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The effective deep recharge to the Hutton Sandstone, a major confined aquifer of the Surat Basin, Australia, has been quantified for the first time with the aid of environmental tracers. A factor of ten discrepancy was found when deriving groundwater flow velocities from applying the environmental tracers 14C and 36Cl. It was possible to reconcile these contradictory results describing the Hutton Sandstone as a dual porosity system, in which a significant part of the tracer is not only lost by radioactive decay, but also by diffusion into stagnant zones of the aquifer. The conceptual and mathematical description of this process allowed for quantification of the effective deep recharge into this aquifer. The resulting recharge value is only a small percentage (~3%) of earlier estimates using chloride mass balance. The chloride mass balance probably gives a correct shallow infiltration rate but most of that infiltration is diverted to springs and surface water nearby (“rejected recharge”). Only a small fraction of recharge finally reaches the deeper system. These results are significant for water resource quantification from groundwater in deep confined systems. The presented dual porosity reconceptualization is likely applicable to a significant number of earlier studies that apply environmental tracers to old groundwater, and indicates that those original results may actually give too small values for groundwater velocity and too large estimates of recharge. This reconceptualization may be particularly valid for systems that include old groundwater and that have limited spatial and temporal coverage of tracer data such as the Great Artesian Basin.

Environmental tracers were used to characterize the origin and determine the age of the groundwater in the Motril–Salobreña aquifer (south-eastern Spain). The stable isotope concentrations (δ18O/δ2H), compared to the results obtained in previous studies, indicate that most of the recharge during the sampling period was from irrigation return flow and the carbonate Escalate aquifer. The combined dating of 3H, 3He, 4He, 85Kr, and 39Ar allowed establishing the presence of modern water throughout the aquifer, although with different mixing percentages. Thus, there is a large zone characterized by a fluvial domain with 100% young waters (< 5 years) due to the circulation of water through an area of high permeability sediments. In the discharge zone of the aquifer, older water is located (age > 170 years), and the percentage of young water is reduced (22.5%). This is explained by the greater distance that groundwater travels (aquifer thicknesses is over 250 m) and the lower permeability of the aquifer in the deeper sectors.

In Gyeongju, South Korea, local magnitude ML5.1 and ML5.8 earthquakes occurred on 12 September 2016; ML4.5 aftershocks and >500 aftershocks with ML > 1.5 were observed over the next 3 months. Responses of the aquifers were compared using hydrological and environmental tracer data (noble gases, δ18O, δD, 3H, and 13C). To assess the hydrologic response to the earthquake activity by the shallow (alluvial) and deep (bedrock) aquifers, time series data from the national groundwater monitoring wells were compared. Groundwater-level changes were not observed in most alluvial wells, while groundwater level and electrical conductivity (EC) increased in the confined igneous rock for several days to months after the earthquake activity. Noble gas anomalies in groundwater were closely related to the epicentral distance, lithology, and aquifer type. The relatively low concentration of 3H (<0.8 TU) and depleted values of δ18O and δD in the alluvial and bedrock aquifers suggest they were affected by upwelling of deep and old water. Elevated values of δ13C and 222Rn were observed in wells close to the epicenter. Groups resulting from cluster analysis using environmental tracer data were closely related to the responses of the earthquake on the aquifers of different types (alluvial and bedrock), lithologies, and distances from the epicenter. Groundwater-level change and geochemical response after the earthquake activity showed different correlations depending on aquifer and fault types. Combined use of groundwater level, EC, and environmental tracer data in groundwater can be useful to understand the origin and preferential flow paths of water during and after earthquakes.

A two-dimensional numerical groundwater flow model was established and calibrated for the hyperarid Najd region in southern Oman. The results indicate that recent recharge rates are required to sustain the observed groundwater heads in the Najd. The model was also used to estimate possible ranges of past recharge rates and the effective porosity of the main aquifer unit. Recharge rates during past humid periods were estimated to be no more than 1–3 times modern rates. The effective porosity was estimated to be between 0.06 and 0.093. Insight into the nature of the long-term transport within the aquifer was gained by using transient model runs over the last 350 ka and (1) varying the recharge intensity (from 0.1 to 2.5 times modern), and (2) the timing and duration of humid and dry periods. Finally, results indicate that although recharge rates and the flow conditions have likely changed over time, a steady-state model is capable of reproducing the observed groundwater residence times in the Najd based on carbon-14, helium and chlorine-36 dating.

One of the main sources of reactive nitrogen pollution is animal manure. The disposal of digestate (material remaining after the anaerobic digestion of a biodegradable feedstock) in agricultural soils could solve both the problems of soil fertilization and waste removal, but the fate of digestate in the environment must be assessed carefully before its massive utilization. To investigate whether digestate could be safely employed as a soil fertilizer, an agricultural field located in Monastier di Treviso (Northern Italy) and characterized by the presence of low hydraulic conductivity clay soils, was selected to be amended with bovine digestate. The experimental site was intensively monitored by a three-dimensional array of probes recording soil water content, temperature, and electrical conductivity, to solve the water and bulk mass fluxes in the unsaturated zone. High-resolution soil coring allowed the characterization of soil water composition over two hydrological years. Chloride, found in high concentrations in the digestate, was used as environmental tracer to track the fate of the percolating water. The study concluded that digestate could be confidently employed in short rotation buffer areas at an average rate of 195 ± 26 kg-N/ha/year in low hydraulic conductivity soils not affected by diffuse fracturing during dry periods.

Geogenic groundwater arsenic (As) contamination is pervasive in many aquifers in south and southeast Asia. It is feared that recent increases in groundwater abstractions could induce the migration of high-As groundwaters into previously As-safe aquifers. Here we study an As-contaminated aquifer in Van Phuc, Vietnam, located ~10 km southeast of Hanoi on the banks of the Red River, which is affected by large-scale groundwater abstraction. We used numerical model simulations to integrate the groundwater flow and biogeochemical reaction processes at the aquifer scale, constrained by detailed hydraulic, environmental tracer, hydrochemical and mineralogical data. Our simulations provide a mechanistic reconstruction of the anthropogenically induced spatiotemporal variations in groundwater flow and biogeochemical dynamics and determine the evolution of the migration rate and mass balance of As over several decades. We found that the riverbed–aquifer interface constitutes a biogeochemical reaction hotspot that acts as the main source of elevated As concentrations. We show that a sustained As release relies on regular replenishment of river muds rich in labile organic matter and reactive iron oxides and that pumping-induced groundwater flow may facilitate As migration over distances of several kilometres into adjacent aquifers.The interface between riverbed and aquifer is a biogeochemical reaction hotspot for arsenic release from river sediments, according to numerical simulations of groundwater flow and biogeochemical reaction processes.

Groundwater dating studies rely on environmental tracers to estimate residence times, but most available reliable tracers cover either short (days to decades; e.g., 222Rn, 3H/3He, 85Kr) or extended timescales (millennia to millions of years; e.g., 4He, 36Cl, 81Kr). This leaves a critical gap in age information for intermediate residence times (50–30,000 years), which are essential for groundwater resources management. Argon‐39 (t1/2 = 269 years) and Carbon‐14 (t1/2 = 5,730 years) could fill this gap, yet apparent groundwater ages estimated with these tracers often show systematic discrepancies, with 39Ar‐ages appearing younger than 14C‐ages. While mixing and geochemical reactions have been suggested as possible explanations, these mechanisms alone do not fully resolve the observed differences. Despite numerous studies using 39Ar–14C dating, no approach has fully reconciled these inconsistencies, particularly in dual‐permeability systems. This study addresses this gap by explicitly modeling tracer transport and production processes, integrating both numerical simulations and field observations to improve groundwater age interpretations. We combined explicit numerical simulations of reactive tracer transport with multi‐tracer field data from Denmark to systematically evaluate the physical and chemical processes affecting 39Ar and 14C activities. Our results demonstrate the systematic biases introduced by depth‐dependent underground production of 39Ar, mixing processes, and diffusive exchange between mobile and immobile groundwater zones in dual‐permeability media. Thus, this study provides a quantitative framework to address transport biases in 14C and 39Ar groundwater dating, allowing for more accurate groundwater residence time estimation and better‐informed decision‐making in water management in both semi‐arid and humid regions.

Groundwater model parameters need to be inferred on the basis of limited observation data, resulting in prediction uncertainty. The reduction of this uncertainty via future complementary observations is of high importance for many problems and can be guided by Bayesian experimental design. We employ a novel combination of Bayesian inversion, accelerated via multilevel methods, and Bayesian experimental design for this purpose. For a synthetic aquifer, we analyze the effect of including or excluding environmental tracer observations besides groundwater heads in two scenarios. In both scenarios, we study the effect of available data on distributions of model predictions after Bayesian inversion and subsequent experimental design. We demonstrate that posterior samples from Bayesian inversion can be reused to perform experimental design without additional model evaluations. In both scenarios, uncertainties and biases of flux‐related and groundwater age‐related predictions are substantially reduced through experimental design. Compared to the scenario with groundwater heads alone, including environmental tracer data in the observation data set leads to less uncertainty and bias in model outputs after Bayesian inversion, greater reduction of uncertainty and bias through experimental design, and reduced overestimation of complementary observation worth. Including environmental tracer observations at the beginning of combined Bayesian inversion and experimental design leads to more reliable predictions and more effective future data acquisition.

Optimizing empirical baseflow filters using environmental tracers (e.g., specific electrical conductance (SEC), turbidity) is an effective and efficient way to quantify the contribution of baseflow to total flow. To execute this baseflow separation, three key components are needed: The tracer, the method to estimate tracer concentration in different flow components, and the empirical baseflow filter. However, a comprehensive evaluation of the various combinations of these components, especially with a large sample of catchments, is currently lacking in the literature. Therefore, our study assembles 16 hybrid baseflow filters from two tracers, two concentration estimation methods, and four empirical baseflow filters, and evaluated their performance in baseflow separation and producing two long‐term baseflow signatures for 1,100 catchments in the Contiguous United States. Our results suggest that SEC is a superior tracer to turbidity for baseflow separation. Additionally, using monthly maximum and minimum values to represent tracer concentration in flow components produces better separation than using a power function relationship between flow rate and concentration. The four empirical baseflow filters offer a similar level of performance, regardless of the other options used. Yet, some of these filters produce inconsistent results in calculating the baseflow signatures for the catchments. Our analysis shed light on the optimization of hybrid baseflow filters for the accurate quantification of baseflow contribution. Plain Language Summary River flow can be broken down into two components: fast flow and slow flow. The latter is usually known as baseflow, and it represents the stable portion of river flow that comes from stored water sources, such as groundwater or snowpack. It is crucial to understand the proportion of baseflow in river flow for effective water resource management. A commonly used method to separate baseflow from river flow is by filtering streamflow data with empirical baseflow filters. These filters contain some parameters that are often optimized using geochemical data, such as specific electrical conductance (SEC) and turbidity, to ensure reasonable performance of baseflow separation. This study examined how SEC and turbidity can be used to optimize four empirical baseflow filters for quantitative assessment of baseflow contribution to streamflow. Our analysis of 1,100 catchments across the Contiguous United States revealed that SEC is a more reliable indicator of baseflow than turbidity. Interestingly, the choice of empirical baseflow filter had minimal impact, though some filters produced inconsistent results for the quantification of baseflow contribution. This research enhances our ability to accurately estimate baseflow, aiding in water resource planning and management. Key Points Evidence suggests that specific electrical conductance is a better tracer for baseflow separation compared to turbidity Using monthly extreme values to describe tracer signature in flow components is better than using a power function relationship The smooth minima method provides the most consistent estimation of baseflow contribution across various combinations

Abstract Silica nanoparticles have become an important tool in material sciences, nanomedicine, biotechnology, and pharmaceutics, with recent suggested applications also in environmental sciences. In life and environmental sciences, the application field is usually aqueous media; however, the crucial issue of silica nanoparticle dissolution behavior and rate in the target medium is often neglected, overlooked, or taken for granted. Silica nanoparticles are not stable in aqueous solutions until equilibrium silica concentrations are reached. While for life science applications, the degradability of silica nanoparticles is prerequisite for biocompatibility, this characteristic impedes the successful application of silica nanoparticles as environmental tracer, where long-term stability is needed. In this study, the impact of external (temperature, pH values, salinity, availability of silica) and internal (degree of condensation, size, porosity) parameters on the stability of ~ 45-nm-sized silica nanoparticles is characterized. Results show that external factors such as elevated temperature and alkaline pH-values accelerate the dissolution, acidic pH, high salinities, and high initial silica concentrations exhibit a contrary effect. Consequently, in applications, where external parameters cannot be controlled (e.g., in vivo, subsurface reservoirs), dissolution control and stability improvement of silica nanoparticles can be achieved by various means, such as adding a protective layer or by condensation of the silanol bonds through calcination.