Laboratory Tracer Experiments Research Articles

Pollutant Dissipation at the Sediment‐Water Interface: A Robust Discrete Continuum Numerical Model and Recirculating Laboratory Experiments

AbstractPollutant exchange in the hyporheic zone is a major process controlling its degradation in river systems. Knowledge of mass transfer processes at the sediment‐water interface (SWI) remains scarce. Accurate predictive modeling of flow driving pollutant fluxes at the SWI is currently limited. We examined mass exchange at the SWI by combining laboratory tracer experiments and the development of a flow reactive transport (FRT) model. NaCl and Foron Blue 291 tracers were used as surrogates of conservative and moderately sorptive organic pollutants, respectively. Tracer experiments in the bench‐scale river channel reproduced the influence of overlying water velocities, the source of the pollutant, and its sorption capacity on pollutant exchange. A methodological framework to calibrate the FRT model against experiments was developed. Good agreement between the experimental and numerical results confirmed the robustness of the experimental setup and numerical model. The pollutant origin, either from the sediment or the overlying water, did not affect the pollutant exchange rates. The exchange rates were quasi‐proportional to the overlying water velocity. The sediment bed caused retention of more than half of the initially injected mass of Foron Blue 291. The moderately sorptive tracer partitioning retarded the equilibrium up to six times compared with the conservative tracer NaCl. Numerical tests, including both overlying and vertical velocities, showed that the latter is the main factor controlling pollutant exchange at the SWI. Altogether, the model allows capturing interactions between pollutant transport and partitioning to the rivers sediment, paving the way for systematic investigations of pollutant behavior in rivers.

Following tracer through the unsaturated zone using a multiple interacting pathways model: Implications from laboratory experiments

AbstractModels must effectively represent velocities and celerities if they are to address the old water paradox. Celerity information is recorded indirectly in hydrograph observations, whereas velocity information is more difficult to measure and simulate effectively, requiring additional assumptions and parameters. Velocity information can be obtained from tracer experiments, but we often lack information on the influence of soil properties on tracer mobility. This study features a combined experimental and modelling approach geared towards the evaluation of different structures in the multiple interacting pathways (MIPs) model and validates the representation of velocities with laboratory tracer experiments using an undisturbed soil column. Results indicate that the soil microstructure was modified during the experiment. Soil water velocities were represented using MIPs, testing how the (a) shape of the velocity distribution, (b) transition probability matrices (TPMs), (c) presence of immobile storage, and (d) nonstationary field capacity influence the model's performance. In MIPs, the TPM controls exhanges of water between pathways. In our experiment, MIPs were able to provide a good representation of the pattern of outflow. The results show that the connectedness of the faster pathways is important for controlling the percolation of water and tracer through the soil. The best model performance was obtained with the inclusion of immobile storage, but simulations were poor under the assumption of stationary parameters. The entire experiment was adequately simulated once a time‐variable field capacity parameter was introduced, supporting the need for including the effects of soil microstructure changes observed during the experiment.

Quantifying physical structure changes and non-uniform water flow in cattle manure during dry anaerobic digestion process at lab scale: Implication for biogas production

The aim of this study was to investigate and quantify non-uniform water flow during dry AD and its implication for biogas production. Laboratory tracer experiments were performed on cattle manure over the course of AD. The evolution of the permeability, the dry bulk density, the dry porosity, the total and volatile solid contents of cattle manure at different stages of AD, revealed waste structure changes, impacting water flow and methane production. Tracer experiments and numerical modeling performed by using a physical non-equilibrium model indicated non-uniform preferential flow patterns during degradation. According to literature, the increase of inoculum recirculation frequency improved methane production rate. However, these results demonstrated that this improvement occurs only at the beginning of manure degradation. After 19days of degradation the inoculum recirculation and the flow patterns modification had no effect on methane production rate.

Field-scale solute transport parameters derived from tracer tests in large undisturbed soil columns

Transport parameters obtained from laboratory tracer experiments were used to evaluate the stochastic form of the equilibrium convection–dispersion equation (CDE) in describing the transition of scale, i.e. from the column or local scale to a larger field scale. Local-scale solute breakthrough curves (BTCs) were measured in 1-m-long and 0.3-m-diameter undisturbed soil columns by means of time-domain reflectometry at six depths for a 79-h input pulse of chloride. The local-scale data were analysed in terms of the equilibrium CDE and the mobile–immobile non-equilibrium transport model (MIM). At the local scale, the MIM transport model better described the observed early breakthrough and the tailing of the BTC than did the CDE. A linear regression analysis indicated that the relationship between the hydrodynamic dispersion D and pore-water velocity v was of the form D = 31vl.92 (correlation ρv,D = 0.74). Averaging of the local-scale BTCs across the field produced a large-scale or field-scale mean BTC; at the greatest observation depth (0.8 m) the field-scale dispersivity <D>/<v> = λ equals 0.656 m. The results further showed that for large values of the mean dispersion coefficient, <D>, local-scale dispersion is an important mechanism for field-scale solute spreading, whereas the standard deviation, σD, and the correlation between v and D, ρvD, have negligible effects on field-scale transport. Stochastic stream tube models supplemented with statistical properties of local-scale transport parameters provide a practical and computationally efficient tool to describe heterogeneous solute transport at large spatial scales.

色素トレーサと画像処理，空間モーメント法を融合した不飽和領域における縦分散と横分散の推定

A new methodology using spatial moment analysis linked with image processing of a dye tracer behavior in porous media was developed to estimate dispersivities not only in longitudinal but in transverse directions. Laboratory tracer experiments using a relatively mobile dye tracer Brilliant Blue FCF were conducted under saturated and unsaturated flow conditions. Uniformly packed homogeneous fields, which were comprised by silica sand or andisol taken from a maize field, were of concern under four different infiltration rates as well as under saturated flow conditions. An image processing technique based on digitalized spatial distributions of dye tracer allowed to link with a spatial moment approach to identify the transverse and longitudinal dispersivities. Dispersivities exhibited an increasing and decreasing tendency associated with infiltration rates and showed a marked difference between estimates under saturated and unsaturated conditions. Laboratory study was extended by a comprehensive literature search to compare the new results with earlier work, demonstrating a good agreement between the experimental and published results. Developed methodology was applied to a field with approximately 100 cm depth from the ground surface under unsaturated conditions. Dye tracer moved through the soils in a preferential path pattern, which induced higher dispersivities in more irregular pore patterns as compared with estimates obtained in laboratory tracer experiments. Experimental results demonstrated the effectiveness of the developed methodology for simultaneous assessment of transverse and longitudinal dispersion in unsaturated soils.

In silico labeling reveals the time-dependent label half-life and transit-time in dynamical systems

BackgroundMathematical models of dynamical systems facilitate the computation of characteristic properties that are not accessible experimentally. In cell biology, two main properties of interest are (1) the time-period a protein is accessible to other molecules in a certain state - its half-life - and (2) the time it spends when passing through a subsystem - its transit-time. We discuss two approaches to quantify the half-life, present the novel method of in silico labeling, and introduce the label half-life and label transit-time. The developed method has been motivated by laboratory tracer experiments. To investigate the kinetic properties and behavior of a substance of interest, we computationally label this species in order to track it throughout its life cycle. The corresponding mathematical model is extended by an additional set of reactions for the labeled species, avoiding any double-counting within closed circuits, correcting for the influences of upstream fluxes, and taking into account combinatorial multiplicity for complexes or reactions with several reactants or products. A profile likelihood approach is used to estimate confidence intervals on the label half-life and transit-time.ResultsApplication to the JAK-STAT signaling pathway in Epo-stimulated BaF3-EpoR cells enabled the calculation of the time-dependent label half-life and transit-time of STAT species. The results were robust against parameter uncertainties.ConclusionsOur approach renders possible the estimation of species and label half-lives and transit-times. It is applicable to large non-linear systems and an implementation is provided within the PottersWheel modeling framework (http://www.potterswheel.de).

Assessment of a UV Excited Fluorescent Dye Technique for Estimating Solute Dispersion in Porous Media

Dyes are important tracers to investigate subsurface water movement and solute transport processes. Laboratory tracer experiments with a fluorescent dye, Brilliant Sulfaflavine, are carried out in a two-dimensional water flow tank uniformly packed with glass beads over a wide range of steady state water flow rates. Under ultraviolet (UV) illumination conditions the fluorescent dye emits visible light and is imaged by a digital camera. An imaging processing technique based on UV excited spatial distributions of the fluorescent dye tracer allows to link with a spatial moment approach to identify the longitudinal and transverse microdispersivities. The experimental results show that the mean values of longitudinal and transverse microdispersivities range from 0.095 cm to 0.40 cm and from 0.027 cm to 0.061 cm, respectively. Laboratory study is extended by a random walk particle tracking approach to reconstruct the estimated microdispersivity transitions, demonstrating a good agreement between the experimental and numerical results in homogeneous porous formations of concern. The methodology applied herein represents a potentially valuable tool in the assessment of microdispersion phenomena in porous media.

Influence of distributed flow losses and gains on the estimation of transient storage parameters from stream tracer experiments

Interactions between mobile stream water and transient storage zones have been the subject of careful attention for decades. However, few studies have considered explicitly the influence of water exchange between the channel and neighbouring hydrological units when modelling transient storage processes, especially the lateral inflow coming from hillslope contributions and outflow to a deep aquifer or to hyporheic flow paths extending beyond the study reach. The objective of this study was to explore the influence of different conceptualizations of these hydrologic exchanges on the estimation of transient storage parameters. Slug injections of sodium chloride (NaCl) were carried out in eight contiguous reaches in the Cotton Creek Experimental Watershed (CCEW), located in south-east British Columbia. Resulting breakthrough curves were subsequently analysed using a Transient Storage Model (TSM) in an inverse modelling framework. We estimated solute transport parameters using three distinct, hypothetical spatial patterns of lateral inflow and outflow, all based on variations of the same five-parameter model structure. We compared optimized parameter values to those resulting from a distinct four-parameter model structure meant to represent the standard application of the TSM, in which only lateral inflow was implemented for net gaining reaches or only lateral outflow for net losing reaches. In the five-parameter model, solute mass was stored predominantly in the transient storage zone and slowly released back to the stream. Conversely, solute mass was predominantly removed from the stream via flow losses in the four-parameter model structure. This led to contrasting estimates of solute transport parameters and subsequent interpretation of solute transport dynamics. Differences in parameter estimates across variations of the five-parameter model structure were small yet statistically significant, except for the transient storage exchange rate coefficient α, for which unique determination was problematic. We also based our analysis on F med 200 , the fraction of median transport time due to transient storage. Differences across configurations in F med 200 estimates were consistent but small when compared to the variability of F med 200 among reaches. Optimized parameter values were influenced dominantly by the model structure (four versus five parameters) and then by the conceptualization of spatial arrangement of lateral fluxes along the reach for a set model structure. When boundary conditions are poorly defined, the information contained in the stream tracer breakthrough curve is insufficient to identify a single, unambiguous model structure representing solute transport simulations. Investigating lateral fluxes prior to conducting a study on transient storage processes is necessary, as assuming a certain spatial organization of these fluxes might set ill-defined bases for inter-reach comparisons. Given the difficulty in quantifying the spatial patterns and magnitudes of lateral inputs and outputs, we recommend small-scale laboratory tracer experiments with well-defined and variable boundary conditions as a complement to field studies to provide new insights into stream solute dynamics.

Experimental determination of three-dimensional dispersivities in homogeneous porous medium

Laboratory tracer experiments were conducted to investigate longitudinal dispersivity (α x ) as well as the transversal (α y ) and vertical (α z ) dispersivities in homogeneous 3–5 mm sandy aquifer. The experiments were carried out in a channel 12-m long, 1.35-m wide and 0.60-m high which was built in the Hydraulics Laboratory of Civil Engineering Department in Dokuz Eylul University. NaCl was used as a tracer and conductivity values were measured at 220 measurement points. Mass Transport 3 Dimensional (Zheng and Wang in SERDP-99-1, US Army Engineer Research and Development Center, Vicksburg, MS, 1999; MT3DMS code) which is a three-dimensional solute transport simulation model incorporating finite differences solution option was used to solve the three-dimensional advective–dispersive transport equation. The estimated dispersivity values were modified until an acceptable compatibility between the observed and calculated concentrations at measurement points was reached. The best match was obtained for α x = 12 cm, α y /α x = 0.2 and α z /α x = 0.05. These values are compatible with those encountered in the literature.

Evolution of porosity and diffusivity associated with chemical weathering of a basalt clast

Weathering of rocks as a result of exposure to water and the atmosphere can cause significant changes in their chemistry and porosity. In low‐porosity rocks, such as basalts, changes in porosity, resulting from chemical weathering, are likely to modify the rock's effective diffusivity and permeability, affecting the rate of solute transport and thus potentially the rate of overall weathering to the extent that transport is the rate limiting step. Changes in total porosity as a result of mineral dissolution and precipitation have typically been used to calculate effective diffusion coefficients through Archie's law for reactive transport simulations of chemical weathering, but this approach fails to account for unconnected porosity that does not contribute to transport. In this study, we combine synchrotron X‐ray microcomputed tomography (μCT) and laboratory and numerical diffusion experiments to examine changes in both total and effective porosity and effective diffusion coefficients across a weathering interface in a weathered basalt clast from Costa Rica. The μCT data indicate that below a critical value of ∼9%, the porosity is largely unconnected in the basalt clast. The μCT data were further used to construct a numerical pore network model to determine upscaled, effective diffusivities as a function of total porosity (ranging from 3 to 30%) for comparison with diffusivities determined in laboratory tracer experiments. By using effective porosity as the scaling parameter and accounting for critical porosity, a model is developed that accurately predicts continuum‐scale effective diffusivities across the weathering interface of the basalt clast.

Point-to-point connectivity, an abstract concept or a key issue for risk assessment studies?

Connectivity of high/low-permeability areas has been recognized to significantly impact groundwater flow and solute transport. The task of defining a rigorous quantitative measure of connectivity for continuous variables has failed so far, and thus there exist a suite of connectivity indicators which are dependent on the specific hydrodynamic processes and the interpretation method. Amongst the many existing indicators, we concentrate on those characterizing connectivity between the points involved in a hydraulic or tracer test. The flow connectivity indicator used here is based on the time elapsed for hydraulic response in a pumping test (e.g., the storage coefficient estimated by the Cooper–Jacob method, S est ). Regarding transport, we select the estimated porosity from the breakthrough curve ( ϕ est ) . According to Knudby and Carrera [Knudby C, Carrera J. On the relationship between indicators of geostatistical, flow and transport connectivity. Adv Water Resour 2005;28(4):405–21] these two indicators measure connectivity differently, and are poorly correlated. Here, we use perturbation theory to analytically investigate the intrinsic relationship between S est and ϕ est . We find that ϕ est can be expressed as a weighted line integral along the particle trajectory involving two parameters: the transmissivity point values, T, and the estimated values of S est along the particle path. The weighting function is linear with the distance from the pumping well, thus the influence of the weighting function is maximum at the injection area, whereas the hydraulic information close to the pumping well becomes redundant (null weight). The relative importance of these two factors is explored using numerical simulations in a given synthetic aquifer and tested against intermediate-scale laboratory tracer experiments. We conclude that the degree of connectivity between two points of an aquifer (point-to-point connectivity) is a key issue for risk assessment studies aimed at predicting the travel time of a potential contaminant.

EXPERIMENTAL INVESTIGATION OF LONGITUDINAL DISPERSION PHENOMENA IN HETEROGENEOUS POROUS MEDIA

Dispersivity is a medium-dependent hydrodynamic parameter that characterizes solute dispersion in a porous medium. Under saturated unidirectional flow condition, laboratory tracer experiments with a pulse source were conducted in homogeneous flow field as well as in less heterogeneous porous formation, which was comprised by a few combinations with three types of soil particles. Mean grain sizes and flow velocities in a region were varied in order to investigate a linear relation between grain size and longitudinal dispersivity under the approximately one order range of Reynolds and Peclet numbers, which were non-linearly expressed as a function of longitudinal dispersion coefficient. Laboratory study was extended by a comprehensive literature search to compare the new results with earlier work, demonstrating good agreement between the experimental and published empirical relations in heterogeneous porous media of concern.

DISPERSIVITY ESTIMATION USING SPATIAL CONCENTRATION DISTRIBUTION AND SPATIAL MOMENT ANALYSIS

Laboratory tracer experiments are implemented in homogeneous flow field under saturated steady flow condition with a pulse source. Application of dye tracer with Brilliant Blue FCF is allowed to visualize solute behavior and to obtain the images of plume evolution. Through an image processing, spatial concentration distributions of dye tracer with 0.125, 0.250 and 0.500 of initial concentration as well as plume outlines are extracted to ensure the further development of parameter identification. Based on these image data, an image analysis with plume outline and a spatial moment analysis linked with extracted concentration distributions are applied to the estimation of longitudinal and transverse dispersivites. Results show that spatial moment analysis provides an adequate estimate of dispersivities as compared to the image analysis, indicating the improvement of estimation accuracy. Whereas estimates using spatial moment analysis depend on the number of spatial concentration contours, spatial moment analysis represents a potentially useful alternative for estimating longitudinal and transverse dispersivities.

Destruction of conspecific bioturbation structures by Amphiura filiformis (Ophiuroida): evidence from luminophore tracers and in situ time-lapse sediment-profile imagery

Bioturbation processes of a dense ophiuroid assemblage were studied by in situ time-lapse sediment-profile imagery (t-SPI) and laboratory tracer-experiments. The burrows of 4 Amphiura fili- formis (Ophiuroida) individuals were visible throughout the t-SPI deployment. Mean burrow excavation was 2.9 cm 2 h -1 . Mounding ranged from 0 to 7.12 cm 2 h -1 . Loss of sediment from the sediment-water interface ranged from 0 to 7.0 cm 2 h -1 . Bioturbation at the station was dominated by A. filiformis, whose presence appeared to mask the activity of conspecifics. Species interactions were investigated in terms of particle reworking by laboratory luminophore-tracer experiments of monocultures and species mix- tures using A. filiformis, Leptopentacta elongata (Holothuroida) and Nephtys hombergii (Polychaeta). Bioturbation by these large species fit a symmetric non-local model well. Species interactions led to underyielding with respect to predicted additive species-effects. Treatments containing A. filiformis were significantly different from treatments without A. filiformis, and not significantly different from each other. When the data was processed on a per capita basis, multiculture treatments containing N. hombergii underyielded due to the high biomass of N. hombergii and its relatively low bioturbation activity. Ordination of a distance matrix derived from tracer profiles showed that tracer profiles produced by multispecies treatments were most similar to the strongest bioturbator, due to destruction of con- specific structures by the strongest bioturbator. Species interactions are not additive in terms of particle bioturbation, and may be difficult to predict without knowing in situ densities.

Fracture‐Controlled Nitrate and Atrazine Transport in Four Iowa Till Units

Fractures in till may provide pathways for agricultural chemicals to contaminate aquifers and surface waters. This study was conducted to quantify the influence of fractures on solute fate and transport using three conservative and two nonconservative tracers. The conservative tracers were potassium bromide (KBr), pentafluorobenzoic acid (PFBA), and 1,4-piperazinediethanesulfonic acid disodium salt (PIPES); the nonconservative tracers were nitrate and atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine]. Three sites in Iowa were investigated, including four late Wisconsinan and Pre-Illinoian tills. Laboratory tracer experiments were conducted using eight large (0.4-0.45 m long by 0.43 m in diameter), undisturbed columns of till collected from depths of 1 to 28 m. The tills were densely fractured, with fracture spacing ranging from 3.8 to 10.4 cm. First arrival velocities of Br- ranged from 0.004 to 64.8 m d(-1), 10 to 100 times faster than predicted for unfractured media. Nitrate behaved as a conservative tracer in weathered till columns, but degraded during experiments using deeper tills. Sorption caused retardation of atrazine in the shallowest four columns. Atrazine degradation occurred in deeper columns as demonstrated by deviations between atrazine and the conservative tracers. Mobile-immobile model (MIM) simulations estimated first-order exchange coefficients (alpha) ranging from 1 x 10(-8) to 1.7 x 10(-2) s(-1), sorption coefficients (K(d)) for atrazine ranging from 2.6 x 10(-5) to 1 x 10(-3) m3 kg(-1), and degradation half-lives ranging from 0.24 to 67 d (nitrate) and 1.6 to 277 d (atrazine). This study suggests that aquifers and surface waters associated with thin, fractured till units may be vulnerable to contamination, yet deeper aquifers may be protected by these materials due to increased residence times provided by matrix diffusion.

Simultaneous analyses and applications of multiple fluorobenzoate and halide tracers in hydrologic studies

An analytical method that employs ion chromatography has been developed to exploit the use of fluorobenzoic acids (FBAs) and halides more fully as hydrologic tracers. In a single run, this reliable, sensitive, and robust method can simultaneously separate and quantify halides (fluoride, chloride, bromide, and iodide) and up to seven FBAs from other common groundwater constituents (e.g. nitrate and sulphate). The usefulness of this analytical method is demonstrated in both field and laboratory tracer experiments. The field study examines the hydrologic response of fractures and the matrix to different flow rates and the contribution of matrix diffusion in chemical transport. Laboratory tracer experiments with eight geologic media from across the USA—mostly from Department of Energy facilities where groundwater contamination is prevalent and where subsurface characterization employing tracers has been ongoing or is in need—reveal several insights about tracer transport behaviour: (1) bromide and FBAs are not always transported conservatively; (2) the delayed transport of these anionic tracers is likely related to geologic media characteristics, such as organic matter, pH, iron oxide content, and clay mineralogy; (3) use of iodine as a hydrologic tracer should take into account the different sorption behaviours of iodide and iodate and the possible conversion of iodine's initial chemical form; (4) the transport behaviour of potential FBA and halide tracers under relevant geochemical conditions should be evaluated before beginning ambitious, large-scale field tracer experiments. Copyright © 2005 John Wiley & Sons, Ltd.

Tracer Penetration into Welded Tuff Matrix from Flowing Fractures

Field and laboratory tracer experiments were conducted to investigate the extent of tracer imbibition and penetration into unsaturated, fractured rock matrix at Yucca Mountain, Nevada. Field experiments were carried out in the Exploratory Studies Facility (ESF), an underground tunnel at Yucca Mountain. Water containing dye was released into horizontal boreholes drilled into the wall of the ESF main drift. The region was then mined to observe the flow pathways and to collect dye-stained rock samples for subsequent laboratory quantification. Dye concentration profiles in the rock, measured using a newly developed sampling technique, showed that liquid flowing through the fractures penetrated into the matrix to a depth of several millimeters. Laboratory studies of tracer penetration into the rock matrix were conducted using tracer-free rock samples, collected from the same hydrogeologic unit and machined into cylindrical cores. Tracer-imbibition tests were performed on cores at two different initial water saturations with both sorbing (dyes) and nonsorbing tracers. The travel distance for sorbing dyes was a few millimeters after ∼16 to 20 h, similar to the extent measured in samples from the field test. The nonsorbing bromide front coincided with the wetting front in the rock core at the initial water saturation of 12%, and the imbibition depth agreed very well with the prediction, using independently measured properties. At the high initial water saturation of 76%, the bromide front lagged significantly behind the wetting front. Sorption coefficients for the dyes in the partially saturated core samples were calculated using two independent approaches, based on tracer travel-distance and mass-distribution calculations, and were found to yield comparable results. Utilization of nonsorbing tracers with different molecular sizes helped to identify the effects of pore-size restriction on tracer transport during imbibition. The results from this work have a direct application to radionuclide transport at Yucca Mountain, and the methods presented are broadly applicable to the investigation of water and solute transport in unsaturated rock.

Tracer Penetration into Welded Tuff Matrix from Flowing Fractures

Field and laboratory tracer experiments were conducted to investigate the extent of tracer imbibition and penetration into unsaturated, fractured rock matrix at Yucca Mountain, Nevada. Field experiments were carried out in the Exploratory Studies Facility (ESF), an underground tunnel at Yucca Mountain. Water containing dye was released into horizontal boreholes drilled into the wall of the ESF main drift. The region was then mined to observe the flow pathways and to collect dye‐stained rock samples for subsequent laboratory quantification. Dye concentration profiles in the rock, measured using a newly developed sampling technique, showed that liquid flowing through the fractures penetrated into the matrix to a depth of several millimeters. Laboratory studies of tracer penetration into the rock matrix were conducted using tracer‐free rock samples, collected from the same hydrogeologic unit and machined into cylindrical cores. Tracer‐imbibition tests were performed on cores at two different initial water saturations with both sorbing (dyes) and nonsorbing tracers. The travel distance for sorbing dyes was a few millimeters after ∼16 to 20 h, similar to the extent measured in samples from the field test. The nonsorbing bromide front coincided with the wetting front in the rock core at the initial water saturation of 12%, and the imbibition depth agreed very well with the prediction, using independently measured properties. At the high initial water saturation of 76%, the bromide front lagged significantly behind the wetting front. Sorption coefficients for the dyes in the partially saturated core samples were calculated using two independent approaches, based on tracer travel‐distance and mass‐distribution calculations, and were found to yield comparable results. Utilization of nonsorbing tracers with different molecular sizes helped to identify the effects of pore‐size restriction on tracer transport during imbibition. The results from this work have a direct application to radionuclide transport at Yucca Mountain, and the methods presented are broadly applicable to the investigation of water and solute transport in unsaturated rock.

Experimental investigation of solute transport in large, homogeneous and heterogeneous, saturated soil columns

Laboratory tracer experiments were conducted to investigate solute transport in 12.5-m long, horizontally placed soil columns during steady saturated water flow . Two columns having cross-sectional areas of 10 x IOcm' were used: a uniformly packed homogeneous sandy column and a heterogeneous column containing layered, mixed, and lenticular formations of various shapes and sizes. The heterogeneous soil column gradually changed, on average, from coarse-textured at one end to fine-textured at the other end . NaCl breakthrough curves (BTC's) in the columns were measured with electrical conductivity probes inserted a t 50- or 100-cm intervals. Observed BTC's in the homogeneous sandy column were relatively smooth and sigmoidal (S-shaped), while those in the heterogeneous column were very irregular, nonsigmoidal, and exhibited extensive tailing. Effective average pore-water velocities (tr,rr) and dispersion coefficients (&r) were estimated simultaneously by tilting an analytical solution of the convection-dispersionequation to the observe d BTC's. Velocity variations in the heterogeneousmedium were found to be much larger than those in the homogeneous sand. Values of the dispersivity, (Y = Derr/ecrrr for the homogeneous sandy column ranged from 0.1 to 5.0cm, while those for the heterogeneous column were as high as 200cm. The dispcrsivity for transport in both columns increased with travel distance or travel time, thus exhibiting scale- dependency. The heterogeneous soil column also showed the effects of preferential flow. i.e., some locations in the column showed earlier solute breakthrough than several locations closer to the inlet boundary. Spatial fluctuations in the dispersivity could bc explained qualitatively by the particular makeup of the heterogeneities in the column.

Fingered preferential flow in unsaturated homogeneous coarse sands

Abstract The occurrence of fingered preferential flow depends on both flow and porous media characteristics. The boundary condition of unsaturated infiltration has been investigated to assess whether it produces unstable wetting in homogeneous coarse sands. Laboratory tracer experiments were conducted in two coarse sand materials; for each sand material two initial conditions of air dry and field capacity were considered. Results indicate that the commonly occurring process of nonponding rainfall can provoke fingered preferential flow in homogeneous sandy soils. This phenomenon is very predominant in dry as compared to wet initial conditions. Models of water flow and solute transport in uniform coarse soils should incorporate the effects of such fingered flow phenomenon.