admixing aggregates, e.g. pumice, pyrite and sand, are interrelated parameters which all influence the generation and accumulation of iron corrosion products. Fe⁰should be characterized in long-term experiments, e.g. 12 months or longer, for Fe dissolution, H₂generation and removal of contaminants in three media, i.e., tap water, spring water and saline water, to allow reactivity comparison and designing field-scale filters.

Permeable reactive barriers (PRBs) containing metallic iron (Fe⁰) as reactive materials are currently considered as an established technology for groundwater remediation. Fe⁰ PRBs have been introduced by a field demonstration based on the fortuitous observation that aqueous trichloroethylenes are eliminated in Fe⁰-based sampling vessels. Since then, Fe⁰ has been tested and used for treating various biological (e.g. bacteria, viruses) and chemical (organic and inorganic) contaminants from polluted waters. There is a broad consensus on the view that “reactivity loss” and “permeability loss” are the two main problems hampering the design of sustainable Fe⁰ systems. However, the view that Fe⁰ is a reducing agent (electron donor) under environmental conditions should be regarded as a distortion of Corrosion Science. This is because it has been long established that aqueous iron corrosion is a spontaneous process and results in the Fe⁰ surface being shielded by an oxide scale. The multi-layered oxide scale acts as a conduction barrier for electrons from Fe⁰. Accordingly, “reactivity loss”, defined as reduced electron transfer to contaminants, must be revisited. On the other hand, because “stoichiometric” ratios were considered while designing the first generation of Fe⁰ PRBs (Fe⁰ as reductant), “permeability loss” should also be revisited. The aim of this communication is to clarify this issue and reconcile a proven efficient technology with its scientific roots (i.e. corrosion science).

In order to solve the problem of temperature non-uniformity of electric heat storage boiler, this paper constructs a numerical model of flow and heat transfer of electric heat storage boiler, optimizes the boiler structure by setting stratified heating and baffle, and studies the changing trend of internal temperature uniformity of electric heat storage boiler by using the SIMPLE algorithm of QUICK format. The research shows that the temperature distribution of the heat storage tank can be more uniform by adding baffles for stratified heating.

Metallic iron (Fe⁰) is a reactive material for treating polluted water. The effect of water salinity on the efficiency of Fe⁰-based remediation systems is not yet established. This work aims to clarify the reasons why Cl⁻ ions are often reported to improve the efficiency of Fe⁰/H₂O remediation systems. Quiescent batch experiments were carried out to characterize the effect of chloride (Cl⁻) ions on the efficiency of methylene blue (MB) discoloration in the presence of Fe⁰. Cl⁻ was used in the form of NaCl at concentrations ranging from 0 to 40 g L⁻¹. The MB concentration was 10 mg L⁻¹, the Fe⁰ loading was 5 g L⁻¹, and the duration of the experiment varied from 2 to 46 days. Four different Fe⁰ materials were tested in parallel experiments. Tests with different NaCl levels were performed in parallel with three other organic dyes: Methyl orange (MO), orange II (OII), and reactive red 120 (RR 120). The results clearly show that the presence of Cl⁻ reduces the extent of dye discoloration in all systems investigated. The efficiency of the dyes increased in the order MB < MO < RR 120 < OII. In systems with varying NaCl concentrations, dye discoloration initially decreases with increasing NaCl and slightly increases for [NaCl] > 30 g L⁻¹. However, the extent of dye discoloration for [NaCl] = 40 g L⁻¹ remains much lower than for the system with [NaCl] = 0 g L⁻¹. The results clearly demonstrate that the presence of Cl⁻ fundamentally delays the process of contaminant removal in Fe⁰/H₂O systems, thus improving the understanding of the contaminant interactions in Fe⁰-based remediation systems. These results also suggest that the effects of other inorganic anions on the efficiency of Fe⁰/H₂O systems should be revisited for the design of field applications.