The induced polarization (IP) method has significant potential for characterizing and monitoring groundwater contamination, particularly given recent advances in extracting rich spectral information from time domain IP (TDIP) data. However, its effectiveness depends on the quality of full-waveform data, and the impact of electrode noise remains insufficiently understood in field applications, especially at contaminated sites. Sandbox experiments were designed to evaluate electrode durability and IP data errors under field relevant combinations of contaminant type and conductivity at sampling rates of 0.1, 10, and 10,000 Hz. An error assessment method combining probability distributions, Fourier transforms, and the signal to noise ratio was proposed. It reveals the frequency ranges of amplitude and phase spectral information inherent in TD data. Pb-PbCl2 electrodes remained stable across the contaminant and conductivity range. For stainless steel electrodes, low-frequency noise decreased with increasing conductivity. Differences among contaminated media at same target conductivity are interpreted as combined responses to conductivity and ionic composition rather than as isolated contaminant effects. Organic contaminants may cause anomalous trends in electrode noise. Amplitude spectra were reliable across most of the 0.001-1000 Hz range, but for weaker signal, the SNR of stainless steel electrodes fell below 10 dB at <0.004 Hz. Reliable phase ranges were 0.01-1000 Hz (Pb-PbCl2) and 0.042-1000 Hz (stainless steel) for Sample A, and significantly narrowed for Samples B and C. These findings provide insights into electrode noise, thereby improving the reliable extraction of IP information.

Monitoring removal performance of permeable reactive barriers (PRBs) for groundwater nitrate remediation and distinguishing remediation mechanism contributions remains a key challenge. Based on flow-through column experiments, this study integrated spectral induced polarization (SIP) monitoring with reactive transport modeling to investigate the dynamics of removal by zero-valent iron (ZVI) and activated carbon (AC) mixtures. SIP parameters link material changes to removal performance. The strong correlation between normalized chargeability and cumulative removal capacity of constrained reactive transport model errors, with average relative errors of 12.5% and 21% for predicted breakthrough concentrations. The presence of Ca2+ and in solution promoted the corrosion of ZVI and the total -N removal capacity increased from 6.61 to 9.05 mg/g. The remediation enhancement is concentrated primarily in the proximal sections near the contaminant injection point. The reaction term exhibits a substantially increase compared to the adsorption term. Conversely, remediation performance declines in distal sections. This finding highlights the important contribution of regulatory ions to the reaction term and emphasizes the necessity of rational proportioning of remediation materials in different PRB sections for enhanced material utilization efficiency. Transport models calibrated via SIP robustly quantify spatial heterogeneity in adsorption and reaction processes, exhibiting significant potential to guide the design of PRBs.

Subsurface contamination is a significant problem due to excessive fertigation and wastewater discharge. However, depth and spatial extent often make it difficult to understand the nature of contaminant spread and its attenuation. Numerical modeling is an essential tool for exploring less-understood processes. However, there is a gap in the systematic simulation of the relationship between biogeochemical processes and redox potential, a relatively straightforward measure of subsurface biochemical state. Thus, a model is needed to facilitate an understanding of subsurface processes and provide a further theoretical basis for practice. This research aims to simulate subsurface biogeochemical dynamics and their correlation with redox potential. Water flow and biogeochemical dynamics are considered in both physical and biogeochemical models, incorporating the biogeochemical cycles of C, N, O, Mn, Fe, and S. The physical and biogeochemical models are coupled with the redox potential model addressed by the Butler-Volmer equation. A synthetic case study that mimics the conditions in a dynamic capillary fringe under a soil-aquifer treatment (SAT) facility was used to demonstrate the model. We demonstrate that the redox potential can reflect redox species concentrations (e.g., oxygen and nitrate) and reaction rates (i.e., nitrification, denitrification, and DOC aerobic oxidation), because the intrinsic mechanism of this model is superior to traditional simplified models. In our mechanistic model, both redox reactions and potentials are governed by multiple redox species and parameters, and the simulation can be adjusted by varying these parameters, making it more suitable for dynamic systems. This is especially important when saturation conditions change frequently. Further, we demonstrate that nitrification, denitrification, and DOC oxidation coexist at different depths, and that flow dynamics can dictate reaction rates and the depths at which they occur.

Global agricultural dependence on blended saline and freshwater irrigation mandates a mechanistic understanding of how salinity influences microbial biofilms within distribution networks, which are pivotal mediators of water quality and pathogen viability. Here, we examine the architectural, mechanical, and operational reactions of multi-species biofilms to saline exposure (0.6% NaCl) utilizing a regulated laboratory-scale irrigation model. Through a cohesive methodology combining confocal microscopy, atomic force microscopy, 16S rRNA sequencing, and meta-transcriptomics, we elucidate that salinity instigates a pivotal trade-off in biofilm maturation. While salt stress consistently suppressed live and dead cell biovolumes, it induced a significant enhancement of extracellular polymeric substances (EPS), leading to a thicker, EPS-rich biofilm architecture. These saline biofilms exhibited a lower adhesive force and Young's modulus, indicating a softer and less sticky surface. A community analysis revealed a reduction in taxonomic heterogeneity, along with an increase in specialized taxa associated with hydrocarbon decomposition functionalities, such as Hydrogenophaga and Nakamurella. Consequently, transcriptomic characterization revealed substantial upregulation of genes implicated in lipid distribution, ionic equilibrium, and oxidative stress mitigation, in conjunction with a downregulation of intercellular adhesion pathways. Our findings reveal that salinity drives biofilm adaptation towards a protected, EPS-dominated state with a functionally specialized community, suggesting a potential increase in the resilience of biofilms and risk of pathogen shielding in saline irrigation systems.

The soil's dry density is of crucial importance for many soil-related processes. Nevertheless, there is no direct and reliable method for its measurement. We propose a simple method for assessing the dry density based on time domain reflectometry measurement of the bulk dielectric constant of the soil and the temperature, coupled with gravimetric water content measurement (a small disturbed soil sample of about 50 g). These measurements, supported by two parameters that can easily be approximated, yield a robust estimation of the dry density that is of comparable accuracy to other methods but is much more straightforward. The novelty of the proposed method lies in its foundation upon fundamental physics principles, minimizing soil disturbance. Its superiority over existing measurement techniques is underscored by several key advantages: (1) no calibration is required for any of the individual measurements, (2) it is independent of empirical relationships, (3) there is no need for a high level of expertise to perform the measurement, (4) minimal soil disruption during measurement, and (5) swift measuring procedure. Moreover, this method is adept at measuring soil density at depth, making it particularly suitable for limited access to narrow boreholes.

The long‐term performance of permeable reactive barriers may diminish due to oxidation, adsorption on the particle surfaces, and pore space clogging. Offering warnings about performance reduction through geophysical techniques enables timely remedial measures. The impact of volume content of conductive particles, particle size, and pore water conductivity on the spectral induced polarization (SIP) response of mixtures comprising zero‐valent iron (ZVI), activated carbon (AC), and silica sand was revealed through column experiments. The electrical conductivity and the chargeability are correlated with the surface area of ZVI and AC per unit pore volume. The relaxation time is positively correlated with particle sizes and electrolyte conductivity. The polarization response resulting from the mixture of two types of filled particles exhibits a superposition relationship governed by the particle size distribution. SIP monitoring for 720 hr was conducted on a sand‐ZVI‐AC column saturated with NaCl or NaNO 3 solutions. Compared to the NaCl solution, under NO 3 − contamination, a thicker adsorption layer (∼0.05 mm) was observed on the ZVI surface in the microstructure. This resulted in a doubling of the parameters related to the relaxation time. Chargeability indicates changes in the mixture's active surface area, reflecting the reactivity of ZVI and the adsorption capacity of AC. Our findings illustrate that the induced polarization parameters record the changes in the reactive surface and particle size of the conductive particles in the sand, ZVI, and AC mixture, indicating the potential for long‐term monitoring of the reaction barrier.

The necessity for secure wastewater recycling is escalating due to worldwide water shortages, putting the effectiveness of traditional treatment techniques to the test against contaminants of emerging concern (CECs). Biofilm-mediated bioremediation (BMB) utilizes microbial communities encapsulated within an extracellular polymeric substance matrix to enhance pollutant removal through mechanisms of biosorption and biomineralization. This review systematically examines the foundations of BMB, including biofilm formation, microbial diversity, and reactor configurations (e.g., MBBR, MBR), highlighting their effectiveness in eliminating organics, heavy metals, and pathogens. This study explores advanced hybrid systems, like photocatalytic-MBBR, which improve the removal of persistent pollutants and help reduce risks such as the spread of antibiotic resistance genes (ARGs). However, obstacles in sustained stability, ARG propagation, and scalable economic efficiency persist as paramount to the technology's implementation. Uniquely, this work bridges the gap between fundamental science and practical implementation by evaluating how BMB aligns with circular economy principles and United Nations Sustainable Development Goals (SDGs), thereby providing a mechanistic-to-policy roadmap to guide the scaling of biofilm-based solutions for sustainable wastewater management.

Soil aquifer treatment (SAT) systems are used to remove pollutants from treated wastewater and store freshwater for reclamation and reuse. However, the accumulation of microbial biomass in the soil pore space, bioclogging, reduces water infiltration and hinders SAT efficiency. Since SAT systems play a crucial role in maintaining water resilience by providing an alternative to freshwater supply, optimizing their operation is essential to ensure their effectiveness. However, SAT systems are complex and dynamic systems that involve coupled interactions between microbial activity, water infiltration, and bioclogging in unsaturated media. This work proposes a continuum model that accounts for all these processes while distinguishing between active and inactive biomass, with the latter split into labile and recalcitrant fractions. The model is used to replicate a laboratory column experiment of bioclogging under unsaturated conditions and to explore how to optimize the operation of SAT systems. Specifically, we determined optimal wetting and drying periods that maximize water input to the SAT system while maintaining nutrient transformation rates. Our simulations show that the dry/wet time ratio controls biomass spatial distribution over depth. In contrast, the dry time extent dictates the degree of recovery of the soil relative to its initial (clean) infiltration capacity. We discuss the potential of this model to be extended to larger-scale experiments and to inform daily SAT operations in the field.

The long-term performance of the permeable reactive barriers in remediating contaminated groundwater may diminish as a result of oxidation, precipitation on the particle surfaces, and pore space clogging. Evaluating its performance through monitoring could address this dilemma. We investigate the spectral induced polarization (SIP) response of zero valent iron (ZVI)-activated carbon (AC)-sand mixtures.The chargeablity exhibits a perfect linear relation to the volumetric concentration of ZVI (2.5-50%) and AC (2.5%-75%) with r = 0.99. However, the low-frequency electrical conductivity shows low sensitivity to the volumetric content of ZVI and AC. The relaxation time increases with the particle sizes. When these two particles are mixed, chargeablity is approximated as a superposition of their individual values. In terms of phase values and frequencies of the phase peaks, it also exhibits this superposition effect. Furthermore, we conducted 720-hour SIP measurements on ZVI-AC-sand columns flushed with NaCl or NaNO3 solutions. It suggests that precipitation of 0.06 mm thick sedimentation onto the ZVI surface induced by changes in redox chemistry observed in micromorphology images, resulting an increase in the normalized chargeability by 44.05%, the scaled relaxation time and Cole–Cole model exponent by 1098.99% and 23.11%. Compared to flow-through by NaCl solution, changes in these parameters are more pronounced for columns saturated with NaNO3 solution, indicating the corrosion of ZVI. Our findings illustrate that induced polarization parameters vary in response to the chemical alteration of ZVI-AC-sand mixed media, showing the potential for noninvasive long-term monitoring of the reactive barriers.

A mystery has emerged as to why patterns of increasing extreme rainfall have not been accompanied by similar levels of flooding, garnering growing attention given concerns over future flood risks. Antecedent moisture conditions have been proposed as the missing explanatory factor. Yet, reasons for moisture variability prior to flooding remain largely unstudied. Here, we evaluate the potential utility of precipitation intermittency, defined as the dry spell length prior to a flood, to explain the variability of flooding over 108 watersheds from 1950 to 2022. Flood magnitude is shown to be sensitive to intermittency, particularly in arid and semi-arid regions (PET/P > 0.84) and for basins with low soil field capacity (<0.31 m3/m3). Following extended dry spells >20 days, floods are only possible from the most intense storms, whereas a wider range of storms can produce flooding for shorter intermittency. The flood probability decreases by approximately 0.5 % for each additional day of dry spell, with overall flood probabilities being up to 30 % lower following extended dry periods. These results underscore the potential utility of precipitation intermittency for diagnosing current and future flood risks.

In agricultural ecosystems, nitrate (NO3-) leaching is the most widespread loss pathway and non-point source of nitrogen (N) to surface water and groundwater. NO3- leaching in rain-fed and irrigated agricultural systems has been extensively modeled using different numerical approaches of varying complexity. Most numerical modeling studies use simplified zero-order or first-order kinetics when optimizing N budgets. Few studies consider the impacts of temperature, soil moisture and other environmental conditions on biogeochemical reaction rates. In this study, we simulate NO3- leaching and biogeochemical processes in two soil columns under large water application events (groundwater recharge events) using a 1D non-equilibrium (e.g. mobile-immobile) reactive transport HP1 (HYDRUS-1D and PHREEQC) model. We compare this calibrated model to several other calibrated models representing simpler HYDRUS nitrate leaching modeling approaches (uniform flow, non-reactive, zero- and first-order kinetics). Results show that the incorporation of conditional environmental factors such as temperature, soil moisture, and a proxy for oxic/anoxic conditions (percent pore-space filled) results in superior model performance representing the timing and magnitude of important biogeochemical processes when estimating cumulative NO3- leached from the shallow vadose zone. In addition, using a physical non-equilibrium (i.e. dual-porosity type, mobile-immobile solute trans-port) approach improves model performance when estimating residual NO3- in the soil profile after water application events.

The capillary fringe (CF) exhibits steep chemical and redox gradients, representing a critical reactive interface in the subsurface. The spatial extent of the CF is temporally dynamic because of water table and soil moisture fluctuations. To simulate the spatio-temporal responses of the CF's microbial community structure and activity, a 91 cm long sandy soil column experiment was conducted by imposing three different sequential hydrologic regimes: a stable water table separating the unsaturated and saturated zones (static regime), periodic pulse infiltration events with a constant water table position (infiltration regime), and water table fluctuations under three drainage-imbibition cycles of 6, 12, and 18 days (fluctuating regime). The investigation focused on the microbially-mediated turnover of nitrogen (N) and carbon (C). Geochemical profiles in the soil column were monitored continuously, while microbial DNA was extracted for 16S rDNA sequencing at the start, middle and end of the experiment. The observed microbial community compositions corresponded to five categories: (1) initial soil, (2) near-surface and (3) CF and saturated soil under the infiltration regime, (4) unsaturated and (5) saturated soil in the fluctuating regime. Based on co-occurrence network analysis, microbial physiologies matched the predominant geochemical conditions, and relatively stable community structures prevailed throughout the experiment. NO3- and NH4+ were more available to the microbes in the fluctuating regime's alternating oxic/anoxic zone, leading to a more flexible network. The prevalent microbial metabolic pathways predicted using FAPROTAX were mainly associated with N and C metabolisms and significantly correlated with the taxonomic compositions. However, the dominant microbial groups were shared among all the soil samples. The limited spatial differences in microbial community structure under the fluctuating regime were likely due to a more pronounced role of microbial activity than microbial diversity, and similar drying-wetting conditions between the soil column and the filed site. Internally consistent geochemical and microbial depth stratifications were identified by (1) the redox potential and nitrogen species distributions, (2) spatial moment analysis of NO3- and NH4+ concentrations, (3) microbial community composition and (4) function. The five imposed hydrologic regimes generated distinct and coupled geochemical and microbial signatures. These signatures resulted from variations in the soil microbial community's activity, rather than diversity or abundance.

This work uses spectral-induced polarization (SIP) to monitor the transport of citrate-coated gold nanoparticles (AuNPs) in soil. The experimental setup includes batch experiments with different NP-to-soil ratios, designed to determine the NP concentration at which electrodic polarization is significant, as well as a flow-through experiment in a natural soil column. The analyzed SIP measurements allow not only the noninvasive monitoring of NP progression through the soil but also the deduction of NP-soil interactions through combination with elemental analysis. The real conductivity increased in a breakthrough pattern in response to the AuNP transport due to CaCO3 dissolution and subsequent Ca2+ release. The imaginary conductivity presented a more complex pattern and reflected a combination of NP-soil interactions (NP retention and accumulation, CaCO3 dissolution with subsequent Ca2+ release, and exchange processes). Overall, we show that the progression of NPs through soil can be tracked using SIP even at low concentrations due to their effect on pore water, sediments, and NP-soil and NP-CaCO3 interactions. Nevertheless, many parts of the observed phenomena require further research to gain better understanding and quantification.

Knowledge concerning the redox potential (Eh) conditions in the vadose zone of a soil aquifer treatment (SAT) system constitutes valuable information for assessing water quality and operational efficiency. The complex nature of Eh conditions in SAT limits the ability to predict and quantify them using detailed models. Alter-natively, data-driven models can be used for predictions and relationship analysis. Hourly measurements of Eh, volumetric water content (theta), soil temperature (T), and gaseous oxygen (O2) were obtained at multiple depths of a SAT vadose zone. A correlation analysis showed that O2 correlated with Eh for most temporal components. Only the monthly component of T and the daily component of theta were correlated with Eh. A detailed multiple linear regression (MLR) analysis illustrated that the gaseous O2, at shallow depths, can explain the majority (above 80%) of the Eh variability. The MLR curve demonstrated breakpoints in the Eh response to O2 at shallow depths, which were identified using a piecewise regression. These breakpoints explain, in part, the different stages of microbial activity throughout the SAT wetting and drying cycles. Combining an oxygen transport analytical model with the piecewise regression enabled the Eh prediction through easy-to-acquire T and theta measurements. At deeper depths, the Eh-O2 relationship demonstrates a step-function characteristic, which indicates that the changes in Eh occur due to the arrival of a low-Eh solution. Thus, the Eh dynamic in the SAT vadose zone is mostly controlled by aerobic conditions.

The paper deals with three issues relevant to the application of TDR methods for monitoring water content in compacted clay soils: ( i) the dependence on dry density, ( ii) the ability to monitor changes in water content in clay soil approaching saturation, and ( iii) difficulties in performing efficient and reliable calibrations. A calibration scheme which inherently controls dry density of the specimen has been developed and applied. The methodology creates continuous calibration curves over a range of volumetric water content from a single test specimen. The new approach has been applied in monitoring the advance of a wetting front through a compacted swelling clay in a laboratory environment.

Soil aquifer treatment (SAT) is an effective and sustainable technology for wastewater or stormwater treatment, storage, and reuse. During SAT, the vadose zone acts as a pseudo-reactor in which physical and biochemical processes are utilized to improve the infiltrated-water quality. Dissolved oxygen (DO) is necessary for aerobic microbial oxidation of carbon and nitrogen species in the effluent. Therefore, to enhance aeration, SAT is generally operated in flooding and drying cycles. While long drying periods (DPs) lead to better oxidizing conditions and improve water quality, they reduce recharge volumes. As the population grows, the quantity of effluent directed to SAT sites increases, and increasing recharge volumes become a concern and often a limiting factor for SAT usage. In this study, direct subsurface air injection SAT (Air-SAT) was tested as an alternative to long-DP operation. Six long-column experiments were conducted (2 m column) that aimed to examine the effect of air injection on the soil's water content, oxidation-reduction potential (ORP), DO concentrations, infiltrated amounts, and ultimate outflow quality. In addition to basic parameters, such as dissolved organic C (DOC) and N species, the effluent quality analysis also included an examination of three emerging water contaminants: ibuprofen, carbamazepine, and 1H-benzotriazole. Pulsed-air-injection experiments were conducted during continuous flooding using different operation modes (i.e., air pulse durations, frequencies, and airflow rates). Our results show that Air-SAT operation doubled the time during which infiltration was possible (i.e., the infiltration was continuous with no downtime) and allowed up to a 46 % higher mean infiltration rate in some cases. As a result, the infiltration volumes in the Air-SAT modes were 47 %-203 % higher than conventional flooding-drying operation (FDO). A longer air pulse duration (60 min vs. 8 min) and higher airflow rate (similar to 2 L min(-1) vs. similar to 1 L min(-1)) led to a higher mean infiltration rate, whereas a high pulse frequency (4.5 h(-1)) led to a lower mean infiltration rate compared with low-frequency operation (24 h(-1)). Air injection also allowed good recovery of the ORP and DO levels in the soil, especially in the high-frequency Air-SAT experiments, where steady aerobic conditions were maintained during most of the flooding. Consequently, the mean DOC, total Kjeldahl N (TKN), and ibuprofen removal values in these experiments were up to 9 %, 40 %, and 65 % higher than those with FDO, respectively. However, high-frequency Air-SAT during continuous flooding also led to significant deterioration of the mean infiltration rate, probably due to enhanced biological clogging. Hence, it may be more feasible and beneficial to combine it with conventional FDO, allowing a steady infiltration rate and increased recharge volumes while sustaining high effluent quality. While these results still need to be verified at full scale, they highlight the possibility of using air injection to minimize the DP length and alleviate the pressure on existing SAT sites.

The demand for agricultural water is a growing problem in irrigated regions across the globe, particularly in arid and semi-arid regions. Changes in the level of groundwater in irrigation districts will affect the flow of surface water connected to the aquifer, which may damage the sustainability of water resources and ecosystems. In this study, a two-dimensional unsteady flow model based on MODFLOW was constructed and three scenarios were established to assess the demand for agricultural water in the Jiaokou Irrigation District. The results show that the groundwater in the study area is basically balanced. However, the supply of irrigation water for summer irrigation is insufficient. The results of the model prediction indicate that when groundwater is primarily used for irrigation (scenario 1), the maximum water level decrease is 25 m, which is beyond this limit (15 m). When the ratio of groundwater to surface water is 2:1 for irrigation (scenario 2), the largest decrease in water level is approximately 10 m. Scenario 3 is proposed based on the Hanjiang-to-Weihe River Valley Water Diversion Project to prevent the salinization of soil owing to the rise in water level, and its result shows that the maximum decrease and buried depth are approximately 5 m and above 3 m, respectively, indicating that the scenario is more reasonable and sustainable. These findings provide theoretical guidance to protect water resources and prevent water pollution and should serve as a reference for rationally allocating water resources in other irrigation districts in arid and semi-arid areas.

Future agriculture calls for increased input (e.g., water, nutrients, pesticides) use efficiency while maintaining or improving productivity, minimizing environmental impacts, and increasing profitability. Complete understanding of complex irrigation systems requires laborious, time-consuming, and expensive field investigations, which invariably involve only a limited number of treatments. On the other hand, fully calibrated process-based models, such as HYDRUS, can quickly evaluate different irrigation management strategies without the need for labor-intensive fieldwork and have become valuable research tools for predicting complex and interactive water flow and solute transport processes in and below the root zone. HYDRUS codes have been used worldwide in several hundreds of studies evaluating various types of irrigation (e.g., sprinkler, furrow, basin, and surface and subsurface drip), their scheduling (e.g., the timing of irrigation and its amount), and solute-related factors (e.g., fertigation, chemigation, salinization, and sodification). The objective of this manuscript is to review the current modeling capabilities of HYDRUS to evaluate various irrigation methods and related processes. The manuscript starts with a section describing governing flow and transport equations solved numerically by the HYDRUS codes, the corresponding initial and boundary conditions, and related factors such as soil hydraulic properties and root water and nutrient uptake. Modeling of different irrigation techniques is described in subsequent sections, followed by sections dealing with solute-related topics such as fertigation, chemigation, and salinization/sodification. Topics, including the effects of spatial variability, optimization of irrigation systems, and special irrigation methods, are covered in the later sections. The manuscript emphasizes the advantages and opportunities of HYDRUS in describing various processes in the root zone of irrigated plants that support sustainable irrigated agriculture. All the project files of the discussed examples and their descriptions are available for download at https://www.pc-progress.com/en/Default.aspx?hyd5-AdvancesInAgronomy.