Natural Hydraulic Gradient Research Articles

Life cycle assessment of rainbow trout farming in the temperate climate zone based on the typical farm concept

Fish from aquaculture has the ability to meet consumer demand for a healthy and sustainable diet. The environmental footprint of fish farming depends largely on the method of production. Trout production in the temperate climate zone in open flow-through systems using the natural hydraulic gradient of streams as freshwater supply is energy efficient relative to more intensive systems. To investigate the possibility of reducing global warming potential, eutrophication potential, acidification potential, and ozone layer depletion associated with rainbow trout (Oncorhynchus mykiss) production in southern Germany, a comprehensive dataset collected under the typical farm framework was analyzed and a cradle-to-gate life cycle assessment was conducted. The impact of feed source (plant- and fish-based) was analyzed, as well as that of photovoltaic panels installed over the production area. Calculated emissions per kg fish live weight were 1.18 kg CO2eq, 7.89e−8 kg CFC11eq, 0.00552 kg SO2eq, and 0.0257 kg PO4eq. Full covering of the production area with photovoltaic panels, compared to the current ∼40% coverage, was estimated to provide a reduction of 1.04 kg CO2eq, resulting in emissions of 0.773 kg CO2eq per kg fish live weight. Feeds containing 35% and 61.8% fishmeal were associated with lower emissions compared to 100% plant-based, with a reduction in GWP of 0.79 kg CO2eq in the 61.8% fishmeal variant. Results showed that the use of photovoltaic panels can significantly reduce rainbow trout culture impact on the analyzed environmental impact categories. Feed containing fishmeal has a lower impact on the analyzed environmental impact categories. Alternatives such as insect meal and sustainable plant alternatives should be the focus of future research.

Consensus Equilibrium for Subsurface Delineation

Heterogeneity and insufficient site characterization limit our knowledge of the subsurface. Inversion techniques, which minimize the mismatch between observations and model predictions, have become an essential tool of subsurface characterization. Most optimization-based approaches fail to incorporate various implicit priors and capture the geological complexity. We overcome these limitations by deploying the plug-and-play and consensus equilibrium (CE) strategies, which provide a flexible framework for image reconstruction. Our CE methodology for spatial delineation of geologic formations consists of an image denoiser and a variational auto-encoder (deep learning-based emulator). The former ameliorates the reconstruction noise, yielding well-defined geological structures; its mathematical equivalence with the proximal operator allows the deployment of advanced denoisers (e.g., CNN-based denoiser) that do not correspond to a regularization objective. The latter defines a geology prior that imposes a geological constraint, for example, continuity and shape of geological features, onto the reconstructed image. We conduct a series of numerical experiments dealing with transient two-dimensional flow driven by a pumping well and natural hydraulic head gradient. They demonstrate the CE framework's ability to delineate, both probabilistically and deterministically, complex subsurface environments with sufficient quality.

Breakthrough curves of dye tracing tests in karst aquifers: Review of effective parameters based on synthetic modeling and field data

Breakthrough curves (BTCs) are known as adequate evidence to characterize karst aquifers which mainly specified by dual flow systems (matrix and conduit) as well as some degree of connection between dual media. Conduit Flow Process version 2 (CFPv2) is a discrete conduit continuum model applying which it is possible to simulate such characteristics. In this research, first CFPv2 was employed to evaluate the effect of several factors including hydraulic conductivity, exchange flow, tracing distance, hydraulic gradient, and conduit diameter and roughness on BTCs in a synthetic karst aquifer. Then, 253 BTCs from 24 dye tracing tests, conducted in karstic aquifers of Iran, were analyzed to evaluate the effect of hydraulic gradient and tracing distance on the BTCs with taking into account the effect of medium type (matrix or conduit) and karst development. These tests were conducted under either natural or elevated man-made hydraulic gradient conditions. The results revealed that peak concentration decreased with enlarging conduit diameter because of considerable flow rate. Likewise, it decreases over longer distances specifically in the elevated man-made hydraulic conditions. However, with increasing hydraulic gradient, peak concentration and its timing have upward and downward trends, respectively. When BTCs start to demonstrate tailing and become flattened, it could be an indication of low matrix hydraulic conductivity or lower hydraulic gradient in conduit as a result of less exchange flow from the matrix into conduits. Based on this study a new intermixture of flow and solute transport parameters was engineered to classify karst development. High peak concentration, short peak time, high velocity particularly over long distances and under the natural hydraulic gradient conditions and feasible conduit flow regime whether laminar or turbulent suggested as indication of well-developed karstic system.

Probabilistic Forecast of Single‐Phase Flow in Porous Media With Uncertain Properties

AbstractUncertainty about geologic makeup and properties of the subsurface renders inadequate a unique quantitative prediction of flow and transport. Instead, multiple alternative scenarios have to be explored within the probabilistic framework, typically by means of Monte Carlo simulations (MCS). These can be computationally expensive, and often prohibitively so, especially when the goal is to compute the tails of a distribution, that is, probabilities of rare events, which are necessary for risk assessment and decision making under uncertainty. We deploy the method of distributions to derive a deterministic equation for the cumulative distribution function (CDF) of hydraulic head in an aquifer with uncertain (random) hydraulic conductivity. The CDF equation relies on a self‐consistent closure approximation, which ensures that the resulting CDF of hydraulic head has the same mean and variance as those computed with statistical moment equations. We conduct a series of numerical experiments dealing with steady‐state two‐dimensional flow driven by either a natural hydraulic head gradient or a pumping well. These experiments reveal that the CDF method remains accurate and robust for highly heterogeneous formations with the variance of log conductivity as large as five. For the same accuracy, it is also up to four orders of magnitude faster than MCS in computing hydraulic head with a required degree of confidence (probability).

Effects of Artefacts on Natural Gradient Single-Borehole Tracer Dilution Tests

Single-borehole tracer dilution tests are used to estimate the Darcy velocity (vd) which is important for investigating contaminant transport and distribution within the aquifers. Although it is generally acknowledged that artefacts other than the horizontal groundwater flow can affect a tracer dilution rate in single-borehole tracer dilution tests, there are no studies which have been carried out to illustrate the effects of these artefacts. As a result, groundwater practitioners have continued to use single-borehole tracer dilution tests without considering the potential influence of these artefacts when estimating the Darcy velocity. This study is primarily designed to investigate the influence of the effect of artefacts other than the horizontal groundwater flow on the single-borehole tracer dilution tests under natural hydraulic gradient. Three single-borehole tracer dilution tests were performed during the experiment in an alluvial aquifer in the main Karoo Basin of South Africa. The data showed that early stage of tracer dilution can be affected by artefacts other than the horizontal groundwater flow. Data affected by the artefacts appear to show fast early stage tracer dilution rates which can result in overestimation of the Darcy velocity. Higher tracer concentrations with respect to the background levels can be considered as the dominant artefacts. The study proposes a simple qualitative approach to identify and remove the data affected by artefacts prior to the estimation of the Darcy velocity. Further studies that will effectively quantify the extent of the effects of these artefacts on tracer dilution rates are urgently needed.

A modified point dilution test for the assessment of groundwater flux in karst aquifers

A parametrization of heterogeneous karst aquifers represents one of the main challenges for hydrogeologist. The aim of this research was to design an easily applicable test that can be used for the assessment of groundwater flux (velocity) in highly permeable karst aquifers. The modified point dilution test (MPDT) is based on the constant-rate injection of tracer solution in a borehole (or sub-vertical cave) and continuous measurement of tracer concentration in the same borehole. When the concentration is stabilized then the groundwater inflow in the borehole can be assessed on the basis of the known injection rate and measured concentrations (initial and stabilized). By the obtained inflow and known borehole cross-sectional area, the apparent groundwater velocity under a natural hydraulic gradient can be assessed. Also, Darcy velocity, hydraulic conductivity and an average linear velocity can be derived from the test results if the borehole-distortion factor, hydraulic gradient and porosity are known. The test was applied in Well B1 (Ljesanska Nahija, Montenegro). The obtained apparent groundwater velocity amounts to 1.71 × 10−4 m/s. In addition to wells and piezometers, the MPDT can also be applied to sub-vertical caves that extend below the water table, and as a results groundwater inflow and the volume of a cave filled with water can be obtained. The MPDT provides satisfactory results only in highly permeable aquifers.

Cross-hole fracture connectivity assessed using hydraulic responses during liner installations in crystalline bedrock boreholes

In order to continually improve the current understanding of flow and transport in crystalline bedrock environments, developing and improving fracture system characterization techniques is an important area of study. The presented research examines the installation of flexible, impermeable FLUTe™ liners as a means for assessing cross-hole fracture connectivity. FLUTe™ liners are used to generate a new style of hydraulic pulse, with pressure response monitored in a nearby network of open boreholes drilled in gneissic rock of the Canadian Shield in eastern Ontario, Canada. Borehole liners were installed in six existing 10–15 cm diameter boreholes located 10–35 m apart and drilled to depths ranging between 25–45 m. Liner installation tests were completed consecutively with the number of observation wells available for each test ranging between one and six. The collected pressure response data have been analyzed to identify significant groundwater flow paths between source and observation boreholes as well as to estimate inter-well transmissivity and storativity using a conventional type-curve analysis. While the applied solution relies on a number of general assumptions, it has been found that reasonable comparison can be made to previously completed pulse interference and pumping tests. Results of this research indicate areas where method refinement is necessary, but, nonetheless, highlight the potential for use in crystalline bedrock environments. This method may provide value to future site characterization efforts given that it is complementary to, and can be used in conjunction with, other currently employed borehole liner applications, such as the removal of cross-connection at contaminated sites and the assessment of discrete fracture distributions when boreholes are sealed, recreating natural hydraulic gradient conditions.

Assessment of the barrier effect caused by underground constructions on porous aquifers with low hydraulic gradient: A case study of the metro construction in Barcelona, Spain

Construction of tunnels can impact aquifers because of the changes produced in the natural groundwater behavior. The drain effect, which is one of the most important impacts, can be eliminated using a tunnel-boring machine (TBM) to drill a tunnel with an impervious lining. However, the use of impermeable linings results in aquifer obstruction, giving rise to the barrier effect, which may cause an increase and decrease of the hydraulic head upgradient and downgradient of the tunnel, respectively. This modification of the hydraulic head, which can be predicted analytically and is proportional to the natural hydraulic gradient of the aquifer perpendicular to the tunnel (iN) (before it is constructed), is negligible for aquifers with values of iN that are very small or null (approximately 0). In these cases, the analytical solutions are not useful to estimate the real impact because the head distribution is not largely affected.This study proposes a methodology to evaluate the hydrogeological impact produced by the construction of underground impervious structures in aquifers, which have a small or null iN. The method, which is based on the analysis of the groundwater response to pumping tests performed before and after construction, was tested in a stratified porous aquifer and was used along with numerical modeling to assess the barrier effect in an experimental site (Sant Cosme, El Prat de Llobregat, Barcelona). The impact on the head distribution was negligible. However, the reduction of the connectivity was considerable. Pumping tests can determine the changes in aquifer connectivity caused by the construction of an underground impervious structure. The behavior of the groundwater during the post-tunneling pumping changes with regard to the pre-tunneling tests. A delay in the response to the pumping and a decrease of the drawdown are observed in the piezometers located on the opposite side of the tunnel where the well is placed, whereas an increase of drawdown occurs in the piezometers situated on the same side of the well. The procedure explained in this paper reveals a useful tool for determining the impact caused by underground impermeable constructions in aquifers, where iN is small or even 0.

Integrated Framework for Assessing Impacts of CO₂ Leakage on Groundwater Quality and Monitoring-Network Efficiency: Case Study at a CO₂ Enhanced Oil Recovery Site.

This study presents a combined use of site characterization, laboratory experiments, single-well push-pull tests (PPTs), and reactive transport modeling to assess potential impacts of CO2 leakage on groundwater quality and leakage-detection ability of a groundwater monitoring network (GMN) in a potable aquifer at a CO2 enhanced oil recovery (CO2 EOR) site. Site characterization indicates that failures of plugged and abandoned wells are possible CO2 leakage pathways. Groundwater chemistry in the shallow aquifer is dominated mainly by silicate mineral weathering, and no CO2 leakage signals have been detected in the shallow aquifer. Results of the laboratory experiments and the field test show no obvious damage to groundwater chemistry should CO2 leakage occur and further were confirmed with a regional-scale reactive transport model (RSRTM) that was built upon the batch experiments and validated with the single-well PPT. Results of the RSRTM indicate that dissolved CO2 as an indicator for CO2 leakage detection works better than dissolved inorganic carbon, pH, and alkalinity at the CO2 EOR site. The detection ability of a GMN was assessed with monitoring efficiency, depending on various factors, including the natural hydraulic gradient, the leakage rate, the number of monitoring wells, the aquifer heterogeneity, and the time for a CO2 plume traveling to the monitoring well.

Chlorinated Solvent Treatment Wetland

AbstractA first‐of‐its‐kind wetland restoration project was completed in October 2000 to treat trichloroethene‐(TCE‐)impacted groundwater from a former manufacturing facility prior to discharge into a highly valued recreational surface water body in the upper Midwest. This article summarizes the design, construction, operation, and effectiveness of the restored wetland. The groundwater‐surface water discharge zone at the site was restored as a wetland to improve the natural degradation of TCE and subsequent degradation by‐products. For the past 11 years, the treatment wetland performance was evaluated by monitoring the wetland vegetation, wetland hydraulics, and water chemistry. Water quality data have been used to assess the wetland geochemistry, TCE and TCE‐degradation by‐product concentrations within the wetland, and the surface water quality immediately downgradient of the wetland. The treatment wetland has been performing according to design, with TCE and TCE‐degradation by‐products not exceeding surface water criteria. The monitoring results show that TCE and TCE‐degradation by‐products are entering the treatment wetland via natural hydraulic gradients and that the geochemistry of the wetland supports both reductive dechlorination (anaerobic degradation) and cometabolic degradation (aerobic degradation) of TCE and TCE‐degradation by‐products: cis‐ and trans‐1,2‐dichloroethene and vinyl chloride. © 2013 Wiley Periodicals, Inc.

Remediation of an aquifer polluted with dissolved tetrachloroethylene by an array of wells filled with activated carbon

In this work, an array of deep passive wells filled with activated carbon, namely a Discontinuous Permeable Adsorptive Barrier (PAB-D), has been proposed for the remediation of an aquifer contaminated by tetrachloroethylene (PCE). The dynamics of the aquifer in the particular PAB-D configuration chosen, including the contaminant transport in the aquifer and the adsorption onto the barrier material, has been accurately performed by means of a computer code which allows describing all the phenomena occurring in the aquifer, simultaneously.A PAB-D design procedure is presented and the main dimensions of the barrier (number and position of passive wells) have been evaluated. Numerical simulations have been carried out over a long time span to follow the contaminant plume and to assess the effectiveness of the remediation method proposed. The model results show that this PAB-D design allows for a complete remediation of the aquifer under a natural hydraulic gradient, the PCE concentrations flowing out of the barrier being always lower than the corresponding Italian regulation limit.Finally, the results have been compared with those obtained for the design of a more traditional continuous barrier (PAB-C) for the same remediation process.

Wellbore-Wall Compression Effects on Monitored Groundwater Levels and Qualities

The effects of wellbore-wall compression from rough excavation on monitored groundwater levels and qualities under natural hydraulic gradient conditions were investigated in a shallow clayey Andisol aquifer. Nine wellbores reaching the underlying aquitard at about 2.6-m depth were constructed by dynamic cone penetrometry to mimic rough wellbore construction. Five of these were constructed under wet aquifer soil conditions and the remaining four under dry conditions. A 15-month period monitoring showed that the groundwater levels in the wellbores constructed under wet conditions responded significantly in retard of, and in narrower ranges than, those constructed under dry conditions. The wellbore-wall hydraulic conductivities at the former wellbores were calculated to be more than one to two orders of magnitude lower than those at the latter ones. Furthermore, remarkable nitrate removal attributable to the occurrence of a heterotrophic denitrification was observed in one of the former wellbores. In contrast, the groundwater levels and qualities in the latter wellbores appeared to be generally similar to those monitored in the conventional soil coring and augering-derived wellbores. Our results suggest that the wellbore-wall compression induced by rough excavation under wet and soft aquifer soil conditions leads to a substantial decrease in the wellbore-wall hydraulic conductivity, which in turn can lead to unreliable groundwater levels and qualities. This problem can occur in clayey Andisols whenever the aquifer soil is wet; however, the problem can be largely avoided by constructing the wellbore under dry and hard aquifer soil conditions.

Is One an Upper Limit for Natural Hydraulic Gradients?

Is One an Upper Limit for Natural Hydraulic Gradients?

Controls on groundwater flow in the Bengal Basin of India and Bangladesh: regional modeling analysis

Groundwater for domestic and irrigation purposes is produced primarily from shallow parts of the Bengal Basin aquifer system (India and Bangladesh), which contains high concentrations of dissolved arsenic (exceeding worldwide drinking water standards), though deeper groundwater is generally low in arsenic. An essential first step for determining sustainable management of the deep groundwater resource is identification of hydrogeologic controls on flow and quantification of basin-scale groundwater flow patterns. Results from groundwater modeling, in which the Bengal Basin aquifer system is represented as a single aquifer with higher horizontal than vertical hydraulic conductivity, indicate that this anisotropy is the primary hydrogeologic control on the natural flowpath lengths. Despite extremely low hydraulic gradients due to minimal topographic relief, anisotropy implies large-scale (tens to hundreds of kilometers) flow at depth. Other hydrogeologic factors, including lateral and vertical changes in hydraulic conductivity, have minor effects on overall flow patterns. However, because natural hydraulic gradients are low, the impact of pumping on groundwater flow is overwhelming; modeling indicates that pumping has substantially changed the shallow groundwater budget and flowpaths from predevelopment conditions.

Variable‐density groundwater flow and solute transport in fractured rock: Applicability of the Tang et al. [1981] analytical solution

The effect of fluid density variations on mixed convective (advective and density‐driven) transport in fractured rock is examined. Assuming a representative natural hydraulic gradient in a vertical fracture, breakthrough curves for variable‐density systems are shown to be significantly different from those with constant‐density conditions for even very small solute source concentrations (as low as approximately 2.3 g L−1 total dissolved solids, corresponding to 6.4% of seawater salinity). We compare analytical solutions with results of fully coupled numerical simulations of variable‐density groundwater flow and solute transport in fractured rock. Current analytical solutions for solute transport in fractured rock do not account for fluid density variations and therefore fail to predict variable‐density transport. Using a mixed convection ratio analysis, we determine the hydrogeologic conditions where variable‐density transport is important and hence when analytical solutions fail to predict variable‐density transport in fractured rock. We present a modified velocity term that includes a density‐driven flow component in a vertical fracture to account for fluid density variations. The modified velocity term is incorporated into the standard Tang et al. (1981) analytical solution. The original density‐invariant analytical solution is shown to be applicable near a solute source and at early time, while the velocity‐modified density‐variant solution gives very good results for long‐term and far‐field behavior. These results suggest that analytical solutions may still be useful for analyzing variable‐density transport in fractured rock. This study has implications for the analysis of dense plume transport in fractured rock where reliable long‐term predictions are important.

Is One an Upper Limit for Natural Hydraulic Gradients?

GroundwaterVolume 46, Issue 4 p. 518-520 Is One an Upper Limit for Natural Hydraulic Gradients? D.J. Hart, Corresponding Author D.J. Hart Wisconsin Geological and Natural History Survey, Madison, WI 53705; (608) 262-2307; fax: (608) 262-8086; djhart@wisc.eduSearch for more papers by this authorK.R. Bradbury, K.R. Bradbury Wisconsin Geological and Natural History Survey, Madison, WI 53705.Search for more papers by this authorM.B. Gotkowitz, M.B. Gotkowitz Wisconsin Geological and Natural History Survey, Madison, WI 53705.Search for more papers by this author D.J. Hart, Corresponding Author D.J. Hart Wisconsin Geological and Natural History Survey, Madison, WI 53705; (608) 262-2307; fax: (608) 262-8086; djhart@wisc.eduSearch for more papers by this authorK.R. Bradbury, K.R. Bradbury Wisconsin Geological and Natural History Survey, Madison, WI 53705.Search for more papers by this authorM.B. Gotkowitz, M.B. Gotkowitz Wisconsin Geological and Natural History Survey, Madison, WI 53705.Search for more papers by this author First published: 04 July 2008 https://doi.org/10.1111/j.1745-6584.2008.00433.xCitations: 7Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume46, Issue4July–August 2008Pages 518-520 RelatedInformation

Modeling the Sorption of Fluoride onto Alumina

Fluoride is a potentially toxic ion that occurs in aquifers both naturally and as a result of anthropogenic activity. Sometimes remediation of the aquifer is required. One potential aquifer decontamination strategy is an “interception-sorption trench”—one of a number of “reactive wall” technologies. This remediation strategy relies upon natural hydraulic gradients to transport the fluoride through the aquifer to the interception-sorption trench where it partitions onto a strong sorbent—alumina. In this paper, the focus is on the development and calibration of an equilibrium-based geochemical model that will be employed in the development of a quantitative reactive transport model, which in turn will be used for the design of an interception-sorption trench. The geochemical model described here takes into account a variety of ions likely to be present in a sandy aquifer, chemical activities, and the surface charge on the alumina. The model is calibrated over a wide pH range and for high initial fluoride concentrations using experimental results obtained from batch tests. It is found that pH dependent equilibrium constants are needed to capture the behavior of the experimentally observed fluoride sorption. The presence of sodium sulfate in solution is investigated, and it is found that sodium significantly interferes with the sorption of fluoride onto alumina under alkaline conditions. The geochemical model indicates that under acidic conditions, the alumina may release potentially large and unacceptable concentrations of aluminum into the aquifer. As a way of managing this potential problem, it is proposed that aluminum concentrations in the pore fluid may be mitigated by the inclusion of tree bark within the interception-sorption trench.

Percolation and transport in a sandy soil under a natural hydraulic gradient

Unsaturated flow and transport under a natural hydraulic gradient in a Mediterranean climate were investigated with a field tracer experiment combined with laboratory analyses and numerical modeling. Bromide was applied to the surface of a sandy soil during the dry season. During the subsequent rainy season, repeated sediment sampling tracked the movement of bromide through the profile. Analysis of data on moisture content, matric pressure, unsaturated hydraulic conductivity, bulk density, and soil texture and structure provides insights into parameterization and use of the advective‐dispersive modeling approach. Capturing the gross features of tracer and moisture movement with model simulations required an order‐of‐magnitude increase in laboratory‐measured hydraulic conductivity. Wetting curve characteristics better represented field results, calling into question the routine estimation of hydraulic characteristics based only on drying conditions. Measured increases in profile moisture exceeded cumulative precipitation in early winter, indicating that gains from dew drip can exceed losses from evapotranspiration during periods of heavy (“Tule”) fog. A single‐continuum advective‐dispersive modeling approach could not reproduce a peak of bromide that was retained near the soil surface for over 3 years. Modeling of this feature required slow exchange of solute at a transfer rate of 0.5–1 × 10−4 d−1 with an immobile volume approaching the residual moisture content.

3-D numerical evaluation of density effects on tracer tests

In this paper we present numerical simulations carried out to assess the importance of density-dependent flow on tracer plume development. The scenario considered in the study is characterized by a short-term tracer injection phase into a fully penetrating well and a natural hydraulic gradient. The scenario is thought to be typical for tracer tests conducted in the field. Using a reference case as a starting point, different model parameters were changed in order to determine their importance to density effects. The study is based on a three-dimensional model domain. Results were interpreted using concentration contours and a first moment analysis. Tracer injections of 0.036 kg per meter of saturated aquifer thickness do not cause significant density effects assuming hydraulic gradients of at least 0.1%. Higher tracer input masses, as used for geoelectrical investigations, may lead to buoyancy-induced flow in the early phase of a tracer test which in turn impacts further plume development. This also holds true for shallow aquifers. Results of simulations with different tracer injection rates and durations imply that the tracer input scenario has a negligible effect on density flow. Employing model cases with different realizations of a log conductivity random field, it could be shown that small variations of hydraulic conductivity in the vicinity of the tracer injection well have a major control on the local tracer distribution but do not mask effects of buoyancy-induced flow.

Modeling field-scale multiple tracer injection at a low-level waste disposal site in fractured rocks: Effect of multiscale heterogeneity and source term uncertainty on conceptual understanding of mass transfer processes

Multiple factors may affect the scale-up of laboratory multi-tracer injection into structured porous media to the field. Under transient flow conditions and with multiscale heterogeneities in the field, previous attempts to scale-up laboratory experiments have not answered definitely the questions about the governing mechanisms and the spatial extent of the influence of small-scale mass transfer processes such as matrix diffusion. The objective of this research is to investigate the effects of multiscale heterogeneity, mechanistic and site model conceptualization, and source term density effect on elucidating and interpreting tracer movement in the field. Tracer release and monitoring information previously obtained in a field campaign of multiple, conservative tracer injection under natural hydraulic gradients at a low-level waste disposal site in eastern Tennessee, United States, is used for the research. A suite of two-pore-domain, or fracture-matrix, groundwater flow and transport models are calibrated and used to conduct model parameter and prediction uncertainty analyses. These efforts are facilitated by a novel nested Latin-hypercube sampling technique. Our results verify, at field scale, a multiple-pore-domain, multiscale mechanistic conceptual model that was used previously to interpret only laboratory observations. The results also suggest that, integrated over the entire field site, mass flux rates attributable to small-scale mass transfer are comparable to that of field-scale solute transport. The uncertainty analyses show that fracture spacing is the most important model parameter and model prediction uncertainty is relatively higher at the interface between the preferred flow path and its parent bedrock. The comparisons of site conceptual models indicate that the effect of matrix diffusion may be confined to the immediate neighborhood of the preferential flow path. Finally, because the relatively large amount of tracer needed for field studies, it is likely that source term density effect may exaggerate or obscure the effect of matrix diffusion on the movement of tracers from the preferred flow path into the bedrock.