Tracer Mass Recovery Research Articles

Channel Network Modeling of Flow and Transport in Fractured Rock at the Äspö HRL: Data‐Worth Analysis for Model Development, Calibration and Prediction

AbstractPerformance assessment of nuclear waste disposal in deep crystalline bedrock demands a thorough understanding of the related flow and transport processes. Uncertainties may arise both from the selection of the conceptual model as well as the estimation of the related model parameters. Discrete fracture network (DFN) models are widely used for such modeling while channel network models (CNM) provide an alternative representation, the latter focusing on the fact that flow and transport in deep fractured media often are dominated by a small number of long preferential flow paths. This study applies the principle of channel networks, implemented in the Pychan3d simulator, to analyze the hydraulic and tracer transport behavior in a 450‐m‐deep fractured granite system at the Äspö Hard Rock Laboratory in Sweden, where extensive site characterization data, including hydraulic and tracer test data are available. Semi‐automated calibration of channel conductances to field characterization data (flow rates, drawdowns, and tracer recoveries) is performed using PEST algorithm. It was observed that an optimal CNM connectivity map for channel conductance calibration can only be developed by jointly fitting flow rates, drawdowns and tracer mass recovery values. Results from data‐calibrated CNM when compared to a corresponding calibrated DFN model shows that the CNM calibrates and adapts better than a DFN model with uniform fracture surfaces. This comparative study shows the differences and uncertainties between two models as well as examines the implications of using them for long term model predictions.

Hydrogeological characterization of Dasht-e-Arjan Lake (Zagros Mountains, Iran): clarifying a long-time question

Dasht-e-Arjan Lake is located in the southern Zagros mountain ranges of Iran. It is a tectonic karst polje, fed by a single spring plus local surface runoff, and drained by a single ponor. The fate of sinking water from the ponor had not been determined before this study. Previous investigations suggested two alternatives: (1) underground flow ~ 24 km southwest to Parishan Lake and/or springs in the Kazeron area, and (2) underground flow ~ 20 km southeast to springs at Domeasb Gorge. To clarify this question, an integrated hydrogeological study including a dye tracer test was undertaken. Comparison of the discharge into the Arjan ponor and the discharge of the Domeasb springs shows a reliable correlation. Further, annual water balance analysis reveals that the discharge at the Domeasb springs is too great for its topographic catchment, requiring a major additional source. The dye tracer test, using 5 kg of uranine, was detected only at the Domeasb springs. Flow through time after the dye injection was 210 h. The tracer breakthrough curves are very simple and sharp, with a clear peak concentration of 7 ppb. The total tracer mass recovery and groundwater flow velocity were estimated to be 58% and 105 m h−1, respectively. The results indicate a simple and direct limestone karst hydraulic connection along the geologic strike between Dasht-e-Arjan Lake and the Domeasb springs. The results clearly suggest the possibility of designing a dam around the ponor to protect drying up the lake during drought periods.

Application of single-well push-drift-pull tests using dual tracers (SF6 and salt) for designing CO2 leakage monitoring network at the environmental impact test site in Korea

A single-well push-drift-pull tracer test using two different tracers (SF6 and salt) was performed at the Environmental Impact Test (EIT) site to determine suitable locations for monitoring wells and arrange them prior to artificial CO2 injection and leak tests. Local-scale estimates of hydraulic properties (linear groundwater velocity and effective porosity) were obtained at the study site by the tracer test with two tracers. The mass recovery percentage of the volatile tracer (SF6) was lower than that of the non-volatile tracer (salt) and increased drift time may make degassing of SF6 intensified. The CO2 leakage monitoring results for both unsaturated and saturated zones suggest that the CO2 monitoring points should be located near points at which a high concentration gradient is expected. Based on the estimated hydraulic properties and tracer mass recovery rates, an optimal CO2 monitoring network including boreholes for monitoring the unsaturated zone was constructed at the study site.

Modeling solute reactivity in a phreatic solution conduit penetrating a karst aquifer

A two-dimensional model for solute migration, transformation, and deposition in a phreatic solution conduit penetrating a karst aquifer is presented in which the solute is anthropogenic to the natural system. Transformation of a reacting solute in a solution conduit has generally been accepted as likely occurring but actual physical measurements and mathematical analyses of the suspected process have been generally minimally investigated, primarily because of the logistical difficulties and complexities associated with solute transport through solution conduits. The model demonstrates how a reacting solute might decay or be transformed to a product solute some of which then migrates via radial dispersion to the conduit wall where it may become adsorbed. Model effects vary for laminar flow and turbulent flow in the axial direction. Dispersion in the radial direction also exhibits marked differences for both laminar flow and turbulent flow. Reaction zones may enhance subsequent reactions due to some overlap resulting from the longitudinal dispersion caused by flow in the axial direction. Simulations showed that varying the reaction rate coefficient strongly affects solute reactions, but that varying deposition coefficients had only minimal impacts. The model was applied to a well-known tracer test that used the tracer dye, Rhodamine WT, which readily converts to deaminoalkylated Rhodamine WT after release, to illustrate how the model may be used to suggest one possible cause, in addition to other possible causes, for less than 100 tracer-mass recovery. In terms of pollutants in a karst aquifer the model also suggests one possible mechanism for pollutant transformation in a solution conduit.

The Effect of Hydraulic Gradient and Pattern of Conduit Systems on Tracing Tests: Bench-Scale Modeling.

Tracer breakthrough curves provide valuable information about the traced media, especially in inherently heterogeneous karst aquifers. In order to study the effect of variations in hydraulic gradient and conduit systems on breakthrough curves, a bench scale karst model was constructed. The bench scale karst model contains both matrix and a conduit. Eight tracing tests were conducted under a wide range of hydraulic gradients from 1 to greater than 5 for branchwork and network-conduit systems. Sampling points at varying distances from the injection point were utilized. Results demonstrate that mean tracer velocities, tracer mass recovery and linear rising slope of the breakthrough curves were directly controlled by hydraulic gradient. As hydraulic gradient increased, both one half the time for peak concentration and one fifth the time for peak concentration decreased. The results demonstrate the variations in one half the time for peak concentration and one fifth the time for peak concentration of the descending limb for different sampling points under differing hydraulic gradients are mainly controlled by the interactions of advection with dispersion. The results are discussed from three perspectives: different conduit systems, different hydraulic-gradient conditions, and different sampling points. The research confirmed the undeniable role of hydrogeological setting (i.e., hydraulic gradient and conduit system) on the shape of the breakthrough curve. The extracted parameters (mobile-fluid velocity, tracer-mass recovery, linear rising limb, one half the time for peak concentration, and one fifth the time for peak concentration) allow for differentiating hydrogeological settings and enhance interpretations the tracing tests in karst aquifers.

Study of the effects of the chaser in push–pull tracer tests by using temporal moment analysis

“Push–pull” tracer tests are a suitable tracer test method for hydrochemical characterization of an aquifer in a single-well setting (e.g. in deep geothermal systems). A known amount of selected solutes as conservative and reactive tracers is injected into the aquifer (“push”) and afterwards extracted (“pull”). In many cases, a so-called “chaser”, which is just original groundwater without any added solutes, is injected directly after the injection of the test solution. Its objective is to push the test solution out of the borehole into the aquifer and therefore to minimize the influence of the gravel pack on the shape of the breakthrough curve. The influence of the chaser on the tracer breakthrough curve is unknown so far. Also, the determination of the appropriate volume for the chaser is a difficult task if at all applied. A first experiment was conducted with the objective to compare three push–pull tests with similar injection volumes, two tests with and one without a chaser. Results show that the application of a chaser lowers the main peak concentration. However, it does not alter the tailing of the breakthrough curve nor does it have a negative influence on tracer mass recovery. In a second experiment, a new method was developed to determine the optimal chaser volume by testing seven different chaser injection volumes combined with temporal moment analysis. As a result, the application of a chaser is recommended, when reactions of injected solutes within the open well or the gravel pack should be avoided. If a chaser is used, the new method mentioned above can easily be used to determine the required chaser injection volume. The experiments were conducted at the Hamasato test site in Horonobe (Hokkaido, Japan).

Controls on solute transport in large spring‐fed karst rivers

The spring‐fed rivers of North Florida flow across a karst plain, leading us to predict significant hyporheic transient storage within the secondary porosity of the carbonate media. They also span a gradient of submerged macrophyte cover and density, offering an opportunity to investigate reach‐scale vegetation effects on dispersion and transient storage. Conservative tracer tests using Rhodamine WT were conducted on nine rivers spanning the extant range of vegetative and geomorphologic characteristics. A one‐dimensional advection‐dispersion‐storage model with variable configurations of storage zones was fit to tracer breakthrough curves to determine optimal model coefficients as well as storage‐zone configuration (i.e., single, two in parallel, two in series). In most cases, two zones in series best fit the observed breakthrough curves. Moreover, fitted storage‐zone attributes were significantly correlated across springs with field‐measured analogs (i.e., vegetation area for storage zone 1, sediment area for zone 2), supporting inference that this model configuration physically represents the river systems. While the hyporheic zone area was large, its effect on transient storage was not, presumably because low‐hydraulic‐conductivity sediments and weak hydraulic gradients limit exchange with the secondary porosity of karst matrix; tracer mass recovery uniformly near 100% underscores the absence of significant hydraulic turnover. Vegetation was a significant predictor of transient storage, mean velocity, and mean residence time, suggesting that plant beds exert important controls on reach‐scale hydraulics in these systems.

Hydraulic performance of small free water surface constructed wetlands treating sugar factory effluent in western Kenya

This study investigated, using a lithium salt tracer, how macrophyte species and hydraulic loading rate (HLR) of wastewater influenced hydraulics in constructed wetlands (CWs). Four pilot-scale CWs received 45 mm day−1 of pre-treated sugar factory effluent and another four received 110 mm day−1. Half the CWs were planted with Cyperus papyrus and half with Echinochloa pyramidalis. Results showed a significant negative connection between tracer mass recovery and wetland water leakages. Also, a significant negative relationship between active wetland water volume and macrophyte density was detected. Further, a significant effect of HLR on mass removal rates of NH4+-N was observed. However, no significant effect of either HLR or macrophyte species on wetland hydraulic parameters was found.

Comparison of roughness models to simulate overland flow and tracer transport experiments under simulated rainfall at plot scale

The Saint-Venant equations have consistently proved capable of accurately simulating hydrographs at plot scale. However, recent works showed that even though the hydrograph is satisfyingly reproduced, the flow velocity field within the plot might be wrong, with the highest velocity largely underestimated. Moreover, the choice of roughness models to be used in the Saint-Venant equations is most often done in the purpose of increasing the hydrograph quality, while the actual travel time of water is ignored. This paper presents a tracer experiment made on a 10-m by 4-m rainfall simulation plot, where travel time and tracer mass recovery as well as local flow velocity have been measured. Four roughness models are tested: (i) Darcy-Weisbach's model, (ii) Lawrence's model, (iii) Manning's model with a constant roughness coefficient, and (iv) Manning's model with a variable roughness coefficient which decreases as a power law of the runoff water depth. Models with a constant friction factor largely underestimate high velocities. Moreover, they are not able to simulate tracer travel-times. Lawrence's model correctly simulates low and high velocities as well as tracer breakthrough curves. However, a specific set of parameters are required for each breakthrough curve from the same experiment. The best results are obtained with the Manning's model with a water-depth dependent roughness coefficient: simulated velocities are consistent with measurements, and a single set of parameters captures the entire set of breakthrough curves, as well as tracer mass recovery. The study reported here brings the following findings: (i) roughness coefficient is flow-dependent, (ii) faithful simulation of the velocity fields does not imply a good prediction of travel time and mass recovery, (iii) the best model is a Manning type model with a roughness coefficient which decreases as a power law of water depth. The full dataset used in this work is available on request. It can be used as benchmark for overland flow and transport models.

Tracer Mass Recovery in Fractured Aquifers Estimated from Multiple Well Tests

Forced-gradient tracer tests in fractured aquifers often report low mass recoveries. In fractured aquifers, fractures intersected by one borehole may not be intersected by another. As a result (1) injected tracer can follow pathways away from the withdrawal well causing low mass recovery and (2) recovered water can follow pathways not connected to the injection well causing significant tracer dilution. These two effects occur along with other forms of apparent mass loss. If the strength of the connection between wells and the amount of dilution can be predicted ahead of time, tracer tests can be designed to optimize mass recovery and dilution. A technique is developed to use hydraulic tests in fractured aquifers to calculate the conductance (strength of connection) between well pairs and to predict mass recovery and amount of dilution during forced gradient tracer tests. Flow is considered to take place through conduits, which connect the wells to each other and to distant sources or sinks. Mass recovery is related to the proportion of flow leaving the injection well and arriving at the withdrawal well, and dilution is related to the proportion of the flow from the withdrawal well that is derived from the injection well. The technique can be used to choose well pairs for tracer tests, what injection and withdrawal rates to use, and which direction to establish the hydraulic gradient to maximize mass recovery and/or minimize dilution. The method is applied to several tracer tests in fractured aquifers in the Clare Valley, South Australia.

Quantifying Preferential Flow by Breakthrough of Sequentially Applied Tracers Silt Loam Soil

Field experiments were conducted on tile‐drained plots at the South East Purdue Agricultural Center in Butlerville, Indiana, to quantify contaminant transport via preferential flow paths in a silt loam soil. Breakthrough patterns of three fluorobenzoic acids (pentafluorobenzoic acid [PFBA], o‐trifluoromethylbenzoic acid [o‐TFMBA], and 2,6‐difluorobenzoic acid [2,6‐DFBA]) in a preliminary study indicated that they were transported as conservatively as is bromide (Br−). These four tracers were then sequentially applied, in an adjacent plot, during simulated precipitation (3 mm h−1 intensity, 10‐h duration). Bromide was sprayed shortly before irrigation started, while PFBA, o‐TFMBA, and 2,6‐DFBA were applied at 2, 4, and 6 h thereafter, respectively. Tile flow began increasing at around 3 h, and Br− appeared in tile drain flow ≈4 h after irrigation started, yet benzoic acids, PFBA, o‐TFMBA, and 2,6‐DFBA, were detected in the tile drainage at 102 min, 42 min, and 18 min after their applications, respectively. Tracer mass recovery from tile drainage was Br− (7.04%), PFBA (13.9%), o‐TFMBA, (18.7%), and 2,6‐DFBA (19.7%) of applied mass. The faster arrival time and greater recovery of sequentially applied tracers confirmed that water movement and contaminant transport shifts toward increasingly larger pores of the preferential flow paths as soil becomes wet during a precipitation event. The breakthrough patterns of these tracers can be used to quantify the water flux distributions of preferential paths. Because ≈90% of the chemical leached from this precipitation event occurred during the first day, it was critical to intensively monitor contaminant transport during the first 24 h after a rainfall. A soil sampling protocol based on collecting soil cores at random locations once every several days is unsuitable for determining the deep leaching under field conditions.

Variably Saturated Flow in Storm-Water Partial Exfiltration Trench

Storm water from impervious urban areas can adversely impact water quality and quantity. The partial exfiltration trench (PET) is a control device designed to moderate both the quality and the quantity of urban runoff. This paper uses a 2D numerical model to evaluate variably saturated flow profiles and residence time distributions for a PET subject to storm water loading. Parameters estimated from laboratory experiments and hydrographs measured at a prototype PET are used to calibrate the numerical model. Simulation experiments show that flow through the PET is influenced strongly by the rate and duration of the hydraulic loading and by the type and properties of the surrounding soil. Unless the surrounding soil is nearly saturated or highly impermeable, propagation of the wetting front through an unlined PET occurs as 2D variably saturated flow. Variably saturated 2D flow through the PET is characterized by skewed residence time distributions, long mean travel times (relative to plug flow), high exfiltration losses to the surrounding soil, and low tracer mass recovery at the underdrain. These features of the PET performance are beneficial for storm-water treatment because the first flush of runoff often contains significantly higher pollutant concentrations than those present in later phases of the storm.

Transport of reactive tracers in rock fractures

Transport of tracers subject to mass transfer reactions in single rock fractures is investigated. A Lagrangian probabilistic model is developed where the mass transfer reactions are diffusion into the rock matrix and subsequent sorption in the matrix, and sorption on the fracture surface as well as on gauge (infill) material in the fracture. Sorption reactions are assumed to be linear, and in the general case kinetically controlled. The two main simplifying assumptions are that diffusion in the rock matrix is one-dimensional, perpendicular to the fracture plane, and the tracer is displaced within the fracture plane by advection only. The key feature of the proposed model is that advective transport and diffusive mass transfer are related in a dynamic manner through the flow equation. We have identified two Lagrangian random variables τ and β as key parameters which control advection and diffusive mass transfer, and are determined by the flow field. The probabilistic solution of the transport problem is based on the statistics of (τ, β), which we evaluated analytically using first-order expansions, and numerically using Monte Carlo simulations. To study (τ, β)-statistics, we assumed the ‘cubic law’ to be applicable locally, whereby the pressure field is described with the Reynolds lubrication equation. We found a strong correlation between τ and β which suggests a deterministic relationship β∼τ3/2; the exponent 3/2 is an artifact of the ‘cubic law’. It is shown that flow dynamics in fractures has a strong influence on the variability of τ and β, but a comparatively small impact on the relationship between τ and β. The probability distribution for the (decaying) tracer mass recovery is dispersed in the parameter space due to fracture aperture variability.