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Vertical flexible barriers (VFBs) are widely used for contaminant source separation and risk control. Inherent or externally caused damage to a VFB can affect its effectiveness in pollution containment and control, but its effective detection is a major challenge due to its concealed nature. This paper presents a combined pumping and tracer approach to detect defects in VFBs. A high-performance model is used to model the detection process revealing the migration and distribution behavior of the tracer in the triple media system of wellbore–aquifer–VFB to better understand the effectiveness of the method and its performance indicators, including response time, detectable depth, and defect size, and to optimize its application. Simulation results show that the tracer quickly moves through the defect to the monitoring well, and temporal tracer concentration reveals details of the breach in the wall. Sensitivity analysis reveals influential factors, including defect size, depth, and tracer concentration. Results indicate that the size and depth of defects will affect the penetration and response time. Breakthrough is less clear, and response time is longer if defects are small and deep. However, a higher tracer injection concentration can shorten the response time and increase tracer concentration in the monitoring well. Therefore, it is necessary to design the tracer dosing concentration according to the site conditions. For the Yancheng site, tracer concentration was increased from 1000 ppb to 2660 ppb to ensure the detection of leakage points at 4 m or deeper.

International audience; Temperature logs are an important tool in the geothermal industry. Temperature measurements from boreholes are used for exploration, system design, and monitoring. The number of observations, however, is not always sufficient to fully determine the temperature field or explore the entire parameter space of interest. Drilling in the best locations is still difficult and expensive. It is therefore critical to optimize the number and location of boreholes. Due to its higher spatial resolution and lower cost, four-dimensional (4D) temperature field monitoring via time-lapse Electrical Resistivity Tomography (ERT) has been investigated as a potential alternative. We use Bayesian Evidential Learning (BEL), a Monte Carlo-based training approach, to optimize the design of a 4D temperature field monitoring experiment. We demonstrate how BEL can take into account various data source combinations (temperature logs combined with geophysical data) in the Bayesian optimal experimental design (BOED). To determine the optimal data source combination, we use the Root Mean Squared Error (RMSE) of the predicted target in the low dimensional latent space where BEL is solving the prediction problem. The parameter estimates are accurate enough to use in BOED. Furthermore, the method is not limited to monitoring temperature fields and can be applied to other similar experimental design problems. The method is computationally efficient and requires little training data. For the considered optimal design problem, a training set of only 200 samples and a test set of 50 samples is sufficient.

In both the oil and gas and the environmental remediation industries, the ability to locate and quantify hydrocarbons in the subsurface is of value. Partitioning interwell tracer testing has been shown effective at detecting and measuring uniformly distributed hydrocarbon mass. Difficulty arises when the hydrocarbon is irregularly distributed – that is, when the overall hydrocarbon saturation within the swept pore volume is low but is high locally. The method of moments underestimates the saturation when most of the tracer bypasses the hydrocarbon. The small zone containing the hydrocarbon that is contacted by the tracers produces a long ‘tail’. An alternative method was developed to compute the irregularly distributed hydrocarbon saturation, and the complete mathematical derivation is presented that utilises the exponential decay of each tracer. The method was verified using experimental data from a two-dimensional physical model where the hydrocarbon saturation was high on a local scale but low overall. The proposed analytical approached yielded results that were significantly better than those using the traditional, first-moment method for computing the hydrocarbon volume. It is concluded that the exponential decay method yielded estimates of the irregularly distributed hydrocarbon mass that were both reasonable and significantly more accurate than those from the standard method of moments.

This study deals with numerical modelling of hydraulic and transport phenomena in granite of the Bohemian massif in Bedrichov, Czechia (Czech Republic). Natural tracers represented by stable isotopes δ18O and δ2H were collected at the tunnel outflow points and nearby catchment and their concentrations were monitored for seven years. The study compared transport simulations by a two-dimensional (2D) physically based model (advection-dispersion) developed in Flow123d software and a simpler lumped-parameter model, calculated with FLOWPC. Both variants were calibrated with UCODE software, either fitting the concentration data alone, or including the tunnel inflow rates in the case of the 2D model calibration (either in separate steps or within a single optimization problem). Since each of the models describes the tracer transport with different parameters, the models were compared based on the mean transit time as a postprocessed quantity. Besides this, two different options for processing the recharge data (input for both models) were evaluated. Calibration and data interpretation were possible for three of the four observed places in the tunnel, thus determining the depth limit of applicability of the stable isotopes. The estimates for discharge sampling at 25–35 m depth based on inverse modelling provide reasonable values of mean transit time (20–40 months) for the lumped parameter models, little revising the results of previous studies at the site. The resulting transport parameters of the advection-dispersion model (porosity and dispersivity) are in accordance with the hydrogeological structures present at the sampling sites.

Organic tracers (bulk δ13C and δ15N) are being increasingly used in sediment/particulate organic carbon source apportionment studies at the catchment scale to support sustainable land management decisions. Here, the use of these isotopic tracers in sediment fingerprinting depends on the critical assumption that δ13C and δ15N values remain conservative during the sediment delivery continuum. Such assumption, however, requires critical evaluation, especially since standard tracer conservation tests applied in conjunction with catchment scale studies are prone to masking potential issues at smaller spatial scales including the field scale. Against this background, our study evaluated the subtle isotopic shift associated with sediment redistribution to edge-of-field and assessed the impact on sediment source apportionment. In a C3-grass dominated study field, δ13C and δ15N values exhibited differences (i.e., Δ13C = 1.4 ± 0.7‰ and Δ15N = 0.4 ± 0.4‰) between soil depths of 0–5 cm and 5–10 cm. Sampled sediments at the edge-of-field flume had higher δ13C values than the 0–5 cm soil layer; i.e., Δ13C values were 0.3 ± 0.4‰ and 0.8 ± 0.4‰ for suspended and deposited (materials deposited in a basket downstream of a flume) sediment, respectively. In contrast, δ15N values increased in suspended (Δ15N = 0. 8 ± 0.6‰) but decreased (Δ15N = − 0.4 ± 0.5‰) in deposited sediment, compared to the 0–5 cm soil layer, suggesting that the N isotopes can respond differently in edge-of-field sediment types. Although current fingerprinting work tends to not take explicit account of organic tracer alteration during transport or after deposition, our results demonstrate that correcting sediment δ13C and δ 15N values for tracer alteration in a Bayesian un-mixing model generated robust and reliable estimates of source contributions to both target sediment types. We therefore recommend taking account of the subtle but consistently altered δ13C and δ 15N values along the sediment cascade in un-mixing modelling to help better discriminate sources and to improve un-mixing model estimates at the catchment scale.

Heterogeneity significantly effects the accuracy of flow and contaminant transport prediction in subsurface formations. The spatial correlation structure of hydraulic conductivity (K) is a crucial factor to characterize the heterogeneous architecture. In presented study, the relationship between the spatial correlation structure of K and plume dispersion is analyzed through the integration of experimental and numerical simulation approaches. A detailed description on the sedimentary facies types in a column experiment is obtained to ensure the accuracy of the heterogeneous characterization. The spatial correlation structure of K is analyzed with the components of ln(K) covariance and facies transition probability structures. Lagrangian-based models are developed to estimate solute dispersion in nonreactive tracer injection experiments. The results show that the model can predict plume spreading accurately when the spatial correlation structure is well defined. The dispersivities calculated by the Lagrangian-based model are slightly higher than those obtained from the solute transport experiments. Further, the upscaled dispersivity derived from the transition probability is dominated by the cross-transition probability structure, while the contribution of the auto-transition terms is quite small. The numerical modeling results confirm that the upscaled dispersivity can reproduce the solute breakthrough in the heterogeneous sediment well. The scale dependence of dispersion is strengthened when the flow direction is perpendicular to the bedding plane where the conductivity dramatically changes along the flow path in a layered bedding sediment.

Integrated watershed modeling techniques have been applied in recent years to examine surface and subsurface interactions. Model performance is often evaluated by best fit of the hydrograph, which alone cannot explicitly explain whole catchment dynamics. To overcome this problem, this study incorporated multiple tracers (3H, 85Kr, and groundwater temperature) into a physically-based fully distributed modeling framework for characterizing regional-scale hydrological processes in Kumamoto, southern Japan. First, a simulation performed by a hydrometrically calibrated model showed satisfactory performance for river discharge and groundwater level. However, this model showed poor fitting for isotopic composition and temperature due to the structural uncertainty of the model. A new model was established reflecting recent deep bore log data and incorporating tracer data showed acceptable accuracy for hydrographs and tracers. Thus, more reliable estimates of groundwater storage, groundwater age and water flow paths were depicted over the regional catchment. Comparisons between the two models indicate that the model structure of an area with an uncertain lower boundary can be addressed by incorporating multiple tracer data. Tracer-aided models could be applied for a holistic understanding of contaminant transport dynamics besides flow simulation.

Mercury is a global pollutant of critical concern but its movement through terrestrial upland systems is poorly understood. We investigated whether forest harvesting practices and varying hydrological conditions resulted in different subsurface transport pathways, fluxes and proportions of recent vs. ambient mercury in runoff. In a multiyear field experiment, we measured subsurface lateral mercury fluxes at three adjacent hillslope plots, including one that was not harvested, one where all biomass was removed after harvest, and one where residual biomass was left after harvest. Two different enriched stable mercury isotopes (Hg tracer) were added to the hillslope plots (one pre-harvest and one post-harvest) to help differentiate between recently deposited mercury and ambient mercury in soils, as well as to identify changes between years. More Hg tracer was recovered in runoff post-harvest (16.2–54.0 μg) than pre-harvest (3.7–11.8 μg) from both harvested hillslopes, but differences between harvested hillslopes were not significant. The movement of Hg tracer was governed by periods of near surface preferential flow that was enhanced by forest harvesting. Runoff Hg tracer concentrations were significantly and inversely related to both event runoff magnitude (R2 = 0.85) and runoff ratio (R2 = 0.81). Insignificant relationships between Hg tracer concentrations and both DOC and precipitation pH suggest that for recently deposited mercury, the overarching controls on mobilization were hydrological rather than biogeochemical. The dependence of mercury mobilization on the routing of precipitation to different subsurface flow pathways suggests an important interaction between climate and the age of mercury pools for mercury transport from terrestrial landscapes.

The study is aimed at identifying patterns in distribution of pollutants in the elementary landscape-geochemical systems (ELGS) of the temperate zone. The study used 137Cs as a tracer, which allows a highly detailed analysis of the nature of the heterogeneity of secondary migration in the toposequence: summit—slope—closing depression, treated as the elementary landscape-geochemical system. The study site was located in the Bryansk region in the Chernobyl abandoned area with an initial level of 137Cs contamination exceeding 1480 kBq/m2 (40 Ci/km2). An original technique of repeated 137Cs measurements along cross-sections accompanied by topographic survey and soil cores sampling has been applied. The obtained results showed a complete absence of constant increase of 137Cs concentration downslope but revealed a steady regular variability of 137Cs activity of a cyclical type. Given uniformity of the initial 137Cs fallout within a small-sized plot, variation of 137Cs due to its secondary distribution in ELGS was 2–2.7-fold according to field gamma-spectrometry data which corresponded to the radionuclide contamination density of the top 20-cm layer of the soil containing 96–99% of the total radionuclide amount (correlation between the parameters equaled to r0.01 = 0.782, n = 20). A specifically regular structure obviously formed under the set of radionuclide water migration processes seems to be inherent in all systems of the studied type. The results obtained are believed to be of both theoretical and practical importance, since they can contribute to making decisions on the precise monitoring of zones of technogenic accumulation, as well as solving fundamental problems of soil formation and its restoration after technogenic pollution.

In many areas around the world, phreatic coastal aquifers are vulnerable to contamination and saltwater intrusion. This study aims to explore the hydrodynamic functioning of the coastal phreatic aq...

Thermal tracer has been widely used as a substitute for salt tracers when investigating the subsurface, since NaCl may cause environmental pollution and cannot be used in aquifers already contaminated with salts. Traditional methods for monitoring thermal conditions, like distributed temperature sensing systems, have their strengths in high accuracy; however, such methods are limited by providing discontinuous information about the subsurface with a limited number of boreholes. Recently, electrical resistivity tomography (ERT) has shown great potential in monitoring temperature change and heat transport in the thermal affected zone. Applications of time-lapse ERT for monitoring thermal tracer transport have been undertaken in the field, but laboratory-scale experiments to validate the field observations are still lacking. In this study, a relationship between temperature and resistivity was derived in a preliminary experiment. The experiment quantitatively assessed the capability of electrical resistivity measurements for characterizing the temporal behavior of the thermal tracer in an experimental column, relying on the derived relationship. The results show that resistivity and the reciprocal of Kelvin temperature have a good quasi-exponential relationship. Time-lapse electrical resistivity measurements are able to capture the temperature change in the column during the continuous injection of hot water. There is a small deviation between the resistivity-derived temperature and the temperature obtained by a temperature sensor, suggesting absolute deviation

Tracer methods have been widely used in many fields of environmental and natural sciences, and also in human health sciences. In particular, tracers are used in the study of karst hydrogeology, typically focusing on phenomena such as sinkholes, sinking rivers and large karst springs. It is known that tracers have been used since antiquity. The aim of tracer tests has been to investigate underground flow paths, transport processes and water–rock interactions, and to get an insight into the functioning of a karst aquifer. In karst hydrogeology, tracer methods are the most important investigation tools beside conventional hydrological methods. In early times, tracer methods were applied only to investigate underground flow-paths. Later they were also used to elucidate transport processes associated with water flow, and today they are often the basis, together with detailed hydrological information, of groundwater protection investigations and aquifer modelling. Many substances (spores, microspheres, bacteriophages, salt tracers, fluorescent dyes, radioactive substances) have been investigated for their properties and potential usage in environmental investigations, in particular the often unknown and inaccessible underground systems of karst areas. A great number of analytical techniques is available. This includes instrumentation for laboratory applications and direct online, on-site or in-situ field measurements. Modern instruments have a high capability for data acquisition, storage and transmission in short intervals, as a basis for quantitative evaluation and modelling. This enables research on the hydrological and hydrochemical dynamics of aquifers and their response to different natural or anthropogenic impacts.

The Luxembourg Sandstone is the main strategic aquifer of the Grand-Duchy of Luxembourg, providing one third of the total drinking water resource in the country. In the unconfined part, the aquifer discharges through more than 400 springs, which are used for supply. The long-lasting diffuse contamination by pesticides (more recently Metolachor ESA) has been previously simulated assuming the aquifer as an equivalent porous medium, and using environmental tracers and baseflow characteristics. A statistical analysis of the whole data set of artificial tracer tests made for this aquifer (112 injections involving 102 springs) emphasized the role of the main fissures (NE–SW) for preferential groundwater flow and the importance of including them in a conceptual model involving a multiple-porosity aquifer. The results show that these fissures are influencing the whole flow system. The average peak velocity is 16 m/h and the average first arrival (maximum) velocity is 52 m/h (32 m/h for injection-well to spring traces alone), with a recorded maximum velocity of 350 m/h. The threats of infiltrating streams within catchments have also been characterized, suggesting the usefulness of tracer tests for optimizing protective measures. The main benefit of the study is the improvement in the ability to delineate safeguard zones accurately, according to more representative velocities and the strong anisotropy.

We propose a dye tracer method to characterize fluid and solute fluxes across the sediment-water interface. Zones of groundwater discharge within the streambed are first identified, and small volume slugs of 0.5 to 1 mL fluorescein dye are released at known subsurface depths. Fluorescein dye allows for visual identification of interface breakthrough locations and times, and dye concentrations at the point of discharge are recorded over time by a fluorometer to generate high resolution breakthrough curves. Groundwater velocities and dispersivities at the demonstration site are estimated by numerically fitting dye breakthroughs to the classical advection-dispersion equation, although the methodology is not limited to a specific transport model. Breakthroughs across the stream-sediment interface at the demonstration site are nonlinear with tracer release depth, and velocity estimates from breakthrough analysis are significantly more reliable than visual dye (time to first dye expression) and Darcy methods which tend to overestimate and underestimate groundwater velocity, respectively. The use of permanent injection points within the streambed and demonstrated reproducibility of dye breakthroughs allow for study of fluid and solute fluxes under seasonally varying hydrologic conditions. The proposed approach also provides a framework for field study of nonconservative, reactive solutes and allows for the determination of characteristic residence times at various depths in the streambed to better understand chemical and nutrient transformations.

Uncertainties associated in tracer transport arising through various geological formations are often sorted by laboratory experiments on representative elementary volume (REV). To understa...

For slug-injection tracer tests, tracer concentrations below the detection limit of the measurement instrument can cause truncation of the observed data. This study investigated the truncation effect on the estimation error of parameters based on analytical solutions and the results of a laboratory-scale experiment. Spatial moment analysis was performed to estimate the measured total mass and transport parameters, including the pore velocity and the longitudinal and transverse dispersivities. Increasing the travel distance and detection limit caused the measured mass and dispersivities to be underestimated regardless of the dimensionality because hydrodynamic dispersion occurs with increasing travel distance, which smoothens the concentration front. The one- and two-dimensional cases showed that the truncation effect on the measured mass and longitudinal dispersivity depended on dimensionality. In contrast, the pore velocity showed no such dependence; the center of mass did not change as the unmeasured portion due to truncation was increased because the plume, which exhibited a Gaussian distribution, was truncated symmetrically. In the experiment, the measured mass and dispersivities likewise depended on the travel distance and detection limit, but there were large differences in the detection limit at which the dimensionless parameter reached a value of zero between the experimental results and analytical solution. This is because the initial plume in the experiment was of a finite size. Thus, experimental design factors such as the scale, device, and dimensionality should be considered to minimize the estimation error of transport parameters, excluding the pore velocity.

Processes in coastal aquifers involving variable-density fluids are often investigated using laboratory sand tanks. However, experimental monitoring of the interface between liquids of different densities is challenging, typically requiring dye tracers and photographic analysis. These are susceptible to problems of tracer adsorption and image inhomogeneity, and experiments are often limited to two-dimensional cross-sections. In addition, the water-table position is an important factor, but its accurate measurement is often difficult. A device has been developed for sand-tank experiments to obtain high-frequency, high-precision measurements of both the freshwater–saltwater interface (inferred from density) and the water-table depth, using a float and a laser sensor in a monitoring well. The proposed device was tested using a two-column experiment, allowing for the estimation of measurement errors, which were generally less than 1 mm. The measurement device can be modified to allow for the monitoring of multiple salinity values, and is well suited to sand-box experiments where salinity dynamics are otherwise difficult to visualize.

Interaction between surface water and groundwater plays a fundamental role in influencing aquatic chemistry, where hyporheic exchange processes, distribution of flow paths and residence times within the hyporheic zone will influence the transport of mass and energy in the surface-water/groundwater system. Geomorphological conditions greatly influence hyporheic exchange, and heterogeneities such as rocks and clay lenses will be a key factor for delineating the hyporheic zone. Electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) were used to investigate the streambed along a 6.3-m-long reach in order to characterise geological layering and distinct features which may influence parameters such as hydraulic conductivity. Time-lapse ERT measurements taken during a tracer injection demonstrated that geological features at the meter-scale played a determining role for the hyporheic flow field. The penetration depth of the tracer into the streambed sediment displayed a variable spatial pattern in areas where the presence of highly resistive anomalies was detected. In areas with more homogeneous sediments, the penetration depth was much more uniformly distributed than observed in more heterogeneous sections, demonstrating that ERT can play a vital role in identifying critical hydraulic features that may influence hyporheic exchange processes. Reciprocal ERT measurements linked variability and thus uncertainty in the modelled resistivity to the spatial locations, which also demonstrated larger variability in the tracer penetration depth, likely due to local heterogeneity in the hydraulic conductivity field.

A Bayesian chemistry-assisted hydrograph separation (BACH) approach was previously demonstrated using 15 years of monthly total phosphorus (TP) and total nitrogen (TN) data from eight mesoscale catchments in New Zealand’s North Island. Calibration was done separately for three 5-year data periods, and in each period, concentrations of the two tracers (TP and TN) discharged from each of the three separated flow paths—fast (event-response near-surface flow), medium (seasonal shallow local groundwater flow), and slow (persistent deeper regional groundwater flow)—were assumed to be constant. This approach has now been extended to reveal non-linear trends in the tracer concentrations in each flow path, each represented using a four-parameter curve (initial and final values of a linear trend plus two harmonics). The extended method (called BACH2) identified clear TP and TN concentration trends in the medium and slow flow paths in most of the eight catchments. TP and TN concentration trends in the fast flow path were generally uncertain, however, due to the infrequency and inherent variability of concentrations sampled during high flow conditions. Concentrations closely matched previously published results from the constant-concentration BACH model calibrated to shorter data series. The BACH2 approach is a powerful tool for revealing concentration trends in the different pathways that sustain stream flow using commonly available water quality and flow data. This type of analysis has not previously been available outside of complex distributed simulation models.

Over-exploitation of groundwater in the regions favorable for human settlements around the world has raised uncertainties on the sustainability of this precious resource in the future. However, the stress on the fresh groundwater zones can be reduced substantially by managing inland brackish/saline aquifers. Freshwater when available in excess can be stored judiciously in such aquifers for later on use; though a certain quantity of fresh water is lost during its storage and recovery depending upon the in situ groundwater gradient and buoyancy effect (caused due to density difference in the injected and ambient waters). Hence, increasing the efficiency of freshwater recovery was our prime concern, and for which there was a need to know the interface boundaries of freshwater stored in the ambient water. For the purpose, a new method has been developed and proposed in this paper that is based on the hydraulics of an injection well and the breakthrough curve of the injected tracer through the storage medium (i.e. aquifer). The close agreement between the outcome of the proposed model and method with the field observations has validated the acceptance of this innovative concept. The model results show that the shape acquired by the injected freshwater in the brackish/saline aquifer depends upon the screened location of the well about its centerline. Additionally, the storage of freshwater in the upper portion of the aquifer is the most efficient way to minimize its contact with the ambient water and subsequently its loss due to the development of the dispersion zone.

The application of accurate transport models is critical in tracer routing through river reaches. Reactive models are widely used because of the reaction and degradation properties of some pollutan...

Coastal lagoons are significant wetland environments found on coastlines throughout the world. Groundwater seepage may be a key component of lagoon water balances, though only a few studies have investigated large (>100 km²) coastal lagoons. In this study, we combined airborne thermal infrared imagery with continuous measurements of radon (²²²Rn—a natural groundwater tracer), conductivity, water temperature and dissolved oxygen to map groundwater seepage to a large coastal lagoon in New Zealand. We found evidence of seepage along the margins of the lagoon but not away from the margins. Our findings confirmed previously known seepage zones and identified new potential locations of groundwater inflow. Both point source and diffuse seepage occurred on the western and northwestern margins of the lagoon and parallel to the barrier between the lagoon and sea. These observations imply geologic controls on seepage. The combination of remote sensing and in‐situ radon measurements allowed us to effectively map groundwater discharge areas across the entire lagoon. Combined, broad‐scale qualitative methods built confidence in our interpretation of groundwater discharge locations in a large, dynamic coastal lagoon.

Understanding scalar transport in solvents is important in chemical engineering, pollution control, and water remediation, where the longitudinal dispersion coefficient (LDC) is a key parameter for describing solute transport in fluids. For flow in classic conditions such as in a pipe or a regular open channel, formulas for LDC are derived from adopting the ideas of the advection–diffusion equation. However, when the flow encounters large-scale roughness, such as an open channel with vegetation, longitudinal dispersion becomes complicated. This paper aims to estimate LDC in an open channel that is partially covered with artificial vegetation. This kind of artificial vegetation is emergent, which comprises eight floats. A two-zone model is proposed to determine LDC in this channel condition. In validating our model, experiments are conducted with Rhodamine as a tracer, whose time-concentration curves are measured at two locations. The routing procedure is applied to obtain LDC from these curves. Results show that the measured LDCs are consistent with the predicted ones, thereby validating the accuracy and reliability of our proposed two-zone model.

A large set of experimental solute tracer breakthrough data (corresponding to more than 350 individual tracer breakthrough curves) in eight granular filter materials, were used to investigate the links between solute dispersivity and the shape of the pore velocity—relative pore area distributions for the materials. Solute dispersivity values and the shape of the pore velocity—relative pore area distributions were determined by fitting the breakthrough data to the advection–dispersion equation and to the stream tube modeling (STM), respectively. For the STM calculations, stream tube diameter and average stream tube flow rate were allowed to vary between individual stream tubes. Shape parameters e.g., mean, coefficient of variation, skewness and excess kurtosis for the STM-based pore velocity—relative pore area distributions were subsequently calculated. Comparisons between dispersivity and shape parameter values showed strong correlation between dispersivity, coefficient of variation, skewness and excess kurtosis. These results also indicated a universal relationship between dispersivity, skewness and excess kurtosis across all materials and breakthrough curves. This relationship will assist in modelling of experimental data which cannot be characterized by a single or a limited number of pore velocities but require continuous pore velocity distributions. Results further indicated, that the shape of the observed pore velocity—relative pore area distributions could be well approximated by a log-normal distribution.

Distinguishing municipal wastewater effluent (MWWE) from other industrial effluents or through an urbanized watershed can be challenging. In complex receiving environments, linking environmental responses to specific compounds or effluents is not always straight forward. In order to characterize the inherent complexity of tracing MWWE in aquatic systems influenced by multiple stressors, a proposed multi-tracer suite is intended to highlight areas of potential biological concern. Characterization and quantification of effluent exposure to aquatic biota in this manner is essential to shape policies intended to encourage wastewater infrastructure development (i.e. treatment plant upgrade) and broader environmental management. This paper describes the use of a comprehensive suite of tracers that includes isotopes in support of a core surveillance program, demonstrating its effectiveness both empirically and with respect to diagnostic value contributed to monitoring programs.

Peat soils are heterogeneous, anisotropic porous media. Compared to mineral soils, there is still limited understanding of physical and solute transport properties of fen peat soils. In this study, we aimed to explore the effect of soil anisotropy on solute transport in degraded fen peat. Undisturbed soil cores, taken in vertical and horizontal direction, were collected from one drained and one restored fen peatland both in a comparable state of soil degradation. Saturated hydraulic conductivity (Ks) and chemical properties of peat were determined for all soil cores. Miscible displacement experiments were conducted under saturated steady state conditions using potassium bromide as a conservative tracer. The results showed that (1) the Ks in vertical direction (Ksv) was significantly higher than that in horizontal direction (Ksh), indicating that Ks of degraded fen peat behaves anisotropically; (2) pronounced preferential flow occurred in vertical direction with a higher immobile water fraction and a higher pore water velocity; (3) the 5% arrival time (a proxy for the strength of preferential flow) was affected by soil anisotropy as well as study site. A strong correlation was found between 5% arrival time and dispersivity, Ks and mobile water fraction; (4) phosphate release was observed from drained peat only. The impact of soil heterogeneity on phosphate leaching was more pronounced than soil anisotropy. The soil core with the strongest preferential flow released the highest amount of phosphate. We conclude that soil anisotropy is crucial in peatland hydrology but additional research is required to fully understand anisotropy effects on solute transport.

The contamination of groundwater by organic compounds is becoming more common and severe. A diesel tracer, a light non-aqueous phase liquid (LNAPL), was selected to carry out experiments of contaminant migration in fractured porous media. At the same time, trials of contaminant remediation methods were conducted using distilled water. The experimental results showed that the dominant flow was more obvious in fractured porous media than in porous media owing to the effect of the fractures on contaminant migration. The mechanism of contaminant migration and remediation was revealed by the movement of oil-driven air and water-driven oil. The dispersion parameters of these media were calibrated through the calculated areas and the measured contamination and remediation areas, using a numerical method. The predicted results indicate that the greater the injection rate, the shorter the time to penetrate the fractured porous media. However, relative to the LNAPL migration experiments, the water-driven oil processes were slower and the remediation time was longer at the same injection rate.

The occurrence of preferential flow in the subsurface has often been shown in field experiments. However, preferential flow is rarely included in models simulating the hydrological response at the catchment scale. If it is considered, preferential flow parameters are typically determined at the plot scale and then transferred to larger‐scale simulations. Here, we successfully used the optimization algorithm DiffeRential Evolution Adaptive Metropolis (DREAM) to calibrate a 3D physics‐based dual‐permeability model directly at the catchment scale. In order to keep computational costs of the optimization routine at a reasonable level, we limited the number of parameters to be calibrated to the ones that had been shown before to be most influential for the simulation of discharge. We also calibrated parameters of the matrix domain and the macropore domain with a fixed parameter ratio between soil layers instead of calibrating every layer separately. These ratios reflected observed depth profiles of soil hydraulic properties at our study site. The dual‐permeability parameter sets identified during calibration were able to simulate observed discharge time series satisfactorily but did not outperform a calibrated single‐domain reference model scenario. Saturated hydraulic conductivities of the macropore domain were calibrated such that they became very similar to matrix saturated hydraulic conductivities, thereby effectively removing the effect of macropores. This suggests that the incorporation of vertical preferential flow as represented by the dual‐permeability approach was not relevant for reproducing the hydrometric response reasonably well in the studied catchment. We also tested the scale‐invariance of the calibrated dual‐permeability parameter sets by using the parameter sets performing best at catchment scale to simulate plot‐scale bromide depth profiles obtained from tracer irrigation experiments. This parameter transfer proved to be not successful, indicating that soil hydraulic parameters are scale‐variant, independent of the direction of parameter transfer.

The presented study evaluates the effect of water-table depth on groundwater evaporation (Eg) and recharge (Rg) when they occurred alternately. A lysimeter experiment incorporating a 1-year-long bromide tracer test was conducted under conditions with a range of maintained water-table depths. The results revealed that both the Rg and Eg decreased as the water table fell, until it was down to the extinction depth of groundwater evaporation (EDGE, 2.4 m). The annual quantity of Rg started to be stable at 100 mm when the water table was below the EDGE, since the maximum soil-water deficit no longer increased. When the water table was above the EDGE, Rg and Eg restricted each other and thus occurred alternately; in the wet season, >68% of the annual Rg occurred, with only

Tritium presents a natural radioactive isotope which can be used as a tracer in hydrology. This paper presents the usage of tritium in hydrogeological research of the Zagreb aquifer, which presents the country`s strategic water reserves and is protected by the Republic of Croatia. In the research area, higher tritium content in surface water and groundwater is the result of releasing coolant water from the Nuclear power plant Krško. Tritium content was measured in 13 piezometers and one pumping well, in the wider area of the Petruševec and Kosnica well fields. It has been shown that tritium from cooling water is much more pronounced than its natural seasonal variability, and it can be very useful as a tracer. Also, results showed that the intensity and delay of responses in the aquifer depend on their distance from the Sava River and piezometer depth. Furthermore, the results suggest that variable hydrologic conditions can generate rapid change in the groundwater flow direction and affect tritium transport through the aquifer., Tricij je prirodni radioaktivni izotop koji se može koristiti kao traser u hidrologiji. Ovim radom prikazuje se korištenje tricija u hidrogeološkim istraživanjima zagrebačkoga vodonosnika koji je Republika Hrvatska zaštitila i uvrstila u strateške zalihe vode. Na istraživanome području povišen sadržaj tricija u površinskoj i podzemnoj vodi posljedica je ispuštanja rashladne vode iz nuklearne elektrane Krško u rijeku Savu. Sadržaj tricija mjeren je u 13 piezometara i u jednome zdencu na području crpilišta Petruševec i Kosnica. Pokazano je da je tricij podrijetlom iz rashladne vode naglašeniji od njegove sezonske varijabilnosti te može biti korišten kao traser. Također, rezultati su pokazali da intenzitet i zakašnjenje reakcije u vodonosniku izravno ovisi o udaljenosti od rijeke Save i dubini piezometra. Nadalje, rezultati upućuju na to da promjenjivi hidrološki uvjeti uzrokuju brze promjene u smjeru tečenja podzemne vode i utječu na transport tricija kroz vodonosnik.

This study explores by a vertically integrated tracer transport model, hydro-meteorological event characteristics and their influence on solute transport. Changes in Hydro-meteorological processes and increasing frequency of extreme weather events are responsible for changing the lake water balance, influencing streamflow variations, and lake tracer transport. We compare historical data over a long time with model data from a vertically integrated model in Comsol Multiphysics. We consider water balance, sources of data uncertainty, correlations, extreme rain and inflow years, and seasonal variations. The lake transport model has estimated soluble loading and transportation. The results showed that the lake rainfall is seasonal; there are strong correlations between tributary inflows and precipitation, and between lake outflow and water level. It was found that “events” influence lake level fluctuations. The solute transport was shown to vary more in wet periods. Modeled transportations were higher in Kenya and Uganda lake zones than in Tanzanian zones. The major inflow, from the Kagera river, appears to strongly influence lake solute transportation, so the composition of this river must be considered. QCR 20191106

The Baltic Sea receives substantial amounts of hazardous substances and nutrients, which accumulate for decades and persistently impair the Baltic ecosystems. With long half-lives and high solubility, anthropogenic uranium isotopes (236U and 233U) are ideal tracers to depict the ocean dynamics in the Baltic Sea and the associated impacts on the fate of contaminants. However, their applications in the Baltic Sea are hampered by the inadequate source-terms information. This study reports the first three-dimensional distributions of 236U and 233U in the Baltic Sea (2018-2019) and the first long-term hindcast simulation for 236U dispersion in the North-Baltic Sea (1971-2018). Using 233U/236U fingerprints, we distinguish 236U from the nuclear weapon testing and civil nuclear industries, which have comparable contributions (142 ± 13 and 174 ± 40 g) to the 236U inventory in modern Baltic seawater. Budget calculations for 236U inputs since the 1950s indicate that, the major 236U sources in the Baltic Sea are the atmospheric fallouts (∼1.35 kg) and discharges from nuclear reprocessing plants (>211 g), and there is a continuous sink of 236U to the anoxic sediments (589 ± 43 g). Our findings also indicate that the limited water renewal endows the Baltic Sea a strong "memory effect" retaining aged 236U signals, and the previously unknown 236U in the Baltic Sea is likely attributed to the retention of the mid-1990s' discharges from the nuclear reprocessing plants. Our preliminary results also demonstrate the power of 236U-129I dual-tracer in investigating water-mass mixing and estimating water age in the Baltic Sea, and this work provides fundamental knowledge for future 236U tracer studies in the Baltic Sea.

Water transit time is now a standard measure in catchment hydrological and ecohydrological research. The last comprehensive review of transit time modeling approaches was published 15+ years ago. But since then the field has largely expanded with new data, theory and applications. Here, we review these new developments with focus on water-age-balance approaches and data-based approaches. We discuss and compare methods including StorAge-Selection functions, well/partially mixed compartments, water age tracking through spatially distributed models, direct transit time estimates from controlled experiments, young water fractions, and ensemble hydrograph separation. We unify some of the heterogeneity in the literature that has crept in with these many new approaches, in an attempt to clarify the key differences and similarities among them. Finally, we point to open questions in transit time research, including what we still need from theory, models, field work, and community practice., Water Resources Research, 58 (11), ISSN:0043-1397, ISSN:1944-7973

Tracer tests are widely used for characterizing hydrodynamics, from stream-scale to basin-wide scale. In karstic environments, the positioning of field fluorometers (or sampling) is mostly determined by the on-site configuration and setup difficulties. Most users are probably aware of the importance of this positioning for the relevance of data, and single-point tests are considered reliable. However, this importance is subjective to the user and the impact of positioning is not well quantified. This study aimed to quantify the spatial heterogeneity of tracer concentration through time in a karstic environment, and its impact on tracer test results and derived information on local hydrodynamics. Two approaches were considered: on-site tracing experiments in a karstic river, and Computational Fluid Dynamics (CFD) modeling of tracer dispersion through a discretized karst river channel. A comparison between on-site tracer breakthrough curves and CFD results was allowed by a thorough assessment of the river geometry. The results of on-site tracer tests showed significant heterogeneities of the breakthrough curve shape from fluorometers placed along a cross-section. CFD modeling of the tracer test through the associated discretized site geometry showed similar heterogeneity and was consistent with the positioning of on-site fluorometers, thus showing that geometry is a major contributor of the spatial heterogeneity of tracer concentration through time in karstic rivers.

In many developing countries, poorly constructed pit latrines are the primary source of groundwater contamination. Approaches are needed to identify site-specific separation distances between domestic wells and pit latrines. In this study, tracer transport simulations are combined with water quality monitoring data to identify separation distances in peri-urban Dar es Salaam. Bivariate correlation and linear regression models were used to find the relationships between (1) simulated tracer and distances from the wells to the nearest pit latrines (2) simulated tracer and observed contaminants (nitrate, E. coli, and total dissolved solids). The results showed a strong correlation between tracer with nitrate and E. coli, with Pearson coefficient (r) values of 0.80 and 0.79, but a weak correlation with total dissolved solids (TDS) (r = 0.23). A strong correlation between tracer and distance for shallow and deep wells (r = −0.96, −0.76) was found. Based on the soil type and aquifer properties in the area, wells must be placed at least 34 m from a pit latrine to minimize contamination. With recent advances in sensor technologies and the availability of low-cost sensors, linking simulated tracer with observed contaminant levels may provide an alternative first approach to quickly assess human health risks associated with groundwater contamination. HIGHLIGHTS A method to determine site-specific separation distance between wells and pit latrines was developed.; Tracer transport simulations and statistical analyses were used to find relationships between tracer, distance and contaminants.; A strong correlation was found between simulated tracer, distance to the nearest pit latrine and contaminants.; In this area, wells must be placed >34 m from pit latrines to minimize contamination.

Oxidative weathering of sedimentary rocks plays an important role in the global carbon cycle. Rhenium (Re) has been proposed as a tracer of rock organic carbon (OCpetro) oxidation. However, the sources of Re and its mobilization by hydrological processes remain poorly constrained. Here, we examine dissolved Re as a function of water discharge, using samples collected from three alpine catchments that drain sedimentary rocks in Switzerland (Erlenbach and Vogelbach) and Colorado, USA (East River). The Swiss catchments reveal a higher dissolved Re flux in the catchment with higher erosion rates, but have similar [Re]/[Na+] and [Re]/[SO42−] ratios, which indicate a dominance of Re from OCpetro. Despite differences in rock type and hydro-climatic setting, the three catchments have a positive correlation between river water [Re]/[Na+] and [Re]/[SO42−] and water discharge. We propose that this reflects preferential routing of Re from a near-surface, oxidative weathering zone. The observations support the use of Re as a proxy to trace rock-organic carbon oxidation, and suggest it may be a hydrological tracer of vadose zone processes. We apply the Re proxy and estimate CO2 release by OCpetro oxidation of 5.7 +6.6/−2.0 tC km−2 yr−1 for the Erlenbach. The overall weathering intensity was ∼40%, meaning that the corresponding export of unweathered OCpetro in river sediments is large, and the findings call for more measurements of OCpetro oxidation in mountains and rivers as they cross floodplains.

Funding: The ISSS-08 program was supported by the Knut and Alice Wallenberg Foundation, the Far Eastern Branch of the Russian Academy of Sciences and the Swedish Research Council (VR contract 621–e2004-4283). Tracing riverine freshwater transport pathways within the Arctic Ocean is key to understanding changes in Arctic Ocean freshwater inventories. Dissolved Ba concentrations have been used in this capacity but are compromised by non-conservative processes. To assess the potential for Ba isotopes to provide insights into the impact of such processes on Arctic Ocean dissolved Ba inventories, Ba concentration and isotope data for surface seawater samples from the Siberian Shelf and Bering Sea/Strait are presented. These samples capture the mixing of riverine freshwater discharged by the rivers Yenisey, Lena and Ob, with Atlantic and Pacific derived seawater, which are traced by relationships between salinity, Ba concentration and δ138/134Ba. The δ138/134Ba of net river inputs, following modification by estuarine processes, are constrained to be 0.31 ± 0.04‰, 0.20 ± 0.06‰ and 0.23 ± 0.04‰, for the rivers Yenisey, Lena and Ob respectively. These values are used to estimate an average δ138/134Ba for Eurasian river freshwater input to the Arctic Ocean of 0.23 ± 0.04‰. The Ba concentration and δ138/134Ba of Lena River freshwater transported across the Laptev Sea are modified by non-conservative processes. These non-conservative processes do not result in distinctive modification of dissolved Ba concentration-δ138/134Ba mixing relationships between Eurasian riverine freshwater and Arctic seawater, which unfortunately limits the potential of Ba isotopes to improve tracing riverine freshwater sources in the central Arctic Ocean basins using dissolved Ba inventories. More generally the results of this study help advance understanding of Ba isotope cycling in the environment and their development as an emerging tracer of marine processes. Postprint

Experiments focused on pollution transport and dispersion phenomena in conditions of low flow (low water depth and velocities) in sewers with bed sediment and deposits are presented. Such conditions occur very often in sewer pipes during dry weather flows. Experiments were performed in laboratory conditions. To simulate real hydraulic conditions in sewer pipes, sand of fraction 0.6–1.2 mm was placed on the bottom of the pipe. In total, we performed 23 experiments with 4 different thicknesses of sand sediment layers. The first scenario is without sediment, the second is with sediment filling 3.4% of the pipe diameter (sediment layer thickness = 8.5 mm), the third scenario represents sediment filling 10% of the pipe diameter (sediment layer thickness = 25 mm) and sediment fills 14% of the pipe diameter (sediment layer thickness = 35 mm) in the last scenario. For each thickness of the sediment layer, a set of tracer experiments with different flow rates was performed. The discharge ranges were from (0.14–2.5)·10−3 m3·s−1, corresponding to the range of Reynolds number 500–18,000. Results show that in the hydraulic conditions of a circular sewer pipe with the occurrence of sediment and deposits, the value of the longitudinal dispersion coefficient Dx decreases almost linearly with decrease of the flow rate (also with Reynolds number) to a certain limit (inflexion point), which is individual for each particular sediment thickness. Below this limit the value of the dispersion coefficient starts to rise again, together with increasing asymmetricity of the concentration distribution in time, caused by transient (dead) storage zones.

In the oil and gas industry, tracers are used to estimate residual oil saturation, to indicate the location and orientation of fractures in tight reservoirs, to identify and mark the direction of fluid flow in fractured deposits, to locate faults and discontinuities, and to measure fluid movement in injection wells during drilling. The tracers should behave in a mechanically similar manner to the tested substance, e.g., formation waters, oil or gas, and, on the other hand, they should significantly differ from them in terms of chemical properties so that it is possible to identify them. One of the fluorescent tracers used in the oil and gas industry, e.g., for inter-well tests during secondary or tertiary production methods (especially during reservoir hydration), is uranine. In order to assess the effectiveness of fluid movement measurements, it is necessary to determine the uranine content in formation waters. In this study, a method was developed to determine uranine in formation water samples using high-performance liquid chromatography with fluorescence detection (HPLC/FLD). The initial step in preparing samples for chromatographic analysis would be solid phase extraction (SPE). The method was validated and allows for the determination of uranine in formation water samples in the concentration range from 0.030 to 2.80 µg/L. The validation of the method included the analysis of factors influencing the measurement result (sources of uncertainty), determination of the linearity range of the standard curve, determination of the quantification limit of the method, and verification of the reproducibility, selectivity, stability and correctness achieved. The method developed within the study can be successfully applied in the case of the determination of uranine content in formation water samples from the oil and gas mining industry, which are often unstable and characterized by a relatively complex matrix. After validation, the method will also be applicable to the determination of uranine in matrices with a similar physicochemical composition, e.g., to assess groundwater flow in deformed carbonate aquifers or to characterize faults that act as barriers to horizontal groundwater flow.

The study of karstic aquifers is a challenge because of their heterogeneous and anisotropic nature; the groundwater flow in these systems often cannot be explained using Darcian theory. Many techniques (direct and indirect) have been used for the characterization of karstic aquifers. Dissolved oxygen alteration method (DOAM) consists in injecting air into wells and tracing the evolution of this parameter over time; this practice offers advantages because it is cheap and harmless. Previous experiences show acceptable results in the identification of fractures and preferential flow in heterogeneous aquifers. The air injection used during the drilling process (rotary perforation with air cleaning) could be used as a tracer. This research monitored the physical parameters in groundwater after drilling wells, especially the dissolved oxygen. Besides, karstic zones have been identified using video surveys in drilling wells. Finally, the results using the DOAM method were contrasted with the physical evidence of karstic zones to compare the technique’s accuracy. Results show that DOAM offers a qualitative approach to the evidence of karstic zones and preferential groundwater flow. DOAM can be considered as a useful choice to study the karstification influence on the aquifer groundwater flow and transport in karstic aquifers. The identification of these zones areis important for the design of supply and wastewater projects; therefore, this approach represents an improvement in the wells’ design process in aquifers with similar conditions.

We compared field measurements of the biological O2 saturation anomalies, ΔO2/Ar and ΔO2/N2, from simultaneous oceanographic deployments of a membrane inlet mass spectrometer and optode/gas tension device (GTD). Data from the Subarctic Northeast Pacific and Canadian Arctic Ocean were used to evaluate ΔO2/N2 as an alternative to ΔO2/Ar for estimates of mixed layer net community production (NCP). We observed strong spatial coherence between ΔO2/Ar and ΔO2/N2, with small offsets resulting from differences in the solubility properties of Ar and N2 and their sensitivity to vertical mixing fluxes. Larger offsets between the two tracers were observed across hydrographic fronts and under elevated sea states, resulting from the differential time-response of the optode and GTD, and from bubble dissolution in the ship’s seawater lines. We used a simple numerical framework to correct for physical sources of divergence between N2 and Ar, deriving the tracer ΔO2/N2′. Over most of our survey regions, ΔO2/N2′ provided a better analog for ΔO2/Ar, and thus more accurate NCP estimates than ΔO2/N2. However, in coastal Arctic waters, ΔO2/N2 and ΔO2/N2′ performed equally well as NCP tracers. On average, mixed layer NCP estimated from ΔO2/Ar and ΔO2/N2′ agreed to within ∼2 mmol O2 m–2 d–1, with offsets typically smaller than other errors in NCP calculations. Our results demonstrate a significant potential to derive NCP from underway O2/N2 measurements across various oceanic regions. Optode/GTD systems could replace mass spectrometers for autonomous NCP derivation under many oceanographic conditions, thereby presenting opportunities to significantly expand global NCP coverage from various underway platforms.

Aquifers under agricultural areas are considered to be an indirect source of nitrous oxide emission (N2O) to the atmosphere, which is the greenhouse gas (GHGs) characterized with the highest global warning potential and acts as a stratospheric ozone depletion agent. Previous investigations performed in the Cretaceous Hesbaye chalk aquifer in Eastern Belgium suggested that the dynamics of N2O in the aquifer is controlled by overlapping biochemical processes such as nitrification and denitrification. The current study aims to obtain better insight concerning the factors controlling the distribution of N2O concentration along a vertical dimension in the aquifer, and to capture and quantify the occurrence of nitrification and denitrification processes in the groundwater system. Low-flow groundwater sampling technique was undertaken at different depths in the aquifer to collect groundwater samples aiming at obtaining information about ambient aquifer hydrogeochemical conditions and their effect on the accumulation of GHGs. Afterwards, laboratory stable isotope experiments, using NO3- and NH4+ compounds labeled with heavy 15N isotope, were applied to quantify the rates of nitrification and denitrification processes. Ambient studies suggest that the occurrence of N transformation was related to denitrification while laboratory incubation experiments did not detect it. Such controversial results might be explained by the discrepancy between real aquifer conditions and lab design studies. Thus, additional in situ tracer experiments should be carried out in areas where natural groundwater fluxes do not flush the injected tracer too rapidly. In addition, it would be useful to conduct microbiological studies to obtain better insight into the nature of subsurface biofilm biotope., This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 675120. Los Gatos Research OA-ICOS was funded by the Fonds National de la Rechecherche Scientifique (FNRS) (22547486). AVB is a Research Director at the FNRS.

Gadolinium (Gd), a member of the rare earth elements (REE), is becoming an increasingly observed microcontaminant in waters of developed regions. Anthropogenic Gd anomalies were first noted in 1996 and were determined to be sourced from Gd-based contrast agents used in magnetic resonance imaging (MRI). This study investigates Gd anomalies in North Carolina’s Triangle Area, focusing on surrounding wastewater treatment plants (WWTPs). Samples were obtained from upstream and downstream of selected WWTPs as well as a freshwater reservoir that supplies part of the region’s drinking water. The PAAS-normalized samples indicate Gd anomalies in the influent, effluent, and downstream samples. We quantify the anthropogenic Gd in wastewater samples to constitute between 98.1% to 99.8%. Sample comparisons show an average increase of 45.3% estimated anthropogenic Gd between samples upstream and downstream of WWTPs. This research contributes to the existing database demonstrating the presence of anthropogenic Gd in developed regions. Although current Gd concentrations are not near toxic levels, they should be continuously monitored as a micropollutant and serve as a wastewater tracer.