Riparian zones are highly‐dynamic transition zones between surface water (SW) and groundwater (GW) and function as key biogeochemical‐reactors for solutes transitioning between both compartments. ...Infiltration of SW rich in dissolved oxygen (DO) into the riparian aquifer can supress removal processes of redox sensitive compounds like NO3−, a nutrient harmful for the aquatic ecosystem at high concentrations. Seasonal and short‐term variations of temperature and hydrologic conditions can influence biogeochemical reaction rates and thus the prevailing redox conditions in the riparian zone. We combined GW tracer‐tests and a 1‐year high‐frequency dataset of DO with data‐driven simulations of DO consumption to assess the effects of seasonal and event‐scale variations in temperature and transit‐times on the reactive transport of DO. Damköhler numbers for DO consumption (DADO) were used to characterize the system in terms of DO turnover potential. Our results suggest that seasonal and short‐term variations in temperature are major controls for DO turnover and the resulting concentrations at our field site, while transit‐times are of minor importance. Seasonal variations of temperature in GW lead to shifts from transport‐limited (DADO > 1) to reaction‐limited conditions (DADO < 1), while short‐term events were found to have minor impacts on the state of the system, only resulting in slightly less transport‐limited conditions due to decreasing temperature and transit‐times. The data‐driven analyses show that assuming constant water temperature along a flowpath can lead to an over‐ or underestimation of reaction rates by a factor of 2–3 due to different infiltrating water temperature at the SW–GW interface, whereas the assumption of constant transit‐times results in incorrect estimates of NO3− removal potential based on DADO approach (40%–50% difference).
Groundwater tracer‐tests are combined with 1‐year high‐frequency data of dissolved oxygen, water temperature, and water‐levels, and data‐driven simulations to assess the seasonal and event‐term variations of transport and consumption of riparian dissolved oxygen and the further implications for redox processes.
•Numerically evaluated photosensitive tracer as a strategy to separate surface and hyporheic transient storage contributions•Jointly fitting day and night tracer data using two-storage zone model ...delineated hyporheic and surface transient storage•Single storage model or day-only tracer data led to biases in the parameter estimates, yet good model-data fit•Good model fit to tracer test data does not guarantee accurate estimates of stream function
We numerically evaluated photosensitive tracers as a potential strategy for separating the effects of surface and hyporheic storage zones (SSZs and HSZs, respectively) on stream corridor transport. Correctly separating HSZ and SSZ effects is critical to estimating the hydro-biogeochemical function of a stream because HSZs and SSZs expose solutes to significantly different biogeochemical conditions, like sunlight exposure, microbial processes, and oxygen availability. Our numerical experiments used a multiscale river-corridor transport model implemented in the ATS code, which accommodates multiple storage zones with distinct travel time distributions and biogeochemical reactions. For parameter inferences, we used Bayesian inverse modeling. We found that breakthrough curves for photo-decaying tracers from day and night injection can delineate surface and hyporheic transient storage contributions, but only when interpreted jointly through a two-storage zone model. Numerical experiments that used only daytime injection or interpreted breakthrough curves with a single storage zone model yielded good fit to breakthrough curves, but parameter estimates were biased and controlling processes misattributed, examples of good model fits for the wrong reasons. Using those biased parameter estimates in reactive transport simulations resulted in significantly different projections of denitrification, which underscores the potential for stream function to be mischaracterized if tracer tests are interpreted through an inappropriately simplified model for transient storage. More generally, this study highlights the role of modeling in evaluating the experimental design and identifying the potential of system mischaracterization and its implications.
Tracking contaminants in karst aquifers is challenging because of the high heterogeneity encountered in carbonate rocks. Multi-tracer tests, combined with chemical and isotopic analyses, were ...conducted to solve a groundwater contamination incident within a complex karst aquifer in Southwest China. Results showed that: (1) the wastewater from a paper mill, public sewers, and septic tanks were the three main potential contaminant sources identified by chemical and isotopic methods; (2) a direct effect of the paper mill wastewater with high Na+ (up to 2230.5 mg/L) and chemical oxygen demand (COD) concentrations on spring water quality was confirmed by multi-tracer tests, which changed the water type from Ca–HCO3 in the 1970s to Ca–Na–HCO3 in the present study and resulted in a depleted carbon isotope value (−16.5‰); and (3) the studied aquifer is a highly complex karst system, due to two conduits crossed each other without mixing, contaminants traveled a long distance (up to 14 km) within the lower conduit, paper mill-contaminated groundwater flowed across a river bottom and discharged to the opposite bank, and an active subsurface divide occurred. After several months of operation, the groundwater restoration measure based on karst hydrogeologic conditions proved that cutting off contaminant sources for karst aquifer self-restore was effective in practice, which contributed to the decline in NH4+ (from 7.81 mg/L to 0.04 mg/L), Na+ (from 50.12 mg/L to 4.78 mg/L), and COD (from 16.42 mg/L to 0.9 mg/L) concentrations coupled with an increase in δ13C-DIC value (from −16.5‰ to −8.4‰) in the earlier contaminated karst spring. This study's integrated method is expected to screen and confirm contaminant sources within complex karst systems rapidly and effectively, thereby contributing to karst groundwater environmental management.
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•Hydrochemistry and stable isotopes to identify the main potential contaminant sources.•Location and pathway of contaminants were verified by multi-tracer tests.•High NH4+ and COD were directly detected at 14 km distance in a karst spring.•Dual karst conduit structure was evident from the forced-gradient test.
In fractured media, solute transport is controlled by advection in open and connected fractures and by matrix diffusion that may be enhanced by chemical weathering of the fracture walls. These ...phenomena may lead to non-Fickian dispersion characterized by early tracer arrival time, late-time tailing on the breakthrough curves and potential scale effect on transport processes. Here we investigate the scale dependency of these processes by analyzing a series of convergent and push-pull tracer experiments with distance of investigation ranging from 4m to 41m in shallow fractured granite. The small and intermediate distances convergent experiments display a non-Fickian tailing, characterized by a –2 power law slope. However, the largest distance experiment does not display a clear power law behavior and indicates possibly two main pathways. The push-pull experiments show breakthrough curve tailing decreases as the volume of investigation increases, with a power law slope ranging from −3 to −2.3 from the smallest to the largest volume. The multipath model developed by Becker and Shapiro (2003) is used here to evaluate the hypothesis of the independence of flow pathways. The multipath model is found to explain the convergent data, when increasing local dispersivity and reducing the number of pathways with distance which suggest a transition from non-Fickian to Fickian transport at fracture scale. However, this model predicts an increase of tailing with push-pull distance, while the experiments show the opposite trend. This inconsistency may suggest the activation of cross channel mass transfer at larger volume of investigation, which leads to non-reversible heterogeneous advection with scale. This transition from independent channels to connected channels when the volume of investigation increases suggest that both convergent and push-pull breakthrough curves can inform the existence of characteristic length scales.
•Increasing investigated volume in push-pull and convergent tests shows transition from independent to connected channels.•Multi-scale tracer tests may contain information about correlation length in fractured rocks•Non-reversible advective process appears more important in push-pull tests with increasing volume of investigation
The in situ characterization of transport processes in fractured media is particularly challenging due to the considerable spatial uncertainty on tracer pathways and dominant controlling processes, ...such as dispersion, channeling, trapping, matrix diffusion, ambient and density driven flows. We attempted to reduce this uncertainty by coupling push‐pull tracer experiments with single‐hole ground penetrating radar (GPR) time‐lapse imaging. The experiments involved different injection fractures, chaser volumes and resting times, and were performed at the fractured rock research site of Ploemeur in France (H+ network, hplus.ore.fr/en). For the GPR acquisitions, we used both fixed and moving antenna setups in a borehole that was isolated with a flexible liner. During the fixed‐antenna experiment, time‐varying GPR reflections allowed us to track the spatial and temporal dynamics of the tracer during the push‐pull experiment. During the moving antenna experiments, we clearly imaged the dominant fractures in which tracer transport took place, fractures in which the tracer was trapped for longer time periods, and the spatial extent of the tracer distribution (up to 8 m) at different times. This demonstrated the existence of strongly channelized flow in the first few meters and radial flow at greater distances. By varying the resting time of a given experiment, we identified regions affected by density‐driven and ambient flow. These experiments open up new perspectives for coupled hydrogeophysical inversion aimed at understanding transport phenomena in fractured rock formations.
Key points:
Combined single‐hole GPR and push‐pull tracer tests help to infer transport phenomena
Transport length scales are estimated through GPR difference imaging
GPR inform on tracer velocity, fracture channeling, and density driven flow
•Inexpensive and reliable approach for characterisation of fractured aquifers.•Combines uniform and point injection dilution tests on open sections of boreholes.•Proposed workflow to identify nature ...and depth distribution of flowing fractures.•Fracture porosity and permeability distribution determined for specific conditions.•Groundwater velocities determined were validated against borehole-to-borehole tests.
Fractured carbonate aquifers derive their transmissivity essentially from a well-developed network of solutionally-enhanced fractures and conduits that can lead to high groundwater velocities and high vulnerability to contamination of water quality. Characterisation of the variation of hydraulic properties with depth is important for delineating source protection areas, characterising contaminant fate and transport, determination of the effectiveness of aquifer remediation, and parameter estimation for models. In this work, ambient open borehole uniform and point injection dilution tests were conducted on observation boreholes in the unconfined Cretaceous Chalk aquifer of East Yorkshire, UK, and interpreted in conjunction with other data via the implementation of a new work flow. This resulted in the characterisation of flow in these boreholes and the inference of properties such as groundwater flow patterns and velocities in the surrounding aquifer formation. Our workflow allowed sections of open boreholes showing horizontal versus vertical flow to be distinguished, and the magnitude of such flows and exchanges with the aquifer to be determined. Flow within boreholes were then used to characterise: i) presence and direction of vertical hydraulic gradients; ii) nature and depth distribution of flowing features; iii) depth interval porosity and permeability estimation of the flowing features from overall borehole transmissivity and geophysical image or caliper logs; iv) groundwater velocity estimation in the surrounding aquifer. Discrete flowing features were distributed across the range of depths sampled by the observation boreholes (typically up to 45–60 mbgl), but the majority were located in the zone of water table fluctuation marked by solutionally enlarged flow features. Quantitative interpretation of both uniform injection (tracer distributed throughout the open borehole section) and point injection (slug of tracer introduced at targeted depth) yielded vertical velocities within the borehole water column in broad agreement with those measured by flow logging. Depth specific fracture kinematic porosities inferred from the ambient dilution data combined with long-interval pump test and geophysical log data ranged between 3.7 × 10−4–4.1 × 10−3 with an average of 2.1 × 10−3; these values were in excellent agreement with those from other methods applied to the same aquifer such as larger scale pumping tests. A new approach to estimation of groundwater velocities from the dilution test data using externally measured hydraulic gradients gave inferred horizontal groundwater velocities ranging between 60 and 850 m/day, in full agreement with those from previously conducted borehole-to-borehole tracer tests. These results confirm that the studied aquifer is karstic, with rapid preferential pathways which have implication for flow and transport modelling, and pollution vulnerability. Our study results indicate that ambient single-borehole dilution approaches can provide an inexpensive and reliable approach for the characterisation of fractured and karstic aquifers.
Groundwater contamination and transport of viruses and bacteria in aquifers are a major concern worldwide. To ascertain the ability of these aquifers to remove pathogens, tracer tests with microbial ...surrogates are carried out. These tests are laborious and may require special permits, and therefore, column tests are often done instead. Unfortunately, results from column tests tend to grossly overestimate removal rates when compared to the field scale, which can lead to an underestimation of groundwater contamination risks. Scale is an important consideration when examining pathogen transport through porous media, as pathogen removal is rarely a linear process. In this study, field tests were carried out with endospores of Bacillus subtilis and coliphage phiX174 over a distance of 25 m in an alluvial gravel aquifer near Vienna, Austria. The sandy gravel material from the field site was also used in column tests with the same tracers. Both attachment-detachment and colloid filtration theory were used to model these tests, as well as log-removal rates per meter. The results show that the spatial removal rate (log/m) is approximately 2 orders of magnitude higher on the column scale, when compared to the field. A comparison with the literature showed a correlation between the heterogeneity of the porous media and the difference in removal rates between the column and field scale.
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•A CFD model is developed and validated by means of experimental tracer tests.•Different modelling approaches are successfully applied for reactor elements.•Dead volumes and ...channelling are located and quantified through model simulations.•Mixing quality is confirmed by means of uniformity index analysis.•A deflector is essential for maintaining the anaerobic-anoxic multi-environment.
AnoxAn is a novel multi-environment reactor for biological nutrient removal (BNR) from wastewater. Although its biological efficacy has been demonstrated on a pilot scale, hydrodynamics is observed to significantly affect the performance of AnoxAn. To study its complex hydraulic behaviour, a model based on Computational Fluid Dynamics 3D (CFD) is constructed using the OpenFOAM® open source toolbox and validated by experimental tests of Residence Time Distribution (RTD). Reactor elements represent a key factor in the modelling process. In this sense, the impeller of the anoxic zone is modelled as a flat disk, and the baffle after the anoxic zone as a porous media. According to CFD model simulations, stagnant, short-circuit zones and mixing quality are established and quantified. Finally, the influence on the hydrodynamics of reactor elements is also evaluated. The results of this detailed hydrodynamic analysis will form the basis for the design and optimization of scalable AnoxAn configurations.
Greenhouse nutrient feedwater (GNF) discharge is considered a potential contributor to eutrophication issues in Lake Erie, Ontario. Land application of GNF is an accepted legislated management ...response to mitigate the impact of such nutrient loads. To assess the potential environmental impacts of this management practice, field infiltration experiments were conducted at four different greenhouse operations near Leamington, Ontario. Over a three-year study, GNF was applied on agricultural land adjacent to the greenhouse operations in the fall during the first year, and along with a bromide tracer in the summer and fall in Years 2 and 3, respectively. The GNF was applied at the maximum allowable rates as defined in legislation. Chemical constituents (nutrients, metals and the conservative tracer bromide) were monitored within the soil profile matrix and pore water above the water table. The results showed that, even with the GNF being applied at the highest permissible rates, the species of interest remained within the unsaturated soil zone at low concentrations over three to six months sampling intervals. The bromide tracer test demonstrated that highly mobile species could move through permeable soils to the water table depth in a potential worst-case application scenario. However, considering the low initial concentrations, long vadose zone residence time and the low mass flux, it would appear that land application of GNF, when applied in accordance with Ontario's Regulations, is a feasible and environmentally reasonable treatment option for managing GNF.
•Environmental impacts of waste greenhouse nutrient feed waters (GNF).•Controlled plot-scale field infiltration testing and monitoring over 3 seasonal cycles.•Quantitative assessment of GNF mobility in the unsaturated zone.•Bromide tracer test helped quantify the potential worst case infiltration scenario.•Land application of GNF determined to be an environmentally feasible disposal and treatment option.
Spreading of conservative solutes in groundwater due to aquifer heterogeneity is quantified by the macrodispersivity, which was found to be scale dependent. It increases with travel distance, ...stabilizing eventually at a constant value. However, the question of its asymptotic behavior at very large scale is still a matter of debate. It was surmised in the literature that macrodispersivity scales up following a unique scaling law. Attempts to define such a law were made by fitting a regression line in the log‐log representation of an ensemble of macrodispersivities from multiple experiments. The functional relationships differ among the authors, based on the choice of data. Our study revisits the data basis, used for inferring unique scaling, through a detailed analysis of literature marcodispersivities. In addition, values were collected from the most recent tracer tests reported in the literature. We specified a system of criteria for reliability and reevaluated the reliability of the reported values. The final collection of reliable estimates of macrodispersivity does not support a unique scaling law relationship. On the contrary, our results indicate, that the field data can be explained as a collection of macrodispersivities of aquifers with varying degree of heterogeneity where each exhibits its own constant asymptotic value. Our investigation concludes that transport, and particularly the macrodispersivity, is formation‐specific, and that modeling of transport cannot be relegated to a unique scaling law. Instead, transport requires characterization of aquifer properties, e.g., spatial distribution of hydraulic conductivity, and the use of adequate models.
Key Points:
Collection of macrodispersivity from literature and evaluation of reliability
Demonstration of inapplicability of an unique scaling law for macrodispersivity
Emphasis on the need for aquifer characterization for transport modeling
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